Photoperiodic changes of soluble sugar levels in Panax ginseng

Abstract

This study measured thylakoid membrane-associated reactions and carbon metabolism to investigate the coordination of these processes in plants that are typical of light-limited environments. The two shade plants Panax quinquefolius and Panax ginseng, American and Korean ginseng, respectively, were compared to the control plants: pea, spinach and Arabidopsis thaliana. In contrast to the control plants, the quantum yield of photosystem II (PSII) declined linearly in ginseng leaves when grown at a light intensity of 500 µmol.m-2.s-1, whereas, the relative quantum yield of PSII in isolated thylakoids was similar for all species. However, the thylakoids of ginseng possessed a large number of inactive PSII centers and methyl viologen was ineffective as an electron acceptor even though PSI levels were normal. The levels of fructose and sucrose were found to be higher in P. quinquefolius and P. ginseng than in pea or spinach. Uniquely, in P. ginseng sugar levels exhibited a marked periodicity during the 12 h/12 h photoperiod: glucose and fructose levels dipped after 9 h in the light and again 12 h later during the dark period; in addition, a corresponding transient increase in sucrose was observed. Remarkably, the level of leaf starch in P. quinquefolius remained elevated throughout the entire dark period and only declined by 25% after an extended dark period of 48 h. Moreover, the starch degrading enzymes, a -amylase and b -amylase, exhibited lower activities in P. quinquefolius than in P. ginseng, pea and A. thaliana. End-product accumulation may therefore contribute to ginseng¿s susceptibility to photoinhibition.