Monocarpic plants require the initiation of whole-plant senescence to remobilize and transfer assimilates pre-stored in vegetative tissues to grains. Delayed whole-plant senescence caused by either heavy use of nitrogen fertilizer or adoption of lodging-resistant cultivars/hybrids that remain green when the grains are due to ripen results in a low harvest index with much nonstructural carbohydrate (NSC) left in the straw. Usually, water stress during the grain-filling period induces early senescence, reduces photosynthesis, and shortens the grain-filling period; however, it increases the remobilization of NSC from the vegetative tissues to the grain. If mild soil drying is properly controlled during the later grain-filling period in rice (Oryza sativa) and wheat (Triticum aestivum), it can enhance whole-plant senescence, lead to faster and better remobilization of carbon from vegetative tissues to grains, and accelerate the grain-filling rate. In cases where plant senescence is unfavorably delayed, such as by heavy use of nitrogen and the introduction of hybrids with strong heterosis, the gain from the enhanced remobilization and accelerated grain-filling rate can outweigh the loss of reduced photosynthesis and the shortened grain-filling period, leading to an increased grain yield, better harvest index and higher water-use efficiency.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2005.01597.x

Grain filling of cereals under soil drying.

MOST temperate plants form phosphoglyceric acid as the first stable product of photosynthesis. These plants also photo-respire, that is, during CO2 fixation they simultaneously release CO2 to the atmosphere from the oxidation of an intermediate of the photosynthetic process1,2. The reaction below, which takes place in the mitochondria3,4, is chiefly responsible for this CO2 release.

Photorespiratory nitrogen cycle

Photorespiration and photoinhibition: Some implications for the energetics of photosynthesis

The CO2 released in photorespiration is believed to result from a complex mitochondrial reaction in which glycine is converted to equimolar amounts of CO2, NH3 and the C-1 group of N5,N10-methylene-tetrahydrofolate 1–3. Because photorespiratory CO2 production may amount to 80 μmol per h per g fresh weight4,5 the stoichiometry of the decarboxylation reaction indicates that NH3 release from glycine exceeds primary NO3− reduction6,7. Leaf cells must therefore be capable of rapid NH3 reassimilation in photorespiratory conditions. It has been suggested8 that NH3 could be refixed directly into glutamate by mitochondrial glutamate dehydrogenase (GDH), utilizing the NADH generated during glycine decarboxylation1,9,10. This seems unlikely11,12 because of the high Km for NH3 exhibited by GDH in vitro. An alternative suggestion11,12, that photorespiratory NH3 could be re-assimilated into glutamate by the sequential action of cytoplasmic glutamine synthetase (GS) and the chloroplast enzyme glutamate synthase (GOGAT)13,14, is supported by circumstantial evidence from in vitro studies12,15. Here we provide direct evidence for such a pathway, based on the results of experiments with mutants of Arabidopsis thaliana (L.) Heynh which are deficient in leaf GOGAT activity.

Inhibition of photosynthesis in Arabidopsis mutants lacking leaf glutamate synthase activity

INTRODUCTION.. . . . ........ . . . . . . . . . . ......... . . . . ........ . . .... 27 MAJOR PHOTO SYNTHETIC O2 EXCHANGE REACTIONS. . . . ......... . . . ........ . . .... 28 Photoreduction of Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 28 Superoxide Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 The Fate of Sup eroxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 29 H202 Peroxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Summary of Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Photoresp iratory-Linked O2 Uptake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1 THE BALANCE BETWEEN OXYGEN EXCHANGE REACTIONS . . ........ . ........ . . . 32 C3 Photosynthetic O2 Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 C4 Photosynthetic Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Algae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 44 CONCLUSIONS . . . . . . . . . ... . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. ....... . . . . . . . . . . ...... . . . . . . . ...... . . . ..... 47

Photosynthetic Oxygen Exchange

Oxidation and phosphorylation associated with the conversion of glycine to serine

In wheat leaf segments carbon can enter the glycolate pathway, via the photosynthetic cycle, either as newly assimilated from CO2 in the atmosphere, from photorespired CO2 that has been refixed, or from recycling of intermediates within the pathway. Furthermore, several inter mediates exist in multiple metabolic pools. All these factors make it difficult to determine directly the flux of carbon along the glycolate pathway during photosynthesis. Following 15 min photosynthesis in 14CC>2 the active metabolic pools of glycine and serine approached saturation with 14C. If the gas mixture is then changed to one without 14C02, in conditions where refixation and recycling are minimized, the loss of 14C from glycine gives an estimate of the flux of carbon through the glycolate pathway. Such measurements showed that in 150 vpm CO2 in air the carbon flow along the glycolate pathway exceeded the assimilation rate; with 325 vpm CO2 both rates were approximately equal. Such fluxes are more than adequate to account for CO2 production by photorespiration assuming the reaction in which glycine is converted to serine is the only one in which CO2 is evolved.

