Introduction

In recent decades, there has been a major expansion of consumption of fruits, nuts, and fruit-derived products (e.g. olive oil, wine) that come from perennial trees and vines (Johnson 2016; Asci and Devadoss 2021) due both to population increases and changing diets to an increasing proportion of consumption coming from fruits, vegetables and nuts (Schmidhuber and Traill 2006). Despite their status as “specialty crops” grown on smaller amounts of land, tree and vine crops management is critical. First, these crops have great importance for human nutrition. Second, tree and vine crops often have greatly increased input costs and economic yields on a per-area basis compared to forage and grain costs (Klonsky 2012). Third, these crops result in hardened water demand (e.g. you cannot usually remove irrigation without causing a major, multi-year, loss of investment) (Feinerman and Tsur 2014; Mall and Herman 2019). Thus, these crops’ relative increase in cultivation area can greatly reduce a region’s capacity to adapt to drought. This is particularly relevant as much specialty crop production occurs in regions with Mediterranean-type climates (e.g. Western United States, Southern Europe, Southern Africa, and Australia) that are seeing increased water scarcity (Feng et al. 2019).

Along with the issues mentioned above, crop quality is critical for the economic value of tree and vine crops along with yield (Fereres and Evans 2006). Factors affecting crop price include fruit size (Lötze and Bergh 2004), color (Ranasingha et al. 2019), uniformity (Calderon-Orellana et al. 2014), sugar content (brix) (Kajikawa 1998), flavor and firmness (Marsal et al. 2010; Diehl et al. 2015), and others. While these factors can vary in importance depending upon the specific crop (Diehl et al. 2015), they share major commonalities across most crops. Irrigation has been shown to affect these properties across a wide range of crops (Zekri et al. 2003; Girona et al. 2006; Zegbe et al. 2007; Pérez-Pastor et al. 2007; Behboudian et al. 2010; Lopez et al. 2012; Laribi et al. 2013; Falagán et al. 2015; Gucci et al. 2019; Suarez et al. 2019; Carrasco-Benavides et al. 2020; Palai et al. 2022a, b). Given the need to optimize irrigation water use, finding and disseminating practices that improve and maintain fruit quality while increasing water productivity in tree and vine crops is imperative. Irrigation practices which degrade fruit size or quality are not likely to be adopted by growers. In this special issue of Irrigation Science, we focus on this crucial challenge.

Papers in this special issue

Papers in this special issue were split closely between those focusing on vines and those on tree crops, with four papers focusing on grapes and six focusing on fruit and nut trees. Papers on vine crops spanned a wide range of topics. Caruso et al. (2022) evaluated the impact of pre and post-veraison deficit irrigation on Sangiovese grapes with differing rootstocks. Among the key findings were that pre-veraison deficits reduced fresh berry weight and acidity. Additionally, the 1103P rootstock showed greater sensitivity to irrigation deficits, particularly pre-veraison. Martínez-Moreno et al. (2022) evaluated the impact of irrigation with brackish water with different anions (chloride versus sulphates) in full season and post-veraison treatments compared to no added salinity and no irrigation (rainfed only). Over three seasons, salinity did not have a major effect on crop quality compared to the no-irrigation treatment in their well-drained soils, thus suggesting that brackish water (EC of 5 dS/m) could be a useful short and medium-term substitute in the absence of higher quality water. However, chloride and sodium accumulation in the leaves in the third year indicate this strategy may not be sustainable in the longer term. Bianchi et al. (2023) examined the impact of a multifaceted irrigation system that applied water through both drip irrigation and mini-sprinklers, with the sprinklers designed to provide protection against frost and extreme heat waves. They found that the mini-sprinklers increased bud temperature by 1 °C during frost events, suggesting protection. Sprinkler operation during the summer increased malic and titratable acids and reduced soluble solids in the must. Finally, Oliver-Manera et al. (2023) evaluated how summer pruining (forcing) may reduce heat stress on berries via delay of veraison under different irrigation strategies. While veraison was successfully delayed with forcing and must acidity increased, forced vines showed greater sensitivity to deficit irrigation both in yield and must quality.

Concerning tree crops, each of the papers in this special issue evaluated a different crop. There was maximum ecosystem water use efficiency in the fall but relatively little sensitivity to the deficit irrigation. Sperling et al. (2022) evaluated the impact of plant mineral status on transpiration in almonds. Low nitrogen and phosphorus and very high nitrogen limited plant transpiration to below 3 mm/day. The results indicate that fertilizer status could be used to predict almond water demands. Calvo et al. (2022) conducted a mini-review of irrigation, growth, and crop quality in walnuts. Their synopsis reported that most walnut groves have yearly ET between ~ 1000 and 1200 mm. Moderate water deficits (midday stem water potential greater than − 0.8 MPa) had little impact on production, but there is concern about kernel and oil quality. Panigrahi (2023) presented the results of a deficit irrigation experiment in mandarin using different irrigation levels and triggers using the Crop Water Stress Index (CWSI). This study found that irrigation at 60% of calculated crop ET (ETc) with a CWSI trigger of 0.4 resulted in superior quality fruits and the highest water productivity. Plavcová et al. (2023) assessed the water-yield relations of two apple cultivars grown under rainfed conditions and with supplemental irrigation in Czechia. They found that the yields were similar between rainfed and irrigated conditions, but the irrigated treatments had larger sized fruits. Finally, Ahumada-Orellana et al. (2022) presented the yield, leaf-gas exchange, and water status of olive orchards subjected to two lengths of irrigation cutoff and water stress. They found that the oil yield of the most severe treatment was slightly greater than 60% of the control treatment, thus indicating considerable susceptibility of olives to deficit irrigation and irrigation cutoff.

Concluding thoughts

Proper irrigation management will be increasingly vital for the production of specialty crops, given the increasing area of tree and vine crops and decreasing availability and consistency of irrigation water. This special issue of Irrigation Science presents ten studies across a wide array of crops and strategies to increase water productivity while maintaining yield and crop quality. With the advancement of sensor technologies and data analysis tools (e.g. artificial intelligence) and increasing prevalence of drip and micro-sprinkler irrigation systems, there will be increased opportunities to precisely control the timing, rate, and amount of irrigation water applied to tree and vine crops to maximize water productivity and crop quality.