Photosynthetic responses to variable light: a comparison of species from contrasting habitats

Photosynthetic responses to variable light were compared for species from habitats differing in light availability and dynamics. Plants were grown under the same controlled conditions and were analysed for the kinetics of photosynthetic induction when photon flux density (PFD) was increased from 25 to 800 mol m^-2s^-1. Gas exchange techniques were used to analyse the two principal components of induction, opening of stomata and activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). On average, 90% of the final photosynthetic rate was attained after 7 min for obligate shade plants (two species), 18 min for fast-growing sun plants (seven species from productive habitats) and 32 min for slow-growing sun plants (nine species from unproductive habitats). The rapidity of response of the shade plants was explained by stomata remaining more open in the low-light period prior to induction. This was also observed in two species of deciduous trees, which therefore resembled shade plants rather than other fast-growing sun plants. The slow response of the slow-growing sun plants was the result of lower rates of both Rubisco activation and stomatal opening, the latter being more important for the final phase of induction. The lower rate of Rubisco activation was confirmed by direct, enzymatic measurements of representative plants. With increasing leaf age, the rate of stomatal opening appeared to decrease but the rate of Rubisco activation was largely conserved. Representative species were also compared with respect to the efficiency of using light-flecks relative to continuously high light. The shade plants and the slow-growing sun plants had a higher efficiency than the fast-growing sun plants. This could be related to the presence of a higher electron transport capacity relative to carboxylation capacity in the former group, which seems to be associated with their lower photosynthetic capacities. Representative species were also compared with respect to the ability to maintain the various induction components through periods of low light. Generally, the fast-growing sun plants were less able than the other two categories to maintain the rapidly reversible component. Thus, although the rate of induction appears to be related to the ecology of the plant, other aspects of photosynthetic dynamics, such as the efficiency of using lightflecks and the ability to maintain the rapidly reversible component, seem rather to be inversely related to the photosynthetic capacity.     

Photosynthetic induction responses of two rainforest tree species in relation to light environment

Photosynthetic induction of in situ saplings of two Costa Rican rainforest tree species wre compared in relation to their light environment, using infrared gas analysis and hemispherical photography. The species studied were Dipteryx panamensis, a climax species found in bright microsites, and Cecropia obtusifolia, a pioneer species. In the morning, when leaves were most responsive, induction time necessary to reach 90% of the lightsaturated rate of photosynthesis was on average 16 min for Dipteryx and 10 min for Cecropia. However, induction times for both species increased in the afternoon resulting in shorter daily average induction times for Dipteryx than for Cecropia. Dipteryx also maintained higher levels of induction for a longer period under low light conditions than did Cecropia. The two species differed in the way they adjusted to light availability. Dipteryx saplings growing in shady sites had faster rates of induction than saplings growing in bright sites, with no difference in light-saturated photosynthetic rate. In contrast, Cecropia saplings growing in bright sites had higher light-saturated photosynthetic rates than saplings growing in shady sites, with no difference in rates of induction. Dipteryx appears to exploit temporal variation in light availability by refining the quickness of the induction response to the light environment, while Cecropia adjusts its scale of exploitation by realizing a higher lightsaturated photosynthetic rate in sites of higher light.     

Photosynthetic light and CO2 utilization and C4 traits of two novel super-rice hybrids

Characteristics of photosynthetic light and CO2 use efficiency from seedling to heading stage, and C4 pathway enzyme activities in both flag leaves and lemma were compared between two newly developed super-rice hybrids (Oryza sativa L.), Liangyoupeijiu and Hua-an 3, and a traditional rice hybrid, Shanyou 63. At seedling and tillering stages, Liangyoupeijiu and Hua-an 3 had higher net photosynthetic rates (Pn) and light saturated assimilation rates (Asat) than did Shanyou 63, at both normal (360 micromol mol(-1)) and doubled (720 micromol mol(-1)) CO2 concentrations. At the heading stage, the flag leaves of all three rice hybrids had similar Pn and Asat. However, the two super-rice hybrids had higher apparent quantum yield (AQY) and carboxylation efficiency (CE) during all three typical developmental stages, and higher quantum yield of CO2 fixation (PhiCO2) at the tillering and heading stages. In addition, Liangyoupeijiu showed significantly higher activities of the C(4) pathway enzymes in both flag leaves and lemmas than did Shanyou 63. As a result, flag leaves of the two super-rice hybrids had higher Pn at morning, noontime and late afternoon during the daily cycle. Since most of the grain yield of rice comes from the photosynthesis of flag leaves, the similar Asat and much higher AQY, CE and PhiCO2 at heading stage of the two super-rice hybrids indicates that higher photosynthetic efficiency rather than higher photosynthetic capacity may be the primary factor contributing to their higher grain yields.     