The Flux of Carbon through the Glycolate Pathway during Photosynthesis by Wheat Leaves

/pdf/adenosine-triphosphate-synthesis-and-the-natural-electron-u0fkfwk770.pdf

Adenosine triphosphate synthesis and the natural electron acceptor for synthesis of serine from glycine in leaves.

When solutions of [14C]glycollate, glycine, serine, glycerate, or glucose were supplied to segments of wheat leaves through their cut bases in the light, most of the 14C was incorporated into sucrose in air but in C02-free air less sucrose was made. The synthesis of sucrose was decreased because metabolism of serine was partly blocked. Sucrose synthesis from glucose and glycerate in C02-free air was decreased but to a smaller extent ; relatively more CO2 was evolved and serine accumulated. The effects of DCMU and light of different wavelengths on metabolism by leaves of L-[U-14C]serine confirmed that simultaneous photosynthetic assimilation of carbon was necessary for the conversion of serine to sucrose. Of various products of photosynthesis fed exogenously to the leaves a-keto acids were the most effective in promoting photosynthesis of sucrose and release of 14C02 from 14C-labelled serine. This suggests that in C02-free air the metabolism of serine may be limited by a shortage of a-keto acid acceptors for the amino group. In C02-free air added glucose stimulated production of CO2 and sucrose from d-[U-14C] glycerate and no competitive effects were evident even though glucose is converted rapidly to sucrose under these conditions. In addition to a supply of keto acid, photosynthesis may also provide substrates that can be degraded and provide energy in the cytoplasm for the conversion of glycerate to sugar and phosphates and sucrose.

Effects of Carbon Dioxide on Metabolism by the Glycollate Pathway in Leaves

The effect on the carbon metabolism of wheat leaf segments of certain inhibitors of photo respiration was studied. Sodium 2-hydroxy-3-butynoate supplied for 40 min resulted in accumulation of 14C in glycolic acid with only a 7% inhibition of photosynthesis ; when supplied for 90 min, photosynthesis was inhibited by 47%. When 14C02 was replaced by 1000 vpm 12C02, radioactivity in glycine decreased but increased more rapidly in sucrose with less release of 14C02. Isonicotinyl hydrazide (INH) inhibited photosynthesis from 14C02 by 50% and glycine replaced sucrose as the main product. When, after 15 min, 14C02 was replaced by 150 vpm 12C02, in the presence of INH less 14C02 was released, 14C in glycine decreased more slowly, and less [14C]sucrose accumulated. Glycidate (potassium 2,3-epoxypropionate) at 2 mM had no effect on photosynthetic rate and little effect on carbon metabolism ; 20 mM glycidate inhibited photosynthesis by 64% and resulted in less radioactivity in glycine, more in phosphate esters, and less 14C02 released. When photosynthesis was measured in 1000 vpm C02 the inhibitors gave smaller effects on metabolism than during photosynthesis from 150 vpm 14C02 but 20 mM glycidate still resulted in a 42% inhibition of photosynthesis. When U - [14C]glycerate was applied to leaf segments in air with 320 vpm 12C02 the total uptake of glycerate was not changed by the inhibitors. INH and glycidate both decreased the amount of glycerate metab olised. More 14C accumulated in glycine in the presence of INH and in phosphate esters and serine in the presence of glycidate. Hydroxybutynoate increased the production of glycolate from glycerate but did not affect the total amount of glycerate metabolised. Although all three inhibitors affected photorespiratory metabolism none stimulated photo synthesis. The results are consistent with the main release of C02 in photorespiration arising from the conversion of glycine to serine.

C. P. Whittingham

Papers

Oxidation and phosphorylation associated with the conversion of glycine to serine

The Flux of Carbon through the Glycolate Pathway during Photosynthesis by Wheat Leaves

Adenosine triphosphate synthesis and the natural electron acceptor for synthesis of serine from glycine in leaves.

Effects of Carbon Dioxide on Metabolism by the Glycollate Pathway in Leaves

Effects of Certain Inhibitors on Photorespiration by Wheat Leaf Segments