The seasonal photosynthetic responses of seedlings of the endangered plant Cathaya argyrophylla to different growth light environments
濒危植物银杉幼树对生长光强的季节性光合响应

Cathaya argyrophylla, an endangered species endemic to China, belongs to a single species genus of the pine family. Previous ecological studies indicated that the specific light requirement was one of the key limiting factors for its survival. Here we investigated the adaptation of its seedlings to different light availabilities created by different layers of black nylon net over the nursery site. The treatment of light avail- abilities included three levels, 100%, 45% and 3% of full daylight. The photosynthesis-related parameters under these three light environments were measured in summer and winter by the technique of gas exchange and chlorophyll fluorescence. Results showed that the low light environment decreased the maximal rate of photosynthesis(Pnmax)and carboxylation efficiency(CE)in the summer growing season, but the degree of depression depended on leaf age. Shading also caused a decrease of light compensation poin(tLCP)and light saturation poin(tLSP)to some degree. The accumulation of time that light intensity was higher than the light compensation poin(tLCP)in low ligh(t3% daylight)on sunny days was not longer than six hours in an entire day. These results indicate that the growth of C. argyrophylla was greatly depressed under low light condi- tions. Its growth was the best under full daylight conditions and declined when light was lower than 45% daylight. The Pnmax, CE, LCP, and LSP in winter were lower than in summer, which may be attributed todown-regulation of the photosynthetic apparatus by the combined effect of low light and low temperature. Both current-year leaves and one-year-old leaves showed slight photoinhibition in winter under illumination, and moderate shade favored the adaptation to photoinhibition in winter. Compared with summer, the leaf of C. argyrophylla in winter dissipated a higher proportion of absorbed light energy in the form of heat, which could be indicated from the higher coefficients of non-photochemical fluorescence quenching (NPQ).