Olive Leaves Tissues’ and Cells’ Structure under Simulated Drought Conditions

Drought is a multidimensional factor that affects morphological, physiological, biochemical, and molecular characteristics. For Olea europaea L. plants, despite the presence of mechanisms of resistance to drought conditions, high temperature and moisture deficit remain the major factors affecting their growth and development. Published data in a sufficient extent describes olives’ morphological and anatomical adaptations, while records on the leaf tissues and cells are still limited. Therefore, the aim of this work was to study the reaction of leaves of some cultivars and hybrids to simulated drought conditions at the tissue and cellular level and to determine drought-resistant genotypes. Leaves of O. europaea cultivars (Corregiolo, Ascolano) and hybrids (35-11/25, 35-16/13) cultivated at the collection plots of the Nikita Botanical Gardens were used as the object of our research. Anatomical and ultrastructural analyses were performed according to generally accepted methods. Statistical analysis was carried out using Past software version 4.03. According to obtained results, investigated cultivars and hybrids of O. europaea had a similar leaves’ structure, but differed in palisade / spongy mesophyll ratio. Study of the tissue structure under the influence of the simulated drought showed the membranes’ separation from the cell wall, starting in the spongy mesophyll (palisade—in 35-16/13 hybrid), which affected the twisting of leaf blades. The greatest reactivity of chloroplasts and mitochondria in cells was revealed after 72 h of the drought stress influence. Determination of the palisade / spongy mesophyll ratio based on the structural analysis of O. europaea leaf blades allowed us to classify the hybrid 35-16/13 as the very promising candidate to include in the list of the drought-resistant genotype. At the ultrastructural level, significant changes were shown in the chloroplasts and mitochondria of leaf cells of the investigated plants. The hybrid 35-16/13 demonstrated greater preservation of organelles after 72 h of simulated drought exposure. Based on the obtained results, we propose that the leaf folding degree under drought is considered as the morphological criterion of drought-resistance, while numerical values of the chlorenchyma types’ ratio can be used as an additional parameter for olive plant selection.