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.     

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.     

Seasonal water availability predicts the relative abundance of C3 and C4 grasses in Australia

Aim Numerous studies have examined the climatic factors that influence the abundance of C₄ species within the grass flora (C₄ relative species richness) in various regions throughout the world, but very few have examined the relative abundance of C₄ vs. C₃ grasses (C₄ relative abundance). We sought to determine the climatic factors that influence C₄ relative abundance throughout Australia. Location Australia (including Tasmania). Methods We measured C₄ relative abundance at 168 locations and measured δ¹³C (the abundance of ¹³C relative to ¹²C) of the bone collagen of 779 kangaroos collected throughout Australia, as bone collagen δ¹³C was assumed to be proportional to the relative abundance of C₄ grasses in the diet. Results Both C₄ relative abundance and kangaroo bone collagen δ¹³C were found to have a strong positive relationship with seasonal water availability, i.e. the distribution of rainfall in the C₄ vs. C₃ growing seasons (76% and 69% of deviance explained, respectively). There was clear evidence that seasonal water availability was a better predictor of both C₄ relative abundance and bone collagen δ¹³C than other climate variables such as mean annual temperature and January daily minimum temperature. However, seasonal water availability appeared to be a relatively poor predictor of C₄ relative species richness, which was most closely related to January daily minimum temperature (90% of deviance explained). Main conclusions Our results highlight the relatively poor relationship between C₄ relative abundance and C₄ relative species richness, and suggest that these two variables may be related to different climatic factors. They also suggest that caution is required when using C₄ relative species richness to infer the relative biomass and productivity of C₄ grasses on a global scale.     

δ 13C variation of C3 and C4 plants across an Asian monsoon rainfall gradient in arid northwestern China

We have investigated carbon isotopic compositions of four plant genus/species, Bothriochloa ischaemum (C₄), Stipa bungeana (C₃), Lespedeza sp. (C₃) and Heteropappus less (C₃), along a precipitation gradient in northwest China in order to assess the impact of water availability on the carbon isotopic discrimination against ¹³C during carbon assimilation in this area. This information is necessary for reconstruction of paleovegetation, particularly paleo‐C₃/C₄ plant ratios using δ¹³C value of organic matter in loess and paleosols in the Chinese Loess Plateau. The δ¹³C of C₃ plants, as a group, exhibits a negative correlation with the annual precipitation amount with a total change and sensitivity of 5‰ and ?1.1‰/100 mm, respectively, for the precipitation range from 200 to 700 mm. The C₄ grass, B. ischaemum responds to aridity by decreasing 1.7‰ for over the precipitation range from 350 to 700 mm; the plant δ¹³C is significantly correlated with annual precipitation with a slope ?0.61‰/100 mm. This result implies that without considering the effect of water availability on the plant δ¹³C values, reconstruction of percent C₄ vegetation during the last glaciation can be overestimated by about a factor of two.     

C4 photosynthesis in a single C3 cell is theoretically inefficient but may ameliorate internal CO2 diffusion limitations of C3 leaves

Attempts are being made to introduce C₄ photosynthetic characteristics into C₃ crop plants by genetic manipulation. This research has focused on engineering single‐celled C₄‐type CO₂ concentrating mechanisms into C₃ plants such as rice. Herein the pros and cons of such approaches are discussed with a focus on CO₂ diffusion, utilizing a mathematical model of single‐cell C₄ photosynthesis. It is shown that a high bundle sheath resistance to CO₂ diffusion is an essential feature of energy‐efficient C₄ photosynthesis. The large chloroplast surface area appressed to the intercellular airspace in C₃ leaves generates low internal resistance to CO₂ diffusion, thereby limiting the energy efficiency of a single‐cell C₄ concentrating mechanism, which relies on concentrating CO₂ within chloroplasts of C₃ leaves. Nevertheless the model demonstrates that the drop in CO₂ partial pressure, pCO₂, that exists between intercellular airspace and chloroplasts in C₃ leaves at high photosynthetic rates, can be reversed under high irradiance when energy is not limiting. The model shows that this is particularly effective at lower intercellular pCO₂. Such a system may therefore be of benefit in water‐limited conditions when stomata are closed and low intercellular pCO₂ increases photorespiration.     

