UC Davis

ABSTRACTIrrigation indices in almond: a comparison with an improved sap flow method Heather K. Vice, Master of Science, 2021 Reliable strategies to assess crop water use are a persistent challenge across the agricultural sector with predicted shifts in annual precipitation patterns. Sap flow techniques provide valuable measurements of transpiration within a given plant, and thus may be useful for informing irrigation, but there are many methods from which to choose. It is also unclear how to best use sap flow data to effectively detect stress thresholds required to trigger irrigation events. The aim of the present work was to assess the potential to use continuous estimates of plant water use provided by sap flow as a complement to, or substitute for other plant-based irrigation predictors. I examined the suitability of three heat pulse methods — the double-ratio, heat-ratio and compensation heat-pulse methods (DRM, HRM and CHPM) — to estimate normalized sap velocity (NV) and normalized isothermal canopy conductance (NG, [∝ NV/VPD]) in almond [Prunus dulcis (Miller) D.A. Webb] orchards located in a semi-arid Mediterranean environment. Measurements of sap flow in twelve 20-year-old trees and three irrigation demand indices — estimated crop evapotranspiration (ETc), soil water content (θ) and stem water potential (Ψstem) — were conducted during two growing seasons, from June to September 2016 and 2017. During well-watered periods of July, transpiration measures given by all irrigation indices demonstrated the limitations of Ψstem and θ as reliable indicators of plant water stress. Both sap flow and canopy conductance elicited clear patterns of the fluctuations in transpiration responses to changes in both θ and high evaporative demand in August, when irrigation was reduced for harvest, declining in response to low θ in comparison to relatively high unchanging rates of ETc. Ψstem followed a similar, yet slower, pattern illustrating the relationship between VDRM and Ψstem, consistent with the concept of the “baseline” water potential varying with VPD. NG plateaued early in the day and decreased much earlier than VDRM in the afternoon, indicating stomatal closure in response to elevated VPD. These data confirm the robust performance of the DRM in almond trees and reveal that stomatal regulation of transpiration in almond is more sensitive to VPD when θ is low. Normalized isothermal canopy conductance inferred from VDRM and VPD is a promising and easily-implemented tool for interpreting short- and long-term dynamics of almond tree water status and their responses to abiotic stress.