Stomatal Number and Size in Horticultural Tree Species: Seasonal and Varietal Variation to face different climatic conditions

Stomatal traits play a fundamental role in plant gas exchange, balancing CO₂ uptake and water loss to optimize photosynthesis and water use efficiency (WUE). This study investigated stomatal characteristics in six horticultural tree species (grape, pomegranate, pecan, carob, mulberry and lemon) grown either in the University of Bari repository or in commercial orchards. Stomatal density (SD) and size (length, width and area) (SS) in Southern Italy over two growing seasons (2019 and 2020) were analysed by coating both the abaxial and adaxial sides of the leaves (and leaflets in the case of pecan and carob) with clear nail polish which was then removed with a transparent adhesive tape. The results revealed significant interspecific and intraspecific variations, highlighting species-specific adaptive strategies of either varieties or clones. Grapevine exhibited the largest stomata area (average 488 μm²), with a wide inter-varietal variability, ranging from 284 μm² in Verdeca #19 to 726 μm² of Negramaro #12, whereas lemon and pomegranate had the smallest ones (120-192 μm²). The SD ranged from 165/mm² in grapevine to 712/mm² in lemon, with intermediate values in carob (214/mm²), mulberry (468/mm²), pecan (505/mm²), and pomegranate (593/mm²). Seasonal effects were species-dependent: pecan and grapevine consistently showed a reduction in SD, decreasing from 555 to 461 and from 178 to 152 stomata/mm² respectively, while grapevine also exhibited a decrease in SS, from 508 μm² in 2019 to 481 μm² in 2020. Conversely, pomegranate increased both SD (576 to 608 stomata/mm²) and SS (156 to 201 μm²) in 2020 with respect to 2019, suggesting a different adaptive strategy since the 2019 was a drier summer season with respect to 2020. These findings underscore the role of stomatal traits in species as a tool for a better resilience to climatic variability. The results also provide valuable insights into optimizing fruit tree selection for the different agricultural areas, with important implications for a sustainable agriculture in the context of climate change where water availability is a key factor.