PHOTOSYNTHETIC RESPONSE OF FLOODED RICE (ORYZA-SATIVA) AND 3 ECHINOCHLOA SPECIES TO CHANGES IN ENVIRONMENTAL-FACTORS

The photosynthetic responses of rice (C-3) and three Echinochloa species (C-4), barnyardgrass, early watergrass, and late watergrass, to changes in CO2 intercellular partial pressure, light intensity, and leaf temperature were investigated under laboratory conditions. The three Echinochloa species exhibited photosynthetic responses characteristic of C-4 plants. The three weedy species showed higher efficiency for CO2 utilization at low CO2 intercellular partial pressure (CO(2)i) than rice. Compensation and saturation of CO(2)i for photosynthesis were lower in the weedy species than in rice. The maximum photosynthetic rates at high light intensity were 33.5, 32.7, 30.5, and 21.5 mu mol CO2 m(-2)s(-1) for barnyardgrass, early watergrass, late watergrass, and rice, respectively. Photosynthesis temperature optimum was 35 to 37 C for the three Echinochloa species and 33 C for rice. Overall, under simulated summer conditions, the four taxa showed a photosynthetic ability hierarchy with regard to gas exchange performance as follows: barnyardgrass greater than or equal to early watergrass > late watergrass > rice.