Two photosynthetic mechanisms mediating the low photorespiratory state in submersed aquatic angiosperms

The submersed angiosperms Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royal exhibited different photosynthetic pulse-chase labeling patterns. In Hydrilla, over 50% of the ¹⁴C was initially in malate and aspartate, but the fate of the malate depended upon the photorespiratory state of the plant. In low photorespiration Hydrilla, malate label decreased rapidly during an unlabeled chase, whereas labeling of sucrose and starch increased. In contrast, for high photorespiration Hydrilla, malate labeling continued to increase during a 2-hour chase. Thus, malate formation occurs in both photorespiratory states, but reduced photorespiration results when this malate is utilized in the light. Unlike Hydrilla, in low photorespiration Myriophyllum, ¹⁴C incorporation was via the Calvin cycle, and less than 10% was in C₄ acids.

Ethoxyzolamide, a carbonic anhydrase inhibitor and a repressor of the low photorespiratory state, increased the label in glycolate, glycine, and serine of Myriophyllum. Isonicotinic acid hydrazide increased glycine labeling of low photorespiration Myriophyllum from 14 to 25%, and from 12 to 48% with high photorespiration plants. Similar trends were observed with Hydrilla. Increasing O₂ increased the per cent [¹⁴C]glycine and the O₂ inhibition of photosynthesis in Myriophyllum. In low photorespiration Myriophyllum, glycine labeling and O₂ inhibition of photosynthesis were independent of the CO₂ level, but in high photorespiration plants the O₂ inhibition was competitively decreased by CO₂. Thus, in low but not high photorespiration plants, glycine labeling and O₂ inhibition appeared to be uncoupled from the external [O₂]/[CO₂] ratio.

These data indicate that the low photorespiratory states of Hydrilla and Myriophyllum are mediated by different mechanisms, the former being C₄-like, while the latter resembles that of low CO₂-grown algae. Both may require carbonic anhydrase to enhance the use of inorganic carbon for reducing photorespiration.

     

The effect on net photosynthesis of pedigree selection for low and high rates of photorespiration in tobacco

A normal appearing plant with a low rate of photorespiration (ratio of ¹⁴CO₂ released light/dark = 1.6) was found in an unselected tobacco (Nicotiana tabacum) cultivar. The plant was self-pollinated, and further selections were made on several successive generations. Excised leaves from the progeny of the selections were examined for photorespiration and net CO₂ assimilation in normal air during photosynthesis. Similar measurements were made of plants derived from selfed parents with high rates of photorespiration (ratio of ¹⁴CO₂ released light/dark = 3.0 or greater). Efficient photosynthetic plants (greater than 22.0 mg of CO₂ dm⁻² hr⁻¹) with low rates of photorespiration produced a larger proportion of efficient progeny (about 25%) than did selfing inefficient plants (about 6%), but this proportion did not increase in successive generations.     

Ammonium formation and assimilation in P(SARK)∷IPT tobacco transgenic plants under low N

Wild Type (WT) and transgenic tobacco plants expressing isopentenyltransferase (IPT), a gene encoding the enzyme regulating the rate-limiting step in cytokinins (CKs) synthesis, were grown under limited nitrogen (N) conditions. We analyzed nitrogen forms, nitrogen metabolism related-enzymes, amino acids and photorespiration related-enzymes in WT and P_SARK∷IPT tobacco plants. Our results indicate that the WT plants subjected to N deficiency displayed reduced nitrate (NO₃⁻) assimilation. However, an increase in the production of ammonium (NH₄⁺), by the degradation of proteins and photorespiration led to an increase in the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle in WT plants. In these plants, the amounts of amino acids decreased with N deficiency, although the relative amounts of glutamate and glutamine increased with N deficiency. Although the transgenic plants expressing P_SARK∷IPT and growing under suboptimal N conditions displayed a significant decline in the N forms in the leaf, they maintained the GS/GOGAT cycle at control levels. Our results suggest that, under N deficiency, CKs prevented the generation and assimilation of NH₄⁺ by increasing such processes as photorespiration, protein degradation, the GS/GOGAT cycle, and the formation of glutamine.     

