Excised Leaf Desiccation (ELD) as a Tool to Distinguish Desiccation Tolerance Among Almond Genotypes

Water scarcity is a major concern for almond growers worldwide. To achieve sustainable almond production, it is crucial to identify strategies for improved water use efficiency including new cultivars compatible with the improved irrigation strategies. Pulsed-irrigation strategies have been shown to provide improved water use efficiency but make orchards highly vulnerable to catastrophic irrigation failures because, while the tree-zone is continuously provided with sufficient water to meet current transpiration needs, insufficient surplus water is available in the root zone to supply tree needs if irrigation is interrupted for as little as a few hours under high heat stress (McClymont and Goodwin, 2016). Excised shoot desiccation is a method for quickly evaluating cultivar differences in rapid leaf water loss that involves exposing excised shoots to desiccation under dry environments to evaluate genetic differences in subsequent leaf tissue damage. This approach appears useful as a rapid screen for tolerance to catastrophic irrigation failures based on early field performance (Appendix A). However, the underlying physiological mechanism as well as the consequence on tree productivity are unknown. To better understand the physiological basis, leaf characteristics of twenty almond genotypes were measured to examine their relationship with plant responses to rapid and severe water stress. We hypothesize that differences in the rate of excised shoot desiccation will be determined by genetic differences in the rate of leaf water loss, primarily by water loss through stomata and secondarily by waterloss through remaining epidermal tissue, and that these basic mechanisms will also determine differences in whole tree rate of water loss under stressful field conditions. Excised shoots of the twenty genotypes were observed over 24 hours and excised shoot desiccation (ESD) scores were determined according to standardized wilting patterns ranging from 0 to 5 (0 for no desiccation, 5 for high desiccation). In subsequent experiments, excised leaves were used as a simplified model-system to evaluate differences in water retention under stress as previously described by McCaig and Romagosa (1991). Excised leaf desiccation (ELD) scores were determined using the same standardized wilting patterns used for ESD scoring. Relative water content (RWC) of sample leaves was recorded over time and the pattern of water loss with time was analyzed to identify underlying physiological mechanisms. This ELD experiment was then repeated with five genotypes selected for high productivity and kernel quality to provide more detailed data for genetic differences in excised leaf water loss patterns under more controlled laboratory environments. A moderate negative correlation was observed between ESD value and stomata density. The pattern of RWC decline differed among genotypes and ELD scores, demonstrating significant genetic differences. ELD score was significantly associated with water loss during the first 180 minutes when stomata closure response has been reported to be the major determinant of rate of water loss (Wang and Clarke, 1993; Darwish and Fahmy, 1997). It was found that the genotypes having lower ELD scores had reduced water loss in the first 180 minutes in the controlled environment. This suggests that genotypes showing low ELD scores were more effective in closing stomata following excision and so more effective in conserving leaf water. ELD scores were also significantly correlated with epidermal conductance (EC) for the five standard genotypes evaluated indicating that post stomata-closure differences in epidermal properties also play an important role in determining rate of water loss. The genotypes that had lower EC were also the genotypes that showed reduced water loss patterns during the later stages of leaf desiccation. Genotypes showing high yields and good quality kernels under good commercial growing conditions expressed both high as well as low ESD scores, indicating that using ESD as a rapid screen for tolerance to catastrophic irrigation failures does not preclude selections for high yields and good kernel quality. While desiccation rates of excised shoots and excised leaves represent a relatively rapid test to identify inherent vulnerabilities in new as well as traditional breeding selections and cultivars, results need to be more fully assessed in both whole-plant as well as whole-orchard trials