| Dendrobium candidum is an epiphytic herb native mainly to regions in South China.As a traditional Chinese medicine,whose main officinal components are polysaccharide and alkaloid,it is used for curing cataract, throat inflammation,peripheral obstruction and chronic superficial gastritis according to the Pharmacopoeia of the People's Republic of China.Polysaccharides have been found to possess immmuno-stimulating, anti-tumor and anti-mutagenic activities.But because of the excessive collection for growing medicinal demands and deterioration of their natural habitats,the wild resources are approached in exhausted or endanger.Hitherto,few researches were studied on the photosynthetic characters and the relationship between photosynthesis and environment conditions about D.candidum.The quality and quantity of plants are usually determined by the dry matter production,which is correlated with photosynthesis.And the photosynthetic rate depends strongly on environmental conditions,such as irradiance,temperature and nutrient supply.Thus,it is very significant for people to find a new,fast way to determine the best mode of cultivation.In this study,a best variety D. candidum of was firstly selected(Kuanye,Zhaiye,Qingeeng),and then cultivated under four different shades(shade intensity:A＞B＞C＞D).The photosynthetic characters,chlorophyll contents,biomass,polysaccharide and alkaloid contents were determined in order to estimated the best-cultivated environment.The main findings are as follows:1.Kuanye has more obvious advantages according to the biomass. The pigment contents in Kuanye of 2-year-old were 2 times more than those 1-year-old plants.There was no statistic difference in Qingeng's 1-year and 2-year leaves.Pigment contents in the 2-year leaves of Zhaiye were about 30%higher than 1-year leaves.Longitudinal comparison,the Chl a,Chl b,Carotenoid,Chl a+b contents of 1-year leaves were the highest in Qingeng,while those of 2-year leaves were the highest in Kuanye.Besides,the highest net photosynthetic rate,longest photosynthesis time,and longest high-photosynthesis-last time were found in Kuanye.2.According to the results,all of the 2-year-old stems had the highest content of polysaccharides,and Kuanye＞Zhaiye＞Qingeng;all of the 2-year-old leaves had the highest content of alkaloid,and Qingeng＞Kuanye＞Zhaiye.3.In accordance with the results of the shade conditions,spring to summer is the peak period for plant growth,and autumn is no longer essential to the growth.Plants grow well under the shade of B,C,and D in spring,while under A,B in summer,B,C,D in autumn.Maximum Pn that was about 2.81μmol m-2s-1in spring were found under B shade,under which high net photosynthetic rate lasted the longest.In summer, maximum Pn and longest high net photosynthetic rate lasted were found under B shade.In autumn,photosynthesis time lasted longest under D shade,while the product of net photosynthetic rate and photosynthetic time were highest in C,followed by D.In winter,the longest photosynthesis time and highest product of net photosynthetic rate and photosynthetic time were found under D shade,followed by the C.4.Polysaccharide contents in all the shade were highest in 2-year stems in autumn.Polysaccharide content of 2-year stems under B,which was about 31.77%shade were highest,followed by D 25.34%,A 22.78%, C 22.52%.The content in leaves in the overall was significantly lower than stems and 1-year leaves from spring to autumn under D shade had highest polysaccharide content.Polysaccharide content in 2-year leaves was less than 1-year leaves.5.2-year-old leave had a higher alkaloid content than 1-year-old leaves,and alkaloids in leave were highest in the autumn,when under B, C shade had higher alkaloid content.In the autumn,the alkaloid in leave under four shade all reach their maximum value,which was 0.039%for A, 0.044%for B,0.045%for C,and 0.035%for D.Compared with the leaves,alkaloids in stem were much lower.The yearly changes of alkaloid in 1-year stems were not very significant.From spring to autumn, the highest alkaloid contents in 1-year stems were found under B shade. 2-year stem had much higher alkaloid contents than 1-year stems,and reach their maximum in autumn,when alkaloid of 2-year stems under A shade was 0.012%,B 0.021%,C 0.021%,D 0.018%.The contents in 3-year stems that were similar with those in 1-year stems were highest under B shade in same season,and the changes were not obvious between seasons.To sum up,in the cultivation of D.candidum,it is better to choose Kuanye as a widely-plant variety.In field cultivation,the suitable shade condition for D.candidum was B,C shade in spring,A-C in summer,B, C in autumn,C,D in winter.Polysaccharide contents were highest in autumn in various shelters,which could be up to 31.77%;alkaloid contents were highest in autumn,which could be up to 0.045%.So autumn is the suitable harvest season.But considering the large-scale cultivation,it is difficult to replace shade net,and the yield and quality of D.candidum under C shade were proved to be good results,so in field cultivation,C shade was suggested. |
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