The relative contributions of reduced photorespiration,and improved water-and nitrogen-use efficiencies,to the advantages of C3−C4 intermediate photosynthesis in Flaveria

Summary Analyses of carbon-assimilation patterns in response to intercellular CO₂ concentrations, and the photosynthetic water-and nitrogen-use efficiencies, were conducted for a C₃, a C₄, and three C₃–C₄ species in the genus Flaveria in order to determine some of the advantages and disadvantages of C₃–C₄ intermediate photosynthesis. Operational intercellular CO₂ partial pressures (pi), determined when the atmospheric CO₂ partial pressure (pa) was approximately 330 bar, in the C₃–C₄ species were generally equal to, or greater than, those observed in the C₃ species under well-watered or water-stressed conditions. This reflects equal, or lower, water-use efficiencies (WUEs) in the C₃–C₄ species. The only case in which higher WUEs were observed in the C₃–C₄ species, compared to the C₃ species, was when photosynthesis rates were limited by available nitrogen and were less than 12.5 mol CO₂ m^-2s^-1. At higher photosynthesis rates, the C₃–C₄ species exhibited lower values of photosynthesis rate for equal values of stomatal conductance (lower WUE), compared to the C₃ species. Comparing slopes for the linear regions of the relationship between leaf nitrogen content and net photosynthesis rate (taken as an index of photosynthetic nitrogen-use efficiency, NUE), the C₄ species exhibited the highest NUE, followed by the C₃–C₄ species, F. ramosissima, with the other two C₃–C₄ species and the C₃ species being equal and exhibiting the lowest NUEs. The lack of consistent advantages in NUE and WUE in the C₃–C₄ species F. pubescens and F. floridana suggest that in some C₃–C₄ Flaveria species C₄-like anatomy and biochemistry do not provide the same gas exchange advantages that we typically attribute to the CO₂-concentrating mechanism of fully-expressed C₄ plants.     

Effect of soil drying on growth,biomass allocation and leaf gas exchange of two annual grass species

Influence of short-term water stress on plant growth and leaf gas exchange was studied simultaneously in a growth chamber experiment using two annual grass species differing in photosynthetic pathway type, plant architecture and phenology:Triticum aestivum L. cv. Katya-A-1 (C₃, a drought resistant wheat cultivar of erect growth) andTragus racemosus (L.) All. (C₄, a prostrate weed of warm semiarid areas). At the leaf level, gas exchange rates declined with decreasing soil water potential for both species in such a way that instantaneous photosynthetic water use efficiency (PWUE, mmol CO₂ assimilated per mol H₂O transpired) increased. At adequate water supply, the C₄ grass showed much lower stomatal conductance and higher PWUE than the C₃ species, but this difference disappeared at severe water stress when leaf gas exchange rates were similarly reduced for both species. However, by using soil water more sparingly, the C₄ species was able to assimilate under non-stressful conditions for a longer time than the C₃ wheat did. At the whole-plant level, decreasing water availability substantially reduced the relative growth rate (RGR) ofT. aestivum, while biomass partitioning changed in favour of root growth, so that the plant could exploit the limiting water resource more efficiently. The change in partitioning preceded the overall reduction of RGR and it was associated with increased biomass allocation to roots and less to leaves, as well as with a decrease in specific leaf area. Water saving byT. racemosus sufficiently postponed water stress effects on plant growth occurring only as a moderate reduction in leaf area enlargement. For unstressed vegetative plants, relative growth rate of the C₄ T. racemosus was only slightly higher than that of the C₃ T. aestivum, though it was achieved at a much lower water cost. The lack of difference in RGR was probably due to growth conditions being relatively suboptimal for the C₄ plant and also to a relatively large investment in stem tissues by the C₄ T. racemosus. Only 10% of the plant biomass was allocated to roots in the C₄ species while this was more than 30% for the C₃ wheat cultivar. These results emphasize the importance of water saving and high WUE of C₄ plants in maintaining growth under moderate water stress in comparison with C₃ species.     

