Effects of the Phosphoenolpyruvate Carboxylase Inhibitor 3,3-Dichloro-2-(Dihydroxyphosphinoylmethyl)propenoate on Photosynthesis: C(4) Selectivity and Studies on C(4) Photosynthesis

The effect of 3,3-dichloro-2-(dihydroxyphosphinoylmethyl)-propenoate (DCDP), an analog of phosphoenolpyruvate (PEP), on PEP carboxylase activity in crude leaf extracts and on photosynthesis of excised leaves was examined. DCDP is an effective inhibitor of PEP carboxylase from Zea mays or Panicum miliaceum; 50% inhibition was obtained at 70 or 350 micromolar, respectively, in the presence of 1 millimolar PEP and 1 millimolar HCO₃⁻. When fed to leaf sections via the transpiration stream, DCDP at 1 millimolar strongly inhibited photosynthesis in C₄ species (79-98% inhibition for a range of seven C₄ species), but only moderately in C₃ species (12-46% for four C₃ species), suggesting different mechanisms of inhibition for each photosynthetic type. The response of P. miliaceum (C₄) net photosynthesis to intercellular pCO₂ showed that carboxylation efficiency, as well as the CO₂ saturated rate, are lowered in the presence of DCDP and supported the view that carboxylation efficiency in C₄ species is directly related to PEP carboxylase activity. A fivefold increase in intercellular pCO₂ over that occurring in P. miliaceum under normal photosynthesis conditions only increased net photosynthesis rate in the presence of 1 millimolar DCDP from zero to about 5% of the maximal uninhibited rate. Therefore, it seems unlikely that direct fixation of atmospheric CO₂ by the bundle sheath cells makes any significant contribution to photosynthetic CO₂ assimilation in C₄ species. The results support the concept that C₄-selective herbicides may be developed based on inhibitors of C₄ pathway reactions.     

Inhibition of photosynthesis ofPopulus euramericana andHelianthus annuus by SO2, NO2 and O3

Populus euramericana cv. I-214 andHelianthus annuus L. cv. Russian Mammoth were exposed to various concentrations of O₃ SO₂ or NO₂ for 2 h in a cylindrical assimilation chamber. The threshold concentrations of air pollutants for inhibition of net photosynthesis differed between the two species and also between the pollutants tested. Furthermore, the lethal concentrations where the net photosynthetic rates were completely inhibited, also differed between species and between pollutants. For SO₂ and NO₂,P. euramericana was more tolerant photosynthetically thanH. annuus when related to the concentration of pollutants used during the experiment. However, when related to the cumulative uptake rate of each pollutant, the photosynthetic tolerance of the two species was similar. In contrast to the effects of SO₂ or NO₂, the influence of O₃ on net photosynthesis was quite different. The relative rates of net photosynthesis in both species showed the same linear relationship with O₃ concentration. However, the relationship between the relative rate of net photosynthesis and the cumulative uptake rate of O₃ differed between the two species, although it was linear in both cases.     

Inhibition of Corn and Sunflower Photosynthesis by Lead

Detached corn and sunflower leaves supplied with PbCl² via the transpiration stream exhibited reduced rates of photosynthesis. The difference between species in the amount of Pb taken up was in direct proportion to their respective transpiration rates. For both species the reduction in photosynthesis and the amount of Pb taken up increased with increasing treatment concentrations. A corresponding reduction occurred in the rate of transpiration suggesting that stomatal resistance may be increased by Pb contamination. The pathways of CO₂ and water vapor exchange are discussed in relation to the effects of Pb on photosynthesis and transpiration.     

Degree of C(4) Photosynthesis in C(4) and C(3)-C(4)Flaveria Species and Their Hybrids : II. Inhibition of Apparent Photosynthesis by a Phosphoenolpyruvate Carboxylase Inhibitor

