Introduction

With a 2024 farm-gate value of over $750 million, potato is an economically critical crop for growers in Washington State’s Columbia Basin. Growing potatoes is also a resource-intensive endeavor. According to Schaible and Aillery (2012) at the United States Department of Agriculture (USDA) Economic Research Service, potatoes and other irrigated crops use approximately 80% of the nation’s consumptive water use and over 90% in many western states. With the ongoing drought in the western US spurring efforts to conserve water, we wanted to explore the possibility that potato growers could use less water and still produce a profitable crop. Additionally, even though climate change may lengthen the growing season for potatoes in the Pacific Northwest (and yields may potentially increase due to increased CO2), water availability may be increasingly unpredictable (Haverkort and Verhagen 2008).

One of the major concerns for both potato growers and processors is the effect reduced irrigation might have on subsequent retention of postharvest quality, such as French fry color. Since its inception after World War Two, potato processing has grown enormously such that, currently, about two-thirds of potatoes go into the processing market; thus, a consumer-desirable product is critical for the industry (Keijbets 2008).

French fry color is critical to the food service industry for at least two reasons. One reason is that commercial frozen French fries must meet specific standards outlined by the United States Department of Agriculture (USDA) Agricultural Marketing Service regulations (1999). The second is the consumer acceptance (Phillips 2017). Color is a major factor in the consumer perception of food quality (Francis 1995). Indeed, Cruz et al. (2018) show how pretreated and fried potato chips have an essential effect on consumer acceptability.

In a test of the acceptance of potato chips based on color across various pre-fry treatments and, frying temperatures and duration treatments, a golden-brown color was found to be the most desirable (Pedreschi et al. 2012). The Maillard reaction strongly affects food color, texture, and flavor acceptance. The color development is mainly due to the formation of polymers called melanoidins (Starowicz and Zieliński 2019). The darkening of French fries is due to the Maillard reaction, where carbonyl sugars, in the presence of various amino acids, form melanoidin pigments and flavor compounds. The result is browning or blackening of the basal end of the fry, often coupled with a bitter flavor (Shallenberger et al. 1959; Kumar et al. 2004). There is also a chance that potentially toxic acrylamides may form as a by-product of the Maillard reaction, especially in the presence of asparagine, an amino acid common in potatoes (Mottram et al. 2002). Glucose and fructose produced the most color change in French fries, while sucrose produced less (Roe and Faulks 1991).

According to Rosen et al. (2018), one of the main factors affecting French fry color is the level of reducing sugars in the tubers; potatoes are ideally harvested for processing when reducing sugars (as well as sucrose, which can act as a precursor for reducing sugars) are at their minimum. Various environmental and agronomic factors influence the levels of reducing sugars in tubers. One is simply ambient growing temperature: sugars increase below eight degrees Celsius and above 25 °C (Rosen et al. 2018). Water stress also increases sugar levels: early stress increases basal-end sugars, while late-season stress increases apical-end sugars (Rosen et al. 2018). Short, intense stresses at any point in the growing season tend to increase sugar levels more than continuous stress (Rosen et al. 2018). Zommick et al. (2014b) describes several other factors, including soil temperature’s effect on specific gravity and sugar levels at harvest and accumulation during storage. Soil temperatures averaging seven degrees Celsius and 13 °C, above ambient temperature (16 °C) during bulking and maturation, respectively, resulted in higher tuber sugar levels after 24 days in storage, as well as higher basal respiration during the bulking stage (Zommick et al. 2014b). In a Pacific Northwest-based study, Eldredge et al. (1996) found that high temperatures and water stress at early tuber bulking resulted in high levels of reducing sugars in Russet Burbank basal ends. They discovered that relieving water stress either before, at maximum, or after stress with sprinkler irrigation resulted in an increased reducing sugar concentrations two or more weeks after stress relief. They found that increased reducing sugar concentrations were positively correlated with drier soil. Shock et al. (1994) previously obtained similar results indicating a delay in the accumulation of sugars after water stress. By omitting furrow irrigation at various times during the growing season, they increased reducing sugar concentrations in harvested tubers. Likewise, Iritani (1981) found that moisture deficit stress early in the growing season was linked to sugar ends on the apical ends of tubers.

In a trial in the Indian state of Punjab, researchers found that specific gravity and starch levels positively correlated with the amount of water applied (Kumar et al. 2007). In comparing partial-root zone drying and deficit irrigation, Elhani et al. (2019) found that partial-root zone drying (PRZD) resulted in a 20% water saving while tuber quality remained high. Sugar and protein content of tubers declined with water restriction under both regimes, but more so with PRZD than deficit irrigation. At the same time, levels of polyphenols and antioxidants increased under both regimes. In a two-year trial in central Wisconsin investigating the effects of irrigation on postharvest tuber quality at 125% of evapotranspiration (ET), 100%, 75%, and 50% ET, Crosby, and Wang (2021) found that overwatering increased nitrate leaching potential and reduced both water-use efficiency and irrigation efficiency. Since no treatment influenced fry color over an 8-month storage period and 75% and 50% ET levels did not affect tuber growth, the researchers found that deficit irrigation has the potential to conserve water. Carli et al. (2014) found that reducing water supply post-tuberization had only a limited effect on yield while significantly increasing water use efficiency. The reduced water supply also correlated with increased starch and dry matter content.

In addition to in-season factors affecting tuber-reducing sugars, postharvest factors such as storage duration and temperature also play a major role in tuber-reducing sugar concentration (Smith 1987). Potato tubers stored under other than optimum temperatures below ten degrees Celsius undergo a phenomenon known as low-temperature sweetening (LTS) (Burton 1969; Ap Rees et al. 1981; Coffin et al. 1987). Prolonged exposure to low temperatures (i.e., < 10 °C) results in the breakdown of starch products, such as sucrose, glucose, and fructose (Blenkinsop et al. 2003).

In addition to reducing sugars, amino acids also play a role in the darkening of fried high-carbohydrate foods (e.g., potatoes) (Roe and Faulks 1991). Roe et al. (1990) found that growing potatoes with high nitrogen fertility resulted in lower free sugars. However, there was more color per unit of sugar, suggesting that amino acids may play a synergistic role. Roe et al.’s statistical analysis found that about 90% of the color variation could be attributed to sugars, but 98% of color could be accounted for when amino acids were added to the regression analysis.

In light of the multifaceted dynamics governing potato fry color, this study aims to shed light on the intricate relationship between late-season irrigation level and potato French fry color. Our overarching objective is to investigate how reducing late-season irrigation practices may impact fry color, with a particular focus on the effects of varying cold storage temperatures after harvest. By addressing this critical research question, we aim to provide practical guidance to potato growers and processors in the Pacific Northwest, allowing them to make informed decisions that align with the imperatives of water conservation, crop profitability, and the stringent quality standards of the frozen potato-processing industry.