The photosynthetic response of C3 and C4 bioenergy grass species to fluctuating light

Bioenergy grass species are a renewable energy source, but their productivity has not been fully realized. Improving photosynthetic efficiency has been proposed as a mechanism to increase the productivity of bioenergy grass species. Fluctuating light, experienced by all field grown crops, is known to reduce photosynthetic efficiency. This experiment aimed to evaluate the photosynthetic performance of both C₃ and C₄ bioenergy grass species under steady state and fluctuating light conditions by examining leaf gas exchange. The fluctuating light regime used here decreased carbon assimilation across all species when compared to expected steady state values. Overall, C₄ species assimilated more carbon than C₃ species during the fluctuating light regime, with both photosynthetic types assimilating about 16% less carbon than expected based on steady state measurements. Little diversity was observed in response to fluctuating light among C₃ species, and photorespiration partially contributed to the rapid decreases in net photosynthetic rates during high to low light transitions. In C₄ species, differences among the four NADP-ME species were apparent. Diversity observed among C₄ species in this experiment provides evidence that photosynthetic efficiency in response to fluctuating light may be targeted to increase C₄ bioenergy grass productivity.     

Growth and water relations in a central North Carolina population of <Emphasis Type="Italic">Microstegium vimineum</Emphasis> (Trin.) A. Camus

The ability of an invasive plant to occupy new areas is often attributed to both morphological and physiological plasticities that allow them to remain viable over a wide range of environmental conditions. Studies addressing the ecological requirements of Microstegium vimineum often consider soil moisture or soil moisture along with other factors as important explanatory components for the establishment and persistence of this invasive monocot. However, controlled studies specifically targeting water relations in M. vimineum are needed. Therefore, the purpose of this study was to determine how different water availabilities influence the growth and physiological performance of M. vimineum. This study utilized experimental microcosms to achieve different water availabilities including low soil moisture (<15% water), moderate soil moisture (ca. 20–30%), and flooded conditions. While both flooded and low soil moisture resulted in diminished growth, M. vimineum still survived under these conditions. Physiological processes including C₄ metabolism, minimum stress under low water conditions, and the ability to increase tissue rigidity may confer some advantages to M. vimineum during periods of limiting water conditions. Similarly, the proportionally low root biomass, shallow root structure, and its ability to maintain stable water relations during flooding and/or soil waterlogging may facilitate M. vimineum’s ability to invade mesic habitats. It is likely, therefore, that the capacity to tolerate both low soil moistures and flooded conditions has enhanced the ability of M. vimineum to populate disturbed systems in central North Carolina.     

Photosynthetic carbon metabolism of the cool-temperate C4 grass Spartina anglica Hubb.

The aim of this work was to describe the photosynthetic carbon metabolism of the cooltemperate C₄ grass Spartina anglica. With the exception of pyruvate, phosphate dikinase and pyruvate kinase, the maximum catalytic activities in leaves of putative enzymes of the C₄ cycle of a phosphoenolpyruvate-carboxykinase C₄ plant were considerably in excess of the observed, steady-state rate of photosynthesis, and were comparable with the maximum catalytic activities of key enzymes of the reductive pentose-phosphate pathway. Radioactive carbon from ¹⁴CO₂ supplied to attached leaves during steady-state photosynthesis appeared first in malate and aspartate from which it moved to intermediates of the reductive pentose-phosphate pathway, and then to sucrose. These experiments show that photosynthetic carbon metabolism in this cool-temperate C₄ plant is similar to that of C₄ plants of hotter climates.     

