Down-regulation of photosynthesis in apple leaves under elevated CO₂ concentration: a long-term field study with different fruit load

Abstract

　Atmospheric CO₂ concentration ([CO₂]) would double by the end of this century. Although the short-term elevated [CO₂] substantially accelerates photosynthesis in C₃ plants, the photosynthetic response under long-term elevated [CO₂] varies depending on the plant size and sink capacity and is still unclear. In addition, only a few studies have been performed on mature fruit tree species under field conditions. Therefore, this study aimed to examine whether, to what extent, and why long-term (throughout the growing seasons from 2019 to 2021) elevated [CO₂] down-regulates photosynthetic performance in mature field-grown apple trees. Measured leaves were selected in greenhouses with ambient (400 ppm) and elevated (600 ppm) [CO₂]. The temperature of the greenhouses was maintained at 3°C higher than outside, anticipating a warmer climate. The number of bearing fruits was adjusted to 8.3 and 16.7 m^-2 by 2020 and 2021, respectively. Photosynthetic light- or CO₂-response curves, along with photosynthesis parameters, were measured in June, August, and October in both years. Our results revealed that photosynthesis down-regulation occurred clearly only under low fruit load conditions. Namely, the photosynthetic rate of leaves measured at 600 ppm [CO₂] in the elevated greenhouse was 6% lower in the elevated greenhouse compare to the ambient greenhouse, when averaging all measurements. This down-regulation was not associated with stomatal limitation; however, it was a consequence of the reduction in Rubisco carboxylation activity and RuBP regeneration efficiency. In high fruit load conditions, the reduction in photosynthetic rate in the elevated greenhouse was 4%, without showing significant differences in the biochemical parameters of photosynthesis. As a whole, photosynthesis downregulation was not clearly detected in high fruit load condition. In spite of the down-regulation, the leaf photosynthetic rate measured under the prospected future climate condition increased by 25% and 33% under low and high fruit load respectively, compared with current condition.