Arabidopsis plants expressing only the redox‐regulated Rca‐α isoform have constrained photosynthesis and plant growth

Rubisco activase (Rca) facilitates the release of sugar‐phosphate inhibitors from the active sites of Rubisco and thereby plays a central role in initiating and sustaining Rubisco activation. In Arabidopsis, alternative splicing of a single Rca gene results in two Rca isoforms, Rca‐α and Rca‐β. Redox modulation of Rca‐α regulates the function of Rca‐α and Rca‐β acting together to control Rubisco activation. Although Arabidopsis Rca‐α alone less effectively activates Rubisco in vitro, it is not known how CO₂ assimilation and plant growth are impacted. Here, we show that two independent transgenic Arabidopsis lines expressing Rca‐α in the absence of Rca‐β (‘Rca‐α only’ lines) grew more slowly in various light conditions, especially under low light or fluctuating light intensity, and in a short day photoperiod compared to wildtype. Photosynthetic induction was slower in the Rca‐α only lines, and they maintained a lower rate of CO₂ assimilation during both photoperiod types. Our findings suggest Rca oligomers composed of Rca‐α only are less effective in initiating and sustaining the activation of Rubisco than when Rca‐β is also present. Currently there are no examples of any plant species that naturally express Rca‐α only but numerous examples of species expressing Rca‐β only. That Rca‐α exists in most plant species, including many C3 and C4 food and bioenergy crops, implies its presence is adaptive under some circumstances.     

An isoleucine residue acts as a thermal and regulatory switch in wheat Rubisco activase

The regulation of Rubisco, the gatekeeper of carbon fixation into the biosphere, by its molecular chaperone Rubisco activase (Rca) is essential for photosynthesis and plant growth. Using energy from ATP hydrolysis, Rca promotes the release of inhibitors and restores catalytic competence to Rubisco‐active sites. Rca is sensitive to moderate heat stress, however, and becomes progressively inhibited as the temperature increases above the optimum for photosynthesis. Here, we identify a single amino acid substitution (M159I) that fundamentally alters the thermal and regulatory properties of Rca in bread wheat (Triticum aestivum L.). Using site‐directed mutagenesis, we demonstrate that the M159I substitution extends the temperature optimum of the most abundant Rca isoform by 5°C in vitro, while maintaining the efficiency of Rubisco activation by Rca. The results suggest that this single amino acid substitution acts as a thermal and regulatory switch in wheat Rca that can be exploited to improve the climate resilience and efficiency of carbon assimilation of this cereal crop as temperatures become warmer and more volatile.     

Low-temperature photosynthetic performance of a C4 grass and a co-occurring C3 grass native to high latitudes

The photosynthetic performance of C₄ plants is generally inferior to that of C₃ species at low temperatures, but the reasons for this are unclear. The present study investigated the hypothesis that the capacity of Rubisco, which largely reflects Rubisco content, limits C₄ photosynthesis at suboptimal temperatures. Photosynthetic gas exchange, chlorophyll a fluorescence, and the in vitro activity of Rubisco between 5 and 35 °C were measured to examine the nature of the low‐temperature photosynthetic performance of the co‐occurring high latitude grasses, Muhlenbergia glomerata (C₄) and Calamogrostis canadensis (C₃). Plants were grown under cool (14/10 °C) and warm (26/22 °C) temperature regimes to examine whether acclimation to cool temperature alters patterns of photosynthetic limitation. Low‐temperature acclimation reduced photosynthetic rates in both species. The catalytic site concentration of Rubisco was approximately 5.0 and 20 µmol m^?2 in M. glomerata and C. canadensis, respectively, regardless of growth temperature. In both species, in vivo electron transport rates below the thermal optimum exceeded what was necessary to support photosynthesis. In warm‐grown C. canadensis, the photosynthesis rate below 15 °C was unaffected by a 90% reduction in O₂ content, indicating photosynthetic capacity was limited by the capacity of P_i‐regeneration. By contrast, the rate of photosynthesis in C. canadensis plants grown at the cooler temperatures was stimulated 20–30% by O₂ reduction, indicating the P_i‐regeneration limitation was removed during low‐temperature acclimation. In M. glomerata, in vitro Rubisco activity and gross CO₂ assimilation rate were equivalent below 25 °C, indicating that the capacity of the enzyme is a major rate limiting step during C₄ photosynthesis at cool temperatures.     

