Effects of growth irradiance on growth, chlorophyll fluorescence and photosynthesis parameters of Gardenia jasminoides J. Ellis

Light availability is a primary factor influencing the growth and survival of tree seedlings in the forest understory. The effects of growth irradiance on the growth, photosynthesis and chlorophyll fluorescence parameters of an evergreen shrub, Gardenia jasminoides, were investigated with three levels of light: full light, 52.2% light and 18.2% light. The results showed that G. jasminoides grown under 18.2% light attained the lowest plant height and the highest crown diameter. The highest and lowest values of ground diameter were obtained in the 52.2% light and 18.2% light environments, respectively. Leaf area increased in plants grown under shade conditions. G. jasminoides grown under 52.2% light had the greatest leaf biomass, stem biomass and total biomass. Shade resulted in significant reductions in electron transport rate through PSII (ETR), photochemical quenching (q(P)) and nonphotochemical quenching (NPQ). Plants grown under shade conditions had lower heat dissipation in the antenna and higher excess energy. Chlorophyll (Chl) a, Chl b, Chl (a + b) and carotenoids (Car) increased and the Chl a/b ratio decreased in response to shade. Area-based photosynthesis (A(area)), stomatal conductance (g(s)) and transpiration rate (E) decreased significantly with decreasing growth irradiance, while there was no significant difference in mass-based photosynthesis (A(mass)) between full light and 52.2% light. G. jasminoides had the highest whole-plant photosynthesis (A(plant)) value in 52.2% light and the lowest value in 18.2% light. The results suggest that approximately 52.2% light is the optimum growth irradiance for G. jasminoides.