Effects of irradiance on the photosynthetic traits, antioxidative enzymes, and growth of Cryptotaenia japonica

Growth, photosynthetic characteristics, chlorophyll (Chl) fluorescence parameters, and peroxidation of membrane lipids of Cryptotaenia japonica were studied under differing irradiances (15, 35, 60, and 100 % of full irradiance). At full irradiance, C. japonica exhibited a typical decline in net photosynthetic rate (P-N) at midday, which was not observed in the other irradiance treatments. This indicates a possible photoinhibition for C. japonica at the high irradiance. Diurnal patterns of stomatal conductance (g(s)) were remarkably similar to those of P-N in each irradiance treatment, and the intercellular CO2 concentration (c(i)) had the opposite trend. C. japonica growing under 60 % of the full irradiance exhibited the highest plant height, stem diameter, leaf area, and biomass. The initial fluorescence (F-0) value was lowest at 60 % of the full irradiance. Maximal fluorescence (F-m), potential activity of photosystem II (PS II) (F-v/F-0), and maximal photochemical efficiency of PS II (F-v/F-m) values were highest at 60 % of full irradiance and lowest at 15 % of the full irradiance. The malondialdehyde (MDA) content in 15 % and 100 % of the full irradiance were higher than under the other irradiances. During the treatment, catalase, peroxidase, and superoxide dismutase activities firstly increased and then declined under 15 % and 100 % of the full irradiance and were steadily low under 60 % of the full irradiance, indicating a low production of reactive oxygen species. Therefore, C. japonica thrived best under 60 % of the full irradiance.