Naturally low carbonic anhydrase activity in C4 and C3 plants limits discrimination against C18OO during photosynthesis

The ¹⁸O content of CO₂ is a powerful tracer of photosynthetic activity at the ecosystem and global scale. Due to oxygen exchange between CO₂ and ¹⁸O-enriched leaf water and retrodiffusion of most of this CO₂ back to the atmosphere, leaves effectively discriminate against ¹⁸O during photosynthesis. Discrimination against ¹⁸O ( Δ ¹⁸O) is expected to be lower in C₄ plants because of low c_i and hence low retrodiffusing CO₂ flux. C₄ plants also generally show lower levels of carbonic anhydrase (CA) activities than C₃ plants. Low CA may limit the extent of ¹⁸O exchange and further reduce Δ ¹⁸O. We investigated CO₂–H₂O isotopic equilibrium in plants with naturally low CA activity, including two C₄ (Zea mays, Sorghum bicolor) and one C₃ (Phragmites australis) species. The results confirmed experimentally the occurrence of low Δ ¹⁸O in C₄, as well as in some C₃, plants. Variations in CA activity and in the extent of CO₂–H₂O isotopic equilibrium ( θ _eq) estimated from on-line measurements of Δ ¹⁸O showed large range of 0–100% isotopic equilibrium ( θ _eq = 0–1). This was consistent with direct estimates based on assays of CA activity and measurements of CO₂ concentrations and residence times in the leaves. The results demonstrate the potential usefulness of Δ ¹⁸O as indicator of CA activity in vivo. Sensitivity tests indicated also that the impact of θ _eq < 1 (incomplete isotopic equilibrium) on ¹⁸O of atmospheric CO₂ can be similar for C₃ and C₄ plants and in both cases it increases with natural enrichment of ¹⁸O in leaf water.     

Ancient latitudinal gradients of C3/C4 grasses interpreted from stable isotopes of New World Pleistocene horse (Equus) teeth

Carbon and oxygen isotopic data are reported from 116 Pleistocene Equus teeth from sixty-six localities in the New World ranging from 68°N (Alaska, Canada) to 35°S (Argentina). Equus species have been predominantly grazers, and as such, carbon isotopic values of their tooth enamel provide evidence of the Pleistocene distribution of C₃ and C₄ grasses. The carbon data presented here indicate a gradient (δ¹³C range of 10 parts/mil) in the relative proportion of C₃ and C₄ grasses between high latitude and equatorial Equus samples. The largest amount of change from C₃ to C₄ grasses during the Pleistocene occurred in the mid-latitudes between about 30 to 40°. The oxygen data, which vary proportionately with temperature, indicate a latitudinal gradient (δ¹⁸O range of 20 parts/mil) between high-latitude and equatorial Equus samples. The basic pattern of latitudinal gradients of C₃/C₄ grass distribution and temperature as interpreted from these Pleistocene data is similar to the modern-day. The use of stable isotopes of fossil herbivore teeth represents a new means to interpret Pleistocene climates and terrestrial ecology.     

Root responses along a subambient to elevated CO2 gradient in a C3–C4 grassland

Atmospheric CO₂ (C_a) concentration has increased significantly during the last 20 000 years, and is projected to double this century. Despite the importance of belowground processes in the global carbon cycle, community‐level and single species root responses to rising C_a are not well understood. We measured net community root biomass over 3 years using ingrowth cores in a natural C₃–C₄ grassland exposed to a gradient of C_a from preglacial to future levels (230–550 μmol mol^?1). Root windows and minirhizotron tubes were installed below naturally occurring stands of the C₄ perennial grass Bothriochloa ischaemum and its roots were measured for respiration, carbohydrate concentration, specific root length (SRL), production, and lifespan over 2 years. Community root biomass increased significantly (P<0.05) with C_a over initial conditions, with linear or curvilinear responses depending on sample date. In contrast, B. ischaemum produced significantly more roots at subambient than elevated C_a in minirhizotrons. The lifespan of roots with five or more neighboring roots in minirhizotron windows decreased significantly at high C_a, suggesting that after dense root growth depletes soil resource patches, plants with carbon surpluses readily shed these roots. Root respiration in B. ischaemum showed a curvilinear response to C_a under moist conditions in June 2000, with the lowest rates at C_a<300 μmol mol^?1 and peak activity at 450 μmol mol^?1 in a quadratic model. B. ischaemum roots at subambient C_a had higher SRLs and slightly higher carbohydrate concentrations than those at higher C_a, which may be related to drier soils at low C_a. Our data emphasize that belowground responses of plant communities to C_a can be quite different from those of the individual species, and suggest that complex interactions between and among roots and their immediate soil environment influence the responses of root physiology and lifespan to changing C_a.     

