Intraspecific measurements of photorespiration

The relative magnitudes of (a) CO₂ compensation concentration, (b) zero CO₂ intercept of the CO₂ response curve, (c) O₂ suppression of net photosynthesis, (d) differential ¹²CO₂ and ¹⁴CO₂ uptake, and (e) ¹⁴CO₂ efflux into CO₂-free air were determined in the dry bean (Phaseolus vulgaris L.) varieties Michelite-62 (M-62) and Red Kidney (RK). In comparing the two varieties for each of the above processes, there were three categories of response, M-62 > RK, M-62 = RK, and M-62 < RK. Since these processes did not give the same relative difference for the two varieties being studied, it was concluded that these phenomena cannot validly be used to estimate the magnitude of photorespiration, although they do identify its presence. The results suggest that photorespiration is but one component of O₂ inhibition of net photosynthesis and that photorespiration itself has two or more component metabolic pathways.     

Phosphorus recycling in photorespiration maintains high photosynthetic capacity in woody species

Leaf photosynthetic CO₂ responses can provide insight into how major nutrients, such as phosphorus (P), constrain leaf CO₂ assimilation rates (A_net). However, triose‐phosphate limitations are rarely employed in the classic photosynthesis model and it is uncertain as to what extent these limitations occur in field situations. In contrast to predictions from biochemical theory of photosynthesis, we found consistent evidence in the field of lower A_net in high [CO₂] and low [O₂] than at ambient [O₂]. For 10 species of trees and shrubs across a range of soil P availability in Australia, none of them showed a positive response of A_net at saturating [CO₂] (i.e. A_max) to 2 kPa O₂. Three species showed >20% reductions in A_max in low [O₂], a phenomenon potentially explained by orthophosphate (P_i) savings during photorespiration. These species, with largest photosynthetic capacity and P_i > 2 mmol P m^?2, rely the most on additional P_i made available from photorespiration rather than species growing in P‐impoverished soils. The results suggest that rarely used adjustments to a biochemical photosynthesis model are useful for predicting A_max and give insight into the biochemical limitations of photosynthesis rates at a range of leaf P concentrations. Phosphate limitations to photosynthetic capacity are likely more common in the field than previously considered.     

Calculator-assisted measurements of photosynthetic,respiration and photorespiration rates in a closed gas exchange system

A closed gas exchange system has been designed for connection to the Hewlett-Packard programmable calculator controlled data acquisition system to provide a complete process of measuring and control. The system enables routine measurements of photosynthetic and dark respiration rates at different irradiances and different carbon dioxide and oxygen concentrations and leaf temperatures, and also a simple and rapid automatic control of irradiance according to the actual photosynthetic rate.     

Measurement of photorespiration in algae

The rates of true and apparent photosynthesis of two unicellular green algae, one diatom and four blue-green algae were measured in buffer at pH 8.0 at subsaturating concentrations of dissolved inorganic carbon (13-27 micromolar). Initial rates of depletion from the medium of inorganic carbon and ¹⁴C activity caused by the algae in a closed system were measured by gas chromatography and by liquid scintillation counting, respectively. The rate of photorespiration was calculated as the difference between the rates of apparent and true photosynthesis. The three eucaryotic algae and two blue-green algae had photorespiratory rates of 10 to 28% that of true photosynthesis at air levels of O₂. Reduction of the O₂ level to 2% caused a 52 to 91% reduction in photorespiratory rate. Two other blue-green algae displayed low photorespiratory rates, 2.4 to 6.2% that of true photosynthesis at air levels of O₂, and reduction of the O₂ concentration had no effect on these rates.     

Real-time measurements of vesicle-SNARE recycling in synapses of the central nervous system

Following the fusion of synaptic vesicles with the presynaptic plasma membrane of nerve terminals by the process of exocytosis, synaptic-vesicle components are recycled to replenish the vesicle pool. Here we use a pH-sensitive green fluorescent protein to measure the residence time of VAMP, a vesicle-associated SNARE protein important for membrane fusion, on the surfaces of synaptic terminals of hippocampal neurons following exocytosis. The time course of VAMP retrieval depends linearly on the amount of VAMP that is added to the plasma membrane, with retrieval occurring between about 4 seconds and 90 seconds after exocytosis, and newly internalized vesicles are rapidly acidified. These data are well described by a model in which endocytosis appears to be saturable, but proceeds with an initial maximum velocity of about one vesicle per second. We also find that, following exocytosis, a portion of the newly inserted VAMP appears on the surface of the axon.     

