SENSITIVITY AND VARIABILITY OF TWO PLANT WATER STRESS INDICATORS: EXPLORING CRITERIA FOR CHOOSING A PLANT MONITORING METHOD FOR AVOCADO IRRIGATION MANAGEMENT

Avocado (Persea americana Mill.) is a fruit-tree species highly susceptible to water deficit, which makes irrigation management a difficult task for growers. When irrigation is inadequate, trees suffer growth reduction, fruit losses, and roots damage. This study addressed the question of how to assess water stress in avocado trees and the considerations to choose an indicator to measure the plant water stress. In this work the sensitivity and variability of two water stress indicators in response to water deficit were analyzed: stem water potential (SWP) and maximum daily trunk shrinkage (MDTS). During a period of high water demand, avocado trees planted in a clay loam soil were subjected to water stress by withholding irrigation and compared to control trees irrigated according to the maximal crop evapotranspiration. During the study, avocado trees reached a minimum SWP of -0.9 MPa and a maximum MDTS of similar to 285 mu m. To better understand avocado tree response to water deficit, leaf abscisic acid, stomatal conductance, soil moisture, and vapor pressure deficit were also measured. Interestingly, it was found that water stress indicators showed differences with control after 3 d of withholding irrigation. It was possible to observe that MDTS was more sensitive in detecting water stress than SWP, signal strength of 4.5 vs. 1.2 respectively; however, MDTS higher variability counteracted its performance as stress indicator, coefficient of variation of 32% vs. 9%, respectively. This study confirms that monitoring water stress is an important tool for avocado irrigation management and should consider both, the sensitivity and variability of the indicator.