Light exposure reshapes photosynthetic thermal tolerance curves, with idiosyncratic responses across 36 species

Photosynthetic thermal tolerance is commonly quantified by heating leaves in darkness, although extreme leaf temperatures in natural canopies occur under irradiance. If light modifies physiological responses to heat stress, dark-based assays may misrepresent thermal vulnerability, yet the magnitude and predictability of light effects remain poorly resolved across species. Here, I examined how sub-saturating light alters photosynthetic thermal tolerance across 36 tropical plant species by characterizing full temperature response curves of PSII efficiency (Fv/Fm). Light exposure reshaped thermal response curves, but responses varied substantially among species. On average, lower thermal thresholds (T5) declined, upper thresholds (T95) increased, and decline width (DW = T95 - T5) broadened, whereas midpoint tolerance (T50) showed no consistent shift despite significant increases in approximately one quarter of species. These opposing shifts indicate that illumination primarily alters processes at the thermal margins. Responses to light were highly variable among species and were not explained by baseline (dark-assayed) tolerance or evolutionary history. Because heat stress in nature co-occurs with illumination, incorporating ecologically relevant light conditions into thermal tolerance assays may improve assessments of plant vulnerability to climate warming.