Effects of Elevated Temperature and Water Deficiency on Functional Traits of Vitis vinifera L. cv. Assyrtiko Leaves

(1) Background: High temperatures and water scarcity, intensified by climate change, present major challenges to Mediterranean viticulture. In this study morphological, anatomical, and physiological responses of leaves at two developmental stages of <i>Vitis vinifera</i> L. cv. Assyrtiko were examined. (2) Methods: Young expanding and fully expanded leaves of two-year-old grapevines grown in pots in a controlled growth chamber, grown in soil in an open-air vineyard, and an adjacent greenhouse on Aegina Island (Greece), were used. The plants were subjected to water deficit (WD), elevated temperature (ET), and combined water deficit and elevated temperature (WD_ET) conditions for four months, and compared with control plants (C). (3) Results: Stress conditions caused contrasting effects on specific leaf area (SLA), which declined in young expanding leaves, except for a significant increase under elevated temperatures, while it increased in fully expanded leaves exposed to stress treatments. Concerning the anatomical traits, the thickness of spongy and palisade parenchyma did not change in young expanding leaves exposed to the three stress treatments, but increased in fully expanded leaves exposed to WD_ET. Metabolic traits (phenolic compounds, soluble sugars, and starch content) further highlighted these differences. (4) Conclusions: The findings reveal distinct stress responses between young expanding and fully expanded leaves of the grapevine Assyrtiko, underscoring the influence of water deficit and elevated temperatures on leaf developmental stage.