Photosynthetic responses to chilling in a chilling‐tolerant and chilling‐sensitive Miscanthus hybrid

Miscanthus is a C₄ perennial grass being developed for bioenergy production in temperate regions where chilling events are common. To evaluate chilling effects on Miscanthus, we assessed the processes controlling net CO₂ assimilation rate (A) in Miscanthus x giganteus (M161) and a chilling‐sensitive Miscanthus hybrid (M115) before and after a chilling treatment of 12/5 °C. The temperature response of A and maximum Rubisco activity in vitro were identical below 20 °C in chilled and unchilled M161, demonstrating Rubisco capacity limits or co‐limits A at cooler temperatures. By contrast, A in M115 decreased at all measurement temperatures after growth at 12/5 °C. Rubisco activity in vitro declined in proportion to the reduction in A in chilled M115 plants, indicating Rubisco capacity is responsible in part for the decline in A. Pyruvate orthophosphate dikinase activities were also reduced by the chilling treatment when assayed at 28 °C, indicating this enzyme may also contribute to the reduction in A in M115. The maximum extractable activities of PEPCase and NADP‐ME remained largely unchanged after chilling. The carboxylation efficiency of the C₄ cycle was depressed in both genotypes to a similar extent after chilling. Φ_P:Φ_CO2 remained unchanged in both genotypes indicating the C₃ and C₄ cycles decline equivalently upon chilling.     

Effects of elevated CO2 and temperature on photosynthesis and Rubisco in rice and soybean

Rice (Oryza sativa L. cv. IR-72) and soybean (Glycine max L. Merr. cv. Bragg), which have been reported to differ in acclimation to elevated CO₂, were grown for a season in sunlight at ambient and twice-ambient [CO₂], and under daytime temperature regimes ranging from 28 to 40°C. The objectives of the study were to test whether CO₂ enrichment could compensate for adverse effects of high growth temperatures on photosynthesis, and whether these two C₃ species differed in this regard. Leaf photosynthetic assimilation rates (A) of both species, when measured at the growth [CO₂], were increased by CO₂ enrichment, but decreased by supraoptimal temperatures. However, CO₂ enrichment more than compensated for the temperature-induced decline in A. For soybean, this CO₂ enhancement of A increased in a linear manner by 32–95% with increasing growth temperatures from 28 to 40°C, whereas with rice the degree of enhancement was relatively constant at about 60%, from 32 to 38°C. Both elevated CO₂ and temperature exerted coarse control on the Rubisco protein content, but the two species differed in the degree of responsiveness. CO₂ enrichment and high growth temperatures reduced the Rubisco content of rice by 22 and 23%, respectively, but only by 8 and 17% for soybean. The maximum degree of Rubisco down-regulation appeared to be limited, as in rice the substantial individual effects of these two variables, when combined, were less than additive. Fine control of Rubisco activation was also influenced by both elevated [CO₂] and temperature. In rice, total activity and activation were reduced, but in soybean only activation was lowered. The apparent catalytic turnover rate (K_cat) of rice Rubisco was unaffected by these variables, but in soybean elevated [CO₂] and temperature increased the apparent K_cat by 8 and 22%, respectively. Post-sunset declines in Rubisco activities were accelerated by elevated [CO₂] in rice, but by high temperature in soybean, suggesting that [CO₂] and growth temperature influenced the metabolism of 2-carboxyarabinitol-1-phosphate, and that the effects might be species-specific. The greater capacity of soybean for CO₂ enhancement of A at supraoptimal temperatures was probably not due to changes in stomatal conductance, but may be partially attributed to less down-regulation of Rubisco by elevated [CO₂] in soybean than in rice. However, unidentified species differences in the temperature optimum for photosynthesis also appeared to be important. The responses of photosynthesis and Rubisco in rice and soybean suggest that among C₃ plants species-specific differences will be encountered as a result of future increases in global [CO₂] and air temperatures.     

