Effects of Topping Date and Preharvest Irrigation Cut-Off on Some Qualitative and Quanitative Characteristics of Table Grapes (Vitis Vinifera L.) Cv. Askari

Background and Objectives Table grape (Vitis vinifera L.) is one of the most important horticulture crops in Iran. In some regions, due to excess water and good climate, the vegetative growth is more than normal. The canopy plays a key role in light energycapture via photosynthesis apparatus, water use as regulated by transpiration, and microclimate of ripening grapes. Canopy management is an important aspect of viticulture due to its effect on grape yields, quality, vigor, and the prevention of grape diseases. Where vineyards have access to a permanent and unlimited water source, irrigation can be managed so that water stress is imposed during certain periods of time to increase fruit quality and to control canopy. This study was conducted to investigate the effects of different levels of topping times and preharvest cut-off irrigation treatments on the qualitative and Quanitative characteristics of Askari grapes in Dena (Sisakht) region during 2013-2015.
Materials and Methods ‘Askari’ vines were 18 years old on their own roots, trained as a head system. The vines were spaced 2.5 × 3 m. They were pruned as spur pruning which left 60 buds (4-node spurs) on each vine. Vineyard had a drip irrigation system. Shoot topping done at 10th buds of the last cluster. An experiment was arranged as factorial that the first factor included different levels of topping dates with four levels (control, after fruit set, unripe stage and veraison stage), and the second factor involved the preharvest irrigation cut-off date treatments on four levels (Control, 70, 80 and 90 days after full bloom) in a randomized complete blocks design with four replications. Factors relating to the determination of the quality of fruit juices included %TSS, pH, TA%, vitamin C and TSS/TA ratio respectively. Also, bunch weight and yield were measured.
Results Results of the analysis of variance showed that the effect of topping and irrigation cut-off date treatment were significant on the TSS, TSS/TA, pH and Vit-C content (P≤0.01). But the interaction effects were significant only on the TSS/TA and Vit-C (P≤0.01). Finally, mean comparison showed that the highest vitamin C was observed at veraison stage and irrigation cutoff dates treatments at 80 and 90 days after full bloom.
Discussion In some of vineyards, high vigour vines also have too much canopy shading which is detrimental to the light microclimate and has also been shown to lead to increase of TA and decrease of TSS and TSS/TA ratio. Therefore, canopy manipulation is used successfully in grape production to balance the vegetative and reproductive growth of vines. Differences in vine water status before veraison has been shown to have no effect on the onset of veraison, while withholding water after veraison can delay the accumulation of soluble solids under severe water deficits. Certainly, light and temperature are the most important environmental conditions that impact the growth and quality of grapes and unshaded conditions affect better coloring and increase TSS/TA ratio.