Characterization of photosynthesis, photoinhibition and the activities of C4 pathway enzymes in a superhigh-yield rice, Liangyoupeijiu

Characteristics of photosynthetic gas exchange, photoinhibition and C4 pathway enzyme activities in both flag leaves and lemma were compared between a superhigh-yield rice (Oryza sativa L.) hybrid, Liangyoupeijiu and a traditional rice hybrid, Shanyou63. Liangyoupeijiu had a similar light saturated assimilation rate (Asat) to Shanyou63, but a much higher apparent quantum yield (AQY), carboxylation efficiency (CE) and quantum yield of CO2 fixation (φCO2). Liangyoupeijiu also showed a higher resistance to photoinhibition and higher non-radiative energy dissipation associated with the xanthophyll cycle than Shanyou63 when subjected to strong light. In addition, Liangyoupeijiu had higher activities of the C4 pathway enzymes in both flag leaves and lemmas than Shanyou63. These results indicate that higher light and CO2 use efficiency, higher resistance to photoinhibition and C4 pathway in both flag leaf and lemma may contribute to the higher yield of the superhigh-yield rice hybrid, Liangyoupeijiu.     

Photosynthetic characteristics of the shade-adapted C4 grass Muhlenbergia sobolifera (Muhl.) Trin.: control of development of photorespiration by growth temperature

Muhlenbergia sobolifera (Muhl.) Trin., a C₄ grass, occurs in understory habitats in the northeastern United States. Plants of M. sobolifera were grown at 23 and 30°C at 150 and 700 μmol photons m⁻² s⁻¹. The photosynthetic CO₂ compensation point, maximum CO₂ assimilation, dark respiration and the absorbed quantum use efficiency (QUE) were measured at 23 and 30°C at 2 and 20% O₂. Photosynthetic CO₂ compensation points ranged from 4 to 14mm³ dm⁻³ CO₂ and showed limited O₂ sensitivity. The mean photosynthetic CO₂ compensation point of plants grown at 30°C (4·5 mm³ dm⁻³) was 57% lower and 80% less inhibited by O₂ than that of plants grown at 23°C. Photosynthesis was similarly affected by growth temperature, with 70% more O₂ inhibition in plants grown at 23°C; suppression over all treatments ranging from 2 to 11%. Unlike typical C₄ species, plants of M. sobolifera from both temperature regimes exhibited higher CO₂ assimilation rates when grown at low light. Growth temperature and light also affected QUE; plants grown at low light and 23°C had the highest value (0·068 mol CO₂/mol quanta). Measurement temperature and growth light regime significantly affected dark respiration; however, O² did not affect QUE or dark respiration under any growth or measurement conditions. The results indicate that M. sobolifera is adapted to low PPFD, and that complete suppression of photorespiration is dependent upon high growth temperature.     

Photosynthetic responses of a C3 and three C4 species of the genus Panicum (s.l.) with different metabolic subtypes to drought stress

Young plants of Panicum bisulcatum (C(3)), Zuloagaea bulbosa [NADP-malic enzyme (ME)-C(4)], P. miliaceum (NAD-ME-C(4)) and Urochloa maxima [phosphoenolpyruvate carboxykinase (PCK)-C(4)] were subjected to drought stress (DS) in soil for 6?days. The C(3) species showed severe wilting symptoms at higher soil water potential (-1.1?MPa) and relative leaf water content (77?%) than in the case of the C(4) species (-1.5 to -1.7?MPa; 58-64?%). DS decreased photosynthesis, both under atmospheric and under saturating CO(2). Stomatal limitation of net photosynthesis (P (N)) in the C(3), but not in the C(4) species was indicated by P (N)/C (o) curves. Chlorophyll fluorescence of photosystem II, resulting from different cell types in the four species, indicated NADPH accumulation and non-stomatal limitation of photosynthesis in all four species, even under high CO(2). In the NAD-ME-C(4) and the PCK-C(4) species, DS plants showed increased violaxanthin de-epoxidase rates. Biochemical analyses of carboxylating enzymes and in vitro enzyme activities of the C(4) enzymes identified the most likely non-stomatal limiting steps of photosynthesis. In P. bisulcatum, declining RubisCO content and activity would explain the findings. In Z. bulbosa, all photosynthesis enzymes declined significantly; photosynthesis is probably limited by the turnover rate of the PEPC reaction. In P. miliaceum, all enzyme levels remained fairly constant under DS, but photosynthesis can be limited by feedback inhibition of the Calvin cycle, resulting in asp inhibition of PEPC. In U. maxima, declines of in vivo PEPC activity and feedback inhibition of the Calvin cycle are the main candidates for non-stomatal limitation of photosynthesis under DS.     

