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

Land surface temperature is predicted to increase by 0.2 °C per decade due to climate change, although with considerable regional variability, and heatwaves are predicted to increase markedly in the future. These changes will affect where crops can be grown in the future. Understanding the thermal limits of plant physiological functioning and how flexible such limits are is thus important. Here, we report on the measurements of a core foliar thermotolerance trait, T50, defined as the temperature at which the maximum quantum yield (Fv/Fm) of photosystem II declines by 50%, across nine different Malaysian Hevea brasiliensis clones. We explore the relative importance of interclonal versus intraclonal variation in T50 as well as its association with leaf and hydraulic traits. We find very low variation in T50 within individual clones (mean intraclonal coefficient of variation (CoV) of 1.26%) and little variation across clones (interclonal CoV of 2.1%). The interclonal variation in T50 was lower than for all other functional traits considered. The T50 was negatively related to leaf mass per area and leaf dry matter content, but it was not related to hydraulic traits such as embolism resistance (P50) or hydraulic safety margins (HSM50). The range of T50 observed (42.9-46.2 °C) is well above the current maximum air temperatures Tmax,obs (T50 - Tmax,obs >5.8 °C), suggesting that H. brasiliensis is likely thermally safe in this south-east Asian region of Malaysia.