Optimization of shade condition and harvest time for Dendrobium candidum plants based on leaf gas exchange, alkaloids and polysaccharides contents

To meet the increasing demand for Dendrobium candidum plants and its bioactive products, the optimal shade condition and the best harvest time was determined. Gas exchange of D. candidum plants under various shade treatments (93%, 88%, 85% and 75% shading) was measured in four seasons to determine the optimal light intensity for plant growth. Diurnal variations of the net CO 2 assimilation rate showed that D. candidum plants under the 85% and 88% shading had the highest value of PN. Also, the 85% and 88% shading treatments resulted in higher values of areas enclosed by diurnal various curves of net CO2 assimilation rate and x axis (AECX) than other treatments in the first two seasons (spring and summer) and the last two seasons (autumn and winter), respectively. To obtain highest levels of bioactive products, the optimal harvest time of D. candidum was determined to be of two-year-old stems grown under 88% shading treatments which had the highest contents of polysaccharides and alkaloids. The relationships among AECX, increased stem volume, leaf area, leaf perimeter, polysaccharide content and alkaloid content showed that biomass and bioactive substance contents were directly proportional to photosynthesis. Therefore, approximately 88% shading and harvesting two-year-old stems in the autumn season can be recommended for this species as photosynthesis, biomass and bioactive products were comprehensively considered.