Materials and Methods

Experimental Site

In a study spanning from 2018 to 2020, the effects of reduced late-season irrigation on five distinct potato cultivars were evaluated at the Washington State University (WSU) Irrigated Agriculture and Extension Center near Othello, Wash. The study site’s soil is predominantly Shano silt loam. A soil texture analysis revealed a typical composition of 36% sand, 51% silt, and 13% clay. The soil’s bulk density was 1.45 g cm−3 at depths ranging from 20 cm to 50 cm. By using the USDA’s Soil SPAW software (Rawls 1998; Saxton and Rawls 2006) it was determined that the soil exhibited a saturated hydraulic conductivity of 27.23 mm d−1 and volumetric water content values of 0.48 m3 m−3, 0.27 m3 m−3, and 0.10 m3 m−3 at saturation, field capacity, and the permanent wilting point, respectively. On average, the upper 20 cm of the soil profile had 5% less sand and 6% more silt content compared to the lower 30 cm of soil.

Based on the Köppen classification, the region’s climate is identified as a BSk or Arid Steppe cold arid type (Larson and Lohrengel 2011). Weather records from WSU’s weather service note that the site receives an annual precipitation of about 192 mm. Additionally, the site has maintained an average temperature of 10 °C and a relative humidity of 68% over the past decade (Gonzalez et al. 2024).

Potato Cultivars

Five russet potato cultivars were chosen, notable for their relevance in the Pacific Northwest, encompassing factors like canopy, tuber attributes, and maturity levels. The selected cultivars included:

  1. 1)

    Alturas (Novy et al. 2003),

  2. 2)

    Clearwater Russet (Novy et al. 2010),

  3. 3)

    Ranger Russet (Pavek et al. 1992),

  4. 4)

    Russet Burbank (Bethke et al. 2014),

  5. 5)

    Umatilla Russet (Mosley et al. 2000) (Gonzalez et al. 2024).

Irrigation Levels

Five equally spaced irrigation levels (ILs) were devised based on modeled evapotranspiration (ET) to assess their impact on fry color. These ILs included:

  1. 1)

    120% ET: This surpassed the ET model’s recommendation by 20%, acting as a comparative benchmark.

  2. 2)

    100% ET: Matched the ET model’s recommendation.

  3. 3)

    80% ET: Imposed a 20% reduction from the model’s recommended irrigation.

  4. 4)

    60% ET: Implemented a 40% reduction.

  5. 5)

    40% ET: Adopted a significant 60% reduction.

Modeled ET was calculated using Washington State University’s weather service (weather.wsu.edu) and a weather station 200 m north of the study site. The process involved estimating the daily ET by combining the reference ET (ETr) with a potato-specific crop coefficient (Kc), as proposed by Allen et al. (1998). The ETr was derived from the ASCE Standardized Reference Evapotranspiration Equation (Allen et al. 2005), using alfalfa as a reference for the potato crop. The Kc values account for the potatoes’ unique growth attributes.

Given the climatic resemblances between Othello, Wash., and Kimberly, Idaho (Larson and Lohrengel 2011), this study utilized Kc values designated for russet potatoes cultivated in Kimberly, Idaho, to estimate ET (Wright 1981). The Kc values used were 0.3, 0.93, and 0.5 for the initial, mid-season, and end-season, respectively (https://www.usbr.gov/pn/agrimet/cropcurves/POTScc.html). These Kc stages typically range between 0 to 45 days after planting (DAP; sprout dev.), 65 to130 DAP (tuber initiation-bulking), and 145 to 155 (maturation) DAP, respectively. The choice to use Kimberly’s Kc values stemmed from the lack of russet potato Kc values for the Columbia Basin.

During the three-year investigation, the ILs were implemented between 100 and 105 DAP, at or near peak canopy development and early tuber bulking, corresponding with roughly 1500 day degrees. Around this period, it has been reported that the potato foliage has reached its peak weight, has a mature root system, and is experiencing a deceleration in the water uptake field (Oliveira et al. 2016). The ILs were maintained until the vine termination phase at 150 to 155 DAP.

The ILs were administered through a lateral irrigation system (Reinke Manufacturing Co. Inc., Deshler, NE). The desired ILs were achieved by combining different nozzle sizes, pressure regulators, and precise adjustments in the sprinkler spacing. To attain the 120% ET irrigation level (IL), a specific nozzle and pressure regulator combination was selected to produce an output 20% higher than the standard 100% ET IL. The 100% ET IL represented the irrigation system’s baseline output. To determine the irrigation output for the three deficit irrigation regimes 80%, 60%, and 40% ET, the irrigation output for each irrigation regime was reduced by 20%, 40%, and 60% from the 100% ET IL, respectively.

The actual discharge values from the sprinklers, responsible for replicating the desired irrigation regime, demonstrated varying degrees of success in achieving the target output. Specifically, for the 40%, 60%, 80%, and 120% ET IL, the achieved irrigation output exceeded the desired output by 2%, 2.6%, 2%, and 4.7%, respectively. The 100% ET IL closely mirrored the desired output, deviating by less than 1%.

Irrigation scheduling was determined by considering soil moisture levels and modeled evapotranspiration (ET), drawing data from WSU’s weather service (www.weather.wsu.edu) (Leib et al. 2002). From the onset of the season until the initiation of irrigation treatments, irrigation events were orchestrated to maintain the average soil water content (SWC) of the five cultivars within a range defined by the soil’s field capacity (FC = 0.27 m3 m−3; upper limit) and 65% readily available water (RAW = 0.21 m3 m−3; lower limit). The allowable depletion at this research site was 18 mm, which was calculated by taking the difference between SWC at FC and SWC at 65% RAW, and then multiplied by the 300-mm soil profile depth. This quantity was estimated to sustain a matured potato crop for roughly two days (Gonzalez et al. 2023). Irrigation events were triggered when the cumulative water depletion, derived from the estimated crop evapotranspiration and calculated over complete days, approached or reached 18 mm. Once the ILs were implemented, irrigation scheduling was managed to maintain the average SWC from plots subjected to 100% ET treatment within the soil’s FC and 65% RAW.

Soil water content was monitored two to three times weakly at two depths 20 cm and 50 cm below the surface throughout the season using a Hydroprobe ELITE 503 neutron probe (Instrotek Inc., Raleigh, SC). Before the neutron probe was used in the field, a soil-specific SWC calibration was conducted, following the instructions provided the manufacturer of the neutron probe. Inserting the neutron probe down to the desired depths was facilitated by installing aluminum access tubes in the ground. Each tube was positioned between two healthy plants shortly after emergence, leaving 30 cm protruding above ground to support the neutron probe. For each irrigation level, two access tubes were installed in two cultivars, Russet Burbank and another cultivar randomly selected each year. The decision to only monitor two cultivars stemmed from recognizing that monitoring 50 sites, encompassing five cultivars and ILs, would inevitably inflict damage to the plants due to the repeated traversal through the plots.

