Morpho-anatomical and Physiological Characteristics of Primary and Lateral Shoot Leaves of Cabernet Franc and Trebbiano Toscano Grapevines Under Two Irradiance Regimes

Abstract

The morpho-anatomical and physiological characteristics of leaves from primary and lateral shoots in response to shade during their ontogenetic development were analyzed in two field-grown grapevine varieties (Cabernet Franc and Trebbiano Toscano). Despite their smaller size, lateral leaves had similar relative water content, specific weight, stomatal density and size, chlorophyll a and b, carotenoids, and nitrogen contents as primary leaves. Chloroplast length, number of chloroplasts per spongy cell, and thickness of epidermis and palisade parenchyma were also similar. During post-veraison, harvest, and just before leaf fall, the lateral leaves had higher photosynthetic (Pn) and transpiration (E) rates than the primary leaves. Under low light conditions (< 300 µmol photons m^-2 s^-1) lateral leaves had a lower Pn than did primary leaves as indicated by a lower rate of bending of the photosynthetic light response curves (θ). In response to lowered light availability during growth, leaves modify their pigment composition (significantly higher chlorophyll a and b and carotenoids content and lower chlorophyll a/b ratio) and gas exchange characteristics (higher quantum yield and θ and lower Pn, E, stomatal conductance, dark respiration, and light saturation and compensation points) and reduced stomatal density, epicuticular wax content, specific leaf weight, and hair density in the abaxial leaf surface. Sun leaves, with higher Pn activity, had more nitrogen content than shade leaves. The abscission of leaves from lateral shoots occurred 10 to 14 days after the fall of leaves from primary shoots. The carbohydrates fixed by the lateral leaves during this period may be very useful for the reconstitution of the reserves.