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Dendrobium candidum is one of the most valuable varieties in Dendrobium family,its wild resource has becoming extinct for slow growth,large market demand as well as excessive pick and destroyed habitat by human activities,and thus it is recommended to be one of the endangered medicinal plants under the national protection.In order to protect wild D.candidum resource and to carry out large-scale plantations,the relative researches were conducted from the 1970s.Owing to its high demand of growth environment,the survival rates of D.candidum tissue culture seedlings was very low after transplant to field.Therefore one of the key problems is to complete the transplant acclimation of the tissue culture seedlings and increase its survival rates.It is well known that the survival of seedlings depended on its carbon assimilation,root activity of absorbing water and nutrient,as well as the environmental stress tolerance.In this study,to explore the adaptation changes of D.candidum tissue culture seedlings to field environment,we investigated changes in photosynthesis parameters,chlorophyll fluorescence characteristics,ultrastructure,photosynthetic pigment and soluble sugar in leaves,as well as root vigor and nitrate reductase activity of D.candidum tissue culture seedlings in the course of transplant acclimation,including 0 day,1 day,2 days,4 days and 20 days(had survived) after transplant.The results showed that the photosynthetic quantum yields,the maximum photosynthetic rate,carboxylation efficiency and dark respiratory rate gradually increased,the photosynthetic quantum yields,the maximum photosynthetic rate,carboxylation efficiency reached the maximum at 20 days after transplant among them,yet dark respiratory rate significantly reduced at 20 days after transplant.The general trend of the maximal photochemical efficiency of photosystem Ⅱ and apparent photosynthetic electron transport rate gradually increased in the whole course of transplanting,and their value reached the maximum at 20 days after transplant.There was no significant difference in content of chlorophyll a,the carotenoid content and the solubility sugar content,but content of chlorophyll b reduced significantly when 20 days post-transtplant,the carotenoid content and the solubility sugar content also showed an ascending tendency.On the other hand,internal structure of chloroplasts damaged distinctly in the process of transplant as the shape of chloroplast,lamellar structure and amount of photosynthate had obvious abnormal changes.Moreover,there were many mitochondrias emerging around the chloroplast.However,chloroplast gradually matured,the starch particles and plastoglobulis became big,lamellar structure were complete,clear and orderly at 20 days after transplant.Obviously,the study also stated clearly root vigor and nitrate reductase(NR) activities increased significantly after transplant 20 days.Photosynthetic properties,ultrastructure and root vigor of D.candidum tissue culture seedlings during transplantation have a certain change.Facing new environment,D.candidum tissue culture seedlings improved their photosynthetic capacity to the greatest degree in the process of adapting energetically to the external environment.With the extension of time,the photosynthetic efficiency increased obviously and the leaves accumulated a great quantity of photosynthesis product,and the capacity of roots absorbing water and nutrients,the adversity tolerance also increased significantly.Our results demonstrated that D.candidum tissue culture seedlings have fully adapt to the field environment when 20 days post-transtplant.This study clarified the physiological base of survival after transplant,and would give strong theories for the scale and efficiency artificial cultivation of D.candidum.