At the end of the season, SWC measurements were conducted at a depth of 30 cm below the ground using a ProCheck handheld soil moisture device equipped with a GS3 volumetric water content probe (Metergroup, Inc., Pullman, WA). These SWC measurements were conducted for each cultivar and IL and were replicated twice. Precipitation and irrigation were monitored using a combination of automated rain gauges (TE525L, Campbell Scientific Inc., Logan, UT) and irrigation system logs. The rain gauges were set up with the top of the funnel measuring 76 cm from the ground. The reported precipitation and irrigation measurements should not be considered “effective”, as these values did not reflect losses from deep percolation, evaporation, and runoff (Gonzalez et al. 2024).

Storage Duration and Temperature Regime

Tubers from each cultivar and irrigation level (IL) underwent a series of treatments involving storage durations of 15 days after harvest (DAH), 45 DAH, and 90 DAH. Additionally, these tubers were stored under three distinct temperature conditions: 4.4 °C, 6.7 °C, and 8.9 °C. Subsequently, they were fried in oil to assess the impact of reduced late-season irrigation on tuber fry color induced by low-temperature sweetening. It’s important to note that before entering long-term storage, all tubers initially underwent storage at a temperature of 8.9 °C, accompanied by a relative humidity of 95%. This initial phase served to facilitate wound healing.

Storing potatoes at temperatures below nine degrees Celsius can prolong their marketability, reduce disease risk, and delay sprouting. Still, this practice often leads to an undesirable increase in reducing sugars, leading to darker fries in many cultivars. This phenomenon is referred to as low-temperature sweetening (Workman et al. 1979; Matsuura-Endo et al. 2004; Zommick et al. 2014a). With this in mind, the three equally spaced storage temperatures, namely 4.4 °C, 6.7 °C, and 8.9 °C, were selected to assess the impact of lowering storage temperature on fry color.

In 2018, fry color was assessed after 90 DAH, following two temperature regimes: 4.4 °C and 8.9 °C. In 2019, fry color was assessed in tubers held in storage temperatures of 8.9 °C and 4.4 °C and held for 15 DAH and 90 DAH. Tubers stored at 8.9 °C were fried at 15 DAH, and those stored at 4.4 °C were fried at 90 DAH. In the subsequent year, 2020, tubers underwent varying storage conditions. Fry color was evaluated after tubers were stored for periods of 45 and 90 DAH and held at temperatures of 4.4 °C, 6.7 °C, and 8.9 °C before being fried in oil.

Experiment Design and Statistical Analysis

The experiment design followed a split-plot design with blocks. The study encompassed two fields, acting as blocks, which were further subdivided into five main plots each, resulting in ten main plots spanning both fields. Within each field, five distinct ILs were randomly positioned within the main plots of each field. Subsequently, five different cultivars (Cv) were randomly allocated within each main plot and replicated three times, following a randomized complete block design.

Ten tubers were collected from each plot as subsamples to evaluate the impact of the ILs, storage duration (SD) and temperature (ST) affect fry color.

Each designated plot was comprised of seven rows, spanning a length of 4.3 m and a breadth of 6.2 m. These rows hosted 14 plants, flanked by a 0.6-m walkway at their end. The arrangement featured outer rows primarily for navigation, followed by two protective or boundary rows, culminating in a central observation row. Notably, the Chieftain cultivar, recognizable by its red skin, was planted at the boundaries of the plots. Its role was to ensure consistent competition for bordering plants and to function as an easily identifiable harvest indicator.

Subsequently, data interpretation was conducted using JMP Pro software (version 14.5, SAS Institute Inc.). A multifactorial ANOVA was conducted leveraging the Restricted Maximum Likelihood Method, complemented by the Kenward-Roger correction. This methodological choice enabled us to assess primary effects and their respective interactions, setting a significant threshold at 0.05. This analytical framework treated the primary factors Cv, IL, SD, and ST as fixed-effect variables. Secondary factors, year, and field were also treated as fixed-effect variables. The Tukey–Kramer pairwise comparisons method was used to determine the difference in factor levels. The selection of the Tukey–Kramer method was motivated by its inherent conservative characteristic, effectively minimizing Type I error probabilities (Lane 2010).

Trends between factors were established by fitting the data using polynomial regression. Pearson’s correlation method also measured and quantified the degree and direction of linear relationships and associations. Graphical depictions and tabular presentations of findings were crafted using Sigmaplot (version 14.5, Systat Software) and Excel (Microsoft Corporation) (Gonzalez et al. 2024).

Cultural Management

The study location underwent fumigation every fall to address nematodes and Verticillium wilt, setting the stage for springtime planting. This was achieved by treating the site with 1, 3-Dichloropropene at a rate of 187 L ha−1 and Metam sodium at a rate of 280 L ha−1. These substances were administered with precision using deep tillage shanks, delving to depths of 45 cm and 30 cm, respectively. Seed tubers weighing between 55 g and 70 g were prepared by placing them in an 8.9 °C environment with 95% humidity for two to three weeks before planting. Come mid-April, these seeds were methodically placed in rows, maintaining a distance of 80 cm between each row and a 25 cm gap between individual seed pieces. A compact two-row planter assisted this process. The seeds, treated with a protective fungicide, were placed 20 cm deep, facilitated by a hydraulic planting apparatus and a drag-off tool. Around a month after planting, hilling was done, and unwanted vegetation was curtailed with the help of a Lilliston cultivator. Harvesting of the tubers took place in September, after which they were stored at 8.9 °C for subsequent examination.

The site’s nutrient and pest management aligned with respected agricultural benchmarks. Every year, the plots were enriched with 400 kg ha−1 of nitrogen (N), 250 kg ha−1 of phosphorus pentoxide (P2O5), and 450 kg ha−1 of potassium oxide (K2O) (Lang et al. 1999). Before the planting began, granular fertilizers were spread out and mixed into the top between 15 cm and 20 cm of soil, except for N. Initially, 100 kg ha−1 of N was blended with the soil. The remaining 300 kg ha−1 of N was dispensed in phases, with 20 kg ha−1 to 40 kg ha−1 intervals through the irrigation system, spanning 40 to 105 DAP, during the vegetative and tuber initiation stages (Pavek et al. 2018; Gonzalez et al. 2024).