Modelling O2 and O2 unidirectional fluxes in plants. III: Fitting of experimental data by a simple model

Photosynthetic assimilation of CO₂ in plants results in the balance between the photochemical energy developed by light in chloroplasts, and the consumption of that energy by the oxygenation processes, mainly the photorespiration in C₃ plants. The analysis of classical biological models shows the difficulties to bring to fore the oxygenation rate due to the photorespiration pathway. As for other parameters, the most important key point is the estimation of the electron transport rate (ETR or J), i.e. the flux of biochemical energy, which is shared between the reductive and oxidative cycles of carbon. The only reliable method to quantify the linear electron flux responsible for the production of reductive energy is to directly measure the O₂ evolution by ¹⁸O₂ labelling and mass spectrometry. The hypothesis that the respective rates of reductive and oxidative cycles of carbon are only determined by the kinetic parameters of Rubisco, the respective concentrations of CO₂ and O₂ at the Rubisco site and the available electron transport rate, ultimately leads to propose new expressions of biochemical model equations. The modelling of ¹⁸O₂ and ¹⁶O₂ unidirectional fluxes in plants shows that a simple model can fit the photosynthetic and photorespiration exchanges for a wide range of environmental conditions. Its originality is to express the carboxylation and the oxygenation as a function of external gas concentrations, by the definition of a plant specificity factor Sp that mimics the internal reactions of Rubisco in plants. The difference between the specificity factors of plant (Sp) and of Rubisco (Sr) is directly related to the conductance values to CO₂ transfer between the atmosphere and the Rubisco site. This clearly illustrates that the values and the variation of conductance are much more important, in higher C₃ plants, than the small variations of the Rubisco specificity factor. The simple model systematically expresses the reciprocal variations of carboxylation and oxygenation exchanges illustrated by a “mirror effect”. It explains the protective sink effect of photorespiration, e.g. during water stress. The importance of the CO₂ compensation point, in classical models, is reduced at the benefit of the crossing points Cx and Ox, concentration values where carboxylation and oxygenation are equal or where the gross O₂ uptake is half of the gross O₂ evolution. This concept is useful to illustrate the feedback effects of photorespiration in the atmosphere regulation. The constancy of Sp and of Cx for a great variation of P under several irradiance levels shows that the regulation of the conductance maintains constant the internal CO₂ and the ratio of photorespiration to photosynthesis (PR/P). The maintenance of the ratio PR/P, in conditions of which PR could be reduced and the carboxylation increased, reinforces the hypothesis of a positive role of photorespiration and its involvement in the plant-atmosphere co-evolution.     

Role of Grafting in Resistance to Water Stress in Tomato Plants: Ammonia Production and Assimilation

In general, drought depresses nutrient uptake by the root and transport to the shoot due to a restricted transpiration rate, which may contribute to growth limitation under water deprivation. Moreover, water stress may also restrict the ability of plants to reduce and assimilate nitrogen through the inhibition of enzymes implicated in nitrogen metabolism. The assimilation of nitrogen has marked effects on plant productivity, biomass, and crop yield, and nitrogen deficiency leads to a decrease in structural components. Plants produce significant quantities of NH₄ ⁺ through the reduction of NO₃ ^? and photorespiration, which must be rapidly assimilated into nontoxic organic nitrogen compounds. The aim of the present work was to determine the response of reciprocal grafts made between one tomato tolerant cultivar (Lycopersicon esculentum), Zarina, and a more sensitive cultivar, Josefina, to nitrogen reduction and ammonium assimilation under water stress conditions. Our results show that when cv. Zarina (tolerant cultivar) was used as rootstock grafted with cv. Josefina (ZarxJos), these plants showed an improved N uptake and NO₃ ^? assimilation, triggering a favorable physiological and growth response to water stress. On the other hand, when Zarina was used as the scion (JosxZar), these grafted plants showed an increase in the photorespiration cycle, which may generate amino acids and proteins and could explain their better growth under stress conditions. In conclusion, grafting improves N uptake or photorespiration, and increases leaf NO₃ ^? photoassimilation in water stress experiments in tomato plants.     

