Study On Photosynthesis Pathway And PEG Stress Physiology Of Phalaenopsis Pulcherrima Lindl.

Phalaenopsis pulcherrima,syn.Doritis pulcherrima-an endecmic species to southeast Asia,only distributes in tropical area of Hainan Island in China.It’s one of the important parent-species in the hybridation of Phalaenopsis for its high ornamental value.However,P.pulcherrima has already become endangered recently due to overexploitation and destruction of natural habitats as a result of market benefit orientation.They grow in the litter layer or upon（are attached to）rock under the sparse forest in the tropical monsoon forest at an attitude of 200-500 m,as the microhabitats varies and there was significant difference in the individual number between two ecotypes.We attempt to find the endogenous causes through the research on P.pulcherrima stomata and the difference of photosynthetic carbon metabolism in summer and winter.As P.pulcherrimas have aerial roots and some of them grow on rocks,and the native habitat is featured in obvious dry season and wet season,P.pulcherrimas growing on rocks are vulnerable to drought stress in winter.In the laboratory experiment,P.pulcherrimas were treated with high,medium and low concentration-polyethylene glycol 6000（PEG6000）solutions（the mass fractions were5%,10%,20%respectively）to simulate drought stress,to explore the effect of drought stress on the plant growth,leaf osmotic adjustment substances and photosynthetic carbon metabolism of P.pulcherrima in order to ascertain the law of water demand,which will play a role in the conservation and introduction culture of P.pulcherrima.The main results of the research are as follows:（1）The number of the opening stoma increased obviously in the nighttime compared to daytime in the P.pulcherrima leaves.For two ecotypes,the stoma density in the reverse side of P.pulcherrima leaves were higher than that in the obverse side.And the stoma density of red type P.pulcherrima in the leaves’obverse or reverse side were higher than that of green type.P.pulcherrima was thick-leaf orchid with mesophyll,of which the pulpy index was greater than 1,succulent,suggesting it a CAM plant.（2）The study on the diurnal variation of malic acid,citric acid,starch and soluble sugar content in P.pulcherrima leaves suggested that P.pulcherrima hold a CAM pathway and CO₂was stored in the form of malic acid,two ecotype P.pulcherrimas had stronger expression of CAM in winter than summer,and the green type had stronger expression of CAM than red type no matter in winter or summer.（3）P.pulcherrimas were treated with polyethylene glycol 6000（PEG6000）solution to simulate drought stress.The results showed that the plant fresh weight and the water content decreased gradually with the increase of PEG concentration.And the concentrations which significantly affected the plant water content and the fresh weight ranged from 13.8%to 14.84%.（4）The content of chlorophyll a,b in P.pulcherrima leaves increased first and then decreased as plants were treated with low concentration PEG.（5）The soluble protein content in P.pulcherrima leaves increased first and then decreased as plants were treated with moderate and low concentration PEG,while the content decreased gradually as plants were treated with high concentration PEG.（6）The total free amino acids content in P.pulcherrima leaves tended to increase as the plants were treated with low concentration PEG,and tended to decrease when the plants were treated with moderate and high concentration PEG.In the detected 16kinds of amino acids,10 kinds had the richer content,and the content of Asp,Glu,Ala,Pro,Met increased while Ser,Thr decreased significantly for drought stress caused by PEG.（7）In the leaves of P.pulcherrimas treated with PEG,the malic acid content at daybreak were obviously higher than that at dusk,while the citric acid content at daybreak were lower than that at dusk with smaller difference between the two.The effect of PEG treatment on the malic acid content in leaves at dusk was more obvious than that at daybreak.The PEG treatment resulted in the difference between the soluble sugar content at daybreak and dusk first increasing and then decreasing while the difference of starch content decreasing on the whole.