Vine water relations and quality of 'Muscat of Alexandria' table grapes subjected to partial root-zone drying and regulated deficit irrigation

The effects of several irrigation strategies on vine water relations and the quality of table grapes, Vitis vinifera L. 'Muscat of Alexandria', were evaluated from veraison to harvest. The treatments included: (1) standard practice irrigation (control): re-irrigation when the soil moisture tension reached 15 kPa; (2) regulated deficit irrigation (RDI): re-irrigation 4 to 7 days after reaching a soil moisture tension of 15 kPa; (3) fixed partial rootzone drying (FPRD): one half of the root system was re-irrigated when the soil moisture tension reached 15 kPa; and (4) alternate partial root-zone drying (APRD): one half of the root system was re-irrigated when the soil moisture tension reached 15 kPa, and every week the irrigated half was switched. Treatments were continued for 7 weeks until harvest. During the experiment, RDI vines received 58% less irrigation water than the controls, while 33% less irrigation water was applied to FPRD and APRD vines. The results showed that only RDI vines had a significantly lower midday stem water potential compared with the control. Vines of FPRD and APRD treatments had the highest efficiency of water use (photosynthetic rate/stomatal conductance). At harvest, RDI fruit had markedly higher TSS, sugars, and amino acids, a similar acidity, and lower firmness and smaller size compared with the control. FPRD and APRD fruits had slightly higher TSS, sugars, and amino acids, a similar firmness and size, and lower acidity compared with the control. Aroma concentrations were high in RDI and FPRD fruits.