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 speci es (C-4), barnyardgrass, early watergrass, and late watergrass, to cha nges in CO2 intercellular partial pressure, light intensity, and leaf temperature were investigated under laboratory conditions. The three E chinochloa species exhibited photosynthetic responses characteristic o f C-4 plants. The three weedy species showed higher efficiency for CO2 utilization at low CO2 intercellular partial pressure (CO(2)i) than r ice. Compensation and saturation of CO(2)i for photosynthesis were low er 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 ric e, respectively. Photosynthesis temperature optimum was 35 to 37 C for the three Echinochloa species and 33 C for rice. Overall, under simul ated summer conditions, the four taxa showed a photosynthetic ability hierarchy with regard to gas exchange performance as follows: barnyard grass greater than or equal to early watergrass > late watergrass > ri ce.