Temperature response of photosynthesis in transgenic rice transformed with 'sense' or 'antisense' rbcS

The responses of chlorophyll fluorescence, gas exchange rate and Rubisco activation state to temperature were examined in transgenic rice plants with 130 and 35 of the wild-type (WT) Rubisco content by transformation with rbcS cDNA in sense and antisense orientations, respectively. Although the optimal temperatures of PSII quantum efficiency and CO2 assimilation were found to be between 25 and 32C, the maximal activation state of Rubisco was found to be between 16 and 20C in all genotypes. The Rubisco flux control coefficient was also the highest between 16 and 20C in the WT and antisense lines [0.88 at an intercellular CO2 pressure (Ci) of 28 Pa]. Gross photosynthesis at Ci 28 Pa per Rubisco content in the WT between 12 and 20C was close to that of the antisense lines where high Rubisco control is present. Thus, Rubisco activity most strongly limited photosynthesis at cool temperatures. These results indicated that a selective enhancement of Rubisco content can enhance photosynthesis at cool temperatures, but in the sense line with enhanced Rubisco content Pi regeneration limitation occurred. Above 20C, the Rubisco flux control coefficient declined. This decline was associated with a decline in Rubisco activation. The activation state of Rubisco measured at each temperature decreased with increasing Rubisco content, and the slope of activation to Rubisco content was independent of temperature. We discuss the possibility that the decline in Rubisco activation at intermediate and high temperatures is part of a regulated response to a limitation in other photosynthetic processes.