Photosynthetic and morphological responses of eelgrass (Zostera marina L.) to a gradient of light conditions

Zostera marina L. (eelgrass) from Great Bay Estuary, New Hampshire and Maine (USA), was transplanted in outdoor mesocosms and subjected to four light treatments (100, 58, 34 and 11% surface irradiance, SI) between May and September 2003 to investigate the relationship between light availability and the growth and survival of eelgrass. Evaluating eelgrass seedlings and adult mature plants demonstrated no differences in photosynthetic response after 22 days of acclimation. During at least the first 19 days of shading, maximum electron transport rate (ETR_max) rate of eelgrass did not differ significantly between light treatments. After 40 days, a significant reduction in ETR_max and minimum saturating light was observed in plants growing at 34% SI and below. Morphological responses exhibited a linear increasing trend with greater light. 34% SI exhibited drastic reductions (to less than 25% of control) in rhizome growth, shoot density, shoot production, number of nodes per plant and plant weight at the end of the study (81 days). Shoot to root ratio at 34% SI increased by > 50%. Plants shaded to 58% SI showed no significant difference from the control in plant parameters except an increased rate of rhizome elongation. Our results link the lower shoot densities with shading to the slow growth rate of horizontal rhizomes and a total lack of lateral expansion at 11% SI. ETR_max declined over time in plants at 11% SI resulting in 81% mortality, no lateral branching and no morphological development, indicating that the minimum light required for long-term eelgrass growth and survival is greater than the previously suggested 11% SI. We demonstrate that eelgrass plants at these latitudes can persist at light levels of 58% SI and above, and are light-limited at 34% SI and below.     

Effects of climate and atmospheric CO2 partial pressure on the global distribution of C4 grasses: present, past, and future

C₄ photosynthetic physiologies exhibit fundamentally different responses to temperature and atmospheric CO₂ partial pressures (pCO₂) compared to the evolutionarily more primitive C₃ type. All else being equal, C₄ plants tend to be favored over C₃ plants in warm humid climates and, conversely, C₃ plants tend to be favored over C₄ plants in cool climates. Empirical observations supported by a photosynthesis model predict the existence of a climatological crossover temperature above which C₄ species have a carbon gain advantage and below which C₃ species are favored. Model calculations and analysis of current plant distribution suggest that this pCO₂-dependent crossover temperature is approximated by a mean temperature of 22°C for the warmest month at the current pCO₂ (35 Pa). In addition to favorable temperatures, C₄ plants require sufficient precipitation during the warm growing season. C₄ plants which are predominantly graminoids of short stature can be competitively excluded by trees (nearly all C₃ plants) – regardless of the photosynthetic superiority of the C₄ pathway – in regions otherwise favorable for C₄. To construct global maps of the distribution of C₄ grasses for current, past and future climate scenarios, we make use of climatological data sets which provide estimates of the mean monthly temperature to classify the globe into areas which should favor C₄ photosynthesis during at least 1 month of the year. This area is further screened by excluding areas where precipitation is <25 mm per month during the warm season and by selecting areas classified as grasslands (i.e., excluding areas dominated by woody vegetation) according to a global vegetation map. Using this approach, grasslands of the world are designated as C₃, C₄, and mixed under current climate and pCO₂. Published floristic studies were used to test the accuracy of these predictions in many regions of the world, and agreement with observations was generally good. We then make use of this protocol to examine changes in the global abundance of C₄ grasses in the past and the future using plausible estimates for the climates and pCO₂. When pCO₂ is lowered to pre-industrial levels, C₄ grasses expanded their range into large areas now classified as C₃ grasslands, especially in North America and Eurasia. During the last glacial maximum (∼18 ka BP) when the climate was cooler and pCO₂ was about 20 Pa, our analysis predicts substantial expansion of C₄ vegetation – particularly in Asia, despite cooler temperatures. Continued use of fossil fuels is expected to result in double the current pCO₂ by sometime in the next century, with some associated climate warming. Our analysis predicts a substantial reduction in the area of C₄ grasses under these conditions. These reductions from the past and into the future are based on greater stimulation of C₃ photosynthetic efficiency by higher pCO₂ than inhibition by higher temperatures. The predictions are testable through large-scale controlled growth studies and analysis of stable isotopes and other data from regions where large changes are predicted to have occurred. Received: 3 July 1997 / Accepted: 3 December 1997     