Hybrids have been made between species of Flaveria exhibiting varying levels of C₄ photosynthesis. The degree of C₄ photosynthesis expressed in four interspecific hybrids (Flaveria trinervia [C₄] × F. linearis [C₃-C₄], F. brownii [C₄-like] × F. linearis, and two three-species hybrids from F. trinervia × [F. brownii × F. linearis]) was estimated by inhibiting phosphoenolpyruvate carboxylase in vivo with 3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate (DCDP). The inhibitor was fed to detached leaves at a concentration of 4 mm, and apparent photosynthesis was measured at atmospheric levels of CO₂ and at 20 and 210 mL L⁻¹ of O₂. Photosynthesis at 210 mL L⁻¹ of O₂ was inhibited 32% by DCDP in F. linearis, by 60% in F. brownii, and by 87% in F. trinervia. Inhibition in the hybrids ranged from 38 to 52%. The inhibition of photosynthesis by 210 mL L⁻¹ of O₂ was increased when DCDP was used, except in the C₄ species, F. trinervia, in which photosynthesis was insensitive to O₂. Except for F. trinervia, control plants with less O₂ sensitivity (more C₄-like) exhibited a progressively greater change in O₂ inhibition of photosynthesis when treated with DCDP. This increased O₂ inhibition probably resulted from decreased CO₂ concentrations in bundle sheath cells due to inhibition of phosphoenolpyruvate carboxylase. The inhibition of photosynthesis by DCDP is concluded to underestimate the degree of C₄ photosynthesis in the interspecific hybrids because increased direct assimilation of atmospheric CO₂ by ribulose bisphosphate carboxylase may compensate for inhibition of phosphoenolpyruvate carboxylase.     

Diagnostic parameters for selecting against novel spruce (Picea abies) decline: I. Tree morphology and photosynthesis response to acute SO2 exposures

The dose- and time-response effects of single 4-h day-exposures to 0.50, 0.79, 1.28, 1.58, 2.38 or 3.35 μl l^?1 (ppm) SO₂ followed by single 3-h night-exposures of 0.60, 0.87, 1.54, 1.91, 2.91 or 3.98 μl l^?1 SO₂ on photosynthesis, transpiration and dark respiration were examined for nine East European (Carpatho-Ukrainian, ‘Rachovo’) half-sib families and for two populations, one from the FRG (‘Westerhof’) and one from the GDR (‘Schmiedefeld’) of Norway spruce [Picea abies (L.) Karst.], all in their 4th growing season. Even the lowest SO₂ concentration reduced photosynthesis and transpiration within 1 h. Photosynthesis of the different spruce types was affected significantly differently, the most sensitive spruce being suppressed 2.5 times more than the most tolerant spruce. ‘Westerhof’ was more resistant to SO₂ than the average ‘Rachovo’ half-sibs. Neither transpiration (stomatal reaction), which was affected alike by all SO₂ concentrations, nor SO₂ uptake, explained adequately the effects on photosynthesis. Night transpiration, but not dark respiratin, was stimulated by night SO₂ preceded by day SO₂ exposure. The gradient of different SO₂ sensitivities among young trees from the half-sib families demonstrated a significant negative correlation with the gradient of different sensitivities to novel decline symptoms of their parents growing in a rural seed orchard in Denmark, and with the gradients of four morphology parameters, (height, branching, branch density and the number of Lammas shoots) of the young trees, which in turn demonstrated a positive correlation with decline sensitivity in the seed orchard. The relative photosynthesis sensitivity and the morphology of half-sibs may serve as diagnostic parameters for laboratory selection of the most resistent trees to novel spruce decline in the field. There was a positive correlation between SO₂ induced scorching of Lammas shoots and the inhibition of photosynthesis, but not between the severity of SO₂ scorching and symptoms of novel spruce decline. The two visible types of symptoms looked very different.     

Water use efficiency inFlaveria andMoricandia species

The water use efficiency (WUE) of the C₃?C₄ intermediate speciesFlaveria anomala andF. pubescens was similar to that found inF. cronquistii (C₃). Compared to this values, the value inF. brownii (C₄-like) was significantly increased and was doubled inF. trinervia (C₄).Moricandia arvensis, a species with an enhanced CO₂ reassimilation potential has a very similar water use efficiency asM. moricandioides (C₃ but a lower transpiration rate.     

Ecology of SO2 resistance: I. Effects of fumigations on gas exchange of deciduous and evergreen shrubs

Summary A unique gas exchange system is described in which photosynthesis, transpiration, and stomatal conductance can be measured on leaves during SO₂ fumigations. SO₂ concentrations can be continuously monitored and manipulated between 0 and 2.0 ppm. Rates of total SO₂ uptake and SO₂ absorption through stomates of a fumigated leaf can also be determined.Using this system we compared the effects of SO₂ on the gas exchange rates of two shrub species that co-occur in the Califormian chaparral. Diplacus aurantiacus, a deciduous shrub, was more sensitive to SO₂ fumigation than Heteromeles arbutifolia, an evergreen shrub. The differences in photosynthetic sensitivity could be attributed, in large part, to differential SO₂ absorption rates.     

