Effects of shading on photosynthetic characteristics and chlorophyll fluorescence parameters in leaves of the endangered plant Thuja sutchuenensis

Thuja sutchuenensis Franch.is a critically endangered plant that is endemic to China.Currently,no research has addressed its endangered status from the perspective of its photosynthetic mechanism.Light is one of the most important environmental factors that can affect plant survival and growth and could become a major limiting factor for natural regeneration.Therefore,we focused on the adaptive ability of T.sutchuenensis seedlings under three light levels(L0 = full light;L1 = 50% full light;L2 = 25% full light) by comparing their photosynthetic capacity,chlorophyll fluorescence parameters,and chlorophyll content.We found that shading resulted in increases in the apparent quantum efficiency(α) and maximum leaf net photosynthetic rate(Pmax).At lower light levels,the dark respiration rate(Rd),light compensation point(LCP),and light saturation point(LSP) declined.For the chlorophyll fluorescence parameters,PS II primary energy conversion efficiency(Fv/Fm),PS II potential activity(Fv/Fo),and photochemical efficiency of PSⅡ in the light(Fv′/Fm′) were significantly higher at lower light levels than at L0,with parameters at L2 higher than at L1.Non-photochemical quenching(NPQ) also had a higher value under shade than under full light,and peaked at L1.With a decrease in light intensity,photochemical quenching(PQ) and the electron transport rate(ETR) decreased.Shading also caused a significant increase in leaf chlorophyll content(Chla + Chlb) per cm2.These results indicate that T.sutchuenensis can tolerate a wide range light conditions,especially low light,and has internal regulatory mechanisms for responding to light level.Therefore,low light under canopy cannot be the decisive limiting factor for natural regeneration of T.sutchuenensis populations.