Acclimation to Warming Shapes Gas Exchange and Metabolic Responses to Heat Shock in <i>Pinus massoniana</i> Seedlings

The sensitivity of physiological and metabolic processes in subtropical trees to temperature remains uncertain, limiting our ability to predict how subtropical forests will acclimate to future climates. In particular, our understanding of gas exchange and metabolic activity responses to warming and heat shocks is quite limited. Here, we exposed Pinus massoniana seedlings to three daytime growth temperatures (25°C, 3°C, and 35°C) for 65 days, followed by a heat shock up to 40°C, then immediately reduced to 25°C, to investigate physiological and metabolic responses. The optimal temperature of photosynthesis (T_optA) did not exhibit a significant shift with warming. Metabolism acclimated to rising growth temperature, resulting in enriched levels of key metabolites (tryptophan, indole, indoleacetate, and o-Phospho-L-serine) and key pathways (tryptophan metabolism). At 25°C, leaf dark respiration (R_d) decreased in warm-grown seedlings. At 40°C (heat shock period), warming reduced R_d, accumulated flavonoid metabolites, and upregulated tryptophan metabolism. After recovery to 25°C, higher growth temperatures decreased the net photosynthetic rate (A_sat), accumulated prenol lipid metabolites, and led to enrichment in tryptophan metabolism, flavone, and flavonol biosynthesis pathways. Our findings suggest that photosynthesis in P. massoniana seedlings exhibits limited thermal acclimation, while respiration and metabolism can acclimate under short-term warming. However, acclimation to warming altered both physiological and metabolic responses to heat shock and during the subsequent recovery phase in seedlings.