Ryuichi Ishii

Cultivar differences in leaf photosynthesis of rice bred in Japan

The grain yield of rice (Oryza sativa L.), as well as of other cereal crops, is limited to a large extent, by the supply of photosynthates produced during grain filling period. In this study, flag leaf photosynthesis (LPS) after heading was compared among 32 cultivars bred during the past century in Japan, to determine if the improvement of LPS has occurred with the breeding advance of high yielding cultivars. Measurement of LPS was made for 5 consecutive years in the paddy field, on the flag leaf of the main stem, at heading (LPS-0), and 2 weeks (LPS-2) and 4 weeks (LPS-4) after heading. LPS decreased with advance of leaf senescence from LPS-0 to LPS-2, and then to LPS-4. However, if nitrogen was top-dressed at the heading time, high LPS-2 was maintained, particularly in the newer cultivars. A significant positive correlation between LPS and the released year of cultivar was found at LPS-2, especially in the nitrogen top-dressed plot, but not at LPS-0 or LPS-4. Cultivar difference in LPS of the senescing leaves were not stable through the different years, whereas LPS-0 was stable over years, suggesting that the LPS in the senescent leaf is susceptible to the environmental variation due to the effects on leaf senescence. Cultivar difference in LPS at any stage was closely associated with mesophyll conductance to CO2, and stomatal conductance was also associated with cultivar difference in such a high LPS as LPS-0 and nitrogen top-dressed LPS-2. Significant correlation between LPS and specific leaf weight was not observed at any stage of the flag leaf.

Grain yield and related physiological characteristics of rice plants (Oryza sativa L.) inoculated with free-living Rhizobacteria

Abstract Experiments were conducted to elucidate the effects of the inoculation of a mixture of several free-living rhizobacteria; Azotobacter, Bacillus, Enterobacter and Xanthobacter which were collected and screened for the nitrogen-fixing ability in China, on nitrogen accumulation, growth and grain yield of rice plants. The inoculation of several bacterial species significantly increased acetylene-reducing activity in the roots of rice plants. The total dry matter yield, grain yield and nitrogen accumulation were increased by the bacterial inoculation by 6 to 13%, 15 to 18%, and 10 to 24%, respectively, in the experiments of 1997 and 1998. The increase of grain yield was associated with the increase of root length and leaf area, and also with the increase of chlorophyll content and photosynthetic rate during the grain-filling period. It was hypothesized that the inoculation of free-living rhizobacteria to rice plants led to increase of nitrogen accumulation in the plants, stimulation of the leaf photosynthesis particularly in the grain-filling stage resulting in the increased dry matter and grain yield.

Rice Cultivar Variation in the Growth Response to Inoculation of Free-Living Rhizobacteria

Our previous studies (Alam et aI., 2001) clearly showed that the growth of rice plants was significantly improved by inoculation of mix-cultured rhizobacteria. However, several studies (Garcia and Dobereiner, 1996; Smith et aI., 1984) showed that an inappropriate combination of bacteria and crop plant often resulted in a negative effect on the nitrogen accumulation and growth of the host plant. Moreover, a number experiments showed that the extent of the positive effect of the bacteria on nitrogen accumulation and crop growth varied with the species or variety of the host plant (Bouton and Brooks, 1982; Chanway et aI., 1988). The main objective of the present study was to investigate the effects of inoculating mix cultured free living rhizobacteria onto several rice cultivars including indica and japonica types of Oryza sativa, and Oryza glaberrima, on the nitrogenase activity and growth of the plant.

Leaf Photosynthesis in Rice in Relation to Grain Yield

Differences in leaf photosynthesis and their relationship to grain yield in Oryza species are discussed. During the evolutionary process, improvement of biochemical reactions of photosynthesis took place toward the direction of increase in CO2 fixing reaction catalyzed by RuBPcase. Most advanced Oryza species acquired the characteristics of high leaf photosynthesis for longer duration and efficient partitioning of photosynthetic products to the grains. Amongst the different groups of varieties, higher yield was associated with high respsonse of photosynthesis to nitrogen and maintenance of high photosynthetic activity of the flag leaf during the grain filling period. Studies on F1 heterosis in photosynthesis suggest that high yields could be associated with higher integrated photosynthesis of the flag leaf during the ripening period.

δ 13C Analysis to Approach the Mechanism of Varietal Difference of Photosynthetic Rate in Rice Plants

We have been so far conducting the research of varietal difference of apparent photosynthetic rate per unit leaf area (APS) in rice plants, aiming to clarify the mechanism of APS determination. APS is limited by the following two CO2 diffusion resistances, stomatal (Rs), and mesophyll (Rm) one. However, the process limited by Rm involves so many different kinds of physical and chemical processes. The purpose of this paper is to divide Rm further to two resistances. One is the physical CO2 diffusion resistance from the stomatal cavity to the CO2 fixation site in the chroloplasts, and the other is the chemical reaction resistance at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).