Deficit Irrigation Applied to Lemon Trees Grafted on Two Rootstocks and Irrigated with Desalinated Seawater.

The use of desalinated seawater (DSW) for irrigation in semi-arid regions is taking hold. Citrus tolerance to ions that predominate in DSW and water stress depends on the rootstock. Deficit irrigation was applied to DSW-irrigated lemon trees and grafted on rootstocks with different tolerance (Citrus macrophylla (CM) and sour orange (SO)). Plants were irrigated with DSW or Control treatment (distilled water), and, 140 days later, irrigation treatments were started: full irrigation (FI) or DI (50% of the volume applied to FI). After 75 days, differences between CM and SO plants irrigated with DSW and under DI were found. The higher concentrations of Cl- and Na+ in CM and B in SO were the main causes of shoot growth reduction. The osmotic adjustment of CM plants was made possible by the accumulation of Na+, Cl-, and proline, but SO failed to adjust osmotically. In CM and SO plants, photosynthesis reduction was due to lower chlorophyll levels, but also to stomatal factors (CM plants) or alterations of the photochemical machinery (SO plants). Finally, unlike CM, SO had a good antioxidant system. In the future, knowing the different responses of CM and SO under these stressful conditions could be useful in citrus-growing areas.

Citrus Irrigation With Desalinated Seawater Under a Climate Change Scenario.

In arid and semiarid regions, the current lack of natural water resources is driving the use of alternative sources for crop irrigation, such as desalinated seawater (DSW). However, the use of DSW could affect the crop productivity due to its chemical composition (predominance of phytotoxic ions: Na+, Cl-, and B). Citrus species are classified as salt and boron-sensitive; however, the rootstock plays a fundamental role in the tree's tolerance of abiotic stresses. One-year-old 'Verna' lemon trees grafted on two rootstocks (CM, Citrus macrophylla, and SO, sour orange) were used. These rootstocks differ in their salinity and boron tolerance, SO being more tolerant than CM. The experiment was carried out at high temperature (35/27°C), and the plants were irrigated with three types of water supplemented with Hoagland nutrients: DSW, DLB (DSW with low boron), and Control (distilled water). The plants were irrigated three times per week and harvested 7 months after the treatments started. The response to high levels of Cl-, Na+, and B was rootstock-dependent. Under the high temperature conditions, the growth of plants grafted on SO was not affected by DSW, and these plants did not reach the Cl- threshold of phytotoxicity, so the decrease in the shoot growth of plants grafted on CM due to DSW irrigation was related more to Cl- rather than the foliar Na+ accumulation. Plants grafted on SO and irrigated with DSW accumulated more B than those grafted on CM, surpassing the threshold of phytotoxicity and producing greater oxidative stress. As the growth of these plants was not reduced, the effects of DSW on plant growth were not directly related to the concentration of B and there must be some mechanisms that allow these plants to withstand the negative effects of high foliar B, such as the increased levels of quaternary ammonium compounds. Since the response of citrus plants to DSW depended on the rootstock, the results obtained in this experiment, using DSW at high temperature, could be useful for the future management of citrus crops, because climate change will increase temperatures and exacerbate the scarcity of water resources in citrus-growing areas.

Changes in Berry Tissues in Monastrell Grapevines Grafted on Different Rootstocks and Their Relationship with Berry and Wine Phenolic Content.

Monastrell grapevines grafted on the rootstocks 140Ru, 1103P, 41B, 110R, and 161-49C were subjected to regulated deficit irrigation (RDI) and partial root-zone irrigation (PRI). We analyzed the effects of the rootstock and irrigation method on the phenolic concentration in different berry tissues, its dilution/concentration due to the berry size, the anatomical and morphological traits of berries related to the phenolic compounds concentration, and the relationships of all these parameters with the final berry and wine phenolic content. The rootstock had an important effect on the accumulation of total phenolic compounds and anthocyanins in the skin (berries from 110R and 140Ru had the highest values). Moreover, the rootstock modified some anatomical and morphological characteristics that had a direct relationship with the final phenolic compounds concentration in the must. Large grapes and high must percentages (110R and 140Ru) produced a dilution effect, whereas small berries and a low must percentage increased the concentration (161-49C). For 110R, the small size of the cells of the epidermis and hypodermis in the grapes also could have contributed to the high phenolic compounds concentration in the skin. The percentage of cells in the skin with a uniform coloration was positively correlated with its total phenolic compounds and anthocyanins concentration and also with the phenolic quality of the wine. The PRI modified some specific morphological/anatomical skin/berry traits, and these may have contributed to important changes in the final concentration of phenolic compounds, depending on the rootstock. The better phenolic quality of the must and wines observed in some rootstocks under PRI could be due to smaller cells in the epidermis and hypodermis of the skin (161-49C), a higher percentage of cells with a uniform coloration in the hypodermis (110R), or a lower number of seeds per berry (161-49C). In contrast, the lower phenolic compounds concentration in the must of grapes observed in the most vigorous rootstocks under PRI could be due to a greater thickness of the epidermis (140Ru), greater cuticle thickness (41B), a higher number of seeds (140Ru), a lower skin/pulp ratio and percentage of skin (140Ru), a greater percentage of cells in the epidermis without coloration or with large inclusions, and a lower percentage of cells with a uniform coloration in the epidermis (140Ru). The final quality of the grape is related to some changes in histological and morphological aspects of the grape produced by the rootstock and irrigation strategy.

