Comparative effects of drought stress on three olive cultivars focusing on older leaves

Drought is one of the main abiotic stresses to which plants are exposed during their growth and development under both natural and agricultural conditions. Water shortage can seriously affect also xerophytic species such as olive (Olea europaea L.), despite the numerous anatomical, morphological, and physiological adaptations to drought developed over time. Until now, the olive physiological responses to drought stress have been studied mainly on young leaves, i.e. on leaves formed after or during water stress, while very few studies have been conducted taking into consideration leaves already existing before stress (older leaves). In this work, in order to investigate the effect of drought on 1-year-old leaves (old leaves), young plants of three Italian olive cultivars, namely 'Degli', 'Leccino', and 'Maurino', were subjected to moderate water stress (30 days irrigated with 10 % of the pot capacity, T1); subsequently, the same plants were exposed at severe water stress (30 days, without water supplying, T2). The results indicate that the stress tolerance of cultivars can be due to several favorable morphoanatomical traits, such as higher percentage of palisade tissue (about 50 %) and uniform size of starch grains pre and post stress event. 'Maurino' stood out for its ability to cope with the imposed drought condition, maintaining an acceptable water status all over the experiment, and sustaining vegetative growth. Moreover, this olive cultivar showed a stability in the size of starch inclusions, all over the experiment. On the contrary, 'Degli' and 'Leccino' showed several negative changes under drought conditions, i.e. less growth of main branches, smaller starch grains size, marked reduction of photosynthetic activity, and reduction of leaf succulence, showing a less adaptive behaviour to water stress. Our work provides a more complete insight on olive older leaves response to drought.