Photosynthetic performance at low temperature of Bouteloua gracilis Lag., a high-altitude C4 grass from the Rocky Mountains, USA

The mechanisms controlling the photosynthetic performance of C₄ plants at low temperature were investigated using ecotypes of Bouteloua gracilis Lag. from high (3000 m) and low (1500 m) elevation sites in the Rocky Mountains of Colorado. Plants were grown in controlled‐environment cabinets at a photon flux density of 700 μ mol m^?2 s^?1 and day/night temperatures of 26/16 °C or 14/7 °C. The thermal response of the net CO₂ assimilation rate (A) was evaluated using leaf gas‐exchange analysis and activity assays of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPCase) and pyruvate,orthophosphate dikinase (PPDK). In both ecotypes, a reduction in measurement temperature caused the CO₂‐saturated rate of photosynthesis to decline to a greater degree than the initial slope of A versus the intercellular CO₂ response, thereby reducing the photosynthetic CO₂ saturation point. As a consequence, A in normal air was CO₂‐saturated at sub‐optimal temperatures. Ecotypic variation was low when grown at 26/16 °C, with the major difference between the ecotypes being that the low‐elevation plants had higher A; however, the ecotypes responded differently when grown at cool temperature. At temperatures below the thermal optimum, A in high‐elevation plants grown at 14/7 °C was enhanced relative to plants grown at 26/16 °C, while A in low‐elevation plants grown at 14/7 °C was reduced compared to 26/16 °C‐grown plants. Photoinhibition at low growth temperature was minor in both ecotypes as indicated by small reductions in dark‐adapted F_v/F_m. In both ecotypes, the activity of Rubisco was equivalent to A below 17 °C but well in excess of A above 25 °C. Activities of PEPCase and PPDK responded to temperature in a similar proportion relative to Rubisco, and showed no evidence for dissociation that would cause them to become principal limitations at low temperature. Because of the similar temperature response of Rubisco and A, we propose that Rubisco is a major limitation on C₄ photosynthesis in B. gracilis below 17 °C. Based on these results and for theoretical reasons associated with how C₄ plants use Rubisco, we further suggest that Rubisco capacity may be a widespread limitation upon C₄ photosynthesis at low temperature.     

Rubisco activase requires residues in the large subunit N terminus to remodel inhibited plant Rubisco

The photosynthetic CO₂ fixing enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) forms dead-end inhibited complexes while binding multiple sugar phosphates, including its substrate ribulose 1,5-bisphosphate. Rubisco can be rescued from this inhibited form by molecular chaperones belonging to the ATPases associated with diverse cellular activities (AAA+ proteins) termed Rubisco activases (Rcas). The mechanism of green-type Rca found in higher plants has proved elusive, in part because until recently higher-plant Rubiscos could not be expressed recombinantly. Identifying the interaction sites between Rubisco and Rca is critical to formulate mechanistic hypotheses. Toward that end here we purify and characterize a suite of 33 Arabidopsis Rubisco mutants for their ability to be activated by Rca. Mutation of 17 surface-exposed large subunit residues did not yield variants that were perturbed in their interaction with Rca. In contrast, we find that Rca activity is highly sensitive to truncations and mutations in the conserved N terminus of the Rubisco large subunit. Large subunits lacking residues 1–4 are functional Rubiscos but cannot be activated. Both T5A and T7A substitutions result in functional carboxylases that are poorly activated by Rca, indicating the side chains of these residues form a critical interaction with the chaperone. Many other AAA+ proteins function by threading macromolecules through a central pore of a disc-shaped hexamer. Our results are consistent with a model in which Rca transiently threads the Rubisco large subunit N terminus through the axial pore of the AAA+ hexamer.     