Effects of genetic manipulation of the activity of photorespiration on the redox state of photosystem I and its robustness against excess light stress under CO<Subscript>2</Subscript>-limited conditions in rice

Under CO₂-limited conditions such as during stomatal closure, photorespiration is suggested to act as a sink for excess light energy and protect photosystem I (PSI) by oxidizing its reaction center chlorophyll P700. In this study, this issue was directly examined with rice (Oryza sativa L.) plants via genetic manipulation of the amount of Rubisco, which can be a limiting factor for photorespiration. At low [CO₂] of 5 Pa that mimicked stomatal closure condition, the activity of photorespiration in transgenic plants with decreased Rubisco content (RBCS-antisense plants) markedly decreased, whereas the activity in transgenic plants with overproduction of Rubisco (RBCS-sense plants) was similar to that in wild-type plants. Oxidation of P700 was enhanced at [CO₂] of 5 Pa in wild-type and RBCS-sense plants. PSI was not damaged by excess light stress induced by repetitive saturated pulse-light (rSP) in the presence of strong steady-state light. On the other hand, P700 was strongly reduced in RBCS-antisense plants at [CO₂] of 5 Pa. PSI was also damaged by rSP illumination. These results indicate that oxidation of P700 and the robustness of PSI against excess light stress are hampered by the decreased activity of photorespiration as a result of genetic manipulation of Rubisco content. It is also suggested that overproduction of Rubisco does not enhance photorespiration as well as CO₂ assimilation probably due to partial deactivation of Rubisco.     

Role of Photorespiration and Cyclic Electron Transport in C<Subscript>4</Subscript> Photosynthesis Evolution in the C<Subscript>3</Subscript>–C<Subscript>4</Subscript> Intermediate Species <Emphasis Type="Italic">Sedobassia sedoides</Emphasis>

Plants from two Sedobassia sedoides (Pall.) Aschers populations (Makan and Valitovo) (Chenopodiaceae) with C₂ photosynthesis (precursor of C₄ photosynthesis in phylogenesis) and photorespiratory CO₂-concentrating mechanism were studied. Genetic polymorphism and isotope discrimination (δ¹³С) levels of the plants were determined under natural conditions, and their morpho-physiological parameters such as fresh and dry biomass of the above ground parts of plants, functioning of photosystem I (PSI) and photosystem II (PSII), intensity of net photosynthesis (A), transpiration (E), photorespiration and water use efficiency (WUE) of plants were calculated under control and salinine conditions (0 and 200 mM NaCl). Results of the population-genetic analysis showed that the Makan population is polymorphic (plastic) and the Valitovo population is monomorphic (narrowly specialized). There were no significant differences between the populations based on δ¹³С values or growth parameters, PSII, A, E and WUE under control conditions. Under saline conditions, dry biomass accumulation decreased in the Makan population by 15% and by more than 2- fold in the Valitovo population. Population differences were revealed in terms of photorespiration intensity and P700 oxidation kinetics under control and saline conditions. Under control conditions, Makan plants were characterized by a higher photorespiration intensity, which decreased by 2-fold under saline conditions to the photorespiration level of Valitovo plants. Cyclic electron transport activity was minimal in the control Makan plants, and it increased by almost 2-fold under saline conditions to the level of that in Valitovo plants under control and saline conditions. Under control conditions, photosynthesis in Makan plants can be specified as the proto-Kranz type (transitional type from C₃ to C₂) and that in Valitovo plants can be specified as the C₂ type (C₄ photosynthesis with photorespiratory CO₂-concentrating mechanism), based on their photorespiration level and cyclic electron transport activity. Under saline conditions, Makan plants exhibited features of C₂ photosynthesis. Intraspecific functional differences of photosynthesis were revealed in different populations of intermediate C₃–C₄ plant species S. sedoides which reflect the initial stages of formation of a photorespiratory CO₂-concentrating mechanism during C₄ photosynthesis evolution, accompanied by decrease in salt tolerance.     

