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 d UV-BBE (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 d 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 d UVBBE. The content of carotenoids, flavonoids and ascorbic acid was increased by PAR and 1 and 2 kJ m d UV-BBE but was reduced at 6 kJ m d UV-BBE. 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. INTRODUCTION Quality of vegetables plays an increasing role in human diet and for marketing of horticultural produce. As important determinants of internal quality we consider flavor, texture, minerals, vitamins (e.g. vitamin C) as well as bioactive substances (e.g. carotenoids). Plants are the major source of ascorbate (Vitamin C) in the human diet as humans are unable to synthesize this substance. Ascorbate is the most abundant soluble antioxidant in plant cells among secondary plant metabolites which also possess antioxidative properties. Since many diseases have been related to the occurrence of reactive oxygen species, plant derived antioxidants have gained importance for human nutrition in recent years (Kauer and Kapoor, 2001). Plant metabolism and as a consequence synthesis of vitamins and antioxidants is affected by many environmental parameters. Light conditions under cultivated protection are different from open field due to UV-B and other light/wavelength absorption of the plant coverage. In order to improve the nutritional value of greenhouse vegetables we started an initial experiment on UV-B exposure to spinach since little is known about the effect of dose and duration of UV-B on growth and metabolism in vegetables. We postulate that plants will generate higher amounts of valuable compounds when exposed to UV-B light from lamps installed in a greenhouse. MATERIALS AND METHODS Spinach (Spinacea oleracea L.) ‘Poncho‘ was cultivated in the glasshouse in pots Proc. VII IS on Prot. Cult. Mild Winter Climates Eds. D.J. Cantliffe, P.J. Stoffella & N. Shaw Acta Hort. 659, ISHS 2004 202 (12 cm, 2 plants pot) with peat, ebb-flood irrigated with complete nutrient solution (Flory 9 specifications, EC 2 mS cm) and temperatures set at 19/17°C. On November 7, nine weeks after sowing, the plants were subjected to differing radiation treatments for two weeks (Table 1). Treatments were no additional lighting (standard growing practise, SGP), additional photosynthetically active radiation (PAR), 1 kJ md UV-BBE (field grown condition, FGC), 2 kJ md UV-BBE (UV intensive, INT), 6 kJ md UV-BBE (UV permanent, PER). The treatments contained 150 plants and were not replicated. The UV lamp was a high-pressure metal-halide gas lamp (400 W, MHL400/E40, BLV Licht) emitting UV-B at >294 nm (Figure 1) and the daily doses were obtained by 4, 8 and 24 hours exposure at equal intensity. Since the UV lamp also emits PAR, the resulting different daily PAR sums were supplemented to 14.9 mol md by horticultural PAR lamps (400 W, SON-T AGRO, Philipps) from 7-17 h daily which is equal in quantity to the PAR treatment. The lighted treatments were separated from each other and from SGP by non-transmitting black PE-sheet walls. Plant weight and quality parameters were assessed before and at the end of the treatment period. Fresh weight was cumulated from 10 pots and dry matter content was determined in four replicates by oven drying. Chlorophyll fluorescence (CF) evaluation during growth was done before radiation, after one and two weeks of radiation, using two middle aged leaves/plant of the same 10 plant pots each time. Additionally, chlorophyll fluorescence of 20 detached leaves was measured after one week of storage at 1°C. The fluorescence parameters Fv/Fm and Fo were determinated with PAM-fluorometer (Walz) by emitting a single saturation light pulse on dark adapted tissue of the spinach plants at temperature of 18°C.