Variation in leaf photosynthetic characteristics in wild rice species

Variations in leaf gas-exchange characteristics, leaf pigment content, and other important leaf traits were investigated in seven wild Oryza species, five hybrids, and five improved varieties. The significant variations were observed in photosynthetic pigment contents amongst different species of Oryza. The mean chlorophyll (Chl) content was higher in O. sativa (varieties and hybrids), while O. eichengeri showed the lowest Chl content. The mean carotenoid (Car) content in O. sativa (varieties and hybrids) was higher than in other wild rice species. O. eichengeri and O. barthii had significantly lower Car contents than other rice species. Significant differences were noticed in the rate of photosynthesis (P _N), stomatal conductance (g _s), transpiration rate (E), internal CO₂ concentration (C _i), specific leaf mass (SLM), and leaf thickness amongst different Oryza species. The mean P _N was the highest in O. nivara followed by O. eichengeri. The mean P _N was the lowest in O. glumaepatula, which was lower than that of cultivated varieties and hybrids of O. sativa. High rates of photosynthesis were observed in O. nivara (ACC. No. CR 100097), O. rufipogon (ACC.No. CR 100267), and O. nivara (ACC.No. CR 100008). The O. nivara and O. rufipogon genotypes with high P _N might be used in rice improvement programmes for an increase of leaf photosynthesis in rice. Multiple correlations performed between different gas-exchange characteristics and other physiological traits revealed that the rate of photosynthesis was not dependent on the leaf pigment content or the leaf thickness. A strong positive correlation between P _N and the P _N/C_i ratio, which represents the carboxylation efficiency, indicated that the observed variation in P _N was not based on pigment content or other leaf traits.     

Photosynthetic Characteristics of C(3)-C(4) Intermediate Flaveria Species : III. Reduction of Photorespiration by a Limited C(4) Pathway of Photosynthesis in Flaveria ramosissima

The initial products of photosynthesis by the C₃ species Flaveria cronquistii, the C₄ species F. trinervia, and the C₃-C₄ intermediate species F. ramosissima were determined using a pulse-chase technique with ¹⁴CO₂-¹²CO₂. The intermediate species F. ramosissima incorporated at least 42% of the total soluble ¹⁴C fixed into malate and aspartate after 10 seconds of photosynthesis in ¹⁴CO₂, as compared with 90% for the C₄ species F. trinervia and 5% for the C₃ species F. cronquistii. In both F. ramosissima and F. trinervia, turnover of labeled malate and aspartate occurred during a chase period in ¹²CO₂, although the rate of turnover was slower in the intermediate species. Relative to F. cronquistii, F. ramosissima showed a reduced incorporation of radioactivity into serine and glycine during the pulse period. These results indicate that a functional C₄ pathway of photosynthesis is operating in F. ramosissima which can account for its reduced level of photorespiration, and that this species is a true biochemical intermediate between C₃ and C₄ plants.     

Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways: V. RESPONSE OF PANICUM MAXIMUM, PANICUM MILIOIDES, AND TALL FESCUE (FESTUCA ARUNDINACEA) TO NITROGEN NUTRITION

The response of apparent photosynthesis to N nutrition was studied in the C₃ grass, tall fescue (Festuca arundinacea Schreb.), in the C₄ species Panicum maximum Jacq., and in Panicum milioides Nees ex Trin., a species with characteristics intermediate between C₃ and C₄ photosynthetic types. Plants were grown in culture solution containing 1, 5, 50, and 200 milligrams N per liter. Apparent photosynthesis was measured on the youngest fully expanded leaves at 320 microliters of CO₂ per liter of air and 21% O₂. Leaf conductance was calculated from transpiration measurements, and CO₂ compensation concentrations were also estimated. Several leaf anatomical characteristics were studied on plastic-embedded material. Leaf N content was determined on leaves which were used in photosynthesis measurements.     