Peroxisomal malate dehydrogenase is not essential for photorespiration in Arabidopsis but its absence causes an increase in the stoichiometry of photorespiratory CO2 release

Peroxisomes are important for recycling carbon and nitrogen that would otherwise be lost during photorespiration. The reduction of hydroxypyruvate to glycerate catalyzed by hydroxypyruvate reductase (HPR) in the peroxisomes is thought to be facilitated by the production of NADH by peroxisomal malate dehydrogenase (PMDH). PMDH, which is encoded by two genes in Arabidopsis (Arabidopsis thaliana), reduces NAD⁺ to NADH via the oxidation of malate supplied from the cytoplasm to oxaloacetate. A double mutant lacking the expression of both PMDH genes was viable in air and had rates of photosynthesis only slightly lower than in the wild type. This is in contrast to other photorespiratory mutants, which have severely reduced rates of photosynthesis and require high CO₂ to grow. The pmdh mutant had a higher O₂-dependent CO₂ compensation point than the wild type, implying that either Rubisco specificity had changed or that the rate of CO₂ released per Rubisco oxygenation was increased in the pmdh plants. Rates of gross O₂ evolution and uptake were similar in the pmdh and wild-type plants, indicating that chloroplast linear electron transport and photorespiratory O₂ uptake were similar between genotypes. The CO₂ postillumination burst and the rate of CO₂ released during photorespiration were both greater in the pmdh mutant compared with the wild type, suggesting that the ratio of photorespiratory CO₂ release to Rubisco oxygenation was altered in the pmdh mutant. Without PMDH in the peroxisome, the CO₂ released per Rubisco oxygenation reaction can be increased by over 50%. In summary, PMDH is essential for maintaining optimal rates of photorespiration in air; however, in its absence, significant rates of photorespiration are still possible, indicating that there are additional mechanisms for supplying reductant to the peroxisomal HPR reaction or that the HPR reaction is altogether circumvented.     

A possible role for abscisic Acid in regulation of photosynthetic and photorespiratory carbon metabolism in barley leaves

The influence of abscisic acid (ABA) on carbon metabolism, rate of photorespiration, and the activity of the photorespiratory enzymes ribulose bisphosphate oxygenase and glycolate oxidase in 7-day-old barley seedlings (Hordeum vulgare L. var. Alfa) was investigated. Plants treated with ABA had enhanced incorporation of labeled carbon from ¹⁴CO₂ into glycolic acid, glycine, and serine, while ¹⁴C incorporation into 3-phosphoglyceric acid and sugarphosphate esters was depressed. Parallel with this effect, treated plants showed a rise in activity of RuBP oxygenase and glycolic acid oxidase. The rate of photorespiration was increased twofold by ABA treatment at IO⁻⁶ molar while the CO₂-compensation point increased 46% and stomatal resistance increased more than twofold over control plants.     

Photosynthetic and Photorespiratory Characteristics of Mutants of Hordeum vulgare L

The relationship between photosynthesis and photorespiration was determined in normal and 26 mutants of barley (Hordeum vulgare L. var. Himalaya). The rate of apparent photosynthesis ranged from 1 to 30 milligrams of CO₂ per square decimeter per hour. The variation in rate of photosynthesis was due, in some cases, to differences in chlorophyll content, in others to stomatal resistance, and in still others to unknown factors; but no single factor accounted for the variation. Photorespiratory activity, as determined by the ¹⁴CO₂/¹²CO₂ technique, CO₂ evolution into CO₂-free air, and the response of photosynthesis to low and high O₂ concentrations, was positively and significantly correlated with photosynthesis. This supports the idea that the two processes are integrally and tightly coupled. There appears to be no competition between photosynthesis and photorespiration, and the probability of finding plants with high rates of photosynthesis and low rates of photorespiration measured under natural conditions, appears to be very low.     

Photorespiration in c(3) and c(4) plant tissue cultures: significance of kranz anatomy to low photorespiration in c(4) plants

Photorespiration rates in tissue cultures of a C₄ plant, Portulaca oleracea, were compared to those in tissue cultures of a C₃ plant, Streptanthus tortuosus. The C₄ plant tissue cultures have one-half to one-third the photorespiration rate of the C₃ plant tissue cultures and respond to varying O₂ concentrations in a manner typical of C₄ plants. The results suggest that the lack of detectable photorespiration in C₄ plants is not related to leaf anatomy.     

