Effect of the Matric Potential on Growth and Water, Nitrate and Potassium Absorption of Vegetables under Soilless Culture

To exploit the full potential of plants, it is essential to provide an adequate water balance during critical growth stages. With knowledge of the substrate’s matric potential, it is possible to realistically measure the amount of water available to the plant. The objective of this research is to study the effect of the matric potential of the substrate on the growth of beans, lettuce, sweet peppers, watermelon, and cauliflower plants. The research goal was to evaluate the effect of the matric potential on the main fertigation parameters of horticultural plants. The plants were cultivated at the University of Almería (Spain) in a controlled chamber in a pot with 250 mL of coconut fiber substrate from block propagation. For treatments T15, T30, T45, and T60, new irrigation was applied when easily available water (EAW) depletion levels of 15, 30, 45, and 60% were reached, respectively. The efficiency of the use of water (WUE), nitrate (NUE), and potassium (KUE) was measured. From T15 to T60, a significant effect was found for the absorption of water, nitrate and potassium, WUE, NUE, and KUE. T60 reduced the plant growth by half. The largest growth was between 15 and 30% of the level of use of EAW. Two different models were constructed according to each plant. It is possible to have a small depletion in the available water and still have plant growth, if there is a balance between the air and water needs of the substrate.


Introduction
It is well-known that irrigation and fertigation are key factors influencing plant water status (Lu et al. 2019) and mineral nutrition. Substrate matric potential is a realistic criterion for measuring water availability to plants, as it constitutes the force with water held by its matrix (substrate particles and pore space) (Yadvinder-Singh et al. 2014) and can be measured by the well-known water release curve of the substrate (de Boodt et al. 1974). For a long time, a very low level of substrate matric potential ranging from − 10 to − 100 cm of a water column was recommended (de Boodt and Verdonck 1972) and later confirmed by other researchers for ornamental plants grown in soilless culture (Caron et al. 1998;Jobin et al. 2004;Londra et al. 2018). This range maintained the easily available water (EAW) and provided the necessary air between their solid particles.
Soilless culture represents a valid opportunity for the horticultural production sector, especially in areas characterized by severe soil degradation, and limited water soilless culture is based on the comparison of the inputs (fertigation) with the outputs (drainage), which are volume, EC, and pH (Amalfitano et al. 2017;Carvalho et al. 2018;Kinoshita et al. 2016;Lee et al. 2017;Rodríguez et al. 2014aRodríguez et al. , 2015Urrestarazu et al. 2019). There are few works published in relation to regulated deficit irrigation in vegetable crops under soilless culture (with a very lower matric potential), and the results show growth yield reduction in many cases and an increase in water use efficiency (Wang et al. 2012;Yang et al. 2017). Additionally, vegetable plants grown under soilless culture are very sensitive to water deficit stress (Ahmed et al. 2014;Urrestarazu 2015). However, there are very few works that define the frequency of irrigation based on a certain percentage of consumption of the available water per substrate and plant (Riviere et al. 1990), which is linked to the matric potential.
On the other hand, it is well known that potassium and nitrate are macronutrients that are more controlled and monitored in the practical management of fertigation in soilless culture (Adams and Ho 1989; Adams 1994; Gallegos-Cedillo et al. 2016;Rodríguez et al. 2014aRodríguez et al. , 2014bWamser et al. 2017).
The objective of this study is to evaluate the effects of various levels of the substrate matric potential through the level of use of a determined EAW volume on (1) vegetative growth of vegetable plants, (2) water and mineral absorption, and (3) water and nutrient use efficiency of vegetable plants grown under substrate soilless culture.
The plants were grown in a chamber with a 16-8 h light-dark cycle, 25-20 °C day-night temperature, 55-75% relative humidity, and 250 μmol m −2 s −1 photosynthetic photon flux density (400-700 nm) supplied by LED Sylvania Cool White lamps (Osram Sylvania Inc., Danvers, MA, USA) during a cycle of 60 days. The macro-and micronutrient compositions of the nutrient solution were similar to those reported by Sonneveld and Straver (1994).
The hydrological characteristics of the substrate used are shown in Fig. 1. To generate the water release curve, the concept described by de Boodt et al. (1974) was applied. The following volumes were calculated: effective total porosity (volumetric percentage of pore space), air volume after drainage (air content at 10-cm water suction, air capacity), easily available water (difference in the volumetric water content between 10-and 50-cm water suction), buffering capacity (difference in the volumetric water content between 50 and 100 cm water suction), and hardly available water. Measurements were performed following European substrate regulations (AENOR 2012a). Other physical, physical-chemical, and chemical characteristics were determined under European substrate regulations (AENOR 2002a(AENOR , 2002b(AENOR , 2008a(AENOR , 2008b(AENOR , 2012a(AENOR , 2012b(AENOR , 2012c).