Oxygen Inhibition of Photosynthesis: I. Temperature Dependence and Relation to O(2)/CO(2) Solubility Ratio

The magnitude of the percentage inhibition of photosynthesis by atmospheric levels of O₂ in the C₃ species Solanum tuberosum L., Medicago sativa L., Phaseolus vulgaris L., Glycine max L., and Triticum aestivum L. increases in a similar manner with an increase in the apparent solubility ratio of O₂/CO₂ in the leaf over a range of solubility ratios from 25 to 45. The solubility ratio is based on calculated levels of O₂ and CO₂ in the intercellular spaces of leaves as derived from whole leaf measurements of photosynthesis and transpiration. The solubility ratio of O₂/CO₂ can be increased by increased leaf temperature under constant atmospheric levels of O₂ and CO₂ (since O₂ is relatively more soluble than CO₂ with increasing temperature); by increasing the relative levels of O₂/CO₂ in the atmosphere at a given leaf temperature, or by increased stomatal resistance. If the solubility ratio of O₂/CO₂ is kept constant, as leaf temperature is increased, by varying the levels of O₂ or CO₂ in the atmosphere, then the percentage inhibition of photosynthesis by O₂ is similar. The decreased solubility of CO₂ relative to O₂ (decreased CO₂/O₂ ratio) may be partly responsible for the increased percentage inhibition of photosynthesis by O₂ under atmospheric conditions with increasing temperature.     

Transpiration in seven plant species colonizing a fishpond shore

In 7 species (Eleocharis palustris R. Br.,Juncus bufonius L.,Gypsophila muralis L.,Trifolium repens L.,Agrostis stolonifera L.,Potentilla anserina L. andAchillea millefolium L.) growing in a gradient of habitats from aquatic to terrestrial, on a sandy fishpond shore in Southern Bohemia, Czechoslovakia, the daily course of transpiration rate and water content was assessed gravimetrically in their cut-off transpiring parts on two typical summer days. Transpiration rate was largely controlled by microclimate but depressions occurred inJuncus and in some species growing in the drier habitats. The highest instantaneous transpiration rate and total daily transpiration per unit dry weight, about 100 mg g^?1 min^?1 and 50 g g^?1 d^?1, respectively, were recorded inAchillea andTrifolium. The water turnover rate ranged from 16.1 g g^?1 (H₂O) d^?1 inJuncus to 10.7 inGypsophila. The estimated daily maximum water saturation deficit was highest inEleocharis (45.7%) and lowest inAchillea (15.0%). The adaptational significance of the observed phenomena is discussed.     

CO2浓度倍增对8种作物叶片光合作用、蒸腾作用和水分利用效率的影响

揭示作物光合作用、蒸腾作用和水分利用效率(WUE)对大气CO₂浓度变化的响应, 对预测未来大气CO₂浓度升高条件下作物生产力与需水规律的变化具有重要意义。在自然CO₂浓度、CO₂倍增和倍增后恢复到自然CO₂浓度3种情况下, 对大豆(Glycine max)、甘薯(Ipomoea batatas)、花生(Arachis hypogaea)、水稻(Oryza sativa)、棉花(Gossypium hirsutum)、玉米(Zea mays)、高粱(Sorghum vulgare)和谷子(Setaria italica) 8种作物的气体交换参数进行了研究。结果表明: CO₂浓度倍增可以提高光合速率, 降低蒸腾速率, 从而提高WUE, 其中光合速率提高的贡献更大; C₃比C₄作物的光合速率、WUE增幅大, C₃作物光合速率提高对WUE的贡献大于C₄作物; 通过对比倍增后恢复到自然CO₂浓度时气体交换参数随环境条件变化的响应确定了其内在调控机制; 倍增后恢复到自然CO₂浓度时作物光合速率低于自然CO₂浓度下的光合速率, 而蒸腾速率无明显差异。由此判断: CO₂浓度倍增下存在光合下调现象, 这可能是由于Rubisco酶蛋白含量、活化水平和比活性降低等“非气孔因素”造成的, 并非由气孔导度的降低引起的。     

Oxygen sensitivity of photosynthesis and photorespiration in different photosynthetic types in the genus Flaveria

