Apple tree transpiration during cycles of progressive drought as assessed via continuous gravimetric and xylem sap flux measurements

As climate change is contributing to increase the pressure on available fresh water, there is a need to develop strategies to reduce the irrigation volumes, without adverse effects on crop productivity. Against this background, a study was conducted in South Tyrol (Italy) with the aim of characterizing apple short-term transpiration responses to progressive soil water depletion using sap flow sensors in combination with a weighing system to estimate the whole tree transpiration (T_grav). Two cycles of progressive water drought were induced within three potted apple trees under semi-controlled conditions. Sap flow rate, based on the Heat ratio method (HRM), and actual gravimetric transpiration measurements have been performed constantly throughout the whole experiment. Plant water status was assessed by midday stem water potential (psi_stem) and leaf gas exchange measurements. Tree transpiration ranged from 0.6 to almost 2 L m^-2 day^-1, depending on the daily climatic conditions. Plant water status was significantly affected by soil water availability only under severe drought conditions corresponding to a psi_stem = -1.75 MPa. The sap flow sensors used in this experiment well responded to both the changes in the environmental conditions and in the soil water availability. Daily transpiration values obtained from the sap flow sensors (T_sap) data were well correlated with the actual transpiration rates (T_grav), but largely underestimated it. Under soil water limitations, the sap flow rate was especially low in the central part of the day and in the afternoon, but not in the morning. HRM sensors were able to record nonnegligible nocturnal fluxes representing about 10% of the total daily fluxes.

DOI: 10.26353/j.itahort/2022.2.3546

Keywords: apple, transpiration, water stress, sap flow, nocturnal fluxes, heat ratio method