Kiwifruit ecophysiological adaptations under moderate and severe deficit irrigation

Precision irrigation scheduling in fruit crops requires good knowledge of water relations. Knowing plants’ behavior and the strategies to be adopted under water stress allows irrigation scheduling to maximize water productivity and minimize water losses. Therefore, the aim of this study is (i) to analyze the relationships between sap flux density, leaf stomatal conductance and fruit diameter daily fluctuations, understanding the water dynamics among plant tissues, and (ii) to understand the effect of deficit irrigation on the fruit size and quality. Actinidia chinensis var. chinensis vines were submitted to four irrigation treatments, applied as percentages of crop evapotranspiration (ETc): 100 %, 68 %, 57 % and 40 % of ETc. Four vines per treatment were monitored with sap flow probes, using the Tmax method, and the same vines had fruit gauges installed to obtain continuous measurements of fruit diameter variations. Measurements of leaf gas exchange were performed throughout the day on six days during the season. Fruit dry matter content was also measured three times during the season and at harvest, together with fruit quality parameters (soluble solids content, firmness, and titratable acidity) at harvest. The comparison of the daily dynamics of sap flux density and leaf stomatal conductance reveals that sap flow continues rising after the beginning of stomatal closure, indicating refilling of storage tissues such as branches and leaves. However, fruit refilling starts at night, when there is less competition for water. Fruit average diameter (Øf) was significantly higher at the 68 % ETc treatment in comparison to control (100 % ETc), being this also significantly higher than the other deficit irrigation treatments (57 % and 40 % ETc). A vapor pressure deficit threshold was identified, above which stomata start to close, regardless of irrigation treatment. Deficit irrigation affected negatively kiwifruit vines sap flux density. Optimized irrigation management, avoiding overirrigation, might lead to higher fruit dry matter content without significantly reducing fruit diameter.