Diurnal and seasonal physiological changes in leaves of <i>Vitis vinifera</i> L.: co₂ assimilation rates, sugar levels and sucrolytic enzyme activity

Abstract

Changes in photosynthetic rates, sugar contents, and sucrolytic activity (invertase and sucrose synthase) in young (apical) and mature (basal) leaves of Vitis vinifera L. were investigated throughout the growth season. Photosynthetic activity of basal leaves was predominant before the berries reached pea size, but declined to low rates during grape ripening and after harvest. Apical leaves, on the other hand, showed a more or less uniform pattern of photosynthesis during the whole season with higher assimilation rates than basal leaves after the onset of ripening (veraison). High photosynthetic rates in young material are likely to be sustained by a continuous demand for assimilates from local or adjacent sinks.
Diurnal fluctuations of sucrose, glucose, and fructose were similar in apical and basal leaves. Other than the photosynthetic rates, sucrose levels in young leaves tended to be somewhat higher than in mature leaves early in the season, but this balance was reversed after veraison. Generally, low sucrose concentrations correlated with high hexose values (berry set) and vice versa (post harvest). Increased sucrose levels were observed under conditions where carbon import or export limitation is expected to prevail.
In comparison with the presumably normal situation observed before veraison, apical leaves of grapevines after extended predarkening showed substantially lower sucrose levels than basal leaves and very small amounts of hexose were detected in either group. This suggests that considerable sink demand for hexose and also sucrose was imposed on tile system by the dark period. Rapid enzymic conversion of sucrose to hexose is guaranteed by high sucrose synthase and invertase activities at least ingrowing material and early in the season. Persisting, albeit low, photosynthetic rates and stable invertase activities in mature leaves throughout maturation and after harvest are taken as indicative of the latent assimilatory competence of basal leaves to sustain maintenance metabolism and contribute to the perennial carbohydrate storage pool of the vine.