Mesophyll cell properties for some C3 and C4 species with high photosynthetic rates

Leaves of twelve C₃ species and six C₄ species were examined to understand better the relationship between mesophyll cell properties and the generally high photosynthetic rates of these plants. The CO₂ diffusion conductance expressed per unit mesophyll cell surface area (g_CO2^cell) cell was determined using measurements of the net rate of CO₂ uptake, water vapor conductance, and the ratio of mesophyll cell surface area to leaf surface area (A^mes/A). A^mes/A averaged 31 for the C₃ species and 16 for the C₄ species. For the C₃ species g_CO2^cell ranged from 0.12 to 0.32 mm s^-1, and for the C₄ species it ranged from 0.55 to 1.5 mm s^-1, exceeding a previously predicted maximum of 0.5 mm s^-1. Although the C₃ species Cammissonia claviformis did not have the highest g_CO2^cell, the combination of the highest A^mes and highest stomatal conductance resulted in this species having the greatest maximum rate of CO₂ uptake in low oxygen, 93 μmol m^-2 s^-1 (147 mg dm^-2 h^-1). The high g_CO2^cell of the C₄ species Amaranthus retroflexus (1.5 mm s^-1) was in part attributable to its thin cell wall (72 nm thick).     

Photosynthesis, immediate export and carbon partitioning in source leaves of C3, C3-C4 intermediate, and C4Panicum and Flaveria species at ambient and elevated CO2 levels

Immediate export in leaves of C₃‐C₄ intermediates were compared with their C₃ and C₄ relatives within the Panicum and Flaveria genera. At 35 Pa CO₂, photosynthesis and export were highest in C₄ species in each genera. Within the Panicum, photosynthesis and export in ‘type I’ C₃‐C₄ intermediates were greater than those in C₃ species. However, ‘type I’ C₃‐C₄ intermediates exported a similar proportion of newly fixed ¹⁴C as did C₄ species. Within the Flaveria, ‘type II’ C₃‐C₄ intermediate species had the lowest export rather than the C₃ species. At ambient CO₂, immediate export was strongly correlated with photosynthesis. However, at 90 Pa CO₂, when photosynthesis and immediate export increased in all C₃ and C₃‐C₄ intermediate species, proportionally less C was exported in all photosynthetic types than that at ambient CO₂. All species accumulated starch and sugars at both CO₂ levels. There was no correlation between immediate export and the pattern of ¹⁴C‐labelling into sugars and starch among the photosynthetic types within each genus. However, during CO₂ enrichment, C₄Panicum species accumulated sugars above the level of sugars and starch normally made at ambient CO₂, whereas the C₄Flaveria species accumulated only additional starch.     

On the significance of C3—C4 intermediate photosynthesis to the evolution of C4 photosynthesis