An investigation into the interaction between nitrogen nutrition,photosynthesis and photorespiration

Photosynthetic CO₂ assimilation, photorespiration and levels of glycollate oxidase and ribulose bisphosphate (RuBP) carboxylase were measured in barley, wheat and maize plants grown on media containing nitrate or ammonium or in plants transferred from nitrate to ammonium. The CO₂ compensation point and photorespiratory CO₂ release were not altered by the nitrogen growth regime nor by transfer from nitrate to ammonium. In barley and wheat plants grown on ammonium the levels of glycollate oxidase and RuBP carboxylase per unit leaf area were higher than in nitrate grown material. These differences were not evident when the results were expressed on a protein or chlorophyll basis. The ratio of glycollate oxidase activity to RuBP carboxylase activity was not altered by the nitrogen regime.     

An evaluation of race and sex identification from cranial measurements

A total of 104 adult human crania (95 American Indian and 9 Labrador Eskimo) are used in this evaluation of a discriminant functional analysis for determining race and sex from eight cranial measurements. The methods used are those given by Giles and Elliot ('62). The study shows that non-deformed American Indian crania are racially misclassified as American White and Negro in 35.6% of the cases when using this metrical method. Deformed Indian crania are racially misclassified 60.0% and 4.4% of the time as White and Negro respectively. The determination of sex on male crania, regardless of deformation, is as accurate as, or better than, the visual method of identification. The female crania, however, are shown to be incorrectly sexed in nearly 50% of the cases, with one non-deformed group (Palus) running as high as 80.0%. This evaluation suggests, therefore, that discriminant functional analyses for race and / or sex determinations are not applicable to problems of human identification unless the crania are from that population on which these functions were established.     

Estimating the rate of photorespiration in leaves

The influence of Li⁺ on the circumnutations of hypocotyls of Helianthus annuus L . cv. Californicus was investigated. LiCl at concentration levels from 0 to 40 m M (lethal) was added to intact hypocotyls grown in liquid nutrient medium. The Li⁺ concentration in the hypocotyls was measured by flame photometry. The growth of the hypocotyls was not affected by the LiCl.
Amplitude and frequency of the circumnutations were determined by correlation analysis. The oscillatory pattern of the movements became less regular at concentrations above 10 m M LiCl. The amplitude of the movements was reduced for concentrations above 7 m M LiCl. The frequency of the movements was reduced when LiCl was increased from 0 to 10 m M . Above 10 m M LiCl the frequency of the circumnutations was higher than for control plants. The results showed that circumnutations of sunflower hypocotyls can be added to the group of oscillators in biological organisms that are affected by Li⁺.     

Photosynthesis and photorespiration in algae

The CO₂ exchange of several species of fresh water and marine algae was measured in the laboratory to determine whether photorespiration occurs in these organisms. The algae were positioned as thin layers on filter paper and the CO₂ exchange determined in an open gas exchange system. In either 21 or 1% O₂ there was little difference between ¹⁴CO₂ and ¹²CO₂ uptake. Apparent photosynthesis was the same in 2, 21, or 50% O₂. The compensation points of all algae were less than 10 μl 1⁻¹. CO₂ or ¹⁴CO₂ evolution into CO₂-free air in the light was always less than the corresponding evolution in darkness. These observations are inconsistent with the proposal that photorespiration exists in these algae.     

An association between photorespiration and protein catabolism: Studies with Chlamydomonas

Work demonstrating the operation of a photorespiratory N cycle in Chlamydomonas is described. NH₃ release by this process is light dependent, sensitive to changes in pO₂ and pCO₂, and abolished by a photosystem II inhibitor. Evidence is presented which shows that this NH₃ derives its N from protein rather than from freshly synthesised glutamate. Protein turnover is shown to provide amino-N at a rate sufficient to account for the highest photorespiratory N excretion observed suggesting that changes in excretion can be accounted for by increased catabolism of normally recirculating amino acids. It is equally possible however that a direct link between photorespiration and protein turnover exists, increased NH₃ excretion resulting from enhanced protein turnover. The data suggest that if similar mechanisms operate in higher plants, previous estimates of the amount of N recycled in photorespiration may have been too high.Abbreviations GS glutamine synthetase - PMSF phenyl methyl sulfonyl fluoride - MSO L-methionine-DL-sulfoximine - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea     