Fluctuations in nitrate reductase activity,and nitrate and organic nitrogen concentrations of succulent plants under different nitrogen and water regimes

The CAM (Crassulacean acid metabolism) succulent species Kalanchoe daigremontiana, K. tubiflora and Crassula argentea, and the succulent C₃ species Peperomia obtusifolia, were cultivated in pure culture in open-air conditions under two different regimes of nitrogen and water supply. At specified intervals during the course of vegetative growth, biomass, nitrate reductase activity (NRA), nitrate concentration, and organic nitrogen concentration of whole plants were measured. After 100 days of cultivation the leaf conductance of Crassula and Peperomia was measured at intervals for the duration of a day. Behaviour of all four species was strongly influenced by the cultivation regime. This was apparent in terms of productivity and variable flucturations in NRA, nitrate concentration, and organic nitrogen concentration during the vegetative period. Increase in biomass was mostly connected with a decrease in all other investigated parameters, especially under conditions of water and/or nitrogen deficiency. The typical reaction of the CAM species Crassula to limited netrogen but adequate soil water was to reduce leaf conductance during light, whereas the C₃ plant Peperomia increased conductance in comparison with plants having a nitrogen suppy. The NRA of all plant species was reduced by both soil nitrate deficiency and drought. The succulent plant species, which are specially adapted to drought, neither took up nor used nitrate when water was limited. This was particularly the case for the CAM species, but less so for the C₃ Peperomia, which showed very high concentrations of nitrate and organic nitrogen, but low NRA and biomass gain. A formula was derived to express the nitrogen use efficiency (NUE) of the species, i.e. the ability of a plant to use nitrogen over a specific period of growth. NUE was shown to increase with age for the crassulacean species but to decrease for the C₃ Peperomia. Furthermore, NUE varied with the different nutrient levels in a species-specific manner, with high values for NUE not necessarily coupled to high productivity, and with NUE of the C₃ species generally higher than that of CAM species.     

Differences in drought sensitivities and photosynthetic limitations between co-occurring C3 and C4 (NADP-ME) Panicoid grasses

Background and Aims

The success of C₄ plants lies in their ability to attain greater efficiencies of light, water and nitrogen use under high temperature, providing an advantage in arid, hot environments. However, C₄ grasses are not necessarily less sensitive to drought than C₃ grasses and are proposed to respond with greater metabolic limitations, while the C₃ response is predominantly stomatal. The aims of this study were to compare the drought and recovery responses of co-occurring C₃ and C₄ NADP-ME grasses from the subfamily Panicoideae and to determine stomatal and metabolic contributions to the observed response.

Methods

Six species of locally co-occurring grasses, C₃ species Alloteropsis semialata subsp. eckloniana, Panicum aequinerve and Panicum ecklonii, and C₄ (NADP-ME) species Heteropogon contortus, Themeda triandra and Tristachya leucothrix, were established in pots then subjected to a controlled drought followed by re-watering. Water potentials, leaf gas exchange and the response of photosynthetic rate to internal CO₂ concentrations were determined on selected occasions during the drought and re-watering treatments and compared between species and photosynthetic types.

Key Results

Leaves of C₄ species of grasses maintained their photosynthetic advantage until water deficits became severe, but lost their water-use advantage even under conditions of mild drought. Declining C₄ photosynthesis with water deficit was mainly a consequence of metabolic limitations to CO₂ assimilation, whereas, in the C₃ species, stomatal limitations had a prevailing role in the drought-induced decrease in photosynthesis. The drought-sensitive metabolism of the C₄ plants could explain the observed slower recovery of photosynthesis on re-watering, in comparison with C₃ plants which recovered a greater proportion of photosynthesis through increased stomatal conductance.

Conclusions

Within the Panicoid grasses, C₄ (NADP-ME) species are metabolically more sensitive to drought than C₃ species and recover more slowly from drought.     