Calculating Day Degrees

Following the same procedures described by Gonzalez et al. (2023) and leveraging meteorological data from WSU’s weather service coupled with readings from a weather station sited 200 m north of the research domain, we deduced the thermal accumulation, often referred to as growing degree days in the Pacific Northwest’s potato agroecosystem. This approach resonates with the biophysical methodology delineated by Campbell and Norman (1998). The quantification of day degrees pivots on the diurnal temperature extremes, encapsulated by the subsequent biophysical equation:

$$\mathrm{Day\ degrees}=\sum_{i=1}^{n}\left(\frac{{T}_{xi}+{T}_{ni}}{2}- {T}_{b}\right)\Delta t$$

where:

Tmax:

signifies the maximum temperature,

Tmin:

illustrates the minimum temperature,

Tb:

represents the base temperature,

Δt:

indicates the time increment of one day.

If the mean diurnal temperature (averaging the maximum and minimum) descends beneath the basal threshold, the inferred day degrees default to zero. Our elected basal threshold of 7.2 °C is congruent with conventions embraced by biophysicists specializing in potato thermal response studies (Worthington and Hutchinson 2005).

Furthermore, in congruence with scientific understandings, we instituted an upper-temperature ceiling of 35 °C. This choice is anchored in research that illustrates a marked decrease in photosynthetic activity within potato crops once ambient temperatures exceed this limit (Dwelle et al. 1981). When the observed maximum temperature exceeds 35 °C, the equation above integrates this upper-temperature limit in place of the recorded top temperature (Gonzalez et al. 2024).

Measuring Fry Color

One precise method for measuring tuber fry color, specifically color lightness, was accomplished by measuring light reflectance (LR) on the basal and apical ends using a photovolt reflectance meter. Fry LR shares a significant negative relationship with fry color lightness; the higher the LR value, the lighter the fry (Shock et al. 1994). The USDA has set standards for fry color, separating fry color into six categories: 00, 0, 1, 2, 3, and 4, where zero is lightest, and four is darkest (USDA-Agricultural Marketing Service 1999). The corresponding LR values for USDA 0, 1, 2, 3, and 4 fries are > 31, 25 to 30, 20 to 24, 15 to 19.5, and < 14, respectively (Knowles et al. 2019). Photovolt LR is typically darker on the basal end of fries than the apical end (Kincaid et al. 1993). Tuber fries with LR value ≥ 19.5, equivalent to ≥ USDA 3, are considered unacceptable by the frozen potato-processing industry. Fries with LR values < 19.5 are deemed unacceptable by the frozen potato-processing industry. Furthermore, the frozen potato-processing industry rejects fries with an absolute difference of ≥ 9 between the basal and apical ends of the fry (Knowles et al. 2019).

Fry quality was assessed by preparing six fry planks cut along the apical to the basal end to extract fry planks measuring 9.5 mm thick and 29 mm wide from six tubers. Fry planks were rinsed to remove excess starch and fried in 191 °C vegetable oil for 3.5 min. Fries were then allowed to drain, and excess oil was gently wiped using paper towels. The LR was measured approximately 12 mm from the basal and apical end of each plank using a photovolt reflectometer (Photovolt Model 577, Photovolt, St. Louis Park, Minn), equipped with a tristimulus green filter, which has a transmittance ranging between 450 nm and 620 nm and standardized with a white porcelain calibration plate (26–570-65) with a LR of 73.5% (Fig. 1). The absolute difference between the basal and apical end LR was also calculated. Following the recommendations of Shock et al. (1994), basal end LR measurements were carried out within five minutes of frying.

Fig. 1
figure 1

The United States standard for frozen French-fried potatoes. Note that this fry color index is not calibrated; it is used for illustrative purposes only (Munsell Co 1997)

Full size image

Results

Main Effects and Interactions

The statistical model included the following factors: year, field, cultivar (Cv), irrigation level (IL), storage durations (SD), and storage temperatures (ST), as well as their interactions. Although statistically significant year-by-treatment interactions were found among Cv, IL, and SD, the interaction did not meaningfully alter the core message conveyed by basal end light reflectance data. The same was true for the basal–apical end difference data year by treatment interactions.

As a result, year was treated as a blocking factor for tubers fried 90 days after harvest (DAH) and stored at 4.4 °C and 8.9 °C. Noteworthy interactions were identified among Cv, IL, and ST, but no such interactions were found between Cv, IL, and SD. Due to these significant interactions, the effects of IL were analyzed separately for the two levels of SD and ST to understand their impact better. Given the interaction between Cv and IL and substantial variability among cultivars, the effects of IL will be discussed for each Cv, focusing on tubers fried at 45 and 90 DAH and stored at temperatures of 4.4 °C, 6.7 °C, and 8.9 °C (Table 1).

Table 1 Main effects and interactions’ p-values for the basal end light reflectance and basal–apical end light reflectance difference as influenced by cultivar, irrigation level, and storage temperature. Values shown for 2018 and 2020 combined, and 2019 alone
Full size table

Irrigation and Precipitation

The water inputs from irrigation and precipitation assessed in each irrigation treatment differed each year. In 2018, the cumulative water depth received by the crop from the 120%, 100%, 80%, 60%, and 40% ET irrigation regimes was 509 mm, 552 mm, 595 mm, 638 mm, and 681mm, respectively. In 2019, the cumulative water depth received by the crop from the 120%, 100%, 80%, 60%, and 40% ET irrigation levels was 505 mm, 565 mm, 629 mm, 691 mm, and 793 mm, respectively. Finally, in 2020, the cumulative water depth received by the crop from the 120%, 100%, 80%, 60, and 40% ET irrigation treatments was 557 mm, 625 mm, 693 mm, 761 mm, and 829 mm, respectively. During the period the irrigation treatments were active, water received by the crop from precipitation accounted for 1% in 2018, 14% in 2019, and 1% in 2020 (Gonzalez et al. 2024).

Soil Water Content

During the three years of the study, soil water content (SWC) was monitored throughout the season in two cultivars on the 100% ET irrigation level (IL) to ensure the proper amount of water was added to each IL. Additionally, at the end of the season, SWC was measured on the five ILs and two cultivars.

The SWC monitored throughout the 2018, 2019, and 2020 seasons on Clearwater Russet, Umatilla Russet, and Ranger Russet, respectively, consistently remained within the range of the soil’s field capacity (FC; 0.27 m3 m-3) and 65% of the readily available water (0.18 m3 m-3). In each of these years, Russet Burbank, which served as one of the reference cultivars for SWC monitoring, exceeded the soil’s FC at different time points: 95 days after planting (DAP) in 2018, 115 DAP in 2019, and 120 DAP in 2020.

The SWC measurements collected at the end of each year are detailed as follows. In 2018, Clearwater Russet’s SWC values were 0.24 m3 m−3, 0.23 m3 m-3, 0.22 m3 m−3, 0.19 m3 m-3, and 0.17 m3 m−3 corresponding to ILs 120%, 100%, 80%, 60%, and 40% ET, respectively. In the case of Russet Burbank, the SWC values were 0.37, 0.35, 0.34, 0.33, and 0.33 m3 m−3 for the same ILs.

