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Citrus is a major evergreen fruit crop around the temperate areas and in the tropics. In the Mediterranean area, its production is largely dependent on irrigation, and given the current water supply situation and future scenarios, it is important to delineate strategies for optimizing water use. Regulated deficit irrigation (RDI) strategies have been widely studied in citrus and, although the results have been variable, they are often proposed as a means for improving water use and productivity under irrigation. We report here the effects of two RDI regimes on transpiration rate, yield and quality parameters in a 4-year experiment in two citrus species. Additionally, we present a meta-analysis of past research that investigated RDI applied during summer in citrus. Our results indicate that transpiration rate was decreased in the more stressed treatment as a consequence of the water shortage. Yields of mandarin and late orange were sustained when irrigation was decreased to 50-55% (related to control treatment) during the RDI period, making RDI a valuable tool for decreasing water application in citrus. Nevertheless, it is necessary to characterize the effects of water stress on transpiration and yield for the correct implementation of deficit irrigation strategies. The relationship between yield and relative transpiration observed in this study indicates that mandarin is less sensitive to water stress than late orange. It was probably related to a change in the dynamics of fruit growth and development. The results with different species and cultivars suggest that the RDI strategy must be fine-tuned for each cultivar according to its phenology. Once the sensitive phases are well-identified, citrus species are suitable for successful application of RDI regimes in most situations. [ABSTRACT FROM AUTHOR]

Abstract: Pepper (Capsicum annuum L.) production is normally carried out under irrigation as the crop is very susceptible to water shortage. Deficit irrigation strategies in pepper for paprika could increase production and facilitate mechanical harvest and, at the same time, save water. We conducted a field experiment that imposed water deficits, either during ripening (T1) or throughout the season (T2), and compared them to a fully irrigated control (T3). Stem water potential varied from −0.6MPa in T3, early in the season to −1.5MPa in T2 prior to harvest. Applied irrigation water for T1, T2, and T3 was 456, 346 and 480mm, respectively. Water deficits depress leaf area and biomass production but did not affect the proportion of flowers that set fruit. Dry fruit weight in T2 at harvest was 66% of T3, but did not differ significantly between T1 and T3. However, commercial yield (based on colour production) was significantly higher in T3 than in the other two treatments, as the late water deficits imposed in T1 delayed harvest. We concluded that water deficits, either sustained or applied at fruit ripening, required for mechanical harvest do not hasten ripening and are detrimental to commercial yields and that pepper plants should be well supplied with water until harvest for maximum paprika production. [Copyright &y& Elsevier]

Abstract: Important savings in the levels of irrigation without an associated penalty in yield have been reported for olive under deficit irrigation strategies. Full irrigation (C), continuous deficit irrigation (CDI) and regulated deficit irrigation (RDI), were compared from 2004 to 2006 in Cordoba, southern Spain, in terms of seasonal evapotranspiration (ET), growth and yield in mature olive trees (Olea europaea L. cv. ‘Arbequina’). In deficit treatments, the total amount of irrigation was around 25% of that of the Control while ET was 65–70% of that of C. Deficit treatments strongly reduced vegetative growth, but only slightly reduced the final fruit volume. Water stress caused a higher reduction in fresh fruit yield than oil yield due to a higher oil concentration in deficit irrigated trees, without differences between CDI and RDI. Therefore, both irrigation strategies may be used in olive to save a significant amount of irrigation with moderate reductions (about 15%) in oil yield. The amount of oil produced per unit intercepted PAR was almost the same for all the treatments, which suggests that olive oil yield may be calculated from intercepted radiation even under moderate water stress. [Copyright &y& Elsevier]

Abstract: Water scarcity limits the supply to irrigation schemes worldwide. In many areas, chronic water deficits force irrigation districts to allocate water at levels well below the crop water requirements for maximum yield. We have developed a model that simulates the water balance and the irrigation performance at the plot and scheme levels, to carry out scenario analyses in an irrigation scheme in Southern Spain (Genil–Cabra Irrigation Scheme) that is frequently subjected to water limitation. The major crops in the scheme are winter cereals, sunflower, garlic, and cotton. The model simulated the scheme performance for different allocation levels of 500, 1500, 2500m3/ha, and full irrigation supply in terms of gross and net income, irrigation water productivity (IWP) and labour needs, using a series of 48 years of climatic data. For each level of water allocation, three strategies were considered. The first strategy allocated the water supply equally to all users, while a second one was aimed at maximising IWP, both assuming the same crop distribution that existed in 2000. In a third strategy, the cropping pattern was allowed to change relative to that existing in 2000. The behaviour of individual farmers in relation to irrigation performance was incorporated into the model, based on the characterization of their performance during four seasons. Results showed that at supply levels of 1500m3/ha, the best strategy in terms of net income was the one that allocated the water to crops with high water productivity. However, when water supply is very low (500m3/ha) scheme net income is maximised by adjusting the cropping pattern. As supply increased to 2500m3/ha, there were no differences in scheme net income among the three strategies. The various levels of water allocation influenced the value of IWP for the scheme, leading to average values that varied between 1.12€/m3 for the 500m3/ha supply to 0.50€/m3 under unlimited water supply. The best strategy varied depending on the level of water allocation and on the maximisation objective (income, IWP, labour) in a complex way. Nevertheless, strategies based on allocating water to crops of high water productivity, combined with a shift in the cropping pattern are recommended when water supply is constrained. The analysis emphasized the need to develop simulation tools for optimising water allocation under scarcity at the irrigation scheme level. [Copyright &y& Elsevier]

Abstract: Little information is available on carbon exchange of olive orchards despite their agronomical and ecological importance. Measurements of CO2 and water vapour exchange were performed during 2006 and 2007 with large closed chambers in an olive orchard in Cordoba (Spain) under two irrigation regimes, full and regulated deficit irrigation. Canopy assimilation was higher for full (10.8–21.3gCO2 m−2 day−1) than for deficit irrigation (5.1–17.4gCO2 m−2 day−1). Forty-five percent of gross assimilation was lost in respiration being root respiration less than 30% of total respiration. Water use efficiency (WUE) typically decreased from maximum values around 30gCO2 L−1 soon after sunrise to 2–7gCO2 L−1 in the afternoon when vapour pressure deficit typically exceeded 5kPa. While water stress improved instantaneous WUE only slightly, the effect was dramatic for daily values. The measurements allowed the calibration of a model coupling canopy conductance and assimilation that showed a seasonal variation in the parameters suggesting changes in the physiology of olive trees. The improvement of WUE in olive trees under water stress supports the adoption of deficit irrigation in olive orchards although further research is required to prevent negative side effects. [Copyright &y& Elsevier]