Dynamic photo-inhibition and carbon gain in a C4 and a C3 grass native to high latitudes

C₄ plants are rare in the cool climates characteristic of high latitudes and altitudes, perhaps because of an enhanced susceptibility to photo‐inhibition at low temperatures relative to C₃ species. In the present study we tested the hypothesis that low‐temperature photo‐inhibition is more detrimental to carbon gain in the C₄ grass Muhlenbergia glomerata than the C₃ species Calamogrostis Canadensis. These grasses occur together in boreal fens in northern Canada. Plants were grown under cool (14/10 °C day/night) and warm (26/22 °C) temperatures before measurement of the light responses of photosynthesis and chlorophyll fluorescence at different temperatures. Cool growth temperatures led to reduced rates of photosynthesis in M. glomerata at all measurement temperatures, but had a smaller effect on the C₃ species. In both species the amount of xanthophyll cycle pigments increased when plants were grown at 14/10 °C, and in M. glomerata the xanthophyll epoxidation state was greatly reduced. The detrimental effect of low growth temperature on photosynthesis in M. glomerata was almost completely reversed by a 24‐h exposure to the warm‐temperature regime. These data indicate that reversible dynamic photo‐inhibition is a strategy by which C₄ species may tolerate cool climates and overcome the Rubisco limitation that is prevalent at low temperatures in C₄ plants.     

Distribution of C3 and C4 grasses at different altitudes in a temperate arid region of Argentina

Summary The distribution of native C₃ and C₄ grasses in a temperate arid region of Mendoza, Argentina, was studied in six areas at different altitudes. C₄ species predominate at low elevations in both relative species abundance and plant cover. At high elevations C₃ species are dominant in cover and composition. At medium altitudes (1100–1600 m) grass species composition is balanced but plant cover of C₃ species is greater. Of 31 genera in the whole area, 19 were C₄. Only the genera Stipa (C₃) and Aristida (C₄) were present in all the six areas surveyed. The pattern of grass distribution shows high correlation with evapotranspiration and temperature parameters, but low correlation with rainfall. The relation between grass distribution and different climatic parameters is discussed.     

Grass (Poaceae) richness patterns across China’s nature reserves

Explaining grass richness patterns over broad geographic scales is a central issue of biogeography and macroecology. In this investigation, we document the richness patterns of grasses across China’s nature reserves, and discuss their possible explanations at national and regional scales using grass richness data coupled with information from climatic and topographical variables. Our results suggest that the water- and energy-related variables and elevation range (a surrogate of topographical heterogeneity) represent the primary explanations for the variation in grass richness across China’s nature reserves. Significant relationships were found between energy and all grasses, water and C₃ grasses, and the combination of water and energy and C₄ grasses at the national scale. Topographical heterogeneity is strongly associated with grass richness across China, whereas climatic constraints to grass diversity vary regionally. In regions of high rainfall, the presence of C₄ grasses is determined by annual potential evapotranspiration and sunshine hours; the climatic constraint to C₃ grass richness in a large and varied region is sunshine hours; whereas in regions of low soil temperature, the temperature determines the grass richness pattern. Our results also suggest that water variables alone do not represent the primary predictor of grass richness at the regional scale.     

Relative nutritional quality of C3 and C4 grasses for a graminivorous lepidopteran,Paratrytone melane (Hesperiidae)

Summary We tested the hypothesis that C₄ grasses are inferior to C₃ grasses as host plants for herbivorous insects by measuring the relative performance of larvae of a graminivorous lepidopteran, Paratrytone melane (Hesperiidae), fed C₃ and C₄ grasses. Relative growth rates and final weights were higher in larvae fed a C₃ grass in Experiment I. However, in two additional experiments, relative growth rates and final weights were not significantly different in larvae fed C₃ and C₄ grasses. We examined two factors which are believed to cause C₄ grasses to be of lower nutritional value than C₃ grasses: foliar nutrient levels and nutrient digestibility. In general, foliar nutrient levels were higher in C₃ grasses. In Experiment I, protein and soluble carbohydrates were digested from a C₃ and a C₄ grass with equivalent efficiencies. Therefore, differences in larval performance are best explained by higher nutrient levels in the C₃ grass in this experiment. In Experiment II, soluble carbohydrates were digested with similar efficiencies from C₃ and C₄ grasses but protein was digested with greater efficiency from the C₃ grasses. We conclude (1) that the bundle sheath anatomy of C₄ grasses is not a barrier to soluble carbohydrate digestion and does not have a nutritionally significant effect on protein digestion and (2) that P. melane may consume C₄ grasses at compensatory rates.     

