Long-term response of ‘Clementina de Nules’ citrus trees to summer regulated deficit irrigation

Highlights

• Long-term regulated deficit irrigation (RDI) was assessed in Clementine trees.

• Two summer RDI strategies were tested during six consecutive seasons.

• Mild summer stress did not reduce the economic return over the six seasons.

• Long-term deficit irrigation did not increase soil salinity.

• Midday stem water potential during summer months should not surpass −1.5 MPa.

Abstract

Regulated deficit irrigation (RDI) has been assessed in a wide number of fruit crops. However, few are the studies dealing with long-term RDI strategies. In this work, we assessed the response of ‘Clementina de Nules’ citrus trees to summer RDI treatments [RDI-1, irrigated at 50% of crop evapotranspiration (ETc) and RDI-2, irrigated at 35% ETc] during six consecutive seasons (2007–2012). Although water restrictions were applied according to reductions over ETc, threshold values of stem water potential (−1.3 to −1.5 MPa in the RDI-1 treatment and −1.5 to −1.7 MPa in the RDI-2) were also considered for scheduling deficit irrigation. Results showed that the water stress imposed in the RDI-2 treatment had a noticeable impact on fresh fruit weight, reducing yield and economic return over the six years and more so during the first two years in comparison to a control treatment irrigated at 100% ETc during the whole season. The RDI-1 strategy had a lower impact than the RDI-2 treatment on the fresh fruit weight and did not significantly reduce yield or the economic return over the six seasons. RDI improved fruit quality particularly in the most stressed treatment by increasing the total soluble solids and titratable acidity. RDI also reduced vegetative growth although the reduction was not proportional to the level of stress reached by trees. On average for the six years, the relative trunk growth and pruning weight in both RDI treatments were reduced around 20% in comparison with the control trees. Long-term deficit irrigation did not lead to a noticeable accumulation of salts in the root zone. Overall, these results show that the RDI-1 strategy here studied can be successfully applied during six consecutive seasons in commercial ‘Clementina de Nules’ orchards allowing water savings of 15% without any detrimental effect on tree performance.

Introduction

The study of irrigation water saving techniques such as the regulated deficit irrigation (RDI), which can be performed by growers with a minimal or any negative effect on yield, is gaining importance during the last decades due to the future scenario of increasing water scarcity occurring in a large number of areas all over the world (Fereres and Gonzalez-Dugo, 2009). RDI has been widely studied in different fruit crops including citrus (Carr, 2012, Ruiz-Sánchez et al., 2010). However, the majority of the RDI studies found in the literature deal with the response of crops to RDI during two–three consecutive seasons (see revision by Naor, 2006) while only few of them explored the crop performance when RDI is implemented at longer term (Domingo et al., 1996; Hueso and Cuevas, 2010; Intrigliolo et al., 2013, Johnson et al., 1992).

One of the concerns that horticulturist have about long-term RDI strategies is the possible decrease in the soil potential productivity because of a possible accumulation of salts within the soil root zone (Melgar et al., 2009, Mounzer et al., 2013, Pedrero et al., 2013), which can be particularly important in orchards under drip irrigation systems. Moreover, long-term RDI strategies may reduce tree reserves and root growth leading to a decrease in the fruit bearing capacity, loss of plant vigor and eventually of yield capacity (Intrigliolo et al., 2013). Studies performed by Girona et al. (2005) in almond trees during four seasons, showed a reduction in the dry matter accumulation capacity during the last experimental seasons when RDI was applied during the kernel-filling period. In other four-year experiment in almond trees, Romero et al. (2004) observed that RDI during kernel-filling and post-harvest stages negatively affected yield due to an excessive reduction of vegetative growth. Similarly, Intrigliolo et al. (2013) observed a significant decrease in yield capacity in young Japanese plum trees subjected to seven consecutive seasons of RDI. On the other hand, RDI studies in crops like loquat during ten consecutive seasons (Hueso and Cuevas, 2010) or early maturing peach trees during four years (Johnson et al., 1992) have reported substantial water savings without any reduction in yield or fruit size.

Citrus is one of the most important crops in the world with an annual production in 2011 of almost 129 million tons (Faostat, 2011). Spain is among the main citrus world producers standing out because of its production of oranges and above all mandarins (most of them Clementine), being the second country after China with the largest production of mandarins in the world (Faostat, 2011). With nearly 65% of the Spanish citrus production, the main citrus producing region in Spain is the Valencia Community, where there is evidence that the patterns and areas affected by drought are increasing (Vicente-Serrano et al., 2004). Studies performed in experimental orchards by González-Altozano and Castel (1999) on ‘Clementina de Nules’ citrus trees in Valencia, Spain, identified the months of summer, just after ‘June fruit drop’, as the more appropriate period for applying water restrictions, providing that full water regime is restored in September, around three months before harvest. González-Altozano and Castel (1999) identified a threshold value of water stress determined by measurement of midday stem water potential (Ψ_s) of −1.3 MPa. More recently, Ballester et al. (2011) in a commercial orchard of the same variety proved the validity of a summer RDI strategy applied during three consecutive seasons based on irrigating trees at 50% of the crop evapotranspiration (ETc) but also taking into account threshold Ψ_s values of −1.3 to −1.5 MPa.

The Mediterranean climate of eastern Spain is characterized by irregular annual precipitations that can lead to very different amounts of water available for irrigation among years. It is important, then, to study the viability of long-term RDI strategies in this area to provide growers with strategies to follow in case they were forced to under-irrigate plants during several consecutive seasons. In ‘Salustiana’ trees, Castel and Buj (1993) reported in a seven-year experiment a reduction in yield of 22% when trees were irrigated during the whole season at 40% respect to a control treatment irrigated at full water requirements. These authors, however, did not observe any significant reduction in yield when trees were irrigated either at 20% of the control trees during the whole season or at 40% of control trees during spring or autumn–winter. In other four-season experiment on ‘Fino’ lemon trees, Domingo et al. (1996) reported water savings of 22% when trees were irrigated at 25%, respect to a treatment irrigated at full water requirements, from October to harvest.

In the present work, we studied the viability of the summer RDI-1 strategy recommended by Ballester et al. (2011) for ‘Clementina de Nules’ trees applied over other three additional seasons (six seasons in total from 2007 to 2012). Yield components, fruit quality, vegetative growth and the effect of RDI on soil salt accumulation were evaluated.