The effects of heat and drought stress on the ecophysiological responses and growth of Afrocarpus falcatus and Podocarpus henkelii seedlings

Abstract

Drought and heat are major abiotic stresses that affect recruitment and growth which in turn affects the distribution of species. Plant distribution and forest composition may be determined by species abilities to tolerate and/or avoid drought and heat stress, particularly at the vulnerable seedling stage. Here we investigate the physiological and morphological responses of Afrocarpus falcatus (needle-like leaf) and Podocarpus henkelii (broadleaf) to heat and drought stress. To measure these responses, A. falcatus and P. henkelii seedlings were exposed to heat (ambient, heat wave and heat stress) and drought stress (well-watered and water-stressed) over a 4-week period. Both species showed a significant reduction in maximum quantum yield efficiency of photosystem II (PSII) and chlorophyll content under stressful conditions. Under both stresses P. henkelii was able to maintain minimal fluorescence yield, and maximum activity of PSII. Water use efficiency (WUE) of A. falcatus was significantly reduced under drought stress, while P. henkelii seedlings exposed to drought stress had greater WUE than well-watered seedlings across all heat treatments. Morphologically, leaf senescence and loss were accelerated by the drought and heat treatments in both podocarps. However, the drought and heat treatments did not have a significant effect on biomass allocation pattern, root:shoot ratio and leaf relative water content of A. falcatus and P. henkelii, except for leaf mass fraction in A. falcatus which was significantly reduced by drought and heat stress combined. Overall, the morphological and physiological responses of A. falcatus and P. henkelii to drought and heat stress are indicative of a heat and drought avoidance strategy, allowing both podocarps to persist during heat stress and under limited water availability. This research informs us of the responses of podocarp species to environmental stress, and how this may influence the distribution and potential persistence of these species under current and future environmental conditions.