Precision stressing by UV-B radiation to improve quality of spinach under protected cultivation

Plants possess a number of defence systems against plant-stress related inflictions such as natural, but at times excessive UV-B radiation. One protection mechanism is the altered synthesis of antioxidative substances like flavonoids, carotenoids, and ascorbic acid. Antioxidants have gained wide recognition for their health promoting actions in human body metabolism. Plants grown under protected cultivation do not receive any UV-B light leading to reduced amounts of secondary metabolites as compared to field crops. We concluded that plants will generate higher amounts of valuable antioxidants when exposed to UV-B light from fixtures installed in a greenhouse. Therefore, seven-week old spinach plants were treated with doses of 0, 1, 2 and 6 kJ m(-2) d(-1) UV-B-BE (biologically effective) radiation for two weeks and compared to PAR-radiated (photosynthetically active radiation) and non-radiated (daylight only) plants. The sum of PAR was maintained at 14.9 mmol m(-2) d(-1) for all radiated treatments. Biomass was increased by PAR and by increasing UV-B doses. Chlorophyll fluorescence measurements indicated improved photosynthetic capacities due to PAR and moderate UV-B, while it was reduced at 6 kJ m(-2) d(-1) UV-B-BE. The content of carotenoids, flavonoids and ascorbic acid was increased by PAR and 1 and 2 kJ m(-2) d(-1) UV-B-BE but was reduced at 6 kJ m(-2) d(-1) UV-B-BE. Antioxidative capacity (AC) of carotenoid-extracts was investigated: Moderate UV-B treatment lead to higher AC compared to PAR treatment. Under conditions of high PAR, low and moderate UV-B radiation improved plant growth and the content of valuable secondary plant substances.