Abstract Evidence is drawn from previous studies to argue that C₃—C₄ intermediate plants are evolutionary intermediates, evolving from fully-expressed C₃ plants towards fully-expressed C₄ plants. On the basis of this conclusion, C₃—C₄ intermediates are examined to elucidate possible patterns that have been followed during the evolution of C₄ photosynthesis. An hypothesis is proposed that the initial step in C₄-evolution was the development of bundle-sheath metabolism that reduced apparent photorespiration by an efficient recycling of CO₂ using RuBP carboxylase. The CO₂-recycling mechanism appears to involve the differential compartmentation of glycine decarboxylase between mesophyll and bundle-sheath cells, such that most of the activity is in the bundlesheath cells. Subsequently, elevated phosphoenolpyruvate (PEP) carboxylase activities are proposed to have evolved as a means of enhancing the recycling of photorespired CO₂. As the activity of PEP carboxylase increased to higher values, other enzymes in the C₄-pathway are proposed to have increased in activity to facilitate the processing of the products of C₄-assimilation and provide PEP substrate to PEP carboxylase with greater efficiency. Initially, such a ‘C₄-cycle’ would not have been differentially compartmentalized between mesophyll and bundlesheath cells as is typical of fully-expressed C₄ plants. Such metabolism would have limited benefit in terms of concentrating CO₂ at RuBP carboxylase and, therefore, also be of little benefit for improving water- and nitrogen-use efficiencies. However, the development of such a limited C₄-cycle would have represented a preadaptation capable of evolving into the leaf biochemistry typical of fully-expressed C₄ plants. Thus, during the initial stages of C₄-evolution it is proposed that improvements in photorespiratory CO₂-loss and their influence on increasing the rate of net CO₂ assimilation per unit leaf area represented the evolutionary ‘driving-force’. Improved resourceuse efficiency resulting from an efficient CO₂-concentrating mechanism is proposed as the driving force during the later stages.     

Responses of a C3 and a C4 perennial grass to elevated CO2 and temperature under different water regimes

An experiment was carried out to determine the effects of elevated CO₂, elevated temperatures, and altered water regimes in native shortgrass steppe. Intact soil cores dominated by Bouteloua gracilis, a C₄ perennial grass, or Pascopyrum smithii, a C₃ perennial grass, were placed in growth chambers with 350 or 700 μL L^?1 atmospheric CO₂, and under either normal or elevated temperatures. The normal regime mimicked field patterns of diurnal and seasonal temperatures, and the high-temperature regime was 4 °C warmer. Water was supplied at three different levels in a seasonal pattern similar to that observed in the field. Total biomass after two growing seasons was 19% greater under elevated CO₂, with no significant difference between the C₃ and C₄ grass. The effect of elevated CO₂ on biomass was greatest at the intermediate water level. The positive effect of elevated CO₂ on shoot biomass was greater at normal temperatures in B. gracilis, and greater at elevated temperatures in P. smithii. Neither root-to-shoot ratio nor production of seed heads was affected by elevated CO₂. Plant tissue N and soil inorganic N concentrations were lower under elevated Co₂, but no more so in the C₃ than the C₄ plant. Elevated CO₂ appeared to increase plant N limitation, but there was no strong evidence for an increase in N limitation or a decrease in the size of the CO₂ effect from the first to the second growing season. Autumn samples of large roots plus crowns, the perennial organs, had 11% greater total N under elevated CO₂, in spite of greater N limitation.     

An examination of the advantages of C3-C4 intermediate photosynthesis in warm environments