Increased heat sensitivity of photosynthesis in tobacco plants with reduced Rubisco activase

High temperature inhibits photosynthesis by several mechanisms including deactivation of Rubisco. The inhibition of photosynthesis by high temperature and its relationship to Rubisco deactivation was studied using tobacco (Nicotiana tabaccum L. cv W38) transformed with a Rubisco activase gene inserted in the antisense orientation and untransformed controls. High temperature (42 °C) reduced photosynthesis in both lines of plants. However, photosynthesis recovered nearly completely in wild-type plants and very little in plants lacking Rubisco activase. The F₀ level of chlorophyll fluorescence decreased and q_N increased in the control plants during heating. In the antisense plants, q_N was always high and F₀ increased slightly during heat stress. NADP-malate dehydrogenase activation was unaffected by heat stress in control plants but was increased in the transgenic plants, consistent with a high redox status in the chloroplast. In wild-type plants, the inhibition of photosynthesis could be explained by a reversible decarbamylation of Rubisco and an acceptor-side limitation imposed on photosynthetic electron transport. However, in the anti-activase plants, carbamylation was low and constant and could not explain how photosynthesis was reduced at high temperature. Because ribulose bisphosphate was saturating at high temperature, the reduction in photosynthesis must have been caused by some impairment of Rubisco function not reflected in measurements of activation state or carbamylation status. This in vivo Rubisco impairment was not relieved upon return to lower temperature. We speculate that the reversible decarbamylation of Rubisco at moderately high temperature may be a protective mechanism by which the plant avoids more serious effects on Rubisco and the rest of the photosynthetic apparatus.This revised version was published online in October 2005 with corrections to the Cover Date.     

Geographical distribution and chorology of grasses in the Arabian Peninsula

Arid regions of Saudi Arabia occupy most of the area of the Arabian Peninsula. These areas are at the meeting position of plants from Mediterranean, Irano-Turanian, Saharo-Arabian, and Sudanian phytogegraphical regions. Geomorphology of the area reveals a wide diversity of landforms including coastal lines, desert plains, and high mountains. Grasses are well represented in the flora of Saudi Arabia and form an appropriate group for studying the relation of grass distribution, chorology, and photosynthetic pathways. In this paper, geographical distribution of C₃ and C₄ grasses was studied in an area extending between latitude 17°N and latitude 31°N. Two regions were recognized in the study area, namely; a (relatively) cold region north of latitude 24°N with ample winter rainfall, and a hot region south of latitude 24°N with scarce summer rainfall. Work involved field observations and collection of grass species in the study area. Work also depended on published carbon discrimination values of grasses and biochemical analysis of C₄ species subtypes. Climatic conditions in the study area vary considerably, and the distribution of grass species was found to follow patterns that reveal adaptive advantages of different photosynthetic pathways. Grass species in the cold northern region with ample winter rainfall are generally C₃ grasses belonging mainly to Mediterranean/Irano-Turanean chorotypes. C₃ grass species found in the southern hot region were recorded at high altitudes of southern mountains characterized by low temperatures. Grass species recorded at low altitudes in the south hot region with scarce summer rainfall were mainly C₄ grasses belonging to Tropical and Saharo-Arabian-Sudanean chorotypes. Pronounced spatial variations of temperature profoundly control the geographical distribution of C₃ and C₄ grasses. Low temperatures in the northern cold region and at high altitudes of the southern hot region limit the occurrence of C₄ grasses and shift the ecological balance in favor of C₃ grasses. Results are discussed in terms of heat sensitivity of the CO₂ carboxylating enzyme of C₃ grasses and high temperature optima for CO₂ assimilation of C₄ grasses. Results are also discussed in comparison with geographical distribution of grasses in other parts of the world.     