Drought limitation of photosynthesis differs between C3 and C4 grass species in a comparative experiment

Phylogenetic analyses show that C₄ grasses typically occupy drier habitats than their C₃ relatives, but recent experiments comparing the physiology of closely related C₃ and C₄ species have shown that advantages of C₄ photosynthesis can be lost under drought. We tested the generality of these paradoxical findings in grass species representing the known evolutionary diversity of C₄ NADP‐me and C₃ photosynthetic types. Our experiment investigated the effects of drought on leaf photosynthesis, water potential, nitrogen, chlorophyll content and mortality. C₄ grasses in control treatments were characterized by higher CO₂ assimilation rates and water potential, but lower stomatal conductance and nitrogen content. Under drought, stomatal conductance declined more dramatically in C₃ than C₄ species, and photosynthetic water‐use and nitrogen‐use efficiency advantages held by C₄ species under control conditions were each diminished by 40%. Leaf mortality was slightly higher in C₄ than C₃ grasses, but leaf condition under drought otherwise showed no dependence on photosynthetic‐type. This phylogenetically controlled experiment suggested that a drought‐induced reduction in the photosynthetic performance advantages of C₄ NADP‐me relative to C₃ grasses is a general phenomenon.     

Comparative effects of light during growth on the photosynthetic properties of NADP-ME type C4 grasses from open and shaded habitats. II. Photosynthetic enzyme activities and metabolism*

Abstract The influences of shading during growth upon the activities of several photosynthetic enzymes were examined in NADP-ME type C₄ grasses from open (Zea mays L.) and shaded (Paspalum conjugation Berg.) habitats. The substantial species-difference in maximum photosynthetic rate observed under a high light regime was correlated with large differences in both enzyme activities and leaf protein contents. With the exception of RuBP carboxylase activity, other photosynthetic enzyme activities in Z. mays were reduced by shading to a similar extent as maximum photosynthetic rate. In contrast, only PEP carboxylase and pyruvate, P_i dikinase activities were decreased by shading in P. conjugatum. As with maximum photosynthetic rate, other photosynthetic enzyme activities in P. conjugatum were relatively insensitive to irradiance during growth. Under a low photon flux density of photosynthetically active radiation (50 μmol m^?2 s^?1), the flow of [¹⁴C] label through photosynthetic intermediates in intact, shade-grown leaves of P. conjugatum was typical of C₄ metabolism. This provides incontrovertible proof for the occurrence of C₄ photosynthesis in shaded habitats.     

Photosynthetic and respiratory responses of <Emphasis Type="Italic">Gracilaria salicornia</Emphasis> (C. Ag.) Dawson (Gracilariales,Rhodophyta) from Thailand and Japan

The photosynthetic and respiratory responses of Gracilaria salicornia in the subtropical waters of Japan (in Okinawa) and in the tropical waters of Thailand (in Rayong and Phuket Provinces), were studied under various conditions of irradiance, salinity and temperature. This alga showed adaptability in its photosynthetic and respiratory responses to oceanic salinity as well as to subtropical to tropical temperature. Significant differences in the photosynthetic and respiratory rates among the G. salicornia populations collected from the above sites were observed. The Phuket population showed adaptability to high irradiance and temperature, characteristic of its natural sun-exposed environment, as it exhibited the highest I _k and I _c, and lowest α and higher P _max at 30–35°C. On the other hand, the Okinawa population demonstrated adaptability to low submarine irradiance as it had a lower I _k and I _c than either population from Thailand. Its P _max at 20–25°C was also higher than that of the Phuket population. The Rayong population, however, showed the highest P _max, R_d, α, I _k and I _c, suggesting its adaptability to both sun and shade light conditions in its natural environment. Presented at the 6th Meeting of the Asian Pacific Society of Applied Phycology, Manila, Philippines     

