Effects of water deficit and salinity stress on late mandarin trees.

To maintain the viability of citrus farms in a scenario of scarce water supplies for irrigation purposes, such as in south-eastern Spain, it is necessary to detect the maximum threshold of crop water status and vegetative growth, both of which are vulnerable to the combined effect of water and salinity stress. With this aim, adult late mandarin trees were subjected to a gradual increase in salinity of the irrigation water for four consecutive growing seasons, accompanied by a water deficit for the first three seasons, while the fourth season counted as a recovery period. The treatments tested were: i) control (CTL), irrigated at 100% of the ETc in the first season followed by an additional 33% water leaching requirement in the second and third seasons; (ii) a regulated deficit irrigation (RDI100) treatment, irrigated as the CTL except in phase I and until mid-phase II, coinciding with the beginning of the slowdown in trunk growth, where irrigation was set at 65 and 50% CTL in the first and last two seasons, respectively, and 80% in phase III; (iii) RDI50, irrigated as the CTL during the first season, and from the second season onwards, at 50% RDI100 during the deficit period; (iv) FARMER, irrigated in accordance with the farmer's criteria. A moderate saline water stress combined with the application of RDI100 treatment did not reduce crop yield after one growing season, but did reduce plant growth. However, after two consecutive seasons (in both RDI strategies) water stress drastically reduced both yield and plant growth, mainly due to a high accumulation of sodium and chloride in the leaves. Gas exchange parameters were also greatly reduced. Furthermore, it was found that the water deficit could be prolonged until the trunk growth slowed down during the second fruit growth stage in mandarin trees, leading to a substantial increase in water productivity values.

Leaf water relations and net gas exchange responses of salinized Carrizo citrange seedlings during drought stress and recovery.

BACKGROUND AND AIMS: Since salinity and drought stress can occur together, an assessment was made of their interacting effects on leaf water relations, osmotic adjustment and net gas exchange in seedlings of the relatively chloride-sensitive Carrizo citrange, Citrus sinensis x Poncirus trifoliata. METHODS: Plants were fertilized with nutrient solution with or without additional 100 mm NaCl (salt and no-salt treatments). After 7 d, half of the plants were drought stressed by withholding irrigation water for 10 d. Thus, there were four treatments: salinized and non-salinized plants under drought-stress or well-watered conditions. After the drought period, plants from all stressed treatments were re-watered with nutrient solution without salt for 8 d to study recovery. Leaf water relations, gas exchange parameters, chlorophyll fluorescence, proline, quaternary ammonium compounds and leaf and root concentrations of Cl(-) and Na(+) were measured. KEY RESULTS: Salinity increased leaf Cl(-) and Na(+) concentrations and decreased osmotic potential (Psi(pi)) such that leaf relative water content (RWC) was maintained during drought stress. However, in non-salinized drought-stressed plants, osmotic adjustment did not occur and RWC decreased. The salinity-induced osmotic adjustment was not related to any accumulation of proline, quaternary ammonium compounds or soluble sugars. Net CO(2) assimilation rate (A(CO2)) was reduced in leaves from all stressed treatments but the mechanisms were different. In non-salinized drought-stressed plants, lower A(CO2) was related to low RWC, whereas in salinized plants decreased A(CO2) was related to high levels of leaf Cl(-) and Na(+). A(CO2) recovered after irrigation in all the treatments except in previously salinized drought-stressed leaves which had lower RWC and less chlorophyll but maintained high levels of Cl(-), Na(+) and quaternary ammonium compounds after recovery. High leaf levels of Cl(-) and Na(+) after recovery apparently came from the roots. CONCLUSIONS: Plants preconditioned by salinity stress maintained a better leaf water status during drought stress due to osmotic adjustment and the accumulation of Cl(-) and Na(+). However, high levels of salt ions impeded recovery of leaf water status and photosynthesis after re-irrigation with non-saline water.

Deficit irrigation and rootstock: their effects on water relations, vegetative development, yield, fruit quality and mineral nutrition of Clemenules mandarin.

Differences between rootstocks, 'Cleopatra' mandarin and 'Carrizo' citrange, in soil-plant water relations and the influence of these factors on vigor, crop yield, fruit quality and mineral nutrition were evaluated in field-grown Clemenules mandarin trees irrigated at 100% of potential seasonal evaporation (ET(c)) (control treatment), or irrigated at 100% ET(c), except during Phases I and III of fruit growth and post-harvest when no irrigation was applied (deficit irrigation (DI) treatment), for 3 years. Differences between rootstocks in plant-soil water relations were the primary cause of differences among trees in vegetative development and fruit yield. After 3 years of DI treatment, trees on 'Cleopatra' showed more efficient soil water extraction than trees on 'Carrizo', and maintained a higher plant water status, a higher gas exchange rate during periods of water stress and achieved faster recovery in gas exchange following irrigation after water stress. The DI treatment reduced vegetative development more in trees on 'Carrizo' than in trees on 'Cleopatra'. Cumulative fruit yield decreased more in DI trees on 'Carrizo' (40%) than on 'Cleopatra' (27%). The yield component most affected by DI in 'Cleopatra' was the number of fruit, whereas in 'Carrizo' it depended on the severity of water stress reached in each phase (severe water stress in Phase I affected mainly the number of fruit, whereas it affected fruit size the most in Phase III). In the third year of DI treatment, water-use efficiency decreased sharply in trees on 'Carrizo' (70%) compared to trees on 'Cleopatra' (30%). Thus, trees on 'Cleopatra' were able to tolerate moderate water stress, whereas trees on 'Carrizo' were more sensitive to changes in soil water content.