Abstract. The photosynthetic responses to temperature in C₃, C₃-C₄ intermediate, and C₄ species in the genus Flaveria were examined in an effort to identify whether the reduced photorespiration rates characteristic of C₃-C₄ intermediate photosynthesis result in adaptive advantages at warm leaf temperatures. Reduced photorespiration rates were reflected in lower CO₂ compensation points at all temperatures examined in the C₃-C₄ intermediate, Flaveria floridana, compared to the C₃ species, F. cronquistii. The C₃-C₄ intermediate, F. floridana, exhibited a C₃-like photosynthetic temperature dependence, except for relatively higher photosynthesis rates at warm leaf temperatures compared to the C₃ species, F. cronquistii. Using models of C₃ and C₃-C₄ intermediate photosynthesis, it was predicted that by recycling photorespired CO₂ in bundle-sheath cells, as occurs in many C₃-C₄ intermediates, photosynthesis rates at 35°C could be increased by 28%, compared to a C₃ plant. Without recycling photorespired CO₂, it was calculated that in order to improve photosynthesis rates at 35°C by this amount in C₃ plants, (1) intercellular CO₂ partial pressures would have to be increased from 25 to 31 Pa, resulting in a 57% decrease in water-use efficiency, or (2) the activity of RuBP carboxylase would have to be increased by 32%, resulting in a 22% decrease in nitrogen-use efficiency. In addition to the recycling of photorespired CO₂, leaves of F. floridana appear to effectively concentrate CO₂ at the active site of RuBP carboxylase, increasing the apparent carboxylation efficiency per unit of in vitro RuBP carboxylase activity. The CO₂-concentrating activity also appears to reduce the temperature sensitivity of the carboxylation efficiency in F. floridana compared to F. cronquistii. The carboxylation efficiency per unit of RuBP carboxylase activity decreased by only 38% in F. floridana, compared to 50% in F. cronquistii, as leaf temperature was raised from 25 to 35°C. The C₃-C₄ intermediate, F. ramosissima, exhibited a photosynthetic temperature temperature response curve that was more similar to the C₄ species, F. trinervia, than the C₃ species, F. cronquistii. The C₄-like pattern is probably related to the advanced nature of C₄-like biochemical traits in F. ramosissima The results demonstrate that reductions in photorespiration rates in C₃-C₄ intermediate plants create photosynthetic advantages at warm leaf temperatures that in C₃ plants could only be achieved through substantial costs to water-use efficiency and/or nitrogen-use efficiency.     

The photosynthesis of young Panicum C4 leaves is not C3-like

Evidence is presented contrary to the suggestion that C₄ plants grow larger at elevated CO₂ because the C₄ pathway of young C₄ leaves has C₃-like characteristics, making their photosynthesis O₂ sensitive and responsive to high CO₂. We combined PAM fluorescence with gas exchange measurements to examine the O₂ dependence of photosynthesis in young and mature leaves of Panicum antidotale (C₄, NADP-ME) and P. coloratum (C₄, NAD-ME), at an intercellular CO₂ concentration of 5 Pa. P. laxum (C₃) was used for comparison. The young C₄ leaves had CO₂ and light response curves typical of C₄ photosynthesis. When the O₂ concentration was gradually increased between 2 and 40%, CO₂ assimilation rates (A) of both mature and young C₄ leaves were little affected, while the ratio of the quantum yield of photosystem II to that of CO₂ assimilation (Φ_PSII/Φ_CO2) increased more in young (up to 31%) than mature (up to 10%) C₄ leaves. A of C₃ leaves decreased by 1·3 and Φ_PSII/Φ_CO2 increased by 9-fold, over the same range of O₂ concentrations. Larger increases in electron transport requirements in young, relative to mature, C₄ leaves at low CO₂ are indicative of greater O₂ sensitivity of photorespiration. Photosynthesis modelling showed that young C₄ leaves have lower bundle sheath CO₂ concentration, brought about by higher bundle sheath conductance relative to the activity of the C₄ and C₃ cycles and/or lower ratio of activities of the C₄ to C₃ cycles.     

Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland

In this study, the response of N₂ fixation to elevated CO₂ was measured in Scirpus olneyi, a C₃ sedge, and Spartina patens, a C₄ grass, using acetylene reduction assay and ¹⁵N₂ gas feeding. Field plants grown in PVC tubes (25 cm long, 10 cm internal diameter) were used. Exposure to elevated CO₂ significantly (P < 0·05) caused a 35% increase in nitrogenase activity and 73% increase in ¹⁵N incorporated by Scirpus olneyi. In Spartina patens, elevated CO₂ (660 ± 1 μ mol mol ^{− 1}) increased nitrogenase activity and ¹⁵N incorporation by 13 and 23%, respectively. Estimates showed that the rate of N₂ fixation in Scirpus olneyi under elevated CO₂ was 611 ± 75 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} compared with 367 ± 46 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} in ambient CO₂ plants. In Spartina patens, however, the rate of N₂ fixation was 12·5 ± 1·1 versus 9·8 ± 1·3 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} for elevated and ambient CO₂, respectively. Heterotrophic non-symbiotic N₂ fixation in plant-free marsh sediment also increased significantly (P < 0·05) with elevated CO₂. The proportional increase in ¹⁵N₂ fixation correlated with the relative stimulation of photosynthesis, in that N₂ fixation was high in the C₃ plant in which photosynthesis was also high, and lower in the C₄ plant in which photosynthesis was relatively less stimulated by growth in elevated CO₂. These results are consistent with the hypothesis that carbon fixation in C₃ species, stimulated by rising CO₂, is likely to provide additional carbon to endophytic and below-ground microbial processes.     

