A reporter gene assay revealed that promoters derived from Synechococcus PCC7942 (S.7942) psbAI and Synechocystis PCC6803 (S.6803) psbAII were suitable for the expression of foreign ribulose-bisphosphate carboxylase (RuBisCO; EC 4.1.1.39) in S.7942 cells. Transformational vectors with a promoter and a foreign RuBisCO gene, cvrbc originated from Allochromatium vinosum, were constructed on a binary vector, pUC303, and introduced to S.7942 cells. When the cvrbc was expressed with the S.7942 psbAI promoter, the total RuBisCO activity increased 2.5- to 4-fold than that of the wild type cell. The S.6803 psbAII promoter increased the activity of the transformant 1.5-2 times of that of wild type cell. There was a significant increase in the rate of photosynthesis depending on the increase of RuBisCO activity. The maximum rate of photosynthesis of the transformant cell was 1.63 times higher than that of the wild type under the illumination of 400 micromol m(-2) s(-1), at 20 mM bicarbonate and at 30 degrees C. Although the photosynthesis of the higher plant is limited by the ability of photosystems under high irradiance and the high CO(2 )concentration, that of the S.7942 cell is limited by the RuBisCO activity, even at high CO(2) concentrations and under high irradiance.

The chronic effects of ambient levels of O3 on photosynthetic components and radical scavenging system of leaves at different positions on the main stem of Japanese rice (Oryza sativa L.) were investigated. Two cultivars of Japanese rice (Koshihikari and Kinuhikari) were planted in plastic boxes filled with flooded Andisol in 9 chambers on 15 May, and exposed daily to charcoal-filtered air or O3 at 60 or 100 nl l -1 (ppb) (10:00-17:00) from 30 May to 20 September 2007. The activity and concentration of ribulose 1, 5-bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of both cultivars were not significantly affected by exposure to O3. However, the concentrations of chlorophyll in the lower leaves on the main stem of both cultivars were significantly reduced by the exposure to 100 ppb O3, while the exposure to O3 significantly increased the activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR) and glutathione reductase (GR); regardless of the leaf position in the two cultivars. Although no significant combined effect of O3 and cultivar on the concentration of chlorophyll, the activity and concentration of Rubisco, or the concentration of antioxidants and activity of radical scavenging enzymes was observed, there was a highly positive correlation between the AOT40 (accumulated exposure over a threshold of 40 ppb) of O3 and the relative activity of APX, MDAR or GR in leaves exposed to O3 to that in leaves exposed to charcoal-filtered air of both cultivars. These results suggest that APX, MDAR and GR played important roles in the antioxidative response of Japanese rice to O3. However, the detoxification capacity of reactive oxygen species by APX, MDAR and GR was insufficient to prevent the adverse effects of ambient levels of O3 on the concentration of chlorophyll in the two cultivars of Japanese rice.

When cyanobacterium cells are grown under extremely low CO(2) concentration, the number of carboxysomes, structures containing ribulose-bisphosphate carboxylase (Rubisco; EC 4.1.1.39), is known to increase. This suggests that Rubisco helps to regulate photosynthesis in cyanobacteria. However, no studies have been done on the changes of Rubisco content and activity in response to the extracellular CO(2) concentration, and no information is available on its effect on photosynthesis. To elucidate the relationship between the expression responses of Rubisco and extracellular CO(2), wild-type cells (Synechococcus PCC7942) and carboxysome-lacking cells were grown under various CO(2) concentrations, and Rubisco activity was determined. In both strains, Rubisco activity increased when the cells were grown under a CO(2) concentration around, or less than, K (1/2)(CO(2)) of photosynthesis. In carboxysome-lacking cells, Rubisco activity increased five to six times at most, and a simultaneous increase in the rate of photosynthesis was observed. These results suggest that stimulation of expression of Rubisco occurs to compensate for the decrease in the rate of photosynthesis under CO(2)-limited conditions.

To evaluate the effects of O 3 on the net photosynthetic rate in flag leaves of rice (Oryza sativa L.) based on stomatal O 3 flux and reactive oxygen species (ROS) scavenging enzyme activities, the Japanese rice cultivar Koshihikari was exposed to O 3 for 1 month after heading in O 3 -exposure chambers. During the exposure period, rice plants were exposed to charcoal-filtered air or O 3 at 60 or 100 nl l -1 (ppb) daily (10:00-17:00). Light-saturated net photosynthetic rate (A sat ) and ROS scavenging enzyme activities in the flag leaves were periodically measured during the exposure period. The O 3 exposure significantly reduced A sat on the 20th and 31st days after the beginning of O 3 exposure (DAE). The degree of O 3 -induced reduction in A sat was explained by the cumulative flux of O 3 compared with concentration-based O 3 indices. The activities of ROS scavenging enzymes such as ascorbate peroxidase and catalase at 32 DAE were lower than at 1 DAE. During the latter half of the O 3 exposure period (21-31 DAE), the daily average stomatal O 3 flux was also lower than that during the first half of the period (1-20 DAE), which was mainly caused by lower photosynthetic photon flux density, lower air temperature and leaf senescence. These results suggest that the balance between stomatal O 3 flux and leaf cellular detoxification capacity in the flag leaves of rice, which determines the degree of O 3 damage, might have been similar between the first half and latter half of the O 3 exposure period.

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