Effect of Shading on Vine Morphology and Productivity and Leaf Gas Exchange Characteristics in Grapevines in the Field

Abstract

The photosynthetic activity of grapevine leaves (Sangiovese/Kober 5BB) was evaluated under field conditions on mature vines grown under three different levels of photosynthetically active radiation (PAR, 100%, 60%, and 30% sunlight) by measuring the response of net photosynthesis (Pn) to PAR at flowering and veraison. The diurnal trends of PAR, Pn, leaf transpiration rate (E), stomatal conductance to HL₂O vapor (g_s), substomatal CO₂ concentration (C_i), and leaf water potential (ψ) were determined. Responses of Pn to PAR were analyzed using asymptotic exponential curves, which provided estimates of the radiation saturated rate of Pn (Pn_sat), dark respiration (R_d), light compensation, and saturation points (PAR_c and PAR_sat, respectively) and the apparent quantum yield of CO₂ assimilation (φ_i). At flowering and veraison, the leaves of shade-grown vines (60% and 30% sunlight) showed significantly lower Pn_sat, R_d, PAR_c, and PAR_sat values, whereas the φ_i was significantly higher. In comparison to unshaded vines, the Pn_sat in the vines grown at 60% and 30% sunlight measured between 0900 and 1100 hours was about 62% and 54% at flowering and 81% and 65% at veraison, respectively. At both phenological stages, the diurnal pattern of Pn, g_s, and ψ were positively correlated with PAR. Leaf transpiration rate was significantly reduced by shading in the early morning during flowering and in the early afternoon during veraison, whereas the calculated C_i was unaffected. All of these modifications that occurred under shading are associated with a decrease in specific leaf dry weight, leaf soluble carbohydrates and starch content, vine yield, total soluble solids in the berries, total leaf area per vine, number of axillary shoots per cane, and winter pruning weight. The increase of φ_i, leaf chlorophyll content, and the change in the growth habit to a more open canopy increased the PAR trapping efficiency, which indicates an adaptability of the grapevine to low light intensity. Vine yield and berry quality decreased linearly with increasing shade intensity. A profitable management strategy is, therefore, necessary in order to assure that most of the leaves receive approximately 700 to 900 µmol m^-2 S^-1 of PAR for the greater part of the day during the entire crop cycle. All the factors that could modify the light availability at the canopy level during the growing season, such as different vineyard exposure, cloudiness, windbreak and horizontal hail netting presence, tall trellis system, and excessive shoot vigor, must be correctly evaluated in order to reduce the risks of low photosynthetic activity, low vine yield, and poorer grape quality.