Seasonal maintenance of leaf level carbon balance facilitated by thermal acclimation of leaf respiration but not photosynthesis in three angiosperm species

Thermal acclimation of leaf gas exchange could facilitate the maintenance of positive carbon gain under thermally contrasting environments, yet our knowledge regarding the physiological mechanisms underpinning thermal acclimation remains incomplete. We hypothesize that leaf level carbon balance under warmer growth temperature (Tgrowth) could be sustained through thermal acclimation in leaf net photosynthesis (An) and dark respiration (Rd) in three angiosperm species differing in growth form. To test this hypothesis, the temperature response of An and Rd were measured in two periods characterized by different ambient monthly average air temperatures, 14.8 °C (April) and 26.3 °C (July). Furthermore, the thermal response of the biochemical components of photosynthesis, including the apparent rates of maximum Rubisco carboxylation (Vcmax) and electron transport (Jmax), as well as leaf nitrogen concentration, were examined to reveal the biochemical control over thermal acclimation of leaf gas exchange. Partial thermal acclimation was found for leaf An in two of the three species. In addition, leaf Vcmax and Jmax measured at reference temperature (25 °C) tended to be lower in July compared with April, while parameters defining thermal sensitivity, including the activation energy (Ea) and entropy (ΔSV), were largely invariable in response to increased Tgrowth. By contrast, leaf Rd exhibited strong thermal acclimation potential. At warmer temperatures, Rd measured at 25 °C (Rd25) was lower than at cooler growth temperatures, leading to higher leaf level carbon balance represented by the ratio of An measured at 25 °C (An25) to Rd25. Adjustments in leaf gas exchange were partially governed by the variation in leaf nitrogen concentration, and potentially nitrogen partitioning. Collectively, these findings demonstrate strong acclimation potential in leaf Rd relative to An, and highlight the role of nitrogen in regulating leaf level carbon gain. Our data contribute to the understanding of carbon acquisition strategy in these species, and provide empirical evidence for modeling the carbon flux using ecosystem models.