Leaf heat tolerance in a native Californian shrub responds in expected and unexpected ways to differences in range, season, and sunlight exposure

Societal Impact Statement Earth's ecosystems are experiencing wide‐ranging impacts from global climate change, including increased average and extreme temperatures. These temperature shifts can drastically affect plant physiology, threatening the future of even common plant species. Here, we focus on photosynthetic function in a native Southern California shrub species. We found important site differences in leaf heat tolerance and evidence that prior heat and sun exposure affected responses, highlighting the complexity of climate shift effects. These results come from a collaborative project aimed at accomplishing research in undergraduate courses, providing students in the classroom with real‐world research experience. Summary Global temperatures are increasing, as are heat waves, threatening the future of native plant species. Students from five colleges and universities across southern California sought to determine how leaf heat tolerance varied in a widespread native shrub, Heteromeles arbutifolia (toyon). We were primarily interested in how regional differences in temperature, season, and sunlight exposure would influence leaf heat tolerance. We selected sun leaves from the outer canopy and shade leaves from the lower interior. We assessed leaf heat tolerance by submerging excised leaves in water baths at sequentially increasing temperatures and used a fluorometer to assess damage via dark‐adapted chlorophyll fluorescence, F v /F m . Leaf heat tolerance was compared by calculating T50, the temperature at which F v /F m decreased to 50% of its initial value (T50). We also measured light‐ and heat‐related structural traits such as leaf thickness and leaf mass per area as predictors. Leaf heat tolerance differed significantly across sites and was linked to temperature across a coastal to desert gradient. In addition, leaves collected in the summer/fall had significantly higher T50 than those in the winter/spring. T50 was higher for shade leaves than sun leaves at three campuses but significantly so only at one campus during the winter/spring. Our results highlight the importance of seasonality and prior heat exposure to leaf heat tolerance of toyon. Sun exposure had inconsistent effects on heat tolerance but did affect measured leaf traits related to heat tolerance, raising questions regarding the importance of leaf structural traits and how they interact with exposure to sunlight.