Detection of Water-deficit Stress from Daily Sap Flow Profiles in Peach

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Eight-year-old peach trees (Prunus persica (L.) Batsch cv. ‘Hakuho’ grafted on ‘Ohatsumomo’ peach rootstock), planted in 1500 L beds in a plastic greenhouse to avoid rain and groundwater effects, were used to evaluate the relationships among sap flow (SF), water-deficit stress parameters, and atmospheric conditions. Under a mild water-deficit stress condition, sap-flow velocity (SFV) decreased in the afternoon but not in the morning. Two indices were derived from the daily SFV profile: i) the slope of the regression line of SFV in the daytime (index C) and ii) the decreasing rate of SFV from the morning to each hour in the afternoon (index D). These indices showed higher correlation coefficients to the tree water status expressed by predawn leaf water potential (ΨPD) (r2 = 0.41) and maximum daily shrinkage of the trunk (MDS) (r2 = 0.58**) than to daily total SF (index A) and SFV at each hour in the daytime (index B). In a nonstressed condition, SFV was more highly correlated to photosynthetic photon flux density (PPFD) (r2 = 0.84**) than to temperature, relative humidity, and vapor pressure deficit (VPD). The correlation coefficients (r2) of indices with MDS and ΨPD increased to 0.77** and 0.92**, respectively, when the SFVs of low light intensity (< 800 μmol·m−2·s−1 of PPFD) were eliminated. The index which most precisely detected water-deficit stress was the decreasing rate of SFV from the morning (07:00–11:00 h) to any hour from 12:00 to 15:00 h with the elimination of SFV values when PPFDs were low.