The following year, 2019, Umatilla Russet registered SWC measurements of 0.28 m3 m−3 0.26 m3 m−3, 0.25 m3 m−3, 0.24 m3 m−3, 0.23 m3 m−3 for ILs 120%, 100%, 80%, 60%, and 40% ET, respectively. The SWC observed for Russet Burbank for the same ILs were 0.36 m3 m−3, 0.34 m3 m−3, 0.34 m3 m−3, 0.33 m3 m−3, and 0.32 m3 m−3.

In 2020, Ranger Russet exhibited SWC values of 0.24 m3 m−3, 0.23 m3 m−3, 0.20 m3 m−3, 0.18 m3 m−3, and 0.16 m3 m−3 corresponding to ILs 120%, 100%, 80%, 60%, and 40% ET, respectively. Conversely, Russet Burbank recorded SWC measurements of 0.33 m3 m−3, 0.32 m3 m−3, 0.31 m3 m−3, 0.31 m3 m−3, and 0.30 m3 m−3 for the same ILs (Gonzalez et al. 2024).

2019 Fry Color Response to Reduced Late-Season Irrigation

The mean basal light reflectance (LR) among the five irrigation levels (ILs) were significantly different in cultivars Alturas (p = 0.0036), Clearwater Russet (p < 0.0001), and Russet Burbank (p = 0.0034), for tubers that were fried 15 days after harvest (DAH) and significantly different for the five cultivars for tubers that were fried 90 DAH from 4.4 °C storage (Alturas p < 0.0001; Clearwater Russet p < 0.0001; Russet Burbank p = 0.0054; Ranger Russet p = 0.0017; Umatilla Russet p < 0.0001). Although there was a significant difference among the basal end LR across the five ILs on tubers fried after 15 and 90 DAH from 4.4 °C storage, the difference was insignificant in terms of affecting the fry color one way or another.

When comparing the mean basal end LR of tubers fried 15 DAH (mean = 12.5) to those fried 90 DAH (mean = 46.1), a significant difference emerged (t(498) = 95, p < 0.0001). Tubers fried 90 DAH produced fries with a basal end LR < 19.5; the frozen potato-processing industry rejects these, as they are considered too dark (Knowles et al. 2019). Conversely, tubers fried 15 DAH yielded fries with a highly desirable basal end LR of 46.1 (Table 2).

Table 2 Analysis of variance and post-hoc comparison of the mean basal end light reflectance of tubers fried 15 and 90 days after storage from 4.4 °C storage. Tubers were grown during 2019
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Regarding the mean basal–apical end LR difference, no significant differences were found among the five ILs for tubers fried 15 DAH from 4.4 °C storage. Importantly, this difference remained within the acceptable range (< 9) set by the frozen potato-processing industry (Knowles et al. 2019). However, the mean basal–apical end LR difference on tubers fried 90 DAH from 4.4 °C storage from cultivars Alturas (p < 0.0001) and Clearwater Russet (p = 0.0002) was significantly different among the five ILs. The basal–apical end LR difference increased as with each IL, with the highest value of 12.2 when reducing late-season irrigation by 60%. In the case of Clearwater Russet, although the basal–apical end LR difference significantly differed among the ILs, it exceeded the industry’s acceptable threshold (Table 3).

Table 3 Analysis of variance and post-hoc comparison of the mean basal–apical end light reflectance difference of tubers fried 15 and 90 days after storage from 4.4 °C storage. Tubers were grown in 2019
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Alturas Fry Color Response to Reduced Late-Season Irrigation

The basal end LR means of Alturas tubers fried 45 days after harvest (DAH) from storage at 8.9 °C (p < 0.0001), 6.7 °C (p < 0.0001) and 4.4 °C storage (p < 0.0001), as well as tubers fried 90 DAH from storage temperatures 8.9 °C (p < 0.0001), 6.7 °C (p < 0.0001) and 4.4 °C storage (p < 0.0001), exhibited significant differences among the five irrigation levels (ILs) utilized late in the season. It was clear that reducing late-season irrigation improves basal end LR, a metric of fry lightness, in tubers fried 45 DAH, regardless of the storage temperature in which tubers were held. A Pearson correlation analysis revealed a robust negative linear relationship between the ILs and the basal LR values on tubers fried at 8.9 °C (r(30) = -0.89, p < 0.0001), 6.7 °C (r(30) = -0.91, p < 0.0001) and 4.4 °C storage (r(30) = -0.88, p < 0.0001), indicating a robust inverse association between the two factors. The linear trend suggests that across the three storage temperatures, each level of reduced irrigation resulted in an 18% improvement in fry color (Fig. 2 and 3).

Fig. 2
figure 2

Analysis of variance and post-hoc comparison analysis of the mean basal end light reflectance (LR) of Alturas tubers grown in 2018 and 2020 held in 4.4 °C, 6.7 °C, and 8.9 °C storage fried 45 and 90 days after harvest as influenced by five irrigation levels. The red line is the frozen-processing industry fry rejection threshold (< 19.5 basal end LR). The error bars are standard error. For each irrigation level, LR with different letters is significantly different

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Fig. 3
figure 3

An illustration showing fried Alturas potato planks sourced from tubers harvested in 2020, showcasing their responses to various late-season irrigation levels after being stored at three distinct temperatures for 90 days after harvest

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This trend persists in Alturas tubers fried 90 DAH from the three storage temperatures, with a strong inverse linear relationship between the ILs and the basal end LR of said tubers fried from 6.7 °C (r(30) = -0.89, p < 0.0001) and 4.4 °C storage (r(80) = -0.81, p < 0.0001). Notably, the improving trend on basal LR from reducing late-season irrigation is negated when irrigating with 60% and 40% ET for tubers fried from 6.7 °C and 4.4 °C storage (Fig. 2).

However, for Alturas tubers fried from 8.9 °C storage, an analysis employing non-linear regression unveiled a distinct negative curvilinear relationship between the ILs and the basal end LR. The empirical model elucidated a well-fitted curve marked by a declining trend in basal end LR as the ILs were reduced below 60% ET (p < 0.0001). This relationship signifies a diminishing benefit in basal end LR as the ILs are further reduced. Furthermore, Alturas tubers fried from the three storage temperatures, at 45 and 90 DAH, demonstrated significantly improved basal end LR when late-season irrigation was reduced by at least 40 and 20%, respectively (Fig. 2).

Alturas tubers subjected to ILs ranging from 100% to 120% ET and subsequently fried at 90 DAH from storage at 6.7 °C and 4.4 °C exhibited basal end LR values that fell below the minimum threshold of 19.5 basal end LR, as dictated by the frozen-processing industry (Knowles et al. 2019). Increased late-season irrigation beyond the recommended ILs, specifically at 120% ET, exacerbated the negative impact on basal end LR in tubers fried at 45 and 90 DAH from 6.7 °C and 4.4 °C storage (Fig. 2 and 3).