Preference for C4 shade grasses increases hatchling performance in the butterfly,Bicyclus safitza

The Miocene radiation of C₄ grasses under high‐temperature and low ambient CO₂ levels occurred alongside the transformation of a largely forested landscape into savanna. This inevitably changed the host plant regime of herbivores, and the simultaneous diversification of many consumer lineages, including Bicyclus butterflies in Africa, suggests that the radiations of grasses and grazers may be evolutionary linked. We examined mechanisms for this plant–herbivore interaction with the grass‐feeding Bicyclus safitza in South Africa. In a controlled environment, we tested oviposition preference and hatchling performance on local grasses with C₃ or C₄ photosynthetic pathways that grow either in open or shaded habitats. We predicted preference for C₃ plants due to a hypothesized lower processing cost and higher palatability to herbivores. In contrast, we found that females preferred C₄ shade grasses rather than either C₄ grasses from open habitats or C₃ grasses. The oviposition preference broadly followed hatchling performance, although hatchling survival was equally good on C₄ or C₃ shade grasses. This finding was explained by leaf toughness; shade grasses were softer than grasses from open habitats. Field monitoring revealed a preference of adults for shaded habitats, and stable isotope analysis of field‐sampled individuals confirmed their preference for C₄ grasses as host plants. Our findings suggest that plant–herbivore interactions can influence the direction of selection in a grass‐feeding butterfly. Based on this work, we postulate future research to test whether these interactions more generally contribute to radiations in herbivorous insects via expansions into new, unexploited ecological niches.     

Performance of a generalist grasshopper on a C<Subscript>3</Subscript> and a C<Subscript>4</Subscript> grass: compensation for the effects of elevated CO<Subscript>2</Subscript> on plant nutritional quality

The increasing CO₂ concentration in Earths atmosphere is expected to cause a greater decline in the nutritional quality of C₃ than C₄ plants. As a compensatory response, herbivorous insects may increase their feeding disproportionately on C₃ plants. These hypotheses were tested by growing the grasses Lolium multiflorum C₃) and Bouteloua curtipendula C₄) at ambient (370 ppm) and elevated (740 ppm) CO₂ levels in open top chambers in the field, and comparing the growth and digestive efficiencies of the generalist grasshopper Melanoplus sanguinipes on each of the four plant × CO₂ treatment combinations. As expected, the nutritional quality of the C₃ grass declined to a greater extent than did that of the C₄ grass at elevated CO₂; protein levels declined in the C₃ grass, while levels of carbohydrates (sugar, fructan and starch) increased. However, M. sanguinipes did not significantly increase its consumption rate to compensate for the lower nutritional quality of the C₃ grass grown under elevated CO₂. Instead, these grasshoppers appear to use post-ingestive mechanisms to maintain their growth rates on the C₃ grass under elevated CO₂. Consumption rates of the C₃ and C₄ grasses were also similar, demonstrating a lack of compensatory feeding on the C₄ grass. We also examined the relative efficiencies of nutrient utilization from a C₃ and C₄ grass by M. sanguinipes to test the basis for the C₄ plant avoidance hypothesis. Contrary to this hypothesis, neither protein nor sugar was digested with a lower efficiency from the C₄ grass than from the C₃ grass. A novel finding of this study is that fructan, a potentially large carbohydrate source in C₃ grasses, is utilized by grasshoppers. Based on the higher nutrient levels in the C₃ grass and the better growth performance of M. sanguinipes on this grass at both CO₂ levels, we conclude that C₃ grasses are likely to remain better host plants than C₄ grasses in future CO₂ conditions.     

