Thriving in the tropics: spatial variation in heat resilience in the early diverging land plant, Marchantia inflexa.

Increasing frequency and intensity of global warming pose a profound threat to plant species persistence. Most investigations on plants' resilience to heat events focus on few genotypes of model species. Novel insights into resilience mechanisms will be gained by focusing on natural variation in thermotolerance and its relationship to local-abiotic factors. Additionally, studying species that survived 'ancient periods' of high temperatures provides insight into resilience mechanisms. Within a species, we assessed spatial thermotolerance variation, its association with temperature and light, while testing for thermotolerance sex differences and its relationship with population sex ratios. We used Marchantia inflexa, a species with unisexual individuals exhibiting spatial variation in physiologies and life histories. To assess field basal thermotolerance (field BT), we examined the efficiency of photosystem II recovery following a heat treatment (53°C for 45 min) in over 200 field-collected plants from seven sites. We further examined whether field BT is linked to initial physiological traits or environmental factors and assessed its potential as a predictor of sex ratios. Following the heat treatment, plants exhibited damage and were still recovering by day ten; recovery was generally higher in road- relative to stream-collected plants with notable variation among sites. Thermotolerance was positively associated with light and tended to be negatively associated with temperature. This light-thermotolerance relationship was more pronounced in males, and thermotolerance differences between females and males tended to be positively related to the proportion of females. The positive light-thermotolerance association suggests that light is a key factor driving heat stress resilience in M. inflexa. The light-thermotolerance relationship for males vs. females implies sex-specific strategies for coping with abiotic stress. There were subtle thermotolerance impacts on population sex ratios. These insights broaden the understanding of the thermotolerance diversity present within a species.