Photosynthetic pathway alters hydraulic structure and function in woody plants

Xylem structure and function is proposed to reflect an evolutionary balance between demands for efficient movement of water to the leaf canopy and resistance to cavitation during high xylem tension. Water use efficiency (WUE) affects this balance by altering the water cost of photosynthesis. Therefore species of greater WUE, such as C₄ plants, should have altered xylem properties. To evaluate this hypothesis, we assessed the hydraulic and anatomical properties of 19 C₃ and C₄ woody species from arid regions of the American west and central Asia. Specific conductivity of stem xylem (K_s ) was 16%–98% lower in the C₄ than C₃ shrubs from the American west. In the Asian species, the C₃ Nitraria schoberi had similar and Halimodendron halodendron higher K_s values compared with three C₄ species. Leaf specific conductivity (K_L ; hydraulic conductivity per leaf area) was 60%–98% lower in the C₄ than C₃ species, demonstrating that the presence of the C₄ pathway alters the relationship between leaf area and the ability of the xylem to transport water. C₄ species produced similar or smaller vessels than the C₃ shrubs except in Calligonum, and most C₄ shrubs exhibited higher wood densities than the C₃ species. Together, smaller conduit size and higher wood density indicate that in most cases, the C₄ shrubs exploited higher WUE by altering xylem structure to enhance safety from cavitation. In a minority of cases, the C₄ shrubs maintained similar xylem properties but enhanced the canopy area per branch. By establishing a link between C₄ photosynthesis and xylem structure, this study indicates that other phenomena that affect WUE, such as atmospheric CO₂ variation, may also affect the evolution of wood structure and function.     

Genotypic and within canopy variation in leaf carbon isotope discrimination and its relation to short-term leaf gas exchange characteristics in cassava grown under rain-fed conditions in the tropics

In a field rain-fed trial with 15 cassava cultivars, leaf gas exchanges and carbon isotope discrimination (Δ) of the same leaves were determined to evaluate genotypic and within-canopy variations in these parameters. From 3 to 7 months after planting leaf gas exchange was measured on attached leaves from upper, middle, and lower canopy layers. All gas exchange parameters varied significantly among cultivars as well as canopy layers. Net photosynthetic rate (P _N) decreased from top canopy to bottom indicating both shade and leaf age effects. The same trend, but in reverse, was found with respect to Δ, with the highest values in low canopy level and the lowest in upper canopy. There were very significant correlations, with moderate and low values, among almost all these parameters, with P _N negatively associated with intercellular CO₂ concentration (C _i), ratio of C _i to ambient CO₂ concentration C _i/C _a, and Δ. Across all measured leaves, Δ correlated negatively with leaf water use efficiency (WUE = photosynthesis/stomatal conductance, g _s) and with g _s, but positively with C _i and C _i/C _a. The later parameters negatively correlated with leaf WUE. Across cultivars, both P _N and correlated positively with storage root yield. These results are in agreement with trends predicted by the carbon isotope discrimination model.     

Effects of irradiance and temperature on photosynthesis in C3, C4 and C3/C4 Panicum species