Inhibition of photosynthesis by high temperature in oak (Quercus pubescens L.) leaves grown under natural conditions closely correlates with a reversible heat-dependent reduction of the activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase

Inhibition of the net photosynthetic CO₂ assimilation rate (P_n) by high temperature was examined in oak (Quercus pubescens L.) leaves grown under natural conditions. Combined measurements of gas exchange and chlorophyll (Chl) a fluorescence were employed to differentiate between inhibition originating from heat effects on components of the thylakoid membranes and that resulting from effects on photosynthetic carbon metabolism. Regardless of whether temperature was increased rapidly or gradually, P_n decreased with increasing leaf temperature and was more than 90% reduced at 45 °C as compared to 25 °C. Inhibition of P_n by heat stress did not result from reduced stomatal conductance (g_s), as heat‐induced reduction of g_s was accompanied by an increase of the intercellular CO₂ concentration (C_i). Chl a fluorescence measurements revealed that between 25 and 45 °C heat‐dependent alterations of thylakoid‐associated processes contributed only marginally, if at all, to the inhibition of P_n by heat stress, with photosystem II being remarkably well protected against thermal inactivation. The activation state of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) decreased from about 90% at 25 °C to less than 30% at 45 °C. Heat stress did not affect Rubisco per se, since full activity could be restored by incubation with CO₂ and Mg²⁺. Western‐blot analysis of leaf extracts disclosed the presence of two Rubisco activase polypeptides, but heat stress did not alter the profile of the activase bands. Inhibition of P_n at high leaf temperature could be markedly reduced by artificially increasing C_i. A high C_i also stimulated photosynthetic electron transport and resulted in reduced non‐photochemical fluorescence quenching. Recovery experiments showed that heat‐dependent inhibition of P_n was largely, if not fully, reversible. The present results demonstrate that in Q. pubescens leaves the thylakoid membranes in general and photosynthetic electron transport in particular were well protected against heat‐induced perturbations and that inhibition of P_n by high temperature closely correlated with a reversible heat‐dependent reduction of the Rubisco activation state.     

Intracellular position of mitochondria in mesophyll cells differs between C<Subscript>3</Subscript> and C<Subscript>4</Subscript> grasses

In C₃ plants, part of the CO₂ fixed during photosynthesis in chloroplasts is released from mitochondria during photorespiration by decarboxylation of glycine via glycine decarboxylase (GDC), thereby reducing photosynthetic efficiency. The apparent positioning of most mitochondria in the interior (vacuole side of chloroplasts) of mesophyll cells in C₃ grasses would increase the efficiency of refixation of CO₂ released from mitochondria by ribulose 1,5-bisphosphate carboxylase/?oxygenase (Rubisco) in chloroplasts. Therefore, in mesophyll cells of C₄ grasses, which lack both GDC and Rubisco, the mitochondria ought not to be positioned the same way as in C₃ mesophyll cells. To test this hypothesis, we investigated the intracellular position of mitochondria in mesophyll cells of 14 C₄ grasses of different C₄ subtypes and subfamilies (Chloridoideae, Micrairoideae, and Panicoideae) and a C₃–C₄ intermediate grass, Steinchisma hians, under an electron microscope. In C₄ mesophyll cells, most mitochondria were positioned adjacent to the cell wall, which clearly differs from the positioning in C₃ mesophyll cells. In S. hians mesophyll cells, the positioning was similar to that in C₃ cells. These results suggest that the mitochondrial positioning in C₄ mesophyll cells reflects the absence of both GDC and Rubisco in the mesophyll cells and the high activity of phosphoenolpyruvate carboxylase. In contrast, the relationship between the mitochondrial positioning and enzyme distribution in S. hians is complex, but the positioning may be related to the capture of respiratory CO₂ by Rubisco. Our study provides new possible insight into the physiological role of mitochondrial positioning in photosynthetic cells.     