Two major indicators were used to access the degree of photorespiration in various photosynthetic types of Flaveria species (C₃, C₃-C₄, C₄-like, and C₄): the O₂ inhibition of photosynthesis measured above the O₂ partial pressure which gives a maximum rate, and O₂- and light-dependent whole-chain electron flow measured at the CO₂ compensation point (). The optimum level of O₂ for maximum photosynthetic rates under atmospheric levels of CO₂ (34 Pa) was lower in C₃ and C₃-C₄ species (ca. 2 kPa) than in C₄-like and C₄ species (ca. 9 kPa). Increasing O₂ partial pressures from the optimum for photosynthesis up to normal atmospheric levels (ca. 20 kPa) caused an inhibition of photosynthesis which was more severe under lower CO₂. This inhibition was calculated as the O₂ inhibition index (_A, the percentage inhibition of photosynthesis per kPa increase in O₂). From measurements of 18 Flaveria species at atmospheric CO₂, the _A values decreased from C₃ (1.9–2.1) to C₃-C₄ (1.2–1.6), C₄-like (0.6–0.8) and C₄ species (0.3–0.4), indicating a progressive decrease in apparent photorespiration in this series. With increasing irradiance at under atmospheric levels of O₂, and increasing O₂ partial pressure at 300 mol quanta·m^–2·s^–1, there was a similar increase in the rate of O₂ evolution associated with whole-chain electron flow (J_{o 2}, calculated from chlorophyll fluorescence analysis) in the C₃ and C₃-C₄ species compared to a much lower rate in the C₄-like and C₄ species. The results indicate that there is substantial O₂-dependent electron flow in C₃ and C₃-C₄ species, reflecting a high level of photorespiration compared to that in C₄-like and C₄ species. Consistent with these results, there was a significant decrease in from C₃ (6–6.2 Pa) to C₃-C₄ (1.0–3.0 Pa), to C₄-like and C₄ species (0.3–0.8 Pa), indicating a progressive decrease in apparent photorespiration. However, C₃ and C₃-C₄ species examined had high intrinsic levels of photorespiration with the latter maintaining low apparent rates of photorespiration and lower values, primarily by refixing photorespired CO₂. The C₄-like and C₄ Flaveria species had low, but measurable, levels of photorespiration via selective localization of ribulose-1,5-bisphosphate carboxylase in bundle sheath cells and operation of a CO₂ pump via the C₄ pathway.Abbreviations and Symbols A CO₂ assimilation rate - CE carboxylation efficiency - C_i intercellular CO₂ partial pressure - I_a absorbed PPFD - J_{o 2} oxygen evolution from PSII - PPFD photosynthetic photon flux density (mol · m^–2· s^–1) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - VPD water-vapor pressure difference between the leaf and atmospheric air - CO₂ compensation point - _{CO 2} quantum yield of CO₂ assimilation - _PSII quantum yield of photosystem II - _A O₂ inhibition index for photosynthesis (percentage inhibition of photosynthesis per kPa increase in O₂) This research was supported by the National Science Foundation Grant IBN 9317756 and Equipment (Grant DMB-8515521 and DOE/USDA/NSF Triagency Plnat Biochemistry Research Training Grant Program.     

Photosynthetic and photorespiratory characteristics of flaveria species

The genus Flaveria shows evidence of evolution in the mechanism of photosynthesis as its 21 species include C₃, C₃-C₄, C₄-like, and C₄ plants. In this study, several physiological and biochemical parameters of photosynthesis and photorespiration were measured in 18 Flaveria species representing all the photosynthetic types. The 10 species classified as C₃-C₄ intermediates showed an inverse continuum in level of photorespiration and development of the C₄ syndrome. This ranges from F. sonorensis with relatively high apparent photorespiration and lacking C₄ photosynthesis to F. Among the intermediates, the photosynthetic CO₂ compensation points at 30°C and 1150 micromoles quanta per square meter per second varied from 9 to 29 microbars. The values for the three C₄-like species varied from 3 to 6 microbars, similar to those measured for the C₄ species. The activities of the photorespiratory enzymes glycolate oxidase, hydroxypyruvate reductase, and serine hydroxymethyltransferase decreased progressively from C₃ to C₃-C₄ to C₄-like and C₄ species. On the other hand, most intermediates had higher levels of phosphenolpyruvate carboxylase and NADP-malic enzyme than C₃ species, but generally lower activities compared to C₄-like and C₄ species. The levels of these C₄ enzymes are correlated with the degree of C₄ photosynthesis, based on the initial products of photosynthesis. Another indication of development of the C₄ syndrome in C₃-C₄ Flaveria species was their intermediate chlorophyll a/b ratios. The chlorophyll a/b ratios of the various Flaveria species are highly correlated with the degree of C₄ photosynthesis suggesting that the photochemical machinery is progressively altered during evolution in order to meet the specific energy requirements for operating the C₄ pathway. In the progression from C₃ to C₄ species in Flaveria, the CO₂ compensation point decreased more rapidly than did the decrease in O₂ inhibition of photosynthesis or the increase in the degree of C₄ photosynthesis. These results suggest that the reduction in photorespiration during evolution occurred initially by refixation of photorespired CO₂ and prior to substantive reduction in O₂ inhibition and development of the C₄ syndrome. However, further reduction in O₂ inhibition in some intermediates and C₄-like species is considered primarily due to the development of the C₄ syndrome. Thus, the evolution of C₃-C₄ intermediate photosynthesis likely occurred in response to environmental conditions which limit the intercellular CO₂ concentration first via refixation of photorespired CO₂, followed by development of the C₄ syndrome.     

