Abstract:

Gas exchange, chlorophyll a fluorescence kinetics, chloroplast pigments and spectral reflectance were investigated to explore the variations of photosynthetic capacity, primary photochemical reactions and development of photoprotective mechanisms in chestnut leaves from emergence to full expansion under field conditions. The results showed that during leaf development, net photosynthetic rate (P_n) gradually increased. Although the maximum quantum yield of PSII photochemistry (φPo) was quite high at the initial stages of leaf development, P _F was markedly lower than that in fully expanded leaves. When the relative leaf area (RLA) was about 10%, φPo and electron transport per active reaction centers (ETO/RC) were just a little lower than that in fully expanded leaves, indicating that its primary photochemical capability had been nearly completed. Due to low chlorophyll concentration, absorption flux per excited crosssection (ABS/CS) in a young leaf was less，nevertheless the absorption flux per reaction center (ABS/RC) was more than that in a mature leaf because of lower density of reaction centers (RC/CS) in the young leaf. High ABS/RC as well as low Pnresulted in more heat dissipation per reaction center (DI O/RC).The changes of chlorophyll were in opposition to the deepoxidating degree of xanthophyll cycle pigments (ΔPRI) at various expanding stages of leaf. The content of chlorophyll per leaf area (ChlNDI) increased steadily with the progress of leaf expansion. When exposed to high irradiance at midday, the deepoxidation of xanthophyll cycle pigments in newly developing leaves was considerably enhanced compared with fully expanded ones, which indicated that xanthophyll cycle might play a key role in excessive energy dissipation in young chestnut leaves. Anthocyanin accumulation was observed in newly emerging leaves of chestnut. The foliage was not photostable and was vulnearable to photodamage because levels of light utilization and its capacity to dissipate excess energy were low. Above results implied that anthocyanins could protect newlyemerging leaves against high light stress, which could interprey why the deepoxidating degree of xanthophyll cycle pigments (ΔPRI) at 10% RLA leaves was appreciably lower than that of 30% RLA leaves. We propose that the timely development of the above photoprotective mechanisms in young leaves is a reason of protecting their photosynthetic apparatus from photodestruction.

Key words: chestnut (Castanea mollissima), leaf expansion, photosynthesis, chlorophyll a fluorescence transient, photo protection