Fertigation Treatments
Four treatments were used. For the T 15 , T 30 , T 45 , and T 60 treatments, new irrigation was applied when easily available water depletion levels of 15, 30, 45, and 60% were reached, respectively ( Fig. 1).
Loss transpiration was quantified by weight measurements with a two-tenth precision analytical scale model, Ohaus Adventurer AX, with a resolution of 0.1 mg. Each new fertigation consisted of the supply transpired volume of the nutrient solution plus 20% to maintain the nutrient balance in the substrate (Rodríguez et al. 2014b).

Analysis and Harvest Sampling
The volume, pH, electrical conductivity (EC), and nitrate and potassium contents of the drainage were measured daily in each pot with a container that was adapted to its morphology. The pH and electrical conductivity (EC) were  et al. 1974). HAW, hardly available water; BCW, buffering capacity water; EAW, easily available water; AC, air capacity; SM, solid matter monitored using systems from HORIBA Ltd. (LAQUAact PC110-K).
The contents of nitrate and potassium were measured with systems from HORIBA Ltd. Japan (LAQUAtwin B-741 and LAQUAtwin B-731, respectively). The absorption of nitrates and potassium in mmol·plant -1 was quantified based on the balance between input and output (Urrestarazu et al. 2005(Urrestarazu et al. , 2008b. Water (WUE), nitrate (NUE), and potassium (KUE) use efficiencies were calculated by dividing the total dry weight of plants by the uptake volumes of water (L), nitrate (mmol), and potassium (mmol), respectively.

Experimental Design and Statistical Analysis
The experiment was conducted using a split-plot design with three blocks per treatment and four plants per treatment in each treatment using a completely random design.
Statistical analysis of different parameters was performed using the SPSS software (Statistics Package). ANOVA was performed and a Tukey test at the P ≤ 0.05 level of significance was used to determine if significant parameter differences existed among different treatments. A quadratic linear equation and its R 2 and P were also considered ( Fig. 4). Table 1 shows the pH and EC of drainage and the nitrate and potassium absorption. There was no significant difference in pH for any crop. For all crops, significant differences between the treatments were recorded in the EC of drainage and absorption of water, nitrate, and potassium. The highest EC values found were consistent with the highest potassium and nitrate absorption. The highest water absorption was recorded when new fertigation was supplied after 15 (T 15 ) or 30% (T 30 ) (in the case of cauliflower) of easily available water was lost by plant transpiration. Except for the pepper, in general, from T 15 to T 60 , a significant correlation between water absorption and mineral absorption (expressed as nitrate and potassium uptake) was found ( Fig. 2) (Amalfitano et al. 2017). In pepper, while the reduction in water was higher (34%, from T 15 to T 60 ), the lowest variation in EC and reduction in absorption nitrate and potassium (11 and 15%, respectively) of all plants were recorded for the same treatments.

Effect on Growth Parameters
In general, as the water needed for the plants was absorbed from a greater depletion of the EAW (greater matric potential, from T 15 to T 60 ), the growth parameters decreased significantly (Table 2). Except for cauliflower, the greatest total fresh and dry weight and leaf area were found in T 15 .
From T 15 to T 60 , the smallest and largest decreases in growth were recorded in bean and pepper, respectively (Figs. 2 and 3). There were 49, 51, and 59% average decrease in five vegetables for their root, stem, and leaf dry weights, respectively.

Effect on the Use Efficiency of Fertigation Inputs
Except for pepper, the best use efficiency of water (WUE) and mineral absorption (NUE and KUE) was found with T 15 and/or T 30 . In pepper plants, T 60 had the best values for WUE, NUE, and KUE (Table 3). An et al. (2021) found similar results in the crop of Cymbidium in the WUE using coir as the substrate. Figure 4 shows a clear and significant trend toward growth reduction even when lower matric potential in soilless culture is used. Except for cauliflower, T 15 recorded the best vegetative growth. Cauliflower had its best growth at T 30 , probably because a benefit is found when root aeration is improved (Heuberger et al. 2001) and because cauliflower is the plant most sensitive to air capacity within the evaluated crops. Figure 4E shows the average dry weight reduction as fertigation was delayed as a function of the depletion of EAW, while some plants, such as cauliflower, reached their optimum at 30% depletion.