The basal–apical end LR difference in Alturas remained consistently below nine across all five ILs, two storage durations, and three storage temperatures. According to the standards set by the frozen potato-processing industry, this level is deemed acceptable (Knowles et al. 2019) (Table 4).

Table 4 Analysis of variance and post-hoc comparison of Alturas fries’ basal–apical end light reflectance difference of tubers grown in 2018 and 2020 under varying storage durations and temperatures
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Clearwater Russet Fry Color Response to Reduced Late-Season Irrigation

The basal end LR means of Clearwater Russet tubers fried 45 days after harvest (DAH) out of 8.9 °C (p < 0.0001), 6.7 °C (p < 0.0001) and 4.4 °C storage (p < 0.0001), as well as tubers fried 90 DAH out of 8.9 °C (p < 0.0001), 6.7 °C (p < 0.0001) and 4.4 °C storage (p < 0.0001) were significantly different among the five irrigation levels (ILs) used late in the season. Tubers of Clearwater Russet fried at 45 and 90 DAH from 8.9 °C, 6.7 °C, and 4.4 °C storage produced significantly improved basal end LR following at least a 20% reduction in late-season irrigation (Fig. 4 and 5).

Fig. 4
figure 4

Analysis of variance and post-hoc comparison of the mean basal end light reflectance (LR) of Clearwater Russet tubers grown in 2018 and 2020 held in 4.4 °C, 6.7 °C, and 8.9 °C storage fried 45 and 90 days after harvest as influenced by five irrigation levels. The red line is the frozen-processing industry fry rejection threshold (< 19.5 basal end LR). The error bars are standard error. For each irrigation level, LR with different letters is significantly different

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Fig. 5
figure 5

An illustration showing fried Clearwater Russet potato planks sourced from tubers harvested in 2020, showcasing their responses to various late-season irrigation levels after being stored at three distinct temperatures for 90 days after harvest

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Tubers fried 45 DAH from the three storage temperatures produced significant curvilinear trends, with the basal end LR of tubers fried from 4.4 °C storage having the greatest impact by the five ILs. On the three storage temperatures, lower ILs did not continue to improve the tuber’s basal end LR, as is evident by the leveling off near the 60% ET IL. In the case of tubers fried at 90 DAH, a curvilinear trend was observed in tubers fried from 4.4 °C and 6.7 °C. The basal end LR of tubers fried from these two storage temperatures did not continue to benefit from the lowest two ILs. In the case of tubers fried from 8.9 °C storage, an analysis of Pearson’s correlation revealed a robust negative linear relationship between the ILs and the tuber fry’s basal LR values (r(80) = -0.77, p < 0.0001). This indicates that each 20% reduction in IL improved fry color by nearly 7% (Fig. 4).

Tubers fried from 45 and 90 DAH, from the three storage temperatures, irrigated with 120% ET, produced the least desirable fry color. Specially, tubers fried from 4.4 °C storage grown fry colors that are unacceptable (< 19.5 basal end LR), according to the frozen potato-processing industry. Additionally, tubers of Clearwater Russet fried at 90 DAH were 12% darker than tubers fried at 45 DAH (Fig. 4 and 5).

The basal–apical end LR difference in Clearwater Russet tubers, subjected to varying ILs ranging between 40% and 80% ET and fried at 45 DAH from a storage temperature of 8.9 °C, met acceptable standards. However, tubers irrigated with 100% and 120% ET exhibited a basal–apical end LR difference exceeding nine, which falls outside the acceptable range set by the frozen potato-processing industry (Knowles et al. 2019). In the case of tubers fried at 45 DAH from 6.7 °C storage, an acceptable basal–apical end LR difference was achieved by reducing late-season irrigation by 60% (Table 5).

Table 5 Analysis of variance and post-hoc comparison of Clearwater Russet fries’ basal–apical end light reflectance difference of tubers grown in 2018 and 2020 under varying storage durations and temperatures
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Tubers fried at 45 DAH from 4.4 °C storage achieved an acceptable basal–apical end LR difference when subjected to 80% ET IL late in the season. However, when irrigated with 40%, 100%, and 120% ET, these tubers yielded an average basal–apical end LR difference greater than nine, which is considered unacceptable (Table 5).

Frying the tubers at 90, DAH exacerbated the basal–apical end LR difference in Clearwater Russet tubers across all three storage temperatures. The basal–apical end LR difference in Clearwater Russet tubers fried from 8.9 °C storage met acceptable criteria when subjected to irrigation regimes ranging between 40% and 60% of ET late in the season. However, tubers irrigated with 80%, 100%, and 120% ET produced basal–apical end LR differences deemed unacceptable (Table 5).

For Clearwater Russet tubers fried from 4.4 °C and 6.7 °C storage, the basal–apical end LR difference exceeded nine in all the ILs (Table 5).

Ranger Russet Fry Color Response to Reduced Late-Season Irrigation

The mean basal end LR in Ranger Russet tubers fried 45 days after harvest (DAH) did not show a significant difference when stored at 8.9 °C (p = 0.2802) or 4.4 °C (p = 0.0879) among the five different irrigation levels (ILs) applied late in the season. However, the mean basal end LR of tubers fried after 6.7 °C storage (p < 0.0001) exhibited a significant difference among the same five ILs applied late in the season. When tubers were fried at 90 DAH, the mean basal end LR at 8.9 °C (p = 0.4703), 6.7 °C (p = 0.0901), and 4.4 °C (p = 0.1343) storage did not show a significant difference among the five ILs applied late in the season. Notably, the basal end LR of Ranger Russet tubers fried 45 DAH after storage at 6.7 °C met the acceptable standard of ≥ 19.5, established by the frozen-processing industry (Knowles et al. 2019). The Ranger Russet fried tubers’ basal end LR did not improve after reduced late-season irrigation. Additionally, it was noted that tubers fried after 45 and 90 DAH from 6.7 °C and 4.4 °C storage were 37% and 28% darker, respectively, than those fried after 8.9 °C storage (Fig. 6 and 7).

Fig. 6
figure 6

Analysis of variance and post-hoc comparison analysis of the mean basal end light reflectance (LR) of Ranger Russet tubers grown in 2018 and 2020 held in 4.4 °C, 6.7 °C, and 8.9 °C storage fried 45 and 90 days after harvest as influenced by five irrigation levels. The red line is the frozen-processing industry fry rejection threshold (< 19.5 basal end LR). The error bars are standard error. For each irrigation level, LR with different letters is significantly different

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Fig. 7
figure 7

An illustration showing fried Ranger Russet potato planks sourced from tubers harvested in 2020, showcasing their responses to various late-season irrigation levels after being stored at three distinct temperatures for 90 days after harvest

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The basal–apical end LR difference of Ranger Russet tubers fried at 45 and 90 DAH after storage at 4.4 °C, 6.7 °C, and 8.9 °C was acceptable, except for tubers fried at 90 DAH from 6.7 °C storage, which were irrigated with 40% and 60% ET late in the season. Additionally, tubers fried 90 DAH out of 8.9 °C storage produced unacceptable basal–apical end LR difference when irrigated with ILs ranging between 80% and 120% ET (Table 6).