Differentiation of bundle sheath,mesophyll, and distinctive cells in the C4 grass Arundinella hirta (Poaceae)

The C₄ grass Arundinella hirta is characterized by unusual leaf blade anatomy: veins are widely spaced and files of bundle-sheath-like cells, the distinctive cells, form longitudinal strands that are not associated with vascular tissue. While distinctive cells (DCs) appear to function like bundle sheath cells (BSCs), they differ developmentally in two ways: they are derived from ground meristem rather than procambium and they are formed 1–2 plastochrons later. This study describes ultrastructural features of differentiating of BSCs, DCs, and associated mesophyll cells (MCs) during leaf development. BSCs and DCs differ from adjacent MCs by undergoing earlier cell enlargement, greater rates of chloroplast enlargement, reduction of chloroplast thylakoids at late stages of differentiation, more extensive starch formation, greater wall thickening, and deposition of a suberin lamella. The precocious delimitation of the bundle sheath layer is reflected in earlier BSC enlargement and vacuole growth. Derivation of DCs from ground meristem is correlated with late developmental changes in chloroplast size, wall thickness, and plasmodesmatal density. Despite these differences in timing of events, particularly at early stages, the development of the specialized structural features of BSCs and DCs is essentially similar. Thus, proximity to vascular tissue appears to be nonessential for the coordination and regulation of BSC- and MC-specific developmental events.     

Host-plant quality alters grass/forb consumption by a mixed-feeding insect herbivore, Melanoplus bivittatus (Orthoptera: Acrididae)

Abstract.  1. Factors affecting the nutritional ecology of mixed-feeding, polyphagous herbivores are poorly understood. Mixed-feeding herbivores do better when they consume both forb and grass species although they typically feed primarily on forbs, which are of relatively higher protein content than grasses.
2. In a field experiment, we examined the effects of nitrogen and phosphorus fertilization and associated changes in host-plant C:N:P on proportional grass consumption by a mixed-feeding insect herbivore, Melanoplus bivittatus , using natural abundance stable carbon isotope (¹²C^/13C) methods. We also examined a grass-feeding ( Phoetaliotes nebrascensis ) and forb-feeding ( Hesperotettix viridis ) species.
3. The C isotope signatures of M. bivittatus collected from plots fertilized with nitrogen (+N), phosphorus (+P), nitrogen and phosphorus (+N+P) and no fertilizer were compared with the C isotope signatures of plants in those plots to determine the proportion of assimilated C derived from C₄ grasses and C₃ forbs in each plot. We also examined the relationship between M. bivittatus diets and plant C:N:P stoichiometry.
4. The proportion of grass assimilated approximately doubled in N-fertilized treatments (39.1 ± 0.1%) compared with non-fertilized treatments (19 ± <0.1%), an increase associated with decreased C:N and increased N:P of grasses.
5. These results indicate that mixed-feeding M. bivittatus can selectively feed to balance C:N:P intake even when choosing between two structurally and chemically different groups of plants.
6. The strong relationship between diet selection and grass stoichiometry also suggests that plant nutrient composition may be more important than defensive chemistry in food choice.     

A comparative anatomical and biochemical analysis in salsola (Chenopodiaceae) species with and without a Kranz type leaf anatomy: a possible reversion of C4 to C3 photosynthesis

Leaf anatomy was studied by light and electron microscopy and the leaf activities of RUBP carboxylase, PEP carboxylase, and malic enzyme were assayed in: Salsola australis and S. oreophila grown on the West Pamirs at 1800 m altitude; in S. australis grown on the East Pamirs at 3860 m; and in S. arbusculiformis grown in the Kisil-Kum desert in Middle Asia near 500 m. Carbon isotope fractionation ratio values also were measured on whole leaf tissue for 18 Salsola species field collected in these and other regions of the former USSR. S. australis leaves are cylindrical and in cross section exhibit a peripheral ring of mesophyll and then an inner ring of bundle sheath type cells; and its biochemical characteristics and deltaC values are typical of a C4 species of the NADP-malic enzyme malate-forming group. These traits were expressed independent of the plant growth altitude up to 4000 m. C4 type deltaC values were obtained in 14 of the Salsola species. Anatomical, structural, and biochemical features typical of the C4 syndrome were absent in S. oreophila and S. arbusculiformis. Four Salsola species, including these two, had C3-type deltaC values. Their cylindrical leaves in cross section exhibited two to three peripheral rings as layers of palisade parenchyma. Although their vascular bundles were surrounded by green bundle sheath cells, their organelle numbers were comparable to those in mesophyll cells. Neither bundle sheath cell wall thickenings nor dimorphic chloroplasts in two leaf cell types were observed. In S. oreophila, there was a high activity of RuBP carboxylase, but a low activity of C4 cycle enzymes. Interpretation of these data lends evidence to the hypothesis that a small group of C3 Salsola species, including S. oreophila, S. arbusculiformis, S. montana, and S. pachyphylla, arose as the result of a reversion of a C4 to a C3 type of photosynthetic CO2 fixation in the cooler climates of Middle Asia.     

Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions

C4 plants contribute ≈ 20% of global gross primary productivity, and uncertainties regarding their responses to rising atmospheric CO₂ concentrations may limit predictions of future global change impacts on C4-dominated ecosystems. These uncertainties have not yet been considered rigorously due to expectations of C4 low responsiveness based on photosynthetic theory and early experiments. We carried out a literature review (1980–97) and meta-analysis in order to identify emerging patterns of C4 grass responses to elevated CO₂, as compared with those of C3 grasses. The focus was on nondomesticated Poaceae alone, to the exclusion of C4 dicotyledonous and C4 crop species. This provides a clear test, controlled for genotypic variability at family level, of differences between the CO₂-responsiveness of these functional types. Eleven responses were considered, ranging from physiological behaviour at the leaf level to carbon allocation patterns at the whole plant level. Results were also assessed in the context of environmental stress conditions (light, temperature, water and nutrient stress), and experimental growing conditions (pot size, experimental duration and fumigation method). Both C4 and C3 species increased total biomass significantly in elevated CO₂, by 33% and 44%, respectively. Differing tendencies between types in shoot structural response were revealed: C3 species showed a greater increase in tillering, whereas C4 species showed a greater increase in leaf area in elevated CO₂. At the leaf level, significant stomatal closure and increased leaf water use efficiency were confirmed in both types, and higher carbon assimilation rates were found in both C3 and C4 species (33% and 25%, respectively). Environmental stress did not alter the C4 CO₂-response, except for the loss of a significant positive CO₂-response for above-ground biomass and leaf area under water stress. In C3 species, stimulation of carbon assimilation rate was reduced by stress (overall), and nutrient stress tended to reduce the mean biomass response to elevated CO₂. Leaf carbohydrate status increased and leaf nitrogen concentration decreased significantly in elevated CO₂ only in C3 species. We conclude that the relative responses of the C4 and C3 photosynthetic types to elevated CO₂ concur only to some extent with expectations based on photosynthetic theory. The significant positive responses of C4 grass species at both the leaf and the whole plant level demand a re-evaluation of the assumption of low responsiveness in C4 plants at both levels, and not only with regard to water relations. The combined shoot structural and water use efficiency responses of these functional types will have consequential implications for the water balance of important catchments and range-lands throughout the world, especially in semiarid subtropical and temperate regions. It may be premature to predict that C4 grass species will lose their competitive advantage over C3 grass species in elevated CO₂.     

Genetic transformation of the C3–C4 intermediate plant, Flaveria pubescens (Asteraceae)

The dicot genus Flaveria (Asteraceae), besides species with C₃ or C₄ photosynthesis, contains taxa with a broad range of different states of transition between the two major photosynthetic types. We have developed a reproducible and efficient Agrobacterium-mediated method for the stable genetic transformation of the C₃–C₄ intermediate species F. pubescens. Fusion constructs of the reporter gene β-glucuronidase (uidA, GUS) to several plant promoters, mainly derived from genes encoding subunits of the glycine cleavage system (gdcs), have been used to confirm the reproducibility and efficiency of the method. The stable integration of the foreign DNA has been examined by Southern analysis, kanamycin resistance, GUS enzyme activities and histochemical staining. Transformed shoots can be routinely obtained within 8–10 weeks after co-cultivation with A. tumefaciens. Received: 16 April 1996 / Revision received: 12 July 1996 / Accepted: 28 July 1996