Effect on Fertigation Parameters and Mineral Absorption
The pH and electrical conductivity (EC) of drainage are parameters used to control and feedback the management of nutrient solution and fertigation (Urrestarazu 2015). In tomato plants, similar results to those found in Table 1 were published on pH and the correlation between drainage EC and water uptake (Gallegos-Cedillo et al. 2016;Rodriguez et al. 2014b). Because of Adams and Ho (1989) and Adams (1994), it is well-known that the major absorption of anions and cations are nitrate and potassium, respectively; this is probably the reason for this positive correlation found Fig. 2 Significant percentage of reduction (%) of water, nitrate, and potassium absorption and drainage electric conductivity (EC) that occurs when a volume of fertigation is supplied after 15 (T 15 ) or 30% (T 30 ) of easily available water has been transpired by the vegetable plants between drainage EC and mineral absorption of nitrate and potassium.
Sweet pepper was the vegetable with the lowest loss of nitrate and potassium absorption under the highest matric potential (T 60 ).
The lowest loss of water absorption and growth was recorded for beans (Fig. 2), which agrees with the result published by Karimzadeh-Soureshjani et al. (2019), who attributed various mechanisms of this plant to escape water stress or water deficit.

Effect on Growth Parameters
Water availability mostly affects the growth of leaves and roots, photosynthesis and dry mater accumulation (Blum 1996). Similar results to our vegetables were found by Riviere et al. (1990) in shrubs and conifers, where the best growth was recorded under high frequent irrigation and the adjustment of the amount of volume to the needs of plants and aeration. Additionally, in general, the best growth results of coir-grown Cymbidium were found when the irrigation system was within the EAW (An et al. 2021). The higher energy required for the same water uptake with increasing matric potential between the T 15 and T 60 treatments led to a significant decrease in growth for all vegetables.

Effect on the Use Efficiency of Fertigation Inputs
It is well-known that water and plant nutrients play a very important role in enhancing water and mineral nutrient use efficiency under limited water supply and other agrosystems (Waraich et al. 2011). Therefore, different fertigation regimes for each crop can be regulated to improve the use efficiency of the different fertigation inputs, as published by Wang et al. (2012) and Yang et al. (2017) .   Fig. 3 Significant percentage of reduction (%) in dry weight growth (leaf, stem, root, and total weight) that occurs when a volume of fertigation is supplied after 15% (T 15 ) or 60% (T 60 ) of easily available water has been transpired by the plant

Model of Growth as a Function of Matric Potential
Based on the traditional model for crop salt tolerance by Maas and Hoffman (1977), another model has been described where zero growth and optimal range are reached and a subsequent threshold point with a decrease in growth is reached (Sonneveld and Voogt 2009). If this salinity tolerance model was compared with the matric potential model, only the behavior of cauliflower was coincident, probably to maintain the air-water availability equilibrium, while the rest of the vegetables had the best fit for a quadratic equation, as shown in Fig. 4E, where the water availability under a very lower matric potential was the limiting factor. If only 15-30% is used, the result will depend on the balance between the needs of air and water from the substrate for each plant.
More research on a larger number of ornamental and horticultural crops and their phenological stages according to horticultural objectives is required. A certain degree of stress through matric potential could be a good tool to control the balance between vegetative and fruiting development, especially in crops whose usable products are fruit or seeds.

Conclusion
When 60% of substrate easily available water is used in a crop unit of soilless culture before a new fertigation it supplies, there is a reduction in the vegetative growth of the vegetable by an average of approximately 50%, with the smallest reduction for the root (45%) and the largest for the leaves (51%).
There was a high and significant correlation between the increased energy required to absorb water from treatments T 15 to T 60 and the decrease in growth that occurred in all plants. Based on a regression analysis, a specific model for each vegetable, one medium for all, was developed. The models showed a very close correlation between the different fertigation regimes and growth for all vegetables. Total dry weight (g plant −1 ) as a function of when the new watering is supplied after a lost volume percentage (%) of easily available water from the substrate by plant transpiration. ***, ** indicate P ≤ 0.01 and 0.05, respectively Funding Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature.

Conflict of Interest The authors declare no competing interests.
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