Species in the Laxa and Grandia groups of the genus Panicum are adapted to low, wet areas of tropical and subtropical America. Panicum milioides is a species with C₃ photosynthesis and low apparent photorespiration and has been classified as a C₃/C₄ intermediate. Other species in the Laxa group are C₃ with normal photorespiration. Panicum prionitis is a C₄ species in the Grandia group. Since P. milioides has some leaf characteristics intermediate to C₃ and C₄ species, its photosynthetic response to irradiance and temperature was compared to the closely related C₃ species, P. laxum and P. boliviense and to P. prionitis. The response of apparent photosynthesis to irradiance and temperature was similar to that of P. laxum and P. boliviense, with saturation at a photosynthetic photo flux density of about 1 mmol m^-2 s^-1 at 30°C and temperature optimum near 30°C. In contrast, P. prionitis showed no light saturation up to 2 mmol m^-2 s^-1 and an optimum temperature near 40°C. P. milioides exhibited low CO₂ loss into CO₂-free air in the light and this loss was nearly insensitive to temperature. Loss of CO₂ in the light in the C₃ species, P. laxum and P. boliviense, was several-fold higher than in P. milioides and increased 2- to 5-fold with increases in temperature from 10 to 40°C. The level of dark respiration and its response to temperature were similar in all four Panicum species examined. It is concluded that the low apparent photorespiration in P. milioides does not influence its response of apparent photosynthesis to irradiance and temperature in comparison to closely related C₃ Panicum species.Abbreviations AP apparent photosynthesis - I CO₂ compensation point - gl leaf conductance; gm, mesophyll conductance - PPFD photosynthetic photon flux density - PR apparent photorespiration rate - RuBPC sibulose bisphosphate carboxylase     

Leaf characteristics and gas exchange of the mangrove <Emphasis Type="Italic">Laguncularia racemosa</Emphasis> as affected by salinity

Under constant salinity we analysed the leaf characteristics of Laguncularia racemosa (L.) Gaertn. in combination with gas exchange and carbon isotopic composition to estimate leaf water-use efficiency (WUE) and potential nitrogen-use efficiency (NUE). NaCl was not added to the control plants and the others were maintained at salinities of 15 and 30 ‰ (S₀, S₁₅, and S₃₀, respectively). Leaf succulence, sodium (Na), nitrogen (N), and chlorophyll (Chl) contents increased under salinity. Salinity had a negative impact on net photosynthetic rate (P _N) and stomatal conductance (g _s) at high and moderated irradiances. Potential NUE declined significantly (p<0.05) with salinity by 37 and 58 % at S₁₅ and S₃₀, respectively, compared to S₀ plants. Conversely, compared to S₀ plants, P _N/g _s increased under saline conditions by 12 % (S₁₅) and 50 % (S₃₀). Thus, WUE inferred from P _N/g _s was consistent with salinity improved short-term WUE. Long-term leaf WUE was also enhanced by salinity as suggested by significantly increased leaf δ¹³C with salinity. Improved WUE under salinity explains the eco-physiological success of mangrove species under increasing salinity. Conversely, decline in NUE may pose a problem for L. racemosa under hyper-saline environments regardless of N availability.     

The efficiency of water use in water stressed plants is increased due to ABA induced stomatal closure

Gas exchange and abscisic acid content of Digitalis lanata EHRH. have been examined at different levels of plant water stress. Net photosynthesis, transpiration and conductance of attached leaves declined rapidly at first, then more slowly following the withholding of irrigation. The intercellular partial pressure of CO₂ decreased slightly. The concentration of 2-cis(S)ABA increased about eight-fold in the leaves of non-irrigated plants as compared with well-watered controls. A close linear correlation was found between the ABA content of the leaves and their conductance on a leaf area basis. In contrast, the plot of net assimilation versus ABA concentration was curvilinear, leading to an increased efficiency of water use during stress. After rewatering, photosynthesis reached control values earlier than transpiration, leaf conductance and ABA content. From these data it is concluded that transpiration through the stomata is directly controlled by the ABA content, whereas net photosynthesis is influenced additionally by other factors.Possible reasons for the responses of photosynthesis and water use efficiency to different stress and ABA levels are discussed.Abbreviations A net CO₂ assimilation - ABA abscisic acid - C_i intercellular CO₂ concentration - g stomatal conductance - T transpiration - WUE water use efficiency     

Growth and photosynthetic response of nine tropical species with long-term exposure to elevated carbon dioxide

