Optical visualization of embolism spread in drought‐induced leaves: revealing differences across three grapevine genotypes

Abstract

Context and purpose of the study ‐Evaluation of xylem embolism is an important challenge in identifying drought tolerant genotypes within the context of climate change. Visualization methods such as the optical vulnerability technique (Brodribb et al. 2016) has been shown to be a reliable and accessible approach to observe the spread of embolism in dehydrating leaves (Hochberg et al. 2017; Lamarque et al. 2018). In this study we use the optical technique to examine the development of leaf embolism in three grapevine cultivars as a method to characterize their drought‐tolerance strategy.

Material and methods ‐Potted plants of Grenache, Semillon and Syrah were grown outdoors in 2018 under well‐watered conditions. Leaf embolism formation and spread was evaluated in four individuals per genotype by monitoring changes in light transmission through the xylem after the irrigation was cutted‐off. For each plant, a mature leaf was placed on a scanner and imaged every 5 minutes until complete desiccation. Simultaneous measurements of stem water potential (Ystem) were registered using psychrometers properly installed on the main stem. The accuracy of the psychrometers was evaluated by measuring the leaf water potential in adjacent leaves previously bagged with aluminum foil using a Scholander pressure bomb. The stack of images obtained were analyzed using the ImageJ software as described in Lamarque et al. (2018). The percentage of embolism (%emb) was calculated as the cumulative number of embolised pixels normalized to the total number of embolised pixels throughout the dehydration. Finally, the %emb was represented as a function of Ystem and different events were colored using a continuous scale respective to their time of appearance.

Results ‐Embolism formation and spread in the leaves were detected at different times for each cultivar since the beginning of drought. While Grenache showed the first embolism event at around 48 h of desiccation (‐0.48 MPa), Semillon showed its first event after 72 h (‐1.5 MPa). Syrah plants were placed in between the other two genotypes showing the first embolisms at ‐0.68 MPa. The vulnerability curves (%emb vs Ystem) constructed from the data obtained followed a sigmoidal function for all genotypes and showed a great variability between individuals. In spite of this, the time and water potentials at which the main embolisms occurred was significantly different between cultivars where Grenache showed an early cavitation (P50 at ‐1.43 MPa), followed by Syrah (P50 at ‐1.65 MPa) and Semillon (P50 at ‐2.08 Mpa). The optical technique tested in this study revealed genotype differences in the temporal appearance of leaf embolism suggesting a different strategy to tolerate dehydration.