Improving table grape quality with less irrigation water in Japan: Partial root-zone drying versus regulated deficit irrigation

In 2004 and 2005, we investigated the effects of pre- and post-veraison irrigation strategies and schedules on vine water relations and quality of 'Muscat of Alexandria' table grapes (Vitis vinifera L.) grown under rain-protected and root-zone restricted conditions at the Okayama University Experimental Vineyard, Japan. The treatments included: (1) standard practice irrigation (Control): irrigated when the soil moisture tension reached 15 kPa; (2) regulated deficit irrigation (RDI): irrigated four to seven days after reaching a soil moisture tension of 15 kPa; (3) fixed partial root-zone drying (FPRD): one side of the root system was irrigated when the soil moisture tension reached 15 kPa; and (4) alternate partial root-zone drying (APRD): one side of the root system was irrigated when the soil moisture tension reached 15 kPa, and every week or two weeks the irrigated side was alternated. We found 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 significantly higher total soluble solids (TSS), fructose and amino acids, lower malic acid and firmness, and smaller size compared with the control. FPRD and APRD fruits had lower malic acid and a similar firmness and size compared with the control. Aroma volatile concentrations were high in RDI fruit. The effects of the irrigation treatments on the development and survival of the water-stressed roots as well as on the growth of new shoots and clusters in the following season are also presented.