RESPONSES OF RICE AND PEA-PLANTS TO HARDENING WITH LOW-DOSES OF ULTRAVIOLET-B RADIATION

Responses of two rice cultivars (Oryza saliva L. cvv. Er Bai Ai and Le mont) and a pea plant (Pisum sativum L. cv. Greenfeast) to low daily d oses of ultraviolet-B (UV-B) radiation were measured to explore mechan isms underlying UV-B tolerance. On hardening of rice plants, cv. Er Ba i Ai produced more UV-absorbing compounds than cv. Lemont; however, cv . Lemont produced more water-soluble proteins, which may be related to its greater UV-B tolerance. These responses occurred without apparent deleterious effects on the photosynthetic properties of rice leaves. In contrast, the same hardening doses caused damage to pea plants, as indicated by decreases in photosynthetic quantum yield, maximum photos ynthetic capacity and chlorophyll content per unit leaf area, as well as water-soluble and total protein contents. The acquired tolerance of the two rice cultivars to a subsequent high UV-B dose increased with the duration of the hardening treatment. In contrast, the 'hardening' treatment exacerbated the damage to the photosynthetic apparatus of pe a plants subsequently exposed to a high UV-B dose. It appears that for defence against UV-B radiation, cv. Er Bai Ai relies more upon the sy nthesis of UV-absorbing compounds, while cv. Lemont depends more upon the synthesis of protective, water-soluble enzymes; however, neither s trategy seems to operate effectively in pea plants.