Water deficits are considered the primary environmental stress in agriculture, and improving the growth and production of plants under this stress is one of the primary goals of breeding and crop management programs (Cattivelli et al., 2008; Queiroz-Voltan et al., 2014). Cultivating apple trees (Malus domestica) in environments with water deficits affects the apple mass, thus reducing production (Ebel et al., 2001). Biochemical changes can also occur in leaves under water deficit conditions, thereby triggering changes during the initial stages and leading to an increase in the concentration of protective substances (oxidizing substances) (Sircelj et al., 2005). Almond trees grown under water deficit conditions develop several adaptive mechanisms such as osmotic adjustment, changes in the elastic properties of cells and tissues, foliar abscission, stomatal regulation, and a deep root system (Oliveira et al., 2018).
Another peculiarity of plants that grow under water deficit is the development of strategies that allow the plants to perform photosynthesis. One significant strategy is the reduction of the cellular osmotic potential (Batista et al., 2010) through the accumulation of liquid solutes (Morgan 1984; Wright et al., 1997). This mechanism results in the maintenance of cell turgor and the opening of the stomata, allowing the plant to accumulate solutes such as carbohydrates, amino acids, and organic acids (Chartzoulakis et al., 1999; Nunes et al., 1989).
In addition to physiological and biochemical changes, the leaves of the plants under water deficit undergo morphological changes. Changes in the leaf blade area, such as the increased thickening of the cuticle, the adaxial and abaxial epidermis, and the palisade and spongy parenchyma, are evidence that plants can respond adaptively to different environments (Batista et al., 2010; Bosabalidis; Kofidis, 2002; Castanheira et al., 2016; Grisi et al., 2008; Oliveira et al., 2018; Queiroz-Voltan et al., 2014).
Many authors have performed morphological studies primarily of leaves, which are fast-adapting organs, to assist in their selection of cultivars, progenies, or accessions of different species to adapt to water deficits. The results have shown that these changes can be inherited (Batista et al., 2010; Bosabalidis; Kofidis, 2002; Castanheira et al., 2016; Grisi et al., 2008; Oliveira et al., 2018; Queiroz-Voltan et al., 2014; Viana et al., 2018). In fact, one of the main characteristics evaluated for the description of leaves of plants for adaptation to the water deficit involves the stomatal properties and water relations (Zokaee-Khosroshahi et al., 2014). Analyses of genetic diversity through multivariate techniques have been used in breeding programs to assist in assessing genetic predispositions (Azevedo et al., 2013; Ivoglo et al., 2008; Moreira et al., 2009).
Changes in the internal structures of apple leaves in areas with water deficits are important. To assist early pre-breeding programs in introducing cultivars in water deficit regions or in selecting potential progeny for future crosses, the aim of this study was to compare the anatomical characteristics of apple leaves from two different environments (water deficit and precipitation) under tropical conditions.
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