Summary Seedlings of nine tropical species varying in growth and carbon metabolism were exposed to twice the current atmospheric level of CO₂ for a 3 month period on Barro Colorado Island, Panama. A doubling of the CO₂ concentration resulted in increases in photosynthesis and greater water use efficiency (WUE) for all species possessing C₃ metabolism, when compared to the ambient condition. No desensitization of photosynthesis to increased CO₂ was observed during the 3 month period. Significant increases in total plant dry weight were also noted for 4 out of the 5 C₃ species tested and in one CAM species, Aechmea magdalenae at high CO₂. In contrast, no significant increases in either photosynthesis or total plant dry weight were noted for the C₄ grass, Paspallum conjugatum. Increases in the apparent quantum efficiency (AQE) for all C₃ species suggest that elevated CO₂ may increase photosynthetic rate relative to ambient CO₂ over a wide range of light conditions. The response of CO₂ assimilation to internal C_i suggested a reduction in either the RuBP and/or Pi regeneration limitation with long term exposure to elevated CO₂. This experiment suggests that: (1) a global rise in CO₂ may have significant effects on photosynthesis and productivity in a wide variety of tropical species, and (2) increases in productivity and photosynthesis may be related to physiological adaptation(s) to increased CO₂.     

Photosynthesis,water use and growth of a C4 grass stand at high CO2 concentration

Leaf photosynthesis rate of the C₄ species Paspalum plicatulum Michx was virtually CO₂-saturated at normal atmospheric CO₂ concentration but transpiration decreased as CO₂ was increased above normal concentrations thereby increasing transpiration efficiency. To test whether this leaf response led growth to be CO₂-sensitive when water supply was restricted, plants were grown in sealed pots of soil as miniature swards. Water was supplied either daily to maintain a constant water table, or at three growth restricting levels on a 5-day drying cycle. Plants were either in a cabinet with normal air (340 mol (CO₂) mol^-1 (air)) or with 250 mol mol^-1 enrichment. Harvesting was by several cycles of defoliation.With abundant water supply high CO₂ concentration did not cause increased growth, but it did not cause an increase in growth over a wide range of growth-limiting water supplies either. Only when water supply was less than 30–50% of the amount used by the stand with a water-table was there evidence that dry weight growth was enhanced by high CO₂. In addition, with successive regrowth, the enhancing effect under a regime of minimal water allocations, became attenuated. Examination of leaf gas exchange, growth and water use data showed that in the long term stomatal conductance responses were of little significance in matching plant water use to low water allocation; regulation of leaf area was the mechanism through which consumption matched supply. Since high CO₂ effects operate principally via stomatal conductance in C₄ species, we postulate that for this species higher CO₂ concentrations expected globally in future will not have much effect on long term growth.     

Carbon isotope discrimination and stomatal responses of mature Pinus sylvestris L. trees exposed in situ for three years to elevated CO2 and temperature

The Climate Change Experiment (CLIMEX) is a unique large scale facility in which an entire undisturbed catchment of boreal vegetation has been exposed to elevated CO₂ (560 ppm) and temperature (+3°C summer, +5°C winter) for the past three years with all the soil-plant-atmosphere linkages intact. Here, carbon isotope composition and stomatal density have been analysed from sequential year classes of needles of mature Scots pine trees (Pinus sylvestris L.) to investigate the response of time-integrated water-use efficiency (UWE) and stomatal density to CO₂ enrichment and climate change. Carbon isotope discrimination decreased and WUE increased in cohorts of needles developing under increased CO₂ and temperature, compared to needles on the same trees developing in pretreatment years. Mid-season instantaneous gas exchange, measured on the same trees for the past four years, indicated that these responses resulted from higher needle photosynthetic rates and reduced stomatal conductance. Needles of P. sylvestris developing under increased CO₂ and temperature had consistently lower stomatal densities than their ambient grown counterparts on the same trees. The stomatal density of P. sylvestris needles was inversely correlated with δ¹³C-derived WUE, implying some effect of this morphological response on leaf gas exchange. Future atmospheric CO₂ and temperature increases are therefore likely to improve the water economy of P. sylvestris, at least at the scale of individual needles, by affecting stomatal density and gas exchange processes.     

