Abstract:
We analyzed the impact of two days of high-temperature treatment on the leaf photosystem properties of potted seedlings of two mangrove species, i.e., cold-tolerant
Kandelia obovata Sheue, H. Y. Liu & J. Yong and heat-tolerant
Bruguiera gymnorrhiza (L.) Savigny, from two geographical populations in Guangxi, China (lower latitude) and Okinawa, Japan (higher latitude). The xanthophyll cycle components and chlorophyll-
a and -
b contents were measured. The leaf photosystemⅠ(PSⅠ) and photosystemⅡ(PSⅡ) activities were measured using a Dual-PAM-100 system. Results showed that high temperature significantly reduced the maximum photochemical efficiency (
Fv/
Fm) of PSⅡ in both species. The
Fv/
Fm values showed less decrease in the lower latitude populations than in the higher latitude populations, and less decrease in
B. gymnorrhiza than in
K. obovata seedlings. The P700 oxidation-reduction state (
Pm) showed less decrease in the lower than higher latitude populations. The leaf non-photochemical quenching (
NPQ) showed a greater increase in the lower latitude populations than in the higher latitude populations. The xanthophyll pool size (V + A + Z) and de-epoxidation ratio (A + Z)/(V + A + Z) increased following heat treatment. A greater increase was observed in the lower latitude populations than in the higher latitude populations. In addition, the (A + Z)/(V + A + Z) ratio was negatively correlated with
Fv/
Fm, but positively correlated with
NPQ. Compared with other hardy habitats, such as savannas and Mediterranean woodlands, mangroves had the largest xanthophyll pool size. On the second day of recovery,
Fv/
Fm was still at 0.69 in the high-latitude
K. obovata seedlings, indicating irreversible photoinhibition of PSⅡ, while all seedlings maintained a higher de-epoxidation ratio and
NPQ for photosystem protection. In conclusion, the lower latitude populations showed higher adaptability to heat stress, which was partly due to strong photoprotection through the xanthophyll cycle, and there was a tradeoff between cold- and heat-tolerance between species and populations. This study revealed a novel heat tolerance mechanism in mangrove species, which has implications for their physiological responses to future changing climate.