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

Abstract

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. CO 2 response curves of A n were used to model maximal rates of Ru BP (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 CO 2 ‐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 CO 2 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 .

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