Abstract:

Panax ginseng is a famous special herb medicine. As a perennial shade-loving medicine crop, it will be harvested after growing at least 5–6 years. The net photosynthetic rate(P_n) is very low (2–4 µmol CO₂·m^-2·s^-1) in leaves of Panax ginseng , which is an important limitation factor for production. In order to investigate the changes of the characteristics and the limitation factors of photosynthesis in Panax ginseng leaves at different growth stages, net photosynthetic rate (Pn) and related parameters in leaves of 1–6 years old Panax ginseng grown in the field and in phytotron were measured by infrared gas analyzer. The net photosynthetic rate reached the maximum , then decreased with leaf age in fully expanded leaves of one-year old Panax ginseng without forming reproductive organ. In leaves of 2–6 years old plant, the first peak of net photosynthetic rate appeared in the fully expanded leaves, then P_n had little decrease at flowering stages and reached the second peak at green fruit stage (fruit growing stage), at last persistently decreased at red fruit(ripe fruit) and yellowing leaf stages. For the experiment with removing flower bud, the second P_n peak did not appeared at green fruits, and P_n was higher in red fruits and yellowing leaf stages, as compared with control plants. The results showed that the changes of P_n are regulated by reproductive growth at different growth stages. Apparent quantum yield (AQY) did not show obvious changes from leaf fully expanded to green fruit stages, which the maximum AQY was observed in fully expanded leaves. AQY was decreased with the decrease of P_n after green fruit stage. The leaves grown under low light (10% of full light) and optimum light (30% of full light) were showed more slowly decrease of Pn than these under high light (50% of full light) after green fruit stage. The leaf senescence and the decrease of Pn were accelerated in each growth stage under high light and high temperature; conversely the senescence and the decrease of Pn were delayed in each growth stage under low light and low temperature. Stomatal conductance(G_s) and transpiration rate(T_r) had similar change trends to P_n at different growth stages. Intercellular CO₂ concentration(C_i) was kept stable from leaf fully expanded to green fruit, and increased persistently after green fruit. The changes of C_i indicated that the decrease of photosynthesis was not caused by stomatal limitation at red fruit and yellowing leaf stages. There was negative correlation between P_n and specific leaf weight (SLW) from leaf fully expanded to red fruit. So, it can be deduced that changes of photosynthesis are related with accumulation and demand of photosynthate. There were higher WUE and lower C_i at green fruit then at other stages. The results indicate that water supply at green fruit stage is very important to improve the photosynthetic productivity.

Key words: Panax ginseng, Photosynthesis, Growth stages, Apparent quantum yield, Water use efficiency