Temporal dynamics of photosynthetically active radiation in the fruit zone of cv. Istrian Malvasia following early defoliation

Abstract

The aim of this study was to investigate the impact of early defoliation on the temporal dynamics of the photosynthetically active radiation (PAR) in the fruit zone of Istrian Malvasia (Vitis vinifera L.) vines. Defoliation of six basal leaves per shoot was performed either pre-bloom or at berry set on vertically shoot positioned Istrian Malvasia vines. Control treatment without defoliation was also included. Fixed radiation sensors were used to intercept PAR in the fruit zone of each treatment as well as above the top of the canopy in order to measure total incident radiation. Berry set defoliation treatment had the highest mean daily PAR values inside the fruit zone from the phenological stage berries pea size until harvest, while control treatment vines had the lowest mean daily PAR values due to higher shading of leaves from the primary shoots. Pre-bloom defoliation treatment had higher PAR values than control treatment, although considerably lower than berry set treatment. Higher PAR values in berry set treatment in comparison to pre-bloom treatment is a consequence of later and less intensive lateral regrowth in berry set treatment. The highest PAR values in the fruit zone during the day were observed from 9 to 11 am and from 4 to 6 pm. During this period the differences in PAR among treatments were most expressed. From 12 am to 3 am PAR values in the fruit zone were lower and differences among treatments were less expressed. As PAR affects carbon assimilation of leaves and total dry matter production, the results of this study affirm the positive effects of early defoliation in the production of Istrian Malvasia grapes and indicate that this practice is suitable to avoid the formation of shaded canopies which are usual for this cultivar.