Modelling ¹⁸O₂ and ¹⁶O₂ unidirectional fluxes in plants: II. analysis of rubisco evolution

The studies of Rubisco characteristics observed during plant evolution show that the variation of the Rubisco specificity factor only improved by two times from cyanobacteria to modern C3 plants. However we note important variations of the ratio between the maximum rates of oxygenation and carboxylation (V_O/V_C). Modelling in vivo¹⁸O₂ data in plant gas exchange shows that the oxygenation reaction of Rubisco plays a regulating role when the photochemical energy exceeds the carboxylation capacity. A protective index ‘oxygenation capacity’ is postulated, related to the ratio V_O/V_C of Rubisco, and hence to the sink energy effect of photorespiration. Analysing the trends of Rubisco parameters along the evolutionary scale, we show: (1) the increase of both V_C and V_O; (2) the enhancement of CO₂ affinity; and (3) the rise in oxygenation capacity at the expense of the CO₂ specificity. Hence, the factors of evolutionary pressure have not only directed the enzyme towards a more efficient utilisation of CO₂, but mainly to positively use the unavoidable great loss of energy and assimilated carbon in the process of photorespiration. These observations reinforce the hypothesis of plant-atmosphere co-evolution and of the complex role of Rubisco, which seems to be selected to develop both better CO₂ affinity and oxygenation capacity. The latter increases the capacity of sink of photorespiration, in particular, during water stress or under high irradiance, the two conditions experienced by plants in terrestrial environments. These observations help to explain some handicaps of C4 plants, and the supremacy of CAM and C3 perennial higher plants in arid environments.     

Enhanced tolerance to salt stress in transgenic rice that overexpresses chloroplast glutamine synthetase

The potential role of photorespiration in the protection against salt stress was examined with transgenic rice plants. Oryza sativa L. cv. Kinuhikari was transformed with a chloroplastic glutamine synthetase (GS2) gene from rice. Each transgenic rice plant line showed a different accumulation level of GS2. A transgenic plant line, G39-2, which accumulated about 1.5-fold more GS2 than the control plant, had an increased photorespiration capacity. In another line, G241-12, GS2 was almost lost and photorespiration activity could not be detected. Fluorescence quenching analysis revealed that photorespiration could prevent the over-reduction of electron transport systems. When exposed to 150 mM NaCl for 2 weeks, the control rice plants completely lost photosystem II activity, but G39-2 plants retained more than 90% activity after the 2-week treatment, whereas G241-12 plants lost these activities within one week. In the presence of isonicotinic acid hydrazide, an inhibitor of photorespiration, G39-2 showed the same salt tolerance as the control plants. The intracellular contents of NH₄ ⁺ and Na⁺ in the stressed plants correlated well with the levels of GS2. Thus, the enhancement of photorespiration conferred resistance to salt in rice plants. Preliminary results suggest chilling tolerance in the transformant.     

Does long-term cultivation of saplings under elevated CO2 concentration influence their photosynthetic response to temperature?

Background and Aims Plants growing under elevated atmospheric CO₂ concentrations often have reduced stomatal conductance and subsequently increased leaf temperature. This study therefore tested the hypothesis that under long-term elevated CO₂ the temperature optima of photosynthetic processes will shift towards higher temperatures and the thermostability of the photosynthetic apparatus will increase.Methods The hypothesis was tested for saplings of broadleaved Fagus sylvatica and coniferous Picea abies exposed for 4–5 years to either ambient (AC; 385 µmol mol⁻¹) or elevated (EC; 700 µmol mol⁻¹) CO₂ concentrations. Temperature response curves of photosynthetic processes were determined by gas-exchange and chlorophyll fluorescence techniques.Key Results Initial assumptions of reduced light-saturated stomatal conductance and increased leaf temperatures for EC plants were confirmed. Temperature response curves revealed stimulation of light-saturated rates of CO₂ assimilation (A_max) and a decline in photorespiration (R_L) as a result of EC within a wide temperature range. However, these effects were negligible or reduced at low and high temperatures. Higher temperature optima (T_opt) of A_max, Rubisco carboxylation rates (V_Cmax) and R_L were found for EC saplings compared with AC saplings. However, the shifts in T_opt of A_max were instantaneous, and disappeared when measured at identical CO₂ concentrations. Higher values of T_opt at elevated CO₂ were attributed particularly to reduced photorespiration and prevailing limitation of photosynthesis by ribulose-1,5-bisphosphate (RuBP) regeneration. Temperature response curves of fluorescence parameters suggested a negligible effect of EC on enhancement of thermostability of photosystem II photochemistry.Conclusions Elevated CO₂ instantaneously increases temperature optima of A_max due to reduced photorespiration and limitation of photosynthesis by RuBP regeneration. However, this increase disappears when plants are exposed to identical CO₂ concentrations. In addition, increased heat-stress tolerance of primary photochemistry in plants grown at elevated CO₂ is unlikely. The hypothesis that long-term cultivation at elevated CO₂ leads to acclimation of photosynthesis to higher temperatures is therefore rejected. Nevertheless, incorporating acclimation mechanisms into models simulating carbon flux between the atmosphere and vegetation is necessary.     

