Relationships between leaf temperature, stomatal conductance and architecture: potential impact on leaf burning among a range of genotypes in grapevine

In the context of climate change, extreme heatwaves are often observed. The consequences of these events led to leaf and grape burning, as observed in June 2019 in the South of France. Previous observations showed that genotypic variability exists in response to these heatwaves. One of the main hypotheses to explain the differences is that genotypes could differentially regulate their leaf temperature. This temperature is closely associated with stomatal conductance and the amount of energy absorbed by the leaves. This amount of energy is known to be a consequence of plant architecture that determines light interception. This study was performed on a set of 33 genotypes that were selected with different leaf-burning sensitivities to high temperatures. Functional (i.e., stomatal conductance, photosynthesis) and architectural traits (internode length, leaf area and leaf elevation angles) were measured to compute their heritabilities and to determine correlations between these traits. Measurements of stomatal conductance and leaf temperature were performed during 30 measurement periods in 2021 and 2022. Architectural traits were extracted from 3D digitizing. High heritability in architectural traits were observed (around 0.8). Heritability of functional traits, although lower, were not negligible (higher than 0.6) and were partly dependent on the weather conditions during the measurements. A clustering of genotypes based on mean values of their architectural and functional traits revealed that both types of traits could be combined independently. New combinations of traits and their impact on leaf temperature were then examined. Stomatal conductance appeared to be associated with the intensity of the burning symptoms than architectural traits. The genotypes with high stomatal conductance also displayed low leaf temperature in accordance with the evaporative cooling effect. However, these genotypes were also the most sensitive to leaf burn. This likely suggests that leaf burn resulted from a high transpiration rate that could cause embolism under hot and dry weather conditions. For future works, modelling approaches could be of major interest to quantify the relative impact of architectural and functional traits on leaf temperature. Nevertheless, our study shows that leaf temperature is not completely associated with the observed leaf-burning symptoms and that other processes are involved.