Response of Dendrobium officinale Kimura et Migo, a Prized Medicinal Plant, to Continuous UV-B Irradiation at Different C/N Ratios

The response of Dendrobium officinale Kimura et Migo (D. officinale) to continuous UV-B irradiation at different carbon to nitrogen ratios (C/N ratios) was investigated. Seedlings grown for 60 days were incubated under aseptic conditions with UV-B irradiation (15.6 µW cm⁻²) at different C/N ratios: control group (CK; C/N 30 without UV-B), UV-B + CK (C/N 30 with UV-B irradiation, similarly hereafter), UV-B + C/N 120, UV-B + C/N 60, UV-B + C/N 15, UV-B + C/N 10, UV-B + C/N 7.5. Growth parameters (the defoliation rate and the sprout number), photosynthetic pigments (carotenoids, chlorophyll a and chlorophyll b), total polysaccharides, total alkaloids, and activities of antioxidant enzymes were determined following 4, 8, 12, and 16 days of continuous UV-B exposure. Results indicated that UV-B irradiation increased the defoliation rate and the content of carotenoids, total polysaccharides and total alkaloids, as well as the activities of antioxidant enzymes. Conversely, UV-B irradiation reduced the sprout number and chlorophyll content in D. officinale. Compared with UV-B + CK, lower C/N ratio treatments (UV-B + C/N 15, UV-B + C/N 10 and UV-B + C/N 7.5) enhanced the defoliation rate and sprout number, but decreased antioxidant enzyme activities and total polysaccharide content during the whole period, and reduced total alkaloid content after 4 days of UV-B exposure. Following initial UV-B irradiation, lower C/N ratios increased the contents of carotenoid and chlorophyll b, while after 8 days, a reversal in carotenoid content was observed, and after 12 days, a reversal in chlorophyll b content was observed. Optimizing the C/N ratio (C/N 60) resulted in lower defoliation rate, higher photosynthetic pigments and total polysaccharides, and increased activities of antioxidant enzymes, whereas no significant change in sprout number and total alkaloid content was recorded under long-term UV-B irradiation. Furthermore, the UV-B + C/N 120 treatment negatively affected D. officinale in terms of an increased defoliation rate and reduced sprout number, photosynthetic pigments, and total alkaloids. Therefore, results suggested that an appropriate C/N ratio (C/N 60) could ameliorate the adverse effects of continuous UV-B irradiation on D. officinale.