Specific features of photorespiration in photosynthetically active organs of C3 plants

Photorespiration of photosynthetically active organs of C₃ plants (leaf, ear, stem, and leaf sheath) and C₄ plants (leaf, tassel, stem, leaf sheath, ear husk) grown under greenhouse and field conditions was studied. Photorespiration was measured using a PTM-48A high-technology monitor of photosynthesis (Bioinstruments S.R.L., Moldova). It is shown that photorespiration (CO₂ ejection after light turning off — apparent photorespiration) in C₃ plants is characteristic only for their leaves. In other photosynthesizing organs, photorespiration was absent, like in the photosynthesizing organs of C₄ plants. The absence of such after-light CO₂ outburst was observed for 31 genotypes: 18 cereal species belonging to four species (Triticum aestivum L., T. durum Desf., Secale cereale L., and Triticale); 6 grain legumes belonging to 2 species (Pisum sativum L. and Glycine max L.); 7 species of wild and rarely cultivated genotypes (T. boeoticum Boiss., T. dicoccoides Koern., T. dicoccum Schuebl., T. spelta L., T. compactum Host., T. monococcum L., and T. sphaerococcum Persiv.), and 2 genotypes of C₄ plants (Zea mays L. and Sorgum vulgaris L.). In all tested photosynthetically active genotypes, except of the C₃ plant leaves, apparent photorespiration was absent, but rather active glycolate cycle operated. The activity of this cycle was determined from the activity of the key enzyme of this cycle — glycolate oxidase. It was supposed that C₃ plants have two mechanisms of CO₂ assimilation: the first one — the mechanism of C₃ type localized in the leaves and the second one localized in other photosynthesizing organs, similar or with some elements of C₄ mechanism of CO₂ assimilation, limiting after-light CO₂ ejection during the metabolism of glycolate.     

Isolation and bicarbonate transport of chloroplast envelope membranes from species of differing net photosynthetic efficiency

A three-phase discontinuous sucrose gradient yielded two fractions of chloroplast envelope membranes from spinach (Spinacia oleracea L.), sunflower (Helianthus annuus L.), and maize (Zea mays L., mesophyll and undifferentiated chloroplasts). These species were selected to represent plants with fast photorespiration and slow net photosynthesis, fast photorespiration yet fast net photosynthesis, and slow photorespiration and fast net photosynthesis, respectively. Buoyant densities were 1.08 and 1.11 g cm^-3. The light fraction contained primarily single (incomplete) membrane vesicles and the heavy fraction double (complete) ones. Enzymic, chemical, and electron microscopic examination of the complete envelope membranes showed a lack of microbial, microsomal, mitochondrial, and lamellar membrane contamination as well as stromal contamination. Envelope membranes for all species examined were found to contain 2 to 4% of the total chloroplast protein and yields of about 0.2 to 0.4 mg of protein were obtained from 40 g leaves. An Mg²⁺-dependent nonlatent ATPase, a marker enzyme for chloroplast envelope membranes, had the following activities (μmoles of phosphate released/hr^-1 mg protein^-1): spinach, 77; sunflower, 163; old maize, 126; and young maize, 87. Bicarbonate transport was directly correlated with levels of ATPase activity in spinach and sunflower envelope membranes. Transport of HCO₃⁻ with sunflower envelope membranes approached that of young maize.     

