Abstract:

Tangtsinia nanchuanica is a kind of rare and primitive plants, a class Ⅱ protected species in China. Its distribution area is very limited. Together with another primitive orchid, Cephalanthera falcata, they are only found in the understory of subtropical Pinus massoniana forests in Nanchuan, Chongqing, China. Using an open gas-exchange system(LI-COR6400), the response and adaptation of the two orchids to variable sunlight were studied. the results showed that:①Because of the influence of temperature, in order to get more reliable results, the accessory-temperature-control sub-system should be used. ②In any variable light environment from 800 to 200 μmol photons·m^-2s^-1, the two orchids could keep an almost constant net photosynthetic rate(P_n),transpiration rate (E), stomatal conductivity (Con), water use efficiency (WUE=P_n/E) and internal CO₂ concentration (C_i), and manifest the good adaptation to this large light variation. Even shade from 800 to 100 μmol photons·m^-2s^-1 gradually, their E and C I didn' t change abruptly. In most cases, the water wasn't the limit factor influencing the growth of herbs in the understory of P.massoniana forests, but the light was. So, in order to promote the solar energy use efficiency, the two orchids reduced their WUE to adapt the sun/shade transition. ③In the condition of shade to less than 200 μmol photons·m^-2s^-1 discontinuously, at the beginning of shade, their P_n declined to the minimum immediately,E and Con also reduced slowly, but the C_i could raise more than 20%. During 5-min shade periods, their P_n rose slowly, their E and Con still reduced slowly, and their C_i could keep the high level. When the light level increased again, C. falcata's P_n would recover more rapidly than T.nanchuanica 's. Their E and C_i could get back to the original levels rapidly in most cases, but the Con couldn't recover. All those above indicated that T.nanchuanica and C.falcata could adapt the variable light environment in the understory of P.massoniana forests very well. Maybe those were the general properties of the shade-tolerant herbs in the subtropical P.massoniana forests.

Key words: Tangtsinia nanchuanica, Cephalanthera falcata, Photosynthesis, Transpiration, Variable sunlight environment