Regulation of leaf photosynthetic rate correlating with leaf carbohydrate status and activation state of Rubisco under a variety of photosynthetic source/sink balances.

There is evidence suggesting that in plants changes in the photosynthetic source/sink balance are an important factor that regulates leaf photosynthetic rate through affects on the leaf carbohydrate status. However, to resolve the regulatory mechanism of leaf photosynthetic rate associated with photosynthetic source/sink balance, information, particularly on mutual relationships of experimental data that are linked with a variety of photosynthetic source/sink balances, seems to be still limited. Thus, a variety of manipulations altering the plant source/sink ratio were carried out with soybean plants, and the mutual relationships of various characteristics such as leaf photosynthetic rate, carbohydrate content and the source/sink ratio were analyzed in manipulated and non-manipulated control plants. The manipulations were removal of one-half or all pods, removal of one-third or two-third leaves, and shading of one-third or one-half leaves with soybean plants grown for 8 weeks under 10 h light (24 degrees C) and 14 h darkness (17 degrees C). It was shown that there were significant negative correlations between source/sink ratio (dry weight ratio of attached leaves to other all organs) and leaf photosynthetic rate; source/sink ratio and activation ratio (percentage of initial activity to total activity) of Rubisco in leaf extract; leaf carbohydrate (sucrose or starch) content and photosynthetic rate; carbohydrate (sucrose or starch) content and activation ratio of Rubisco; amount of protein-bound ribulose-1,5-bisphosphate (RuBP) in leaf extract and leaf photosynthetic rate; and the amount of protein-bound RuBP and activation ratio of Rubisco. In addition, there were significant positive correlations between source/sink ratio and leaf carbohydrate (sucrose or starch) content; source/sink ratio and the amount of protein-bound RuBP; carbohydrate (sucrose or starch) content and amount of protein-bound RuBP and the activation ratio of Rubisco and leaf photosynthetic rate. The plant water content, leaf chlorophyll and Rubisco contents were not affected significantly by the manipulations. There is a previous report in Arabidopsis thaliana that the amount of protein-bound RuBP in leaf extract correlates negatively with the activation ratio of Rubisco in the leaf extract. Therefore, the results obtained from the manipulation experiments indicate that there is a regulatory mechanism for the leaf photosynthetic rate that correlates negatively with leaf carbohydrate (sucrose and starch) status and positively with the activation state of Rubisco under a variety of photosynthetic source/sink balances.

Analysis of the feed-forward effects of sink activity on the photosynthetic source-sink balance in single-rooted sweet potato leaves. I. Activation of RuBPcase through the development of sinks.

Single-rooted sweet potato leaves having a petiole with a fragment of stem allocated exceptionally large amounts of photosynthates to tuberous roots, the only major storage organ, throughout an experimental period of 50 d. The increase in photosynthetic activity for CO(2) fixation depended exclusively on the development of sink activity due to the growth of tuberous roots. Thus this model expressed a remarkable feed-forward effect on the photosynthetic source-sink balance. The level of ribulose-1,5-bisphosphate carboxylase (RuBPcase) protein in the leaves increased continuously during the period. The lowered initial as well as total activity of RuBPcase observed at the start of the experiment was raised with the cancellation of the sink-limited state due to the development of tuberous roots. The maximum activity determined after removing some inhibitor(s) from the enzyme by treating the leaf extract with SO(4)(2-) was much greater than the total activity and remained approximately constant throughout the experimental period. The clear decrease in the difference between maximum and total activities with the development of tuberous roots might reflect the reactivation of RuBPcase due to the removal of some inhibitor(s) from the enzyme through the cancellation of the sink-limited state.

Determination of sugar phosphates and nucleotides related to photosynthetic metabolism by high-performance anion-exchange liquid chromatography with fluorometric and ultraviolet detection.

A high-performance anion-exchange liquid chromatography system was constructed to identify sugar phosphates and nucleotides involved in photosynthetic metabolism. First sugar phosphates and nucleotides were separated by a gradient elution with boric acid and sodium phosphate, then they were detected by a fluorescence detector (as fluorescent derivatives with arginine) and UV detector, respectively. Eight authentic sugar phosphates and 11 authentic nucleotides could be analyzed using the system. The applicability of the system to the determination of the corresponding sugar phosphates and nucleotides in extracts from only five soybean leaf discs (8.95 cm2) was shown.

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.