Determination of the threshold of soil water potential (psi) at which point irrigation should be considered in horticultural plants is important for efficient water use, especially in drylands. The purpose of this study was to determine the threshold for scheduling irrigation during the last week of stage 1 of berry development. Pione grapevine ( Vitis vinifera L., cv. V. labrasca) was allowed to experience decreasing psi monitored by tensiometers after an irrigation event. Berry diameters were measured every hour for 5 days using photogrammetry, a new methodology for monitoring berry size. Net photosynthesis and transpiration rates were measured at various intervals by a portable photosynthesis system. The results showed that after irrigation, the psi decreased slowly during the first 2 days, but started decreasing rapidly when it decreased -6.2 kPa at 10 cm soil depth. Berry diameters increased at night and decreased during the day, and were sensitive to soil moisture status. Berry diameter increased rapidly after irrigation till psi became -3 kPa. In the psi range of -3 to -5.4 kPa, the growth rate decreased, and as psi decreased beyond -5.4 kPa, the berry started shrinking and the shrinkage showed a strong linear relationship with decreasing psi. However, photosynthesis and transpiration rates were unaffected until psi became -9.3 kPa or less. Thus, berry diameter was a more sensitive indicator of water stress than photosynthesis and transpiration. The value of -5.4 kPa should be considered as the threshold psi for scheduling irrigation in the last phase of stage 1 of berry development in grapevines.
