Sensing water status in peach trees with sap flow meters, fruit gauges and leaf thermal images

Abstract

Recently, measurements of water status, such as stem water potential, have been proposed as irrigation tools in various fruit trees. However, measuring water potential with pressure chambers requires specific skills and a lot of labour and time for a representative sampling in the orchard. The use of proximal sensors may allow for accurate, continuous, and fast acquisition of data that directly and indirectly relate to tree water status. The use of leaf thermal images is also a valuable and non-destructive indicator of plant water status. In this study, crop water stress index (CWSI), sap flow rate (SFR), absolute fruit growth rate (AGR), and greatest daily difference in fruit variation (ΔD) were related to stem water potential (SWP) to identify single or combined non-destructive measures that could serve as indicators of peach tree water status during fruit development. At stage I of fruit growth, no significant relationship between SWP and ΔD, AGR or SFR was detected. On the contrary, at stage III when trees had reached relatively low SWP (-2 to -2.5 MPa), a direct relationship was found between SWP and SFR (R2=0.223) and between SWP and ΔD (R2=0.117), while a negative relationship was found between SWP and CWSI (R2=0.113). AGR was also inversely related to SWP (R2=0.320), confirming the ability of fruit sinks to attract more water and assimilates than other organs during days of high evaporative demand and low SWP. Overall, fruit gauges alone or in combination with sap flow may provide good estimates of peach tree water status and ultimately give useful information to develop an automated and precise irrigation system for peach orchards.