Leaf physiological and biochemical parameters change with stages of its ontogeny. Studies in crops determined that heat sensitivity is highly variable across developmental stages. It is likely that heat tolerance varies between different life stages as well, but this has never been tested on tropical plants. Other morphological traits such as leaf thickness and leaf size strongly influence and mitigate heat damage. However, little is known on the extent of how tropical plants deal with leaf thickness and leaf size to reduce heat stress, and even less across life stages. According to previous studies that demonstrate the high plasticity of heat tolerance and many gaps in thermal physiological responses on tropical plants, we aim to test the plastic responses of heat tolerance and other traits related to the temperature at two extreme life sages of plants. This study focuses on species from the order Zingiberales (the ginger and banana-like plants) at La Selva Biological Station, Costa Rica. To test if the performance of photosynthetic systems of Zingiberales species is acclimated to temperature prevalent in their life zone, we estimated the maximum heat tolerance (T50) of the photosystem II. We hypothesize that heat tolerance will vary among life stages because of different leaf sizes across life stages that change their cooling abilities. Specifically, we predict that heat tolerance will be higher in early life stages than late-life stages if early stages are more susceptible to heat damage than late-stage leaves. We determined heat tolerance at two extreme life stages (early and late stage) of 20 species of Zingiberales at La Selva. We also measured traits related to leaf temperatures such as leaf area and leaf thickness. From these traits and the ambient temperature at La Selva, we determined the leaf energy balance by life stage and species. Heat tolerance does vary between the most extreme life stages of Zingiberales. However, contrary to our prediction, early stages have lower heat tolerance than late stages. Given this differentiation in heat tolerance, our study suggests that early-stage leaves are more susceptible to heat damage. Late-stage leaves are older and have larger leaves than early-stage leaves that in return, might obstruct their cooling process. Heat tolerance is an intricated parameter with high plasticity that is strongly related to other factors such as leaf thickness and leaf size.
thermal tolerance, chlorophyll fluorescence, zingiberales, developmental stages, global warming