RETARDATION OF HYPOCOTYL ELONGATION OF ORNAMENTAL AND VEGETABLE SEEDLINGS BY ULTRAVIOLET-IRRADIATION

Seedlings of cosmos (Cosmos bipinnatus Cav.), lettuce (Lactuca sativa L.), sunflower (Helianthus annuus L.), ornamental kale (Brassica olera cea L. var. acephara), tomato (Lycopersicon esculentum Mill.), bell pe pper (Capsicum annuum L.), and cucumber (Cucumis sativus L.) were irra diated by a UV-B lamp (fluorescent sun lamp) or a three-band fluoresce nt lamp (control) for 72 hr just after sowing. Hypocotyl elongation wa s repressed by all species during the irradiation with UV-B. The retar ding effects of UV-B persisted when these seedlings were placed in dar k or a greenhouse with 30% shade after irradiation. The most effective timing of UV-B irradiation for cosmos and ornamental kale seedlings w as from 48 hr to 72 hr after sowing, when the seedlings were rapidly i ncreasing their surface area to UV-B.