Transpiration rates of rain-fed sweet cherry (Prunus avium L.) using sap flow under warm temperate conditions

Quantifying the amount of water which sweet cherry fruit trees use daily is essential for irrigation planning. The purpose of the study was to determine the sap flow and transpiration rates (water use) of sweet cherry trees under rain-fed conditions in the warm temperate climate area of Ficksburg, South Africa, under adverse midday heatwave conditions. Transpiration for six 25-year old sweet cherry trees was measured using heat pulse velocity (HPV) and thermal dissipation probe (TDP) methods. Measurements were conducted for three months (September - November) during two consecutive growing seasons (2017 and 2018). The daily average sap flow rates measured by HPV and TDP were 3.1 and 1.93 L tree^‑1 day^‑1 and 3.71 and 4.56 L tree^-1 day^‑1, respectively for the two years. Cumulative transpiration was 270 and 420 mm for 2017 and 2018, respectively in an orchard of 3.16 ha with tree spacing of 5×3 m (15 m²). From diurnal patterns, transpiration rates depended on microclimatic conditions. Transpiration rates were very high during scorching days when the midday air temperature reached 35°C, and the incident net radiation reached 790.1 W m^‑2. The lowest transpiration rates were observed on partly cloudy to overcast days. The coefficient of determination between daytime maximum air temperature and sap flow was 0.68, and for vapor pressure deficit (VPD) and transpiration was 0.63. Sap flow increased after rainy days as a result of favorable soil water conditions. Sap flow was sensitive to day time air temperature, solar radiation and vapor pressure deficit during crucial growth stages. There was a noticeable change in sap flow rates among principal growth stages from flowering, fruit development, ripening and maturity of sweet cherry.