Effects of UVB preconditioning on heat tolerance of cucumber (Cucumis sativus L.)

To study the preconditioning effects of exposure to UVB on heat tolerance (44–48°C), 10-day-old cucumber (Cucumis sativus L.) seedlings were irradiated for 6 days with a daily dose of either 3.4, 5.5 or 10.6 kJ m−2 of biologically effective UVB (UVBBE). Cellulose diacetate-filtered UVB lamps were used to supply UVB. These approximated low, ambient (medium) and twice-ambient (high) UVB doses, respectively, at 42°N latitude on a clear day during the summer solstice. A parallel group of control plants, held under polyester (PE)-filtered UVB lamps, received no UVB. UVB treatment increased the level of methanol-extractable UV-absorbing phenols in cotyledons and primary leaves. In a further HPLC analysis of acid-hydrolyzed leaf extract, eight peaks were separated. The two major peaks were co-chromatographed with authentic flavonoids, quercetin and kaempferol. The quercetin-like compound was preferentially enhanced in UVB-exposed plants. When the low and medium UVB-irradiated plants were later heat-stressed (46°C, 1 h), survival improved by 112 and 82% and height growth increased by 35 and 40%, respectively, compared with controls that received no UVB. In contrast to low and ambient levels of UVB, which reduced membrane leakage, twice-ambient levels increased lipid peroxidation and electrolyte leakage. Although exposure to above-ambient levels of UVB was injurious, low and ambient doses of UVB promoted survival and growth of cucumber seedlings. Such a synergism between UVB and heat tolerance could be used to precondition seedlings grown in UVB-deficient greenhouses and may benefit plants under the predicted global warming scenario.