Improving the Quality of Yaghouti Grapes (Vitis vinifera L.) under Drought Stress Conditions in Sistan Region

Yaghouti grape (Vitis vinifera L.) in the Sistan region is subjected to water deficit, which is one of the main causes of reduced fruit quality. To improve the qualitative attributes of Yaghouti grape, an experiment was conducted at the agricultural and natural resources research and education station of Zahak during 2020–2024. The irrigation regimes included: full irrigation (control), irrigation after 35% depletion of available soil water (moderate stress), and irrigation after 70% depletion of available soil water (severe stress). Irrigation stages consisted of: bud swelling to flowering, flowering to berry color change, berry color change to harvest, and harvest to leaf fall. Results indicated that moderate water stress applied during the flowering to berry color change stage increased the soluble solids content (sugars, sweetness index) of the grapes. However, severe stress during this stage enhanced berry sunburn and reduced fruit quality. Severe stress during berry color change to harvest increased titratable acidity and decreased berry pigmentation, resulting in poorer cluster appearance and undesirable taste. Reduced irrigation during the flowering to berry color change stage also lowered cluster compactness; nevertheless, due to increased sunburn and reduced sugar accumulation, the marketability of the fruit declined. Interestingly, the fruit flavor index was enhanced under moderate stress during the berry color change to harvest stage. Overall, the findings suggest that applying moderate water stress during the berry color change to harvest stage in the Sistan region not only improves water-use efficiency but also enhances the flavor quality of Yaghouti grape.