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 elevated CO2 on the capacity for photosynthesis of a single leaf and a whole plant, and on growth in a radish.

The atmospheric concentration of CO2 will probably rise to about 700 micromol mol(-1) by the end of this century. The effects of elevated growth CO2 on photosynthesis are still not fully understood. Effects of elevated growth CO2 on the capacity for photosynthesis of a single leaf and a whole plant were investigated with the radish cultivar White Cherish. The plants were grown under ambient ( approximately 400 micromol mol(-1)) or elevated CO2 ( approximately 750 micromol mol(-1)). The rates of net photosynthesis per leaf area with a whole plant and a single leaf of plants of various ages (15-26 d after planting) were measured under ambient and elevated CO2. The rates of photosynthesis were increased by 20-28% by elevated CO2. There was no effect of elevated growth CO2 on the rate of photosynthesis, clearly indicating no downward acclimation of photosynthesis to elevated CO2. Elevated CO2 increased dry weight accumulation by >27%. The effect of elevated CO2 on other growth characteristics will also be shown.

High intracellular phosphorus contents exhibit a correlation with arsenate resistance in chlamydomonas mutants.

Pi in the medium relieved the toxicity of arsenate against cellular growth of Chlamydomonas reinhardtii. To investigate the relationship between intracellular P contents and arsenate resistance, we determined the intracellular P contents of arsenate-sensitive and arsenate-resistant mutants, which had been generated by random insertional mutagenesis. All 13 arsenate-resistant mutants showed higher P contents than the parent strain, while arsenate-sensitive mutants with high P contents were not found. In one of the arsenate-resistant mutants, AR3, the intracellular P content was about twice that in the wild type during growth in the absence of arsenate. Arsenate incorporation in AR3 was suppressed within 10 min after the addition of 1 mM arsenate, while Pi incorporation continued even after arsenate uptake ceased. Whereas the P content of the wild type decreased to half in the presence of 0.5 mM arsenate, almost the same degree (about 50%) of decrease was observed in AR3 cells grown in the presence of as much as 3 mM arsenate. AR3, in which PTB1, a homolog of a Pi transporter gene, had been disrupted, exhibited a higher activity of a high-affinity Pi transporter, suggesting that it may be due to a compensatory transport activity. These data suggest that the intracellular level of P is one of the important factors of arsenate resistance.

Insertional mutagenesis in a homologue of a Pi transporter gene confers arsenate resistance on chlamydomonas.

An arsenate-resistant mutant AR3 of Chlamydomonas reinhardtii is a recessive mutant generated by random insertional mutagenesis using the ARG7 gene. AR3 shows about 10-fold resistance against arsenate toxicity compared with the wild type. By using a flanking region of an inserted tag as a probe, we cloned the corresponding wild-type allele (PTB1) of a mutated gene, which could completely complement the arsenate-resistance phenotype of AR3. The size of PTB1 cDNA is about 6.0 kb and it encodes a putative protein comprising 1666 amino acid residues. This protein exhibits significant sequence similarity with the yeast Pho89 protein, which is known to be a Na(+)/Pi co-transporter, although the PTB1 protein carries an additional Gln- and Gly-rich large hydrophilic region in the middle of its primary structure. Analyses of arsenic accumulation and release revealed that PTB1-disrupted cells show arsenate resistance due to low arsenate uptake. These results suggest that the PTB1 protein is a factor involved in arsenate (or Pi) uptake. Kinetics of Pi uptake revealed that the activity of high-affinity Pi transport component in AR3 is more activated than that in the wild type.

Photosynthesis with single-rooted Amaranthus leaves. II. Regulation of ribuelose-1,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylase, NAD-malic enzyme and NAD-malate dehydrogenase and coordination between PCR and C4 photosynthetic metabolism in response to changes in the source-sink balance.

We have studied source-sink relationships with a model consisting of single-rooted leaves without petioles. We previously reported that the rate of photosynthesis decreased when C4 model plants prepared from Amaranthus cruentus leaves were subjected to sink-limited conditions by exposure to continuous light for a few days. It was suggested that the inhibition is due to a coordinated decrease in the activity of ribulose-1,5-bisphosphate carboxylase (RuBPcase) and phosphoenol-pyruvate carboxylase (PEPcase), both essential enzymes for photosynthesis in C4 plants. We further investigated the mechanisms behind the decreased activity of RuBPcase, PEPcase, NAD-malic enzyme and NAD-malate dehydrogenase. The results suggested that (1) the initial activity of RuBPcase is suppressed by a lowering of the P(i) level in chloroplasts, (2) the inhibition of PEPcase is due to dephosphorylation of the enzyme via the inhibition of PEPcase kinase and PEPcase phosphatase, (3) the inhibition of NAD-malic enzyme and NAD-malate dehydrogenase is derived from the oxidation of these enzymes, and (4) some proteinous factor(s) may be involved in the inhibition of the activity of these latter three enzymes. The significance of a coordinated decrease in these enzymes in response to a change in the source-sink balance is discussed.