Seedling,made up of small individuals of woody plant species in the understorey,is an important component of many forests and considered as an important source for natural restoration in forest ecosystems.Simultaneously seedling plays an important role in the regeneration of numerous tree species.Based on space-for-time substitution method,we treated coniferous and broad-leaved mixed forest,secondary monsoon evergreen broad-leaved forest(MEBF),primary MEBF in the same area as three different stages of secondary succession of MEBF.Eight sampling plots representing the three different stages distribute in Caiyanghe nature reserve,surrounding the areas of Meizihu and Xinfang reservoir of Puer,in Yunnan Province,the southwest of China.We investigated seedling regeneration characteristics of woody plants of MEBF during secondary succession by looking at species composition,density,height level and the relationship with environmental factors.Overall,we found 2014 individuals of woody seedlings belonging to 101 species(dominated by understory tree species) in the eight plots covering 144-m2 area.The species richness of tree seedling,shrub seedling and liana seedling were 50,29 and 22 respectively,and their corresponding individuals were 1499,299 and 216.Tree seedling was the main component of woody plant seedling,such as Castanopsis echidnocarpa.Pinus kesiya var.langbianensis are dominant species in coniferous and broad-leaved mixed forest,however,their seedlings rarely appeared in the understory.The pine seedling stages need more light than shade-tolerant species which can conserve seedling bank in understory and waited for appropriate chance to regeneration.The woody plant seedling density of coniferous and broad-leaved mixed forest,secondary and mature MEBF was 183±5.69,240±20.31 and 372.5±4.5 individuals per 18m2.As succession progresses,the density of total woody seedlings,tree seedlings and liana seedlings all increased,while the density of shrub seedlings did not change significantly.Celastrus virens as a liana was particularly conspicuous in mature MEBF.Species richness of liana seedlings tended to increase,while species richness of tree seedlings and shrub seedlings remained relatively stable along the succession gradient.The Shannon-Wiener index of woody seedlings was significantly lower in primary MEBF than those in mixed forest and secondary MEBF.The abundance of woody seedlings in secondary and primary MEBF decreased with the increase of height level,while in mixed forest,the abundance of woody seedlings form a unimodal pattern with peak at 20 cm.Species richness,however,showed consistent(increasing or decreasing) pattern with the increase of height level in all these forest types.Distribution of woody seedlings abundance concentrated in 0—20 cm in the three community types.Woody seedlings species richness of mature MEBF was significantly lower than coniferous and broad-leaved mixed forest when height level was at Ⅰand Ⅱ.Srensen index indicated a close link between tree and liana seedlings and the species composition of the community.Moreover,tree seedling density distribution was significantly negatively correlated with the slope of the plot,while shrub seedling density was significantly positively related to soil pH.

Particulate organic carbon(POC) is one of the main forms of carbon and plays an important role in aquatic ecosystems and global carbon cycling.Marine and terrestrial ecosystems are the two important carbon pools for the estuaries on earth,so the investigation on the POC in estuaries is dramatically practical for the research of global carbon cycling.Generally,the POC in estuaries can be classified into two categories by its source(terrestrial input and marine organism production).Stable carbon isotope(δ13C) and C/N are the two recommendatory methods for measuring and analyzing the source of organic carbon,and the precision of stable carbon isotope method(δ13C) is better than that of C/N method because of the stability of δ13C.The present study aimed to investigate the distribution of POC and its sources in the Pearl River Estuary in summer through the POC measurements of 20 sites in August,2010.The results showed that the concentrations of POC ranged from 98.5 to 1929.8 μg/L(the average of 541.9 μg/L),and the concentrations decreased with the salinity gradient in the northern and middle part of the Pearl River Estuary and then increased from the middle part to the southern part of the Pearl River Estuary,and the concentrations of POC in the bottom waters layers were higher than those in the surface water layers.The total suspended matter(TSM),chlorophyll a and hydrodynamics were the major factors that influenced the POC spatial distribution,and the δ13C values of TSM increased with the salinity gradient and varied from-27.05‰ to-21.03‰(the average of-24.57‰),and the values in the bottom water layers were higher than those in the surface water layers.Our results showed that the terrestrial input and marine phytoplankton production contributed to the POC together in the Pearl River Estuary.According to the analysis of binary mixture model,most POC in the northern and middle part of the Pearl River Estuary was subject to the terrestrial organic carbon,which accounted for 50% to 88%(the average of 64%) of the total POC.In contrast,the marine organic carbon was the main contributor to the POC in the southern part of the Pearl River estuary,and it accounted for 50% to 89%(the average of 68%) of the total POC.Compared with two decades ago,the distribution and source of POC varied significantly,which is mainly due to the decrement of sediment discharge and increment of nutrient inputs into the Pearl River estuary.We predict that the sediment discharge rate of the Pearl River will decrease gradually with the construction of more large reservoirs,and then the contribution of terrestrial organic carbon to the POC in the Pearl River estuary will be decreasingly.