Variation in photorespiration. The effect of genetic differences in photorespiration on net photosynthesis in tobacco

The hypothesis that net photosynthesis is diminished in many plant species because of a high rate of CO₂ evolution in the light has been tested further. High rates of CO₂ output in CO₂-free air in comparison with dark respiration were found in Chlamydomonas reinhardi, wheat leaves, tomato leaves, and to a lesser extent in Chlorella pyrenoidosa by means of the ¹⁴C-photorespiration assay. In tobacco leaves high photorespiration was characteristic of a standard variety, Havana Seed, and a possibly still higher rate was found in a yellow heterozygous mutant, JWB Mutant. However, the dark homozygous sibling of the latter, JWB Wild, had a low photorespiration for the tobacco species. The relative rates of photorespiration were in the same sequence when measured by the ¹⁴CO₂ released in normal air from leaf disks supplied with glycolate-1-¹⁴C in the light.

As would be predicted by the hypothesis, the maximal net rate of photosynthesis at 300 ppm of CO₂ in the air in JWB Wild leaves was greater (24%) than in Havana Seed, while JWB Mutant had less CO₂ uptake than the standard variety (21%). At 550 ppm of CO₂ the differences in net photosynthesis were not as great between the 2 siblings as at 200 ppm. The relative leaf expansion rates of seedlings of the 3 tobacco varieties in a greenhouse had the same relationship as their rates of CO₂ assimilation.

Thus within the tobacco species, as in a comparison between tobacco and maize, low photorespiratory CO₂ evolution was correlated with higher photosynthetic efficiency. Therefore it seems that increased CO₂ uptake should be achieved by genetic interference with the process of photorespiration.

     

Photosynthesis and photorespiration in the genus Oryza

Photosynthetic gas exchange has been surveyed in 22 of the 23species currently placed in the genus Oryza and constitutingthe wild relatives of cultivated rice. Unimproved, wild germplasmof a number of species showed light-saturated assimilation ratesin atmospheric air at least as great as cultivars and elitebreeding material of the cultigen O. satlva. One of these specieswas O. australiensis, different accessions of which were significantly(P=0.001) superior in assimilation rate to the 0. satlva genotypestested, including representatives of the Indica, Japonica andJavanica subgroups. Amongst species, assimilation rate was correlatedpositively with light saturation and with carboxylation efficiency.The wild species fell into two distinct groupings accordingto whether they originated from sun or shade habitats, withthe higher assimilation rates being associated with the sunspecies. Assimilation rates were also higher in diploids thanin tetraploids and this was associated with the fact that allsun species are diploids and all tetraploids are shade species.The carbon dioxide compensation concentrations ranged from 28to 43µmol mol^–1 with the two lowest values (28 and32µmol mol^–1) coming from accessions of O. rufipogon.The mean value for the absolute quantum yield of photosynthesismeas ured on attached leaves was 0.060. There was a large rangein the activity of phosphoenolpyruvate (PEP) carboxylase witha number of species having rates several times those of C₃ species.Some species with the highest assimilation rates were assessedfor photorespiratory losses and these were generally around30% and similar to O. satlva cultivars. However, a range ofO. rufipogon accessions had photorespiration rates significantly(P=0.01) lower than the O. sativa genotypes tested. No speciesin the genus possessed C₄ photosynthetic metabolism though somedid overlap with compensation concentrations and phosphoenolpyruvatecarboxylase activities reported for C₃–C₄ intermediatespecies. The potential value of wild relatives to the improvementof cultivated rice is discussed. Key words: Oiyza, photorespiration, photosynthesis, rice, wild rice     

Biochemical dissection of photorespiration.

Progress has been made in the understanding of photorespiration and related proteins (Rubisco, glycolate oxidase and glycine decarboxylase) in the context of recent structural information. Numerous shuttles exist to support transamination, ammonia refixation and the supply or export of reductants generated or consumed (via malate-oxaloacetate shuttles) in the photorespiratory pathway. A porin-like channel that is anion selective represents the major permeability pathway of the peroxisomal membrane.     

