Differential effects of canopy manipulation and shading of <i>Vitis vinifera</i> L. cv. Cabernet Sauvignon. Leaf gas exchange, photosynthetic electron transport rate and sugar accumulation in berries

Abstract

Partial cluster and leaf removals were performed on Vitis vinifera L. cv. Cabernet Sauvignon at veraison during two years to give 4 treatments: control (C), cluster thinning (CT), leaf removal (LR) and cluster thinning + leaf removal (CT+LR). A half of each plot was shaded by a 50 % shading net at veraison (40 % berries coloured).
Shading significantly reduced stomatal conductance but not CO₂ assimilation rate (P_n) and carboxylation efficiency. P_n was decreased by cluster thinning and enhanced by leaf removal. Leaves of CT vines showed a photosynthetic decay 2 days after the treatment while LR leaves presented an afternoon photosynthetic enhancement 3 days after the treatment probably due to a higher photoassimilate requirement of the bunches (sink). Stomatal conductance did not totally contribute to the P-n depression or enhancement. Electron transport rate and maximum Rubisco activity were strongly affected by CT and LR suggesting that photochemical and biochemical processes were affected to a greater extent than physical processes.
Juice sugar concentration was reduced by shading, an effect explained straightforwardly by the lessening of stomatal conductance. CT and LR had large effects on biochemical and photochemical processes but these were in the opposite direction to the expected effects on juice sugar concentration. It is suggested that CT increased sugar concentration because of the low fruit sink load per vine, and that LR decreased sugar concentration because of the low leaf area per vine.
If cluster thinning is to be used by growers to gain increased sugar concentration it is recommended to do it at veraison; the sugar accumulation rate is then high and the adaptation of photosynthetic processes to thinning takes several days.