Comparative ultrastructure and gas exchange characteristics of the C3–C4 intermediate Neurachne minor S. T. Blake (Poaceae)

Abstract Ultrastructural and physiological characteristics of the C₃-C₄ intermediate Neurachne minor S. T. Blake (Poaceae) are compared with those of C₃ and C₄ relatives, and C₃-C₄Panicum milioides Nees ex Trin. N. minor consistently exhibits very low CO₂ compensation points (τ: 1.0, usually 0.3–0.6 Pa) yet has C₃-like δ¹³C values. CO₂ assimilation rates (A) respond like those of C₃ plants to a decrease in O₂ partial pressure (2 × 104–1.9 × 103 Pa) at ambient CO₂ levels, but this response is progressively attenuated until negligible at very low CO₂. By contrast, other species of the Neurachneae are clearly either C₄ (two spp.) or C₃ (seven spp.). For plants grown and measured at different photon flux densities (PFDs), τ for N. minor and P. milioides increases from 0.5 to 1.0, and from 1.0 to 2.1 Pa, respectively, as PFD is decreased from 1860 to 460 μmol m^?2s^?1. In N. minor, the O₂ response of τ is either biphasic as in P. milioides, but much diminished and with a higher transition point, or is very C₄-like. As in C₄ relatives, inner sheath cells contain numerous chloroplasts. Their walls possess a suberized lamella, which may make them more CO₂-tight than bundle sheath cells of P. milioides, contributing to the almost C₄-like τ characteristics of N. minor. The biochemical basis of C₃-C₄ intermediacy is considered.     

Carbon isotope discrimination in C3-C4 intermediates

Carbon isotope discrimination in C₃–C₄ intermediates is determined by fractionations during diffusion and the biochemical fractionations occurring during CO₂ fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO₂ fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO₂ fixation occurring in the bundle sheath, the extent of bundle-sheath leakiness and the contribution which C₄-cycle activity makes to the CO₂ pool there. In most instances, carbon isotope discrimination in C₃–C₄ intermediates is C₃-like because only a small fraction of the total carbon fixed is fixed in the bundle sheath. In particular, this must be the case for Flaveria intermediates which initially fix substantial amounts of CO₂ into C₄-acids. In C₃–C₄ intermediates that refix photorespiratory CO₂ alone, it is possible for carbon isotope discrimination to be greater than in C₃-species, particularly at low CO₂ pressures or at high leaf temperatures. Short-term measurements of carbon isotope discrimination and gas exchange of leaves can be used to study the photosynthetic pathways of C₃-C₄ intermediates and their hybrids as has recently been done for C₃ and C₄ species.     

C3 grasses have higher nutritional quality than C4 grasses under ambient and elevated atmospheric CO2

Grasses with the C₃ photosynthetic pathway are commonly considered to be more nutritious host plants than C₄ grasses, but the nutritional quality of C₃ grasses is also more greatly impacted by elevated atmospheric CO₂ than is that of C₄ grasses; C₃ grasses produce greater amounts of nonstructural carbohydrates and have greater declines in their nitrogen content than do C₄ grasses under elevated CO₂. Will C₃ grasses remain nutritionally superior to C₄ grasses under elevated CO₂ levels? We addressed this question by determining whether levels of protein in C₃ grasses decline to similar levels as in C₄ grasses, and whether total carbohydrate : protein ratios become similar in C₃ and C₄ grasses under elevated CO₂. In addition, we tested the hypothesis that, among the nonstructural carbohydrates in C₃ grasses, levels of fructan respond most strongly to elevated CO₂. Five C₃ and five C₄ grass species were grown from seed in outdoor open‐top chambers at ambient (370 ppm) or elevated (740 ppm) CO₂ for 2 months. As expected, a significant increase in sugars, starch and fructan in the C₃ grasses under elevated CO₂ was associated with a significant reduction in their protein levels, while protein levels in most C₄ grasses were little affected by elevated CO₂. However, this differential response of the two types of grasses was insufficient to reduce protein in C₃ grasses to the levels in C₄ grasses. Although levels of fructan in the C₃ grasses tripled under elevated CO₂, the amounts produced remained relatively low, both in absolute terms and as a fraction of the total nonstructural carbohydrates in the C₃ grasses. We conclude that C₃ grasses will generally remain more nutritious than C₄ grasses at elevated CO₂ concentrations, having higher levels of protein, nonstructural carbohydrates, and water, but lower levels of fiber and toughness, and lower total carbohydrate : protein ratios than C₄ grasses.     