Influence of light intensity,temperature and humidity on photosynthesis and transpiration ofSasa nipponica andArundinaria pygmaea

Photosynthesis and transpiration were simultaneously measured under different light intensity, temperature and humidity conditions inSasa nipponica andArundinaria pygmaea grown in exposed and shaded habitats. Both species showed a saturated light curve for photosynthetic rate. The saturation point was lower in shaded plants. The apparent quantum yields were larger inS. nipponica and in shaded plants, while the maximum photosynthesis was higher inA. pygmaea and exposed plants. The temperature response of photosynthesis showed an optimum curve in both species. The optinum temperatures were 20 C inS. nipponica and 25 C inA. pygmaea. The influence of humidity on photosynthesis was insignificant for both species. The responses of transpiration to light intensity and relative humidity showed a saturated curve and an optimal one, respectively. There was a significant relationship between transpiration and stomatal frequency, both of which were higher inS. nipponica, while water use efficiency was higher inA. pygmaea. These results suggest thatS. nipponica adapts itself better to shaded, low temperature and less water stress habitats as compared withA. pygmaea.     

Physiological advantages of C4 grasses in the field: a comparative experiment demonstrating the importance of drought

Global climate change is expected to shift regional rainfall patterns, influencing species distributions where they depend on water availability. Comparative studies have demonstrated that C₄ grasses inhabit drier habitats than C₃ relatives, but that both C₃ and C₄ photosynthesis are susceptible to drought. However, C₄ plants may show advantages in hydraulic performance in dry environments. We investigated the effects of seasonal variation in water availability on leaf physiology, using a common garden experiment in the Eastern Cape of South Africa to compare 12 locally occurring grass species from C₄ and C₃ sister lineages. Photosynthesis was always higher in the C₄ than C₃ grasses across every month, but the difference was not statistically significant during the wettest months. Surprisingly, stomatal conductance was typically lower in the C₃ than C₄ grasses, with the peak monthly average for C₃ species being similar to that of C₄ leaves. In water‐limited, rain‐fed plots, the photosynthesis of C₄ leaves was between 2.0 and 7.4 μmol m^?2 s^?1 higher, stomatal conductance almost double, and transpiration 60% higher than for C₃ plants. Although C₄ average instantaneous water‐use efficiencies were higher (2.4–8.1 mmol mol^?1) than C₃ averages (0.7–6.8 mmol mol^?1), differences were not as great as we expected and were statistically significant only as drought became established. Photosynthesis declined earlier during drought among C₃ than C₄ species, coincident with decreases in stomatal conductance and transpiration. Eventual decreases in photosynthesis among C₄ plants were linked with declining midday leaf water potentials. However, during the same phase of drought, C₃ species showed significant decreases in hydrodynamic gradients that suggested hydraulic failure. Thus, our results indicate that stomatal and hydraulic behaviour during drought enhances the differences in photosynthesis between C₄ and C₃ species. We suggest that these drought responses are important for understanding the advantages of C₄ photosynthesis under field conditions.     

Dependence of the membrane potential on intracellular ATP concentration in tonoplast-free cells of Nitellopsis obtusa