Biomass and leaf‐level gas exchange characteristics of three African savanna C4 grass species under optimum growth conditions

C₄ savanna grass species, Digitaria eriantha, Eragrostis lehmanniana and Panicum repens, were grown under optimum growth conditions with the aim of characterizing their above‐ and below‐ground biomass allocation and the response of their gas exchange to changes in light intensity, CO₂ concentration and leaf‐to‐air vapour pressure deficit gradient (D_l). Digitaria eriantha showed the largest above‐ and below‐ground biomass, high efficiency in carbon gain under light‐limiting conditions, high water use efficiency (WUE) and strong stomatal sensitivity to D_l (P = 0.002; r² = 0.5). Panicum repens had a high aboveground biomass and attained high light saturated photosynthetic rates (A_sat, 47 μmol m^?2 s^?1), stomatal conductance, (g_sat, 0.25 mol m^?2 s^?1) at relatively high WUE. Eragrostis lehmanniana had almost half the biomass of other species, and had similar A_sat and g_sat but were attained at lower WUE than the other species. This species also showed the weakest stomatal response to D_l (P = 0.19, r² = 0. 1). The potential ecological significance of the contrasting patterns of biomass allocation and variations in gas exchange parameters among the species are discussed.     

Effects of polyploidy on photosynthesis

In polyploid plants the photosynthetic rate per cell is correlated with the amount of DNA per cell. The photosynthetic rate per unit leaf area is the product of the rate per cell times the number of photosynthetic cells per unit area. Therefore, the photosynthetic rate per unit leaf area will increase if there is a less than proportional increase in cell volume at higher ploidal levels, or if cell packing is altered to allow more cells per unit leaf area. In autopolyploids (Medicago sativa, C₃ species, and Pennisetum americanum, C₄ species) there is a doubling of photosynthesis per cell and of cell volume in the tetraploid compared to the diploid. However, there is a proportional decrease in number of cells per unit leaf area with this increase in ploidy such that the rate of photosynthesis per leaf area does not change. There is more diversity in the relationship between ploidal level (gene dosage) and photosynthetic rates per unit leaf area in allopolyploids. This is likely to reflect the effects of natural selection on leaf anatomy, and novel genetic interactions from contributed genomes which can occur with allopolyploidy. In allopolyploid wheat (C₃ species) a higher cell volume per unit DNA at the higher ploidal level is negatively correlated with photosynthesis rate per unit leaf area. Although photosynthesis per cell increases with ploidy, photosynthesis per leaf area decreases, being lowest in the allohexaploid, cultivated bread wheat (Triticum aestivum). Alternatively, doubling of photosynthetic rate per cell with doubling of DNA, with apparent natural selection for decreased cell volume per unit DNA, results in higher rates of photosynthesis per leaf area in octaploid compared to tetraploid Panicum virgatum (C₄) which may be a case of allopolyploidy. Similar responses probably occur in Festuca arundinacea. Therefore, in some systems anatomical factors affecting photosynthesis are also affected by ploidal level. It is important to evaluate that component as well as determining the effect on biochemical processes. Current information on polyploidy and photosynthesis in several species is discussed with respect to anatomy, biochemistry and bases for expressing photosynthetic rates.Abbreviations Chl chlorophyll - RuBPC ribulose-1,5-bisphosphate carboxylase     

Parameterization of the CO2 and H2O gas exchange of several temperate deciduous broad-leaved trees at the leaf scale considering seasonal changes

A combined model to simulate CO₂ and H₂O gas exchange at the leaf scale was parameterized using data obtained from in situ leaf‐scale observations of diurnal and seasonal changes in the CO₂ and H₂O gas exchange of four temperate deciduous broad‐leaved trees using a porometric method. The model consists of a Ball et al. type stomatal conductance submodel [Ball, Woodrow & Berry, pp. 221–224 in Progress in Photosynthesis Research (ed. I. Biggins), Martinus‐Nijhoff Publishers, Dordrecht, The Netherlands, 1987] and a Farquhar et al. type biochemical submodel of photosynthesis (Farquhar, von Caemmerer & Berry, Planta 149, 78–90, 1980). In these submodels, several parameters were optimized for each tree species as representative of the quantitative characteristics related to gas exchange. The results show that the seasonal physiological changes of V_cmax25 in the biochemical model of photosynthesis should be used to estimate the long‐term CO₂ gas exchange. For R_d25 in the biochemical model of photosynthesis and m in the Ball et al. type stomatal conductance model, the difference should be counted during the leaf expansion period.     

