Strong thermal acclimation of photosynthesis in tropical and temperate wet-forest tree species: the importance of altered Rubisco content

Understanding of the extent of acclimation of light-saturated net photosynthesis (A_n) to temperature (T), and associated underlying mechanisms, remains limited. This is a key knowledge gap given the importance of thermal acclimation for plant functioning, both under current and future higher temperatures, limiting the accuracy and realism of Earth system model (ESM) predictions. Given this, we analysed and modelled T-dependent changes in photosynthetic capacity in 10 wet-forest tree species: six from temperate forests and four from tropical forests. Temperate and tropical species were each acclimated to three daytime growth temperatures (T_growth): temperate – 15, 20 and 25 °C; tropical – 25, 30 and 35 °C. CO₂ response curves of A_n were used to model maximal rates of RuBP (ribulose-1,5-bisphosphate) carboxylation (V_cmax) and electron transport (J_max) at each treatment's respective T_growth and at a common measurement T (25 °C). SDS-PAGE gels were used to determine abundance of the CO₂-fixing enzyme, Rubisco. Leaf chlorophyll, nitrogen (N) and mass per unit leaf area (LMA) were also determined. For all species and T_growth, A_n at current atmospheric CO₂ partial pressure was Rubisco-limited. Across all species, LMA decreased with increasing T_growth. Similarly, area-based rates of V_cmax at a measurement T of 25 °C (V_cmax ²⁵) linearly declined with increasing T_growth, linked to a concomitant decline in total leaf protein per unit leaf area and Rubisco as a percentage of leaf N. The decline in Rubisco constrained V_cmax and A_n for leaves developed at higher T_growth and resulted in poor predictions of photosynthesis by currently widely used models that do not account for T_growth-mediated changes in Rubisco abundance that underpin the thermal acclimation response of photosynthesis in wet-forest tree species. A new model is proposed that accounts for the effect of T_growth-mediated declines in V_cmax ²⁵ on A_n, complementing current photosynthetic thermal acclimation models that do not account for T sensitivity of V_cmax ²⁵.