Changes in photosynthetic properties,ultrastructure and root vigor of Dendrobium candidum tissue culture seedlings during transplantation

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.