Recovery of photosynthesis, sucrose metabolism, and proteolytic enzymes in Kandelia obovata from rare cold events in the northernmost mangrove, China

Introduction Understanding how mangroves respond to rare cold events has implications for both restoration and conservation under climate change scenarios. This study investigated the responses of photosynthesis and activities of key enzymes involving carbon and nitrogen metabolism at different ages of Kandelia obovata to a rare cold event in the winter of 2010. Methods This study took place on Ximen Island, Zhejiang Province, China. We measured the physiological recovery of 2–3-, 5–6-, 9–10- and 54–55-year-old K. obovata trees after freezing injury in February and March in 2011 and 2012, respectively. Results Chilling injury index and electrolyte leakage of K. obovata increased with increasing tree age in the winter of 2010, and electrolyte leakage in K. obovata at different ages in the winter of 2010 was far higher than that in the winter of 2011. The rare cold events significantly changed the recoveries of the leaf net photosynthetic rate ( Pn ) and stomatal conductance ( Gs ); ratios of chlorophyll a/chlorophyll b (Chl a/Chl b); contents of total soluble sugar (TSS), sucrose, free amino acid (FAA), and soluble protein; and activities of sucrose phosphate synthase (SPS), endopeptidase, and carboxypeptidase in K. obovata at different ages. These effects were mainly due to changes in the physiological mechanism in the 2-year-old trees. A clear decrease in Pn of the 2-year-old trees was observed in February 2011, as exemplified by reductions in ratios of Chl a/Chl b and chlorophyll/carotenoid (Chl/Car), as well as inhibition of the levels of TSS and FAA (osmotic substances). During recovery in 2011 and 2012, the activities of SPS and sucrose synthase (SS) were responsible for sucrose synthesis after the rare cold events in 2011, but only SPS activity was one of the main factors contributing to the metabolism of stachyose to sucrose without cold damage in 2012. Carboxypeptidase played a more important role than endopeptidase during protein hydrolysis after the rare cold events. Conclusions The results suggest that the recovery of photosynthetic capacity in K. obovata was changed after a rare cold event, which is associated with pigment components and activities of SS, SPS, and carboxypeptidase, especially the seedlings.