Thermal acclimation of <i>Pinus massoniana</i> to warming depends on water conditions and climate of origin

Future climatic conditions are projected to become warmer and drier, critically challenging tree growth and survival. Understanding how photosynthesis and respiration of tree seedlings acclimate to warming is pivotal for predicting seedling responses to climate change and tree carbon dynamics. However, the photosynthetic capacity parameters of thermal acclimation remain unresolved; particularly their interaction with water availability constraints and local climate adaptation needs to be fully explored. Here, we grew seedlings of a widely planted timber species in subtropical China, Pinus massoniana, with two provenances from contrasting climates of origin (cooler-drier and warmer-wetter), under three growth temperatures (22°C, 26°C, and 30°C) and two water conditions (well-watered and drought). Temperature responses of gas exchange and nutrient status were investigated, together with a data synthesis of meta-analysis. The experimental results showed that under well-watered conditions, the optimal temperature for photosynthesis (T_optA), the net photosynthetic rate (A₂₅) and dark respiration rate (R₂₅) at 25°C, and respiratory sensitivity parameters (Q₁₀) remained unexpectedly stable under warming for both provenances, contrasting with our hypotheses; however, data synthesis shows that the T_optA from the cooler region increased with warming. Drought reduced T_optA and A₂₅, largely related to the magnitude of warming. Compared to drought-only conditions, warming and drought in combination induced an increase in T_optA and the optimal temperature for photosynthetic electron transport rate (T_optJ) in the provenance from the warmer-wetter region and a reduction in Q₁₀ for both provenances, but did not affect A₂₅ and R₂₅. Collectively, our results indicated that thermal acclimation of photosynthesis and respiration in P. massoniana seedlings was limited under well-watered conditions. However, drought increased the capacity for thermal acclimation in the warmer-wetter provenance compared to the cooler-drier provenance, suggesting that P. massoniana seedlings from warmer-wetter areas may cope better with future warm and dry environments.