Hideaki Usuda

Hydraulic conductance as well as nitrogen accumulation plays a role in the higher rate of leaf photosynthesis of the most productive variety of rice in Japan

An indica variety Takanari is known as one of the most productive rice varieties in Japan and consistently produces 20–30% heavier dry matter during ripening than Japanese commercial varieties in the field. The higher rate of photosynthesis of individual leaves during ripening has been recognized in Takanari. By using pot-grown plants under conditions of minimal mutual shading, it was confirmed that the higher rate of leaf photosynthesis is responsible for the higher dry matter production after heading in Takanari as compared with a japonica variety, Koshihikari. The rate of leaf photosynthesis and shoot dry weight became larger in Takanari after the panicle formation and heading stages, respectively, than in Koshihikari. Roots grew rapidly in the panicle formation stage until heading in Takanari compared with Koshihikari. The higher rate of leaf photosynthesis in Takanari resulted not only from the higher content of leaf nitrogen, which was caused by its elevated capacity for nitrogen accumulation, but also from higher stomatal conductance. When measured under light-saturated conditions, stomatal conductance was already decreased due to the reduction in leaf water potential in Koshihikari even under conditions of a relatively small difference in leaf–air vapour pressure difference. In contrast, the higher stomatal conductance was supported by the maintenance of higher leaf water potential through the higher hydraulic conductance in Takanari with the larger area of root surface. However, no increase in root hydraulic conductivity was expected in Takanari. The larger root surface area of Takanari might be a target trait in future rice breeding for increasing dry matter production.

Effects of Growth under Elevated CO2 on the Capacity of Photosynthesis in Two Radish Cultivars Differing in Capacity of Storage Root

Abstract The effect of growth under elevated CO2 on the capacity of photosynthesis was assessed in two cultivars of radish, Raphanus sativus L. cv White Cherrish and Kosena, with a large and small storage root, respectively. Plants were grown under ambient (ca. 350 μmol CO2 mol-1) and elevated (ca. 750 μmol CO2 mol-1) CO2 and the first leaf of the plants at various ages, were examined for chlorophyll fluorescence, the maximum photosynthetic rates under saturated CO2 (photosynthetic capacity) and the rates of transpiration simultaneously. Elevated CO2 did not significantly reduce the capacity of photosynthesis, transpiration, quantum yield of electron transport from photosystem II (ΦPSII), and the maximum intrinsic yield of photosystem II at any developmental stage in both cultivars. In other words, growth under elevated CO2 had no effect on the capacity of photosynthesis in either cultivar. These results suggested that not only the storage root but also vigorously growing young leaves play an important role as a sink in utilizing increased photosynthate under elevated CO2. The elevated CO2 accelerated ontogeny and caused a slightly earlier decline in the capacity of photosynthesis. The capacity of carbon metabolism and the photochemical capacity decreased coordinately with advancing age accompanied with the decline of photosynthetic activity under both ambient and elevated CO2.

Evaluation of the Effect of Photosynthesis on Biomass Production with Simultaneous Analysis of Growth and Continuous Monitoring of CO2 Exchange in the Whole Plants of Radish, cv Kosena under Ambient and Elevated CO2

Abstract The effects of elevated CO2 (approximate doubling of atmospheric CO2 concentration) on the rate of photosynthesis estimated from continuous monitoring of CO2 exchange in whole plants were investigated in radish cv. Kosena accompanied with simultaneous analysis of growth for 6 days from 15 to 21 days after planting (DAP). The elevated CO2 increased the dry weights of hydroponically grown radish plants by 59% at 21 DAP. The increase in dry weight was due to a combined effect of increased leaf area and increased photosynthetic rate per unit leaf area. Leaf area and the photosynthetic rate were increased by elevated CO2 by 18-43% and 9-20%, respectively, during 15 to 21 DAP. Namely, an increase in the rate of photosynthesis is accompanied by an increase in leaf area, both having a significant effect on biomass production.

The in vivo Functioning Forms of Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase in Plants

Ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) changes its functioning form depending on the concentration of ribulose bisphos-phate (RuBP) in the in vitro assay (Fig. 1) [1,2]. The specific activity of the carboxylase reaction is very different among the three forms. Binding of RuBP to the regulatory sites of RuBisCO has been inferred to cause the change of the form.

Arsenate uptake by Pi transport system of an arsenate-resistant mutant, AR3, of Chlamydomonas

It is known that arsenate is taken up by the cells via Pi transport systems. In a green alga Chlamydomonas, arsenate was a competitive inhibitor of Pi transport systems, and the toxicity of arsenate disappeared in the presence of higher concentration of Pi. An arsenate-resistant mutant, AR3, which was generated by the random insertional mutagenesis, showed lower arsenic- and higher P-contents in the cells in the presence of arsenate than those of the wild type. The kinetics of Pi uptake demonstrated that AR3 had a high activity of a high-affinity Pi transport system even in the presence of 1mM Pi, where the system was suppressed in the wild type. The higher intracellular P content in AR3 might be due to one of the high-affinity Pi transporters. On the other hand, the kinetics of arsenate uptake indicated that the mutant had a high activity of a high-affinity arsenate transport system, which was suppressed rapidly after the incorporation in of arsenate into the cell. These results suggest that the arsenate resistance in AR3 is attributed to the high P content maintained by the activated Pi transport system and to the specific suppression of arsenate uptake via the Pi transport system.

How Can the C 4 Stromal System Sense Differences in Light Intensity to Adjust Its Activities to the Overall Flux

The amount of light, the driving force of photosynthesis, that each leaf receives varies remarkably and the overall rate of photosynthesis changes significantly throughout the day. The rate at which photochemical reactions occur depends primarily on the intensity of the incident light, therefore, the ability of the stromal system to balance its activities to the overall flux under differing light intensities would seem to be of paramount importance. The means by which the stromal system senses different light intensities is not yet known, however. The main aim of this study was to approach the question of how the stromal system can sense different light intensities and accommodate its activites to the overall flux by evaluating a likely candidate that links PPDK regulatory cascade to light in maize (Zea mays L. variety Chuseishu-B) mesophyll chloroplasts and by estimating thermodynamically active ADP and ATP concentration in a reconstituted model of the stromal system.

Diurnal Changes in Carbon Partitioning in Leaves

Sucrose is the primary transport carbohydrate in higher plants such as maize, soybean and spinach, and the regulation of sucrose formation appears to be one of the primary factors in the control of photosynthate partitioning in leaves. Identification of diurnal changes in carbon partitioning in leaves has important implications for whole plant source-sink interactions, and provides a system to probe the biochemical regulation of leaf photosynthetic metabolism.