Natural abundance of 13C and 15N in C3 and C4 vegetation of southern Africa: patterns and implications

The mean annual rainfall in southern Africa is found to explain over half of the observed variance in the stable nitrogen (N) isotopic signatures of C₃ vegetation in southern Africa (r²=0.54, P<0.01). The inverse relationship between the stable N isotopic signatures of foliar samples from C₃ vegetation and long‐term southern African rainfall is found on a scale larger than previously observed. A modest relationship is found between stable carbon (C) isotopic signatures of C₃ vegetation and rainfall across the region (r²=0.20, P<0.01). No such relationship is found between stable C and N isotopic signatures of C₄ vegetation and rainfall. The explanation of the relationship between ¹⁵N in C₃ vegetation and the mean annual rainfall presented here is that nutrient availability varies inversely with water availability. This suggests that water‐limited systems in southern Africa are more open in terms of nutrient cycling and therefore the resulting natural abundance of foliar ¹⁵N in these systems is enriched. The use of this relationship may be of value to those researchers modeling both the dynamics of vegetation and biogeochemistry across this region. The use of the isotopic enrichment in C₃ vegetation as a function of rainfall may provide an insight into nutrient cycling across the semi‐arid and arid regions of southern Africa. This finding has implications for the study of global change, especially as it relates to vegetation responses to changing regional rainfall regimes over time.     

Oxygen sensitivity of C4 photosynthesis: evidence from gas exchange and chlorophyll fluorescence analyses with different C4 subtypes

Because photosynthetic rates in C₄ plants are the same at normal levels of O₂ (c, 20 kPa) and at c, 2 kPa O₂ (a conventional test for evaluating photorespiration in C₃ plants) it has been thought that C₄ photosynthesis is O₂ insensitive. However, we have found a dual effect of O₂ on the net rate of CO₂ assimilation among species representing all three C₄ subtypes from both monocots and dicots. The optimum O₂ partial pressure for C₄ photosynthesis at 30 °C, atmospheric CO₂ level, and half full sunlight (1000 μmol quanta m^?2 s^?1) was about 5–10 kPa. Photosynthesis was inhibited by O₂ below or above the optimum partial pressure. Decreasing CO₂ levels from ambient levels (32.6 Pa) to 9.3 Pa caused a substantial increase in the degree of inhibition of photosynthesis by supra-optimum levels of O₂ and a large decrease in the ratio of quantum yield of CO₂ fixation/quantum yield of photosystem II (PSII) measured by chlorophyll a fluorescence. Photosystem II activity, measured from chlorophyll a fluorescence analysis, was not inhibited at levels of O₂ that were above the optimum for CO₂ assimilation, which is consistent with a compensating, alternative electron How as net CO₂ assimilation is inhibited. At suboptimum levels of O₂, however, the inhibition of photosynthesis was paralleled by an inhibition of PSII quantum yield, increased state of reduction of quinone A, and decreased efficiency of open PSII centres. These results with different C₄ types suggest that inhibition of net CO₂ assimilation with increasing O₂ partial pressure above the optimum is associated with photorespiration, and that inhibition below the optimum O₂ may be caused by a reduced supply of ATP to the C₄ cycle as a result of inhibition of its production photochemically.