Hydraulics and gas exchange recover more rapidly from severe drought stress in small pot-grown grapevines than in field-grown plants.

Modifications of plant hydraulics and shoot resistances (Rshoot) induced by water withholding followed by rewatering, and their relationships with plant water status, leaf gas exchange and water use efficiency at the leaf level, were investigated in pot-grown and field-grown, own-rooted Syrah grapevines in an arid climate. Water stress induced anisohydric behavior, gradually reducing stomatal conductance (gs) and leaf photosynthesis (A) in response to decreasing midday stem water potential (Ψs). Water stress also rapidly increased intrinsic water-use efficiency (A/gs); this effect persisted for many days after rewatering. Whole-plant (Kplant), canopy (Kcanopy), shoot (Kshoot) and leaf (Kleaf) hydraulic conductances decreased during water stress, in tune with the gradual decrease in Ψs, leaf gas exchange and whole plant water use. Water-stressed vines also had a lower Ψ gradient between stem and leaf (ΔΨl), which was correlated with lower leaf transpiration rate (E). E and ΔΨl increased with increasing vapour pressure deficit (VPD) in non-stressed control vines but not in stressed vines. Perfusion of xylem-mobile dye showed that water flow to petioles and leaves was substantially reduced or even stopped under moderate and severe drought stress. Leaf blade hydraulic resistance accounted for most of the total shoot resistance. However, hydraulic conductance of the whole root system (Kroot) was not significantly reduced until water stress became very severe in pot-grown vines. Significant correlations between Kplant, Kcanopy and Ψs, Kcanopy and leaf gas exchange, Kleaf and Ψs, and Kleaf and A support a link between water supply, leaf water status and gas exchange. Upon re-watering, Ψs recovered faster than gas exchange and leaf-shoot hydraulics. A gradual recovery of hydraulic functionality of plant organs was also observed, the leaves being the last to recover after rewatering. In pot-grown vines, Kcanopy recovered rather quickly following restoration of Ψs, although gas exchange recovery did not directly depend on recovery of Kcanopy. In field-grown vines, recovery of water status, gas exchange and hydraulic functionality was slower than in pot-grown plants, and low gs after rewatering was related to sustained decreased Kplant, Kcanopy and Kshoot and lower water transport to leaves. These results suggest that caution should be exercised when scaling up conclusions from experiments with small pot-grown plants to field conditions.

Influence of deficit irrigation timing on the fruit quality of grapefruit (Citrus paradisi Mac.).

The irrigation necessities for grapefruit production are very high. Due to the scarcity of water resources, growers use deficit irrigation (DI) - which could affect the fruit quality. Different DI strategies were studied: Control (irrigated at 100% ETc) and T1, T2 and T3 (50% ETc at phases I, II and III of fruit growth, respectively). Strategy T1 only delayed external maturation depending on the duration of the water stress. High water stress in T2 delayed fruit maturation, increased acidity and reduced the sugar concentration. Under T2, trees suffering moderate water stress showed increased flavonoid and phenolic contents but decreased lycopene levels. External maturation was delayed in T3 when severe stress occurred during the first part of phase III. Strategy T3 advanced internal ripening when moderate water stress occurred during the first 40 days of phase III, increasing sugar accumulation, promoted by the high acidity of the fruits. Moderate water stress also increased ß-carotene, flavonoids and phenolics levels.

Partial root zone drying exerts different physiological responses on field-grown grapevine (Vitis vinifera cv. Monastrell) in comparison to regulated deficit irrigation.

Regulated deficit irrigation (RDI) and partial root zone irrigation (PRI) were compared for 4 years at two irrigation volumes (110mm year-1 (1) and 78mm year-1 (2)) in field-grown grafted Monastrell grapevines (Vitis vitifera L.) to distinguish the effects of deficit irrigation from specific PRI effects. PRI-1 and RDI-1 vines received ~30% of the crop evapotranspiration (ETc) from budburst to fruit set, 13-15% from fruit set to veraison and 20% from veraison to harvest. RDI-2 and PRI-2 vines received around 20% of ETc from budburst to fruit set, no irrigation from fruit set to veraison, and recovery (21-24% ETc) thereafter. Compared with RDI-1, PRI-1 increased irrigation depth and total soil water (θv) availability in the root zone, and stimulated greater fine root growth and water uptake. Increased soil volume exploration supported greater canopy water use, vegetative development, biomass accumulation and internal water storage capacity. PRI-1 vines had higher stomatal conductance, lower leaf-level water use efficiency and increased leaf xylem sap concentration ([X-ABA]leaf) following reirrigation. Compared with RDI-2, PRI-2 decreased total θv availability, fine root growth and water uptake, gas exchange, leaf water status, [X-ABA]leaf, biomass accumulation and storage capacity. Xylem ABA decreased with total θv availability in PRI-2, probably from limited sap flow when θv in drying soil was low (≈20%). For this rootstock-scion combination, high irrigation volumes applied to the wet part of the roots (θv>30%) are critical for increasing root-to-shoot ABA signalling and growth, and improving performance under semiarid conditions.