Photosynthetic And Water Physiology Characteristics Of Four Typical Plants In Karst Regions Of Northwest Guangxi, China.

Radermachera sinica, Alchornea trewioides, Vitex negundo,Sapium rotundifolium and Cipadessa cinerascens were the typical plants in karst regions of Northwest Guangxi, China. Studying Photosynthetic characteristics of these plants and water physiology can provide scientific basis for vegetation restoration in in karst regions of Northwest Guangxi, China, the photosynthetic rate(Pn) and transpiration rate(Tr) of these four plant were measured in different periods of the growing season by LI-6400 portable photosynthesis system. The response of photosynthesis to rain falling and related mathematical models about the response characteristics of photosynthesis to the light were studyed. Meanwhile, Photosynthetic of the three plant's seedlings(Radermachera sinica, Alchornea trewioides and Cipadessa cinerascens)were measured in different water stress levels. The results are as follows:1 The diurnal changes of photosynthetic active radiation (PAR) and air temperature were single peak curves, and comparing to PAR, the peak time of Temperature curve delayed two hours. in their growth period(June, July), more change scope occurred in the prosperous growth period and less change in the last growth period(October) for PAR. The diurnal changes of relative humidity(RH) and CO2 density(Ca) showed that the value in the morning and evening was higher than that in the noon. More change scope occurred in the whole day of their prosperous growth period, and less change in the whole day of the last growth period for CO2 density.2 The diurnal changes of four plant's photosynthesis were main single peaked curves, but the diurnal change of water use efficiency (WUE) were not significant. The seasonal changes of the four plant's photosynthesis were Identical trend, in July photosynthesis reached the highest value. Photosynthetic rate(Pn), transpiration rate (Tr) and stomatal conductance (Cond) in the prosperous growth period was higher than that in last growth period, but the order of WUE changed. PAR was the main environmental factor influencing Pn of four plant, other environmental factors were different for different plant spieces in growth period. The annual average net photosynthetic rate of Alchornea trewioides was higher than Vitex negundo, then Radermachera sinica ,and Sapium rotundifolium was lowest.3 The leaf nitrogen and Chlorophyll contents for Radermachera sinica, Alchornea trewioides, Vitex negundo and Sapium rotundifolium in prosperous growth period was higher than that in last growth period(P<0.05). The leaf nitrogen and Chlorophyll contents was significant different among the four plant, and the leaf nitrogen and Chlorophyll contents for Alchornea trewioides and Vitex negundo was higher than Radermachera sinica and Sapium rotundifolium (P<0.05).4 According to parameters of the established response characteristic curve of photosynthesis to the light, we can know that the photosynthesis capability of four plant was: Vitex negundo>Alchornea trewioides>Radermachera sinic a>Sapium rotundifolium. Light saturation point was Alchornea trewioides >Vitex negundo >Radermachera sinica>Sapium rotundifolium, light compensation point was Sapium rotundifolium>Radermachera sinica>Vitex negundo>Alchornea trewioides, and dark respiration rate was the same order.5 In dry season the response of photosynthesis to rainfall was significant regular pattern. Pn, Tr, Cond of Radermachera sinica, Vitex negundo and Alchornea trewioides increased with days addition after rainfall, but WUE increased in the first period, then reduced in laster period.6 The results of water stress experiment for Radermachera sinica, Vitex negundo and Cipadessa cinerascens showed that Pn, Tr and Cond decreaced with aggravating drought stress while WUE increaced before Moderate water stress and then decreaced. Pn and WUE of Vitex negundo were higher than Radermachera sinica and Cipadessa cinerascens, but Tr was lower(P<0.05). Vitex negundo was the plant of higher Pn and lower Tr, Sensitive to the change of water stress level. Vitex negundo may be more competitive than Radermachera sinica and Cipadessa cinerascens during the karst ecological restoration.