Table 6 Analysis of variance and post-hoc comparison of Ranger Russet fries’ basal–apical end light reflectance difference of tubers grown in 2018 and 2020 under varying storage durations and temperatures
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The basal–apical end LR difference of Ranger Russet tubers, when fried at both 45 and 90 DAH following storage temperatures of 4.4 °C, 6.7 °C, and 8.9 °C, generally met acceptable standards. However, an exception was noted for tubers fried at 90 DAH from 6.7 °C storage, particularly those that had been subjected to ILs of 40% and 60% ET late in the season (Table 6).

Furthermore, it should be noted that tubers fried at 90 DAH from 8.9 °C storage exhibited unacceptable basal–apical end LR differences when they had been irrigated with ILs ranging between 80% and 120% ET (Table 6).

Russet Burbank Fry Color Response to Reduced Late-Season Irrigation

The mean basal end LR of Russet Burbank tubers fried 45 DAH from 8.9 °C (p < 0.0001), 6.7 °C (p < 0.0001), and 4.4 °C storage (p < 0.0001), as well as tubers fried 90 DAH from 8.9 °C (p < 0.0001), and 6.7 °C storage (p < 0.0001) were significantly different among the five reduced late-season irrigation levels (ILs). The basal end LR of tubers fried 90 DAH from 4.4 °C (p = 0.1456) storage was not significantly different among the five late-season ILs. The basal end LR of Russet Burbank tubers fried 45 DAH from 8.9 °C, 6.7 °C, and 4.4 °C was generally improved following an irrigation reduction late in the season. However, the basal end LR of tubers fried 90 DAH from 6.7 °C and 4.4 °C storage was not improved sufficiently to meet the minimum basal end LR (> 19.5), required by the frozen potato-processing industry (Knowles et al. 2019). The same observation was made on tubers fried 45 DAH from 4.4 °C storage (Fig. 8 and 9).

Fig. 8
figure 8

Analysis of variance and post-hoc comparison of the mean basal end light reflectance (LR) of Russet Burbank tubers grown in 2018 and 2020 held in 4.4 °C, 6.7 °C, and 8.9 °C storage fried 45 and 90 days after harvest as influenced by five irrigation levels. The red line is the frozen-processing industry fry rejection threshold (< 19.5 basal end LR). The error bars are standard error. For each irrigation level, LR with different letters is significantly different

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Fig. 9
figure 9

An illustration showing fried Russet Burbank potato planks sourced from tubers harvested in 2020, showcasing their responses to various late-season irrigation levels after being stored at three distinct temperatures for 90 days after harvest

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Tubers of Russet Burbank fried 45 DAH from 6.7 °C storage and 90 DAH from 8.9 °C storage produced a significantly improved basal end LR following at least a 40% reduction in late-season irrigation. A regression analysis of said tubers fried 45 DAH from 6.7 °C storage, and tubers fried 90 DAH from 8.9 °C storage revealed a plateauing effect near the 60% and 80% ET, indicating that late-season reduced irrigation has a diminishing effect on improving fry color. In the case of tubers fried 45 DAH from 8.9 °C storage, a significant negative linear relationship was found between the fry’s basal end LR and ILs (r(30) = -0.84, p < 0.0001), indicating that each 20% reduction in IL led to a 12% improvement on fry color (Fig. 8 and 9).

The basal–apical end LR differences of Russet Burbank fries, fried at 45 DAH from storage temperatures of 4.4 °C, 6.7 °C, and 8.9 °C, were generally acceptable across the five ILs as per the standards set by the frozen potato-processing industry (Knowles et al. 2019). The only exception to this was observed in the case of tubers irrigated with 120% ET and then fried after being stored at 6.7 °C (Table 7).

Table 7 Analysis of variance and comparison of Russet Burbank fries’ basal–apical end light reflectance difference of tubers grown in 2018 and 2020 under varying storage durations and temperatures
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Similarly, tubers fried 90 DAH from storage temperatures of 6.7 °C and 8.9 °C were acceptable across all five ILs, except when irrigated with 120% ET and subsequently fried after storage at 6.7 °C. Russet Burbank tubers irrigated with ILs ranging between 60% and 100% ET and fried 90 DAH from 8.9 °C storage consistently produced basal–apical end LR differences within acceptable limits (Table 7).

Umatilla Russet Fry Color Response to Reduced Late-Season Irrigation

The basal end LR means of Umatilla Russet tubers fried 45 DAH from 8.9 °C (p < 0.0001), 6.7 °C (p < 0.0001) and 4.4 °C storage (p < 0.0001), as well as tubers fried 90 DAH from 8.9 °C (p < 0.0001), 6.7 °C (p = 0.0062), and 4.4 °C storage (p < 0.0001) were significantly different among the five ILs. The general trend across all treatments was that reducing late-season irrigation on Umatilla Russet improves fry color, illustrated by the increase in basal end LR (Fig. 10 and 11).

Fig. 10
figure 10

Analysis of variance and post-hoc comparison of the mean basal end light reflectance (LR) of Umatilla Russet tubers grown in 2018 and 2020 held in 4.4 °C, 6.7 °C, and 8.9 °C storage fried 45 and 90 days after harvest as influenced by five irrigation levels. The red line is the frozen-processing industry fry rejection threshold (< 19.5 Basal end LR). The error bars are standard error. For each irrigation level, LR with different letters is significantly different

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Fig. 11
figure 11

An illustration showing fried Umatilla Russet potato planks sourced from tubers harvested in 2020, showcasing their responses to various late-season irrigation levels after being stored at three distinct temperatures for 90 days after harvest

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The basal end LR of Umatilla Russet tubers fried 45 and 90 DAH from 8.9 °C was improved following a reduction in late-season irrigation. A Person’s correlation analysis of the basal end LR of tubers fried 45 and 90 DAH from 8.9 °C storage revealed a significant negative relationship between the basal end LR and the ILs, indicating that each reduction in IL increased basal end LR by 9% and 7%, respectively. The basal end LR of tubers fried 45 and 90 DAH out of 4.4 °C and 6.7 °C was improved following a reduction in late-season irrigation of up to 40% from 100% ET. Reducing late-season irrigation on tubers fried at 90 DAH from 4.4 °C and 6.7 °C storage by greater than 40% decreased basal end LR, indicating a darker fry, which is less desirable. Importantly, irrigating Umatilla Russet with 120% ET late in the season was detrimental to the fry color on tubers fried 45 and 90 DAH from 4.4 °C storage, as it led to unacceptable dark fries, with a basal end LR values < 19.5 (Knowles et al. 2019) (Fig. 10 and 11).

