Abstract:

Pometia tomentosa, is a dominant uppermost canopy tree and a symbol species of vallyland forest in Xishuangbanna tropical rainforest. In addition to anatomical and physiological characteristics, diurnal changes in chlorophyll fluorescence parameters were measured with a modulated chlorophyll fluorescence monitoring system (FMS2) in order to explore the difference of diurnal photoinhibition in leaves of two developing stages, young fully expanded leaves and mature leaves of Pometia tomentosa in the rainy season. The difference in the effect of midday strong light and high temperature on the activities of partly protective enzymes was also briefly analyzed. The results showed: leaf thickness, chlorophyll contents, stomatal conductance, carboxylation efficiency, maximum net photosynthetic rate and light saturation point of mature leaves were much higher than those of young fully expanded leaves, while the stomatal density and guard cell length were similar between two leaves. On clear days in the rainy season, the highest photon flux density can exceed 2 200 μmol·m-2·s-1, the highest leaf temperature was 7-8 ℃ higher than air temperature, and the highest leaf temperature of mature leaves was 1.5-2℃ higher than that of young fully expanded leaves. Non-photochemical quenching (NPQ) was increased, the initial photochemical efficiency (Fv/Fm) and actual photochemical efficiency ［(Fm′-Fs)/Fm］ decreased gradually in the morning, and reached the lowest values at about 15∶30. The reverse changes occurred in the afternoon, at dusk these parameters could recover to nearly the original dawn level. The decrease in Fv/Fm was mainly caused by a decrease in Fm, because initial fluorescence (F0) did　not change significantly over the course of the day. This suggested that the photoinhibition observed in Pometia tomentosa was due to photoprotective process and not to photoinhibitory damage. The initial photochemical efficiency (Fv/Fm) of young fully expanded leaves was lower than that of mature leaves, and exhibited more sensitiveto strong light and high temperature treatment, but their diurnal photoinhibition was statistically similar in the field. After field strong light at midday the protective enzymes (SOD, APX) increased, while H2O2 content was stable and the lipid peroxide level was lower in young expanded leaves compared with mature leaves. We concluded that the alleviated photoinhibition in young expanded leaves might be caused by its lower photochemical efficiency and leaf temperature, as well as active protection by higher activities of protective enzymes under the field conditions.