Ammonia emission from rice leaves in relation to photorespiration and genotypic differences in glutamine synthetase activity

Background and Aims

Rice (Oryza sativa) plants lose significant amounts of volatile NH₃ from their leaves, but it has not been shown that this is a consequence of photorespiration. Involvement of photorespiration in NH₃ emission and the role of glutamine synthetase (GS) on NH₃ recycling were investigated using two rice cultivars with different GS activities.

Methods

NH₃ emission (AER), and gross photosynthesis (P_G), transpiration (Tr) and stomatal conductance (g_S) were measured on leaves of ‘Akenohoshi’, a cultivar with high GS activity, and ‘Kasalath’, a cultivar with low GS activity, under different light intensities (200, 500 and 1000 µmol m⁻² s⁻¹), leaf temperatures (27·5, 32·5 and 37·5 °C) and atmospheric O₂ concentrations ([O₂]: 2, 21 and 40 %, corresponding to 20, 210 and 400 mmol mol⁻¹).

Key Results

An increase in [O₂] increased AER in the two cultivars, accompanied by a decrease in P_G due to enhanced photorespiration, but did not greatly influence Tr and g_S. There were significant positive correlations between AER and photorespiration in both cultivars. Increasing light intensity increased AER, P_G, Tr and g_S in both cultivars, whereas increasing leaf temperature increased AER and Tr but slightly decreased P_G and g_S. ‘Kasalath’ (low GS activity) showed higher AER than ‘Akenohoshi’ (high GS activity) at high light intensity, leaf temperature and [O₂].

Conclusions

Our results demonstrate that photorespiration is strongly involved in NH₃ emission by rice leaves and suggest that differences in AER between cultivars result from their different GS activities, which would result in different capacities for reassimilation of photorespiratory NH₃. The results also suggest that NH₃ emission in rice leaves is not directly controlled by transpiration and stomatal conductance.     

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.     

光呼吸研究进展

光呼吸是指植物绿色组织依赖光能吸收O₂并释放CO₂的过程,它被认为是一个浪费能量的过程。正常生长的C₃植物光呼吸可损耗光合产物的25%~30%,在干旱、高温、高光等逆境胁迫下,该损耗可高达50%,因此,显著提高C₃植物的生产力可通过减少光呼吸通量来实现。尽管光呼吸对植物生产力的负面影响明显,但它对植物一些必要生理活动可能起着重要作用,其中包括参与光保护、H₂O₂信号发生、氮代谢、光氧化和抗逆反应等。该文对光呼吸的改造优化需要把握好平衡点与适配度。基于Rubisco改造、CO₂浓缩机制(CCM)和光呼吸支路创建的光呼吸改造研究进展进行了综述。通过了解调控光呼吸提高植物光能转化效率方面的最新进展, 可望为光呼吸代谢的分子调控及改良研究提供指导。     

Photosynthesis, photorespiration and productivity of wheat and soybean genotypes

The results of the numerous measurements obtained during the last 40 years on gas exchange rate, photosynthetic carbon metabolism by exposition in ¹⁴CO₂ and activities of primary carbon fixation enzyme, ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RuBPC/O), in various wheat and soybean genotypes grown over a wide area in the field and contrasting in photosynthetic traits and productivity are presented in this article. It was established that high productive wheat genotypes (7–9 t ha^?1) with the optimal architectonics possess higher rate of CO₂ assimilation during the leaf ontogenesis. Along with the high rate of photosynthesis, high values of photorespiration are characteristic for the high productive genotypes. Genotypes with moderate (4–5 t ha^?1) and low (3 t ha^?1) grain yield are characterized by relatively low rates of both CO₂ assimilation and photorespiration. A value of photorespiration constitutes 28–35% of photosynthetic rate in contrasting genotypes. The activities of RuBPC and RuBPO were changing in a similar way in the course of the flag leaf and ear elements development. High productive genotypes are also characterized by a higher rate of biosynthesis and total value of glycine–serine and a higher photosynthetic rate. Therefore, contrary to conception arisen during many years on the wastefulness of photorespiration, taking into account the versatile investigations on different aspects of photorespiration, it was proved that photorespiration is one of the evolutionarily developed vital metabolic processes in plants and the attempts to reduce this process with the purpose of increasing the crop productivity are inconsistent.