Coordination between drought and heat tolerance in woody species across ecosystems

Denise E. Mitchell, Botany & Plant Sciences, University of California Riverside, Riverside, CA and Louis S. Santiago, Department of Botany and Plant Sciences, University of California, Riverside, CA

Background/Question/Methods
When one studies drought resistance, it is helpful to examine heat tolerance as well, because high heat accelerates drought. Widespread woody plant mortality has been observed across ecosystems as warmer temperatures amplify the effects of arid conditions. The objective of this research is to characterize the coordination of drought tolerance and heat tolerance in deciduous and evergreen woody species from three distinct ecosystems, that differ vastly in both mean annual precipitation (MAP) and mean annual temperature (MAT). We ask the question, “Does there exist a statistically significant correlation between drought tolerance and heat tolerance within ecosystems, across ecosystems and within the leaf growth habits, deciduous and evergreen?” To answer this question, a young mature leaf was collected from approximately five individuals of 20 woody species during the wet season (Feb. 2020/2021) and the dry season (July 2020), during stress. Turgor loss point (πtlp), the metric used to assess drought tolerance, was determined by using a vapor pressure osmometer. The decline in photosynthetic efficiency, the metric used for assessing heat tolerance, was determined using a chlorophyll a fluorescence system (MINI-PAM; Walz, Inc.) The fluorometer reveals the quantum yield of Photosystem II as the ratio of variable to maximum fluorescence (Fv/Fm). Therefore, the decline in photosynthetic efficiency is marked by the temperature at 50 % decline in Fv/Fm. (T50)
Results/Conclusions
Analysis of the data from July 2020 (stress) revealed the strongest correlation of drought tolerance and heat tolerance occurring when species data from all 3 ecosystems was combined, producing a Multiple R-squared value of 0.2602 and a p-value of 0.02158. This result suggests that there may exist suites of traits for tolerating heat and drought that are conserved across ecosystems, regardless of the distinctly different precipitation and temperature regimes. Further, the data suggests a tighter correlation between drought tolerance and heat tolerance exists in the evergreen leaf growth habit, as the plot of evergreen species from the desert and Mediterranean shrubland produced a Multiple R-squared value of 0.819 and a p-value of 0.03467, suggesting that the physiology is driving selection over ecological parameters. We attest that by assessing the correlation of drought tolerance and heat tolerance of woody species across three distinct ecosystems we may gain a deeper understanding of the physiological, morphological, and biochemical traits that enable the plant to sustain respectable water potentials and minimize heat damage to ultimately gain enough carbon to survive stressful periods of drought and extreme heat.