Grapevine Sap Flow in Response to Physio-Environmental Factors under Solar Greenhouse Conditions

Understanding transpiration responses to physiological and environmental factors is essential for efficient water management practices in greenhouse grapevine farms. To determine the driving factors of grapevine sap flow under solar greenhouse conditions in a typical cold climate, the sap flow, greenhouse micro-environmental conditions, and canopy details were measured and analyzed for the 2017–2018 growing season in Northeast China. The results showed that leaf area index controlled the upper boundary of sap flow rate (SFR). Correlations between SFR and meteorological factors obviously varied with time scales. Besides, the correlations at the hourly scale varied across the seasons. Photo-synthetically active radiation (PAR) was the primary control factor of sap flow, irrespective of time scale or season. The start and stop times of sap flow did not change with weather conditions, but SFR had broader peaks with higher peak values during sunny days. The diurnal variation of SFR lagged behind that of PAR, but remained ahead of those of VPD and temperature. Weather condition changed the sizes of the hysteresis loops, but not the rotation direction. The hydrological and physiological processes involved in sap flow are useful for refining transpiration models and improving water use efficiency in the greenhouse environment.