Daily and seasonal CO2 exchange in Scots pine grown under elevated O3 and CO2: experiment and simulation

Starting in early spring of 1994, naturally regenerated, 30-year-old Scots pine (Pinus sylvestris L.) trees were grown in open-top chambers and exposed in situ to doubled ambient O₃,doubled ambient CO₂ and a combination of O₃ and CO₂ from 15 April to 15 September. To investigate daily and seasonal responses of CO₂ exchange to elevated O₃ and CO₂, the CO₂ exchange of shoots was measured continuously by an automatic system for measuring gas exchange during the course of one year (from 1 Januray to 31 December 1996). A process-based model of shoot photosynthesis was constructed to quantify modifications in the intrinsic capacity of photosynthesis and stomatal conductance by simulating the daily CO₂ exchange data from the field. Results showed that on most days of the year the model simulated well the daily course of shoot photosynthesis. Elevated O₃ significantly decreased photosynthetic capacity and stomatal conductance during the whole photosynthetic period. Elevated O₃ also led to a delay in onset of photosynthetic recovery in early spring and an increase in the sensitivity of photosynthesis to environmental stress conditions. The combination of elevated O₃ and CO₂ had an effect on photosynthesis and stomatal conductance similar to that of elevated O₃ alone, but significantly reduced the O₃-induced depression of photosynthesis. Elevated CO₂ significantly increased the photosynthetic capacity of Scots pine during the main growing season but slightly decreased it in early spring and late autumn. The model calculation showed that, compared to the control treatment, elevated O₃ alone and the combination of elevated O₃ and CO₂ decreased the annual total of net photosynthesis per unit leaf area by 55% and 38%, respectively. Elevated CO₂ increased the annual total of net photosynthesis by 13%.     

Diurnal Gas Exchange and Superior Resources Use Efficiency of Typical C4 Species in Hunshandak Sandland,China

Net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), leaf water potential (ψ_leaf), leaf nitrogen content, and photosynthetic nitrogen use efficiency (PNUE) were compared between a typical C₄ plant, Agriophyllum squarrosum and a C₃ plant, Leymus chinensis, in Hunshandak Sandland, China. The plant species showed different diurnal gas exchange patterns on June 12–14 when photosynthetic photon flux density (PPFD), air temperature (T _air), and water potential were moderate. P _N, E, and g _s of A. squarrosum showed distinct single peak while those of L. chinensis were depressed at noon and had two peaks in their diurnal courses. Gas exchange traits of both species showed midday depression under higher photosynthetic photon flux density (PPFD) and T _air when Ψ_leaf was significantly low down on August 6–8. However, those of A. squarrosum were depressed less seriously. Moreover, A. squarrosum had higher P _N, Ψ_leaf, water use efficiency (WUE), and PNUE than L. chinensis. Thus A. squarrosum was much more tolerant to heat and high irradiance and could utilise the resources on sand area more efficiently than L. chinensis. Hence species like A. squarrosum may be introduced and protected to reconstruct the degraded sand dunes because of their higher tolerance to stress and higher resource use efficiency. This revised version was published online in August 2006 with corrections to the Cover Date.     