Acclimation to temperature of the response of photosynthesis to increased carbon dioxide concentration in Taraxacum officinale

The relative stimulation of photosynthesis by elevated carbon dioxide in C₃ species normally increases strongly with increasing temperature. This results from the kinetic characteristics of Rubisco, and has potentially important implications for responses of vegetation to increasing atmospheric carbon dioxide. It is often assumed that because Rubisco characteristics are conservative, all C₃ species have the same temperature dependence of the response of photosynthesis to elevated carbon dioxide. However, in this field study of Taraxacum officinale, there were no significant differences in the relative stimulation of photosynthesis by elevated carbon dioxide among days with temperatures ranging from 15 to 34 °C. Nevertheless, short-term measurements indicated a strong temperature dependence of the stimulation. This suggested that acclimation to temperature caused the lack of variation in the seasonal data. Experiments in controlled environments indicated that complete acclimation of the relative stimulation of photosynthesis by elevated carbon dioxide occurred for growth temperatures of 10 – 25 °C. The apparent specificity of Rubisco for carbon dioxide relative to oxygen at 15 °C, as assayed in vivo by measurements of the carbon dioxide concentration at which carboxylation equalled oxygenation, also varied with growth temperature. Changes in the apparent specificity of Rubisco accounted for the acclimation of the temperature dependence of the relative stimulation of photosynthesis by elevated carbon dioxide. It is premature to conclude that low temperatures will necessarily reduce the relative stimulation of photosynthesis caused by rising atmospheric carbon dioxide. This revised version was published online in June 2006 with corrections to the Cover Date.     

Carbon assimilation by tree stems: potential involvement of phosphoenolpyruvate carboxylase

In woody species, the photosynthesis of stems, especially young branches, occurs by refixing part of the internal respiratory CO₂. The present study aims to improve the physiological characterization of stem photosynthesis by examining enzymatic characteristics. During an entire growing season, three enzymatic activities that are linked to C₃ and C₄ metabolism were investigated in relation to the CO₂ efflux and chlorophyll content of current year stems of European beech and were compared to the corresponding characteristics of leaves. High activities of phosphoenolpyruvate carboxylase (PEPC) and NADP malic enzyme were detected in stems (up to 13 times and 30 times higher in stems than in leaves, respectively), whereas Rubisco activity remained low in comparison with leaves. Stem maximal Rubisco and PEPC activities occurred at the beginning of the season when the total chlorophyll content and the CO₂ assimilation rate were also maximal. Stems were characterized by a PEPC:Rubisco ratio that was equal to 2.5 [an intermediate value between that of C₃-plants (about 0.1) and that of C₄-plants (about 10)], whereas it was equal to 0.1 in leaves. Eight other tree species were also measured and the PEPC:Rubisco ratio was, on average, equal to 3.6. The potential role of PEPC in stem carbon assimilation is discussed in relation to its known involvement in the anaplerotic function of C₃ plants and in the carbon metabolism of the C₄ pathway.     

Modulation of Rubisco activity in leaves of <Emphasis Type="Italic">Prosopis juliflora</Emphasis> in response to tropical conditions in north India

The success of P. juliflora, an evergreen woody species has been largely attributed to temperature acclimation and stomatal control of photosynthesis under wide range of environmental conditions prevalent in India. We studied the contribution of the enzyme ribulose-1,5 bisphosphate carboxylase/oxygenase (Rubisco) in diurnal and seasonal photosynthesis changes in P. juliflora. The changes observed in photosynthesis under natural conditions could be effected by the growth temperatures, which ranged from 10–30 °C in winter to 30–47 °C in summer. However, the Total Rubisco activity displayed a constant diurnal pattern and showed a maximum at 1200 in all seasons namely spring, summer, monsoon and winter irrespective of the changes in temperature. The Total Rubisco activity from two cohorts of leaves produced in spring and monsoon appeared to be down-regulated differentially at low PPFD during the evening. The in vivo and in vitro measurements of carboxylation efficiency of Rubisco showed wide variation during the day and were correlated with the photosynthesis rate. The light activation of Rubisco showed the acclimation to moderately high temperatures in different seasons except in summer. The exceptionally high temperatures (>45 °C) in summer, though not affecting Total activity, severely inhibited the light activation of Rubisco and also modulated the recovery process for the activation of Rubisco. Our studies suggest that the modulation of Rubisco driven by Rubisco activase and not Rubisco per se was crucial for the diurnal regulation of photosynthesis. NBRI Publication No.: 528