However, in the case of tubers fried 45 and 90 DAH from 6.7 °C and 4.4 °C storage, an analysis employing non-linear regression revealed a distinct negative curvilinear relationship between the ILs and the basal end LR. The empirical model illustrated a well-fitted curve marked by a declining trend in basal end LR as the ILs were reduced below 80% ET for tubers fried 45 DAH from 8.9 °C storage and 60% ET for tubers fried 90 DAH from 4.4 °C and 6.7 °C storage (p < 0.0001). A declining trend for tubers fried 45 DAH from 4.4 °C storage was observed when ILs were reduced below 60% ET (Fig. 10 and 11).

When fried 45 DAH from storage temperatures of 8.9 °C and 6.7 °C, the basal–apical end LR of Umatilla tubers was deemed unacceptable when subjected to ILs 40% and 120% ET late in the season. However, tubers irrigated with regimes ranging from 60% to 100% ET exhibited acceptable basal–apical end LR differences, in line with the requirements of the frozen potato-processing industry (Knowles et al. 2019) (Table 8).

Table 8 Analysis of variance and post-hoc comparison of Umatilla Russet fries’ basal–apical end light reflectance difference of tubers grown in 2018 and 2020 under varying storage durations and temperatures
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Tubers fried from storage at 4.4 °C showed an acceptable basal–apical end LR difference when subjected to a late-season irrigation regime of 60% and 80% ET. In general, the basal–apical end LR difference of tubers fried 90 DAH from storage at 4.4 °C, 6.7 °C, and 8.9 °C met acceptable standards in all cases, except for tubers that were irrigated with 40% and 60% ET and fried after storage at 4.4 °C (Table 8).

Discussion

Soil Water Content

During the three years of the study, Russet Burbank’s soil water content (SWC) consistently remained above the soil’s field capacity (FC) across the five irrigation levels (ILs) (Gonzalez et al. 2024). This phenomenon was primarily attributed to Russet Burbank’s lower water consumption when compared to the other four cultivars (Gonzalez et al. 2023). The relatively high SWC of Russet Burbank was further exacerbated by the fact that all five potato cultivars were irrigated using the same quantity of water. Unfortunately, personalized irrigation events for each cultivar were not feasible due to the plot layout (Gonzalez et al. 2024).

Since the irrigation for these five potato cultivars was based on their average modeled evapotranspiration (ET), cultivars with lower water consumption, such as Russet Burbank, tended to have significantly higher SWC levels towards the end of the growing season (Gonzalez et al. 2024).

Fry Color

By reducing late-season irrigation, the expectation was to initiate crop maturation ahead of the usual schedule, as the crop perceived a significant and continuous decrease in irrigation. As late-season irrigation was reduced from 120 to 40% ET in 20% increments, it was observed that tubers of Alturas, Clearwater Russet, Russet Burbank, and Umatilla Russet exhibited significantly lighter and more desirable fry colors when subjected to frying test at 45 and 90 days after harvest (DAH) (Fig. 2, 4, 6, 8, and 10). These findings are in complete agreement with the results of Shock et al. (1992) but contradict the conclusions reported by Iritani and Weller (1978).

These improvements in fry color may have resulted from lowering reducing sugars in tubers following a reduction in late-season irrigation, which induced a gradual physiological maturity of the crop before harvest. However, it’s important to note that this remains speculative without measurements of reducing sugars. Nevertheless, this hypothesis aligns well with Gonzalez et al. (2024) findings on tuber-specific gravity, which showed a negative relationship between tuber-specific gravity and several incremental ILs. Higher tuber specific gravity values, indicative of physiologically mature tubers, were associated with lower ILs. This agrees with findings Iritani (1981), Ap Rees et al. (1981), and Knowles et al. (2008) reported that physiologically mature tubers typically produced higher tuber-specific gravity values and lower reducing sugars than immature tubers.

The findings from the 2019 study revealed nuanced insights into the role of storage duration at 4.4 °C on fry color. While significant differences in basal light reflectance were observed among different ILs for specific cultivars, these differences did not meaningfully impact the color of the fries when fried at 15 DAH. However, tubers stored for 90 days at 4.4 °C and then fried exhibited basal LR values that did not meet industry standards for fry color (Table 2). This suggests that longer storage durations at this temperature may not be beneficial for achieving optimal fry color.

However, this study also found that reducing late-season irrigation by greater than 40% from 100% ET or over-irrigating with 120% ET led to darker fries on four of the five potato cultivars, varying in storage temperature. This suggests a delicate balance in water management during the late growing season. Over-irrigation or extreme deficit irrigation could induce crop stress, affecting starch formation and its subsequent breakdown into sugars, as supported by previous research (Geigenberger et al. 1997, 1999).

Interestingly, as noted in prior studies, reduced irrigation did not improve fry in Ranger Russet, possibly due to its sensitivity to becoming over-mature (Knowles et al. 2009). This highlights the cultivar-specific responses to ILs and suggests that more than one-size-fits-all approach may be needed. Iritani (1981) showed that over-mature tubers accumulate more reducing sugars in storage than properly mature tubers.

The study also examined the basal–apical end light reflectance (LR) difference, a critical factor for fry quality. For instance, Alturas tuber’s basal–apical end LR difference was less than nine, which was deemed acceptable by the frozen potato-processing industry (Knowles et al. 2019). The relatively low basal–apical end LR difference of Alturas’s fries across the three storage temperatures and ILs depicts a uniform color across the entire fry (Table 4). Most importantly, these results illustrate that Alturas tubers’ reducing sugars are well stabilized across the tuber.

However, Clearwater Russet tubers stored at 4.4 °C and irrigated with 100% and 120% ET produced fries with unacceptable (≥9) basal–apical end LR differences (Table 5). This underscores the risk of delaying tuber maturity by over-irrigation, which can lead to undesirable fry quality. Bethke et al. (2009) reported higher reducing sugars in the basal end of tubers that were well-watered through vine senescence, which he attributed to the vigorous growth rates.

Tuber reconditioning could be a potential solution for improving fry color, as suggested by Zommick et al. (2014b). However, the ideal scenario would be optimizing ILs to achieve the desired fry quality without post-harvest interventions.

In summary, this research provides valuable insights for potato growers in the Pacific Northwest. It emphasizes the importance of carefully managing late-season irrigation to improve fry color and quality while highlighting the risks of over- or under-irrigating. These findings could have significant implications for enhancing the quality and marketability of potatoes, thereby benefiting both growers and consumers.