Differences in gas exchange contribute to habitat differentiation in Iberian columbines from contrasting light and water environments

During photosynthesis, respiration and transpiration, gas exchange occurs via the stomata and so plants face a trade‐off between maximising photosynthesis while minimising transpiration (expressed as water use efficiency, WUE). The ability to cope with this trade‐off and regulate photosynthetic rate and stomatal conductance may be related to niche differentiation between closely related species. The present study explored this as a possible mechanism for habitat differentiation in Iberian columbines. The roles of irradiance and water stress were assessed to determine niche differentiation among Iberian columbines via distinct gas exchange processes. Photosynthesis–irradiance curves (P–I curves) were obtained for four taxa, and common garden experiments were conducted to examine plant responses to water and irradiance stress, by measuring instantaneous gas exchange and plant performance. Gas exchange was also measured in ten individuals using two to four field populations per taxon. The taxa had different P–I curves and gas exchange in the field. At the species level, water stress and irradiance explained habitat differentiation. Within each species, a combination of irradiance and water stress explained the between‐subspecies habitat differentiation. Despite differences in stomatal conductance and CO₂ assimilation, taxa did not have different WUE under field conditions, which suggests that the environment equally modifies photosynthesis and transpiration. The P–I curves, gas exchange in the field and plant responses to experimental water and irradiance stresses support the hypothesis that habitat differentiation is associated with differences among taxa in tolerance to abiotic stress mediated by distinct gas exchange responses.     

A New Approach to Distinguishing Photosynthetic Types of Plants. A Case Study in Northeast China Transect (NECT) Platform

Discriminant analysis is an important method in multivariable statistic analysis to show what type an individual should belong to. Based on actual field photosynthetic value set obtained from our research platform, North East China Transect (NECT), a new approach, developed from the concept and principle of discriminant analysts, was proposed to distinguish C₃ and C₄ plants. Indices related to plant photosynthetic capacity measured by an LCA4 photosynthesis system were selected to build the discriminant model which is based on four related parameters: net photosynthetic rate, transpiration rate, stomatal conductance, and difference in temperature between leaf surface and atmosphere. Compared with other approaches, the present one is fast, straightforward, and efficient. This revised version was published online in June 2006 with corrections to the Cover Date.     

Control of photosynthesis by the carbohydrate level in leaves of the C4 plant Amaranthus edulis L.

Photosynthesis was studied in relation to the carbohydrate status in intact leaves of the C₄ plant Amaranthus edulis. The rate of leaf net CO₂ assimilation, stomatal conductance and intercellular partial pressure of CO₂ remained constant or showed little decline towards the end of an 8-h period of illumination in ambient air (340 bar CO₂, 21% O₂). When sucrose export from the leaf was inhibited by applying a 4-h cold-block treatment (1°C) to the petiole, the rate of photosynthesis rapidly decreased with time. After the removal of the cold block from the petiole, further reduction in photosynthetic rate occurred, and there was no recovery in the subsequent light period. Although stomatal conductance declined with time, intercellular CO₂ partial pressure remained relatively constant, indicating that the inhibition of photosynthesis was not primarily caused by changes in stomatal aperture. Analysis of the leaf carbohydrate status showed a five- to sixfold increase in the soluble sugar fraction (mainly sucrose) in comparison with the untreated controls, whereas the starch content was the same. Leaf osmotic potential increased significantly with the accumulation of soluble sugars upon petiole chilling, and leaf water potential became slightly more negative. After 14 h recovery in the dark, photosynthesis returned to its initial maximum value within 1 h of illumination, and this was associated with a decline in leaf carbohydrate levels overnight. These data show that, in Amaranthus edulis, depression in photosynthesis when translocation is impaired is closely related to the accumulation of soluble sugars (sucrose) in source leaves, indicating feedback control of C₄ photosynthesis. Possible mechanisms by which sucrose accumulation in the leaf may affect the rate of photosynthesis are discussed with regard to the leaf anatomy of C₄ plants.Abbreviations and symbols A net CO₂ assimilation rate - Ci intercellular CO₂ partial pressure - PEP phosphoenolpyruvate - RuBP ribulose-1,5-bisphosphate - water potential - osmotic pressure