Leaf mechanisms for drought resistance in Zizyphus jujuba trees

No information exists on the mechanisms developed at the level of leaf water relations by pear-jujube trees (Zizyphus jujuba Mill.) to confront drought. For this reason, the purpose of the present study was to analyse its leaf water relations in order to clarify the resistance mechanisms (avoidance and tolerance) developed in response to a water stress and during recovery. Field-grown 7-year-old pear-jujube trees (cv. Grande de Albatera) were subjected to three irrigation treatments. Control (T0) plants were drip irrigated (112% ETo) in order to guarantee non-limiting soil water conditions, T1 plants (deficit irrigation, 64% ETo) were drip irrigated according to the criteria used by the grower and T2 plants irrigated as T0 but subjected to water withholding for 36 days and a subsequent re-irrigation at the levels used in T0 for 14 days, during the summer of 2011. The results indicated that pear-jujube plants confront water stress by developing stress avoidance and stress tolerance mechanisms. From the beginning of deficit irrigation (T1) and water withholding (T2) to when maximum water stress levels were achieved, leaf turgor was maintained allowing substantial gas exchange levels and, consequently, good leaf productivity. This leaf turgor maintenance was mainly due to two simultaneous and complementary mechanisms. Leaf conductance and the duration of maximum stomatal opening in water stressed plants decreased in order to control water loss via transpiration, contributing to maintain leaf turgor (stress avoidance mechanisms). Also, the gradual recovery of gₗ observed after rewatering the plants can be considered as a mechanism for promoting leaf rehydration. In addition, from the beginning of the stress period, active osmotic adjustment operated, also contributing to the maintenance of leaf turgor (stress tolerance mechanism). The high RWCₐ levels and the possibility of increasing the accumulation of water in the apoplasm in response to water stress, supporting a steeper gradient in water potential between the leaf and the soil, which can be considered another drought tolerance characteristic in pear-jujube.