Intact chloroplasts were obtained from mesophyll protoplasts isolated from Mesembryanthemum crystallinum in the C₃ or Crassulacean acid metabolism (CAM) photosynthetic mode, and examined for the influence of inorganic phosphate (Pi) on aspects of bicarbonate-dependent O₂ evolution and CO₂ fixation. While the chloroplasts from both modes responded similarly to varying Pi, some features appear typical of chloroplasts from species capable of CAM, including a relatively high capacity for photosynthesis in the absence of Pi, a short induction period, and resistance to inhibition of photosynthesis by high levels of Pi. In the absence of Pi the chloroplasts retained 75–85% of the ¹⁴CO₂ fixed and the total export of dihydroxyacetone phosphate was low compared with the rate of photosynthesis. In CAM plants the ability to conduct photosynthesis and retain most of the fixed carbon in the chloroplasts at low external Pi concentrations may enable storage of carbohydrates which are essential for providing a carbon source for the nocturnal synthesis of malic acid. At high external Pi concentrations (e.g. 10 25 mM), the amount of total dihydroxyacetone phosphate exported to the assay medium relative to the rate of photosynthesis was high while the products of ¹⁴CO₂ fixation were largely retained in the chloroplasts which indicates starch degradation is occurring at high Pi levels. Starch degradation normally occurs in CAM plants in the dark; high levels of Pi may induce starch degradation in the light which has the effect of limiting export of the immediate products of photosynthesis and thus the degree of Pi inhibition of photosynthesis with the isolated chloroplast.     

Estimating Photosynthesis and Concurrent Export Rates in C3 and C4 Species at Ambient and Elevated CO2

The ability of 21 C₃ and C₄ monocot and dicot species to rapidly export newly fixed C in the light at both ambient and enriched CO₂ levels was compared. Photosynthesis and concurrent export rates were estimated during isotopic equilibrium of the transport sugars using a steady-state ¹⁴CO₂-labeling procedure. At ambient CO₂ photosynthesis and export rates for C₃ species were 5 to 15 and 1 to 10 μmol C m⁻² s⁻¹, respectively, and 20 to 30 and 15 to 22 μmol C m⁻² s⁻¹, respectively, for C₄ species. A linear regression plot of export on photosynthesis rate of all species had a correlation coefficient of 0.87. When concurrent export was expressed as a percentage of photosynthesis, several C₃ dicots that produced transport sugars other than Suc had high efflux rates relative to photosynthesis, comparable to those of C₄ species. At high CO₂ photosynthetic and export rates were only slightly altered in C₄ species, and photosynthesis increased but export rates did not in all C₃ species. The C₃ species that had high efflux rates relative to photosynthesis at ambient CO₂ exported at rates comparable to those of C₄ species on both an absolute basis and as a percentage of photosynthesis. At ambient CO₂ there were strong linear relationships between photosynthesis, sugar synthesis, and concurrent export. However, at high CO₂ the relationships between photosynthesis and export rate and between sugar synthesis and export rate were not as strong because sugars and starch were accumulated.     

Photosynthesis,carbon allocation,and growth of sulfur dioxide ecotypes ofGeranium carolinianum L.

Summary This study investigated ways in which genetically determined differences in SO₂ susceptibility resulting from ecotypic differentiation inGeranium carolinianum were expressed physiologically. The SO₂-resistant and SO₂-sensitive ecotypes were exposed to a combination of short- and long-term SO₂ exposures to evaluate the responses of photosynthesis, H₂S efflux from foliage (sulfur detoxification), photoassimilate retention, leaf-diffusive resistance to CO₂, and growth. When exposed to SO₂, both ecotypes re-emit sulfur in a volatile, reduced form, presumably as H₂S. Because H₂S efflux rates at various SO₂ concentrations were comparable between ecotypes, genetic differences inG. carolinianum could not be attributed to a re-emission of excess sulfur as H₂S. Incipient SO₂ effects on photosynthesis were observed as cumulative SO₂ flux into the leaf interior excecded 0.40 nmol·m^–2 in the resistant ecotype and 0.26 nmol·m^–2 in the sensitive ecotype. Although initial SO₂-induced changes in photosynthesis in both ecotypes were mediated through an increase in stomatal resistance to CO₂, the ecotype-specific patterns as a function of pollutant concentration and exposure time were associated with marked increases in residual resistance to CO₂. Patterns in photosynthesis, photoassimilate retention, and growth following long-term SO₂ exposures were also ecotype-specific. Although physiological accommodation of SO₂ stress was observed in both ecotypes, it was more pronounced in the resistant ecotype. The physiological mechanisms underlying genetic differences inG. carolinianum in response to SO₂ stress were concluded to be (1) dissimilar threshold levels of response to SO₂ and/or its toxic derivatives and (2) differences in homeostatic processes governing the rate of repair or compensation for physiological injury.Research sponsored by the Office of Health and Environmental Research, U.S. Department of Energy, under contract No. DEAC05-840R21400 with Martin Marietta, Energy Systems, Inc. and the U.S. Environmental Protection AgencyPublication No. 2610, Environmental Sciences Division, Oak Ridge National Laboratory