PS 15-196 - How hot is too hot? High-throughput plant thermal limit phenotyping

Abstract: Earth’s climate is warming at an unprecedented rate in the Anthropocene. Along with chronic warming driven by increased human emissions, acute extreme heatwaves are intensifying and becoming more frequent. Recent heatwaves have even exceeded the thermal limits of plants—temperatures beyond which living tissues cannot survive, even for brief periods of time. Thus, understanding the thermal limits of plants and how they may vary within an individual plant, population, species, or community is urgently needed. In this study, we aim to develop a high-throughput thermal limit phenotyping platform. Coupling chlorophyll fluorescence imaging with custom-designed 3-D printed sample cassettes, and a code platform for image data extraction and model fitting, we set out to increase the efficiency of plant thermal limit measurement by an order of magnitude.

Our objectives were to understand the variation in plant thermal limits across species within an ecosystem, between provenances of the widely distributed species, and between phenological and developmental stages. We hypothesized that plant thermal limits would vary across space, developmental time, and species; revealing the importance of understanding how the timing, intensity, and frequency of heatwaves on our warming planet may further impact ecosystems.

We measured thermal limit traits for 48 species with our platform, including grasses, forbs, shrubs, and trees. We measured thermal limits for multiple genotypes and geographic provenances within grass and tree species, and across developmental axes of intra-seasonal phenology, and developmental life stages. Our study reveals remarkable variation in plant thermal limits. We observed T50, the temperature at which 50% of cell vitality is lost, to range from 46 to 61 degrees C across all species. Within a single species, the widest range observed was 6C in T50. Generally, we observed that expanding/immature foliage of plants is significantly more vulnerable to extreme heat than fully-expanded mature foliage (e.g., 4C lower T50). Between developmental stages of the whole plant, immature foliage was typically more vulnerable to heat stress.

In conclusion, thermal limits of plants are varied within and between species, with important differences depending on developmental stage, tissue age, and climate of adaptation. Our novel high-throughput platform provided a 90% reduction in the time to process samples compared to state-of-the-art methods, allowing the increased sampling between and within species, especially across phenologies and developmental stages, that will be necessary to fully understand the thermal limits that my determine the fate of plants on our warming planet.