Responses of Rice and Pea Plants to Hardening With Low Doses of Ultraviolet-B Radiation

Abstract

Responses of two rice cultivars (Oryza sativa L. cw. Er Bai Ai and Lemont) and a pea plant (Pisum sativum L. cv. Greenfeast) to low daily doses of ultraviolet-B (UV-B) radiation were measured to explore mechanisms underlying UV-B tolerance. On hardening of rice plants, cv. Er Bai 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 photosynthetic 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 pea 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 synthesis of UV-absorbing compounds, while cv. Lemont depends more upon the synthesis of protective, water-soluble enzymes; however, neither strategy seems to operate effectively in pea plants.