Evidence for the occurrence of photorespiration in synurophyte algae

The fluxes of CO(2) and oxygen during photosynthesis by cell suspensions of Tessellaria volvocina and Mallomonas papillosa were monitored mass spectrometrically. There was no rapid uptake of CO(2,) only a slow drawdown to compensation concentrations of 26 μM for T. volvocina and 18 μM for M. papillosa, when O(2) evolution ceased, indicating a lack of active bicarbonate uptake by the cells. Darkening of the cells after a period of photosynthesis did not cause rapid release of CO(2), indicating the absence of an intracellular inorganic carbon pool. However, upon darkening a brief burst of CO(2) was observed similar to the post-illumination burst characteristic of C(3) higher plants. Treatment of the cells of both species with the membrane-permeable carbonic anhydrase inhibitor ethoxyzolamide had no adverse effect on photosynthetic rate, but stimulated the dark CO(2) burst indicating the dark oxidation of a compound formed in the light. In the absence of any active accumulation of inorganic carbon photosynthesis in these species should be inhibited by O(2). This was investigated in four synurophyte species T. volvocina, M. papillosa, Synura petersenii, and Synura uvella: photosynthetic O(2) evolution rates in all four algae, measured by O(2) electrode, were significantly higher (40-50%) in media at low O(2) (4%) than in air-equilibrated (21% O(2)) media, indicating an O(2) inhibition of photosynthesis (Warburg effect) and thus the occurrence of photorespiration in these species.     

Photosynthetic rate control in cotton : photorespiration

The purpose of this research was to determine the magnitude of photorespiration in field-grown cotton (Gossypium hirsutum L.) as a function of environmental and plant-related factors. Photorespiration rates were estimated as the difference between measured gross and net photosynthetic rates.

A linear increase in photorespiration was observed as air temperature increased from 22 to 40°C at saturating photon flux density. At 22°C, photorespiration was less than 15 per cent of net photosynthesis and very comparable to the dark respiration rate. At 40°C, photorespiration represented about 50 per cent of net photosynthesis. Gross photosynthesis had a temperature optimum of 32 to 34°C. Water stress, as indicated by Ψ_L, did not alter the ratio of gross photosynthesis to net photosynthesis when the confounding effects of leaf temperature differences were accounted for in the data analyses. A reduction in both gross and net photosynthesis was apparent as Ψ_L declined from −2.0 megapascals indicating direct effects of water stress on the photosynthetic process. Photorespiration expressed as a proportion of net photosynthesis increased as water stress intensified.

Cotton cultivars possessing a fruit load had significantly higher gross and net photosynthetic rates and lower photorespiration rates than did photoperiod-sensitive cotton strains without a fruit load. Within the fruiting types, which were genetically very similar, only minor differences were observed in the photorespiration:net photosynthesis ratios. However, in the photoperiod-sensitive strains, considerable genetic variability existed when photorespiration was expressed as a proportion of net photosynthesis. These results suggest that the kinetics of ribulose-1,5-bisphosphate carboxylase:oxygenase may be different and, thus, the possibility of genetically reducing photorespiration exists.

     

Photosynthesis and photorespiration in whole plants of wheat

Wheat was cultivated in a small phytotronic chamber. ¹⁸O₂ was used to measure the O₂ uptake by the plant, which was recorded simultaneously with the O₂ evolution, net CO₂ uptake, and transpiration. At normal atmospheric CO₂ concentration, photorespiration, measured as O₂ uptake, was as important as the net photosynthesis. The level of true O₂ evolution was independent of CO₂ concentration and stayed nearly equal to the sum of net CO₂ photosynthesis and O₂ uptake. We conclude that at a given light intensity, O₂ and CO₂ compete for the reducing power produced at constant rate by the light reactions of photosynthesis.     

A C-assay for photorespiration in aquatic plants

The ¹⁴C-assay developed by Zelitch to evaluate photorespiration was modified for use in submersed aquatic plants in the laboratory and in situ. Results in laboratory cultures of Najas flexilis suggest that photorespiration occurs but is limited in comparison to terrestrial C₃ plants and is related to the rate of diffusion of CO₂ from the plant and to the carrying capacity of water for dissolved oxygen.