Leaf thermotolerance of Hevea brasiliensis clones: intra- versus interclonal variation and relationships with other functional traits.

This article focuses on assessing the photosynthetic thermal tolerance (T50) of nine Malaysian clones of Hevea brasiliensis, the rubber tree, to understand intraspecific variation and its relationship with leaf and hydraulic traits under climate change scenarios. The T50, defined as the temperature at which the maximum quantum yield of photosystem II declines by 50%, showed low variation both within and across clones (mean ~44.9 °C), and was well above current maximum air temperatures in north-western Malaysia, indicating a thermal safety margin for these clones. Thermal tolerance was positively associated with larger leaf area and thickness but negatively related to leaf mass per area and leaf dry matter content, while no significant relationship was found between T50 and hydraulic traits such as embolism resistance or hydraulic safety margins. Comparisons with native Amazonian populations revealed higher T50 values in Malaysian clones, suggesting phenotypic plasticity, and the study highlights that thermal tolerance and hydraulic risk appear to be independent traits in H. brasiliensis.