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PubMed1976

Effect of glycidate on glycolate formation and photosynthesis in isolated spinach chloroplasts.

Glycidate (2,3-epoxypropionate) increased CO(2) photoassimilation in intact spinach (Spinacia oleracea L.) chloroplasts in the presence of various inhibitors of photosynthesis, including O(2), arsenite, azide, iodo-acetamide, and carbonylcyanide 3-chlorophenylhydrazone. Although the mechanism by which glycidate enhances photosynthesis is obscure, the stimulatory effect cannot be ascribed to either an inhibition of glycolate formation, a specific interaction with the O(2) inhibition of photosynthesis, or a direct effect on the ribulose 1,5-bisphosphate carboxylase (EC 4.1.1.39) reaction. The lack of a differential effect of glycidate on photosynthesis and glycolate formation in the isolated chloroplast was confirmed in whole leaf studies by the CO(2) compensation concentration assay. These results are at variance with the report that glycidate stimulates net photosynthesis in tobacco leaf disks by irreversibly inhibiting glycolate formation and thus photorespiration (Zelitch, I., 1974, Arch. Biochem. Biophys. 163: 367-377).

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PubMed1976

Effects of sulfur on the photosynthesis of intact leaves and isolated chloroplasts of sugar beets.

Effects of sulfur on photosynthesis in sugar beets (Beta vulgaris L. cv. F58-554H1) were studied by inducing sulfur deficiency and determining changes in the photosynthesis of whole attached leaves and of isolated chloroplasts. The rates of photosynthetic CO(2) uptake by intact leaves, photoreduction of ferricyanide, cyclic and noncyclic photophosphorylation of isolated chloroplasts, and the rate of CO(2) assimilation by ribulose diphosphate carboxylase, decreased with decrease in total leaf sulfur from 2500 to about 500 mug g(-1) dry weight. Sulfur deficiency reduced photosynthesis through an effect on chlorophyll content, which decreased linearly with leaf sulfur, and by decreasing the rate of photosynthesis per unit chlorophyll. There was only a small effect of sulfur deficiency on stomatal diffusion resistance to CO(2) until leaf sulfur decreased below 1000 mug g(-1) when stomatal resistance became a more significant proportion of the total diffusion resistance to CO(2). Light respiration rates were positively correlated with photosynthesis rates and dark respiration was unchanged as leaf sulfur concentrations declined.

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PubMed1976

Diurnal and Seasonal Patterns of Photosynthesis and Respiration by Stems of Populus tremuloides Michx.

The photosynthetic and respiratory rates of 5- to 7-year-old aspen stems (Populus tremuloides Michx.) were monitored in the field for 1 year to determine the seasonal patterns. The stem was not capable of net photosynthesis, but the respiratory CO(2) loss from the stem was reduced by 0 to 100% depending on the time of year and the level of illumination as a result of bark photosynthesis. The monthly dark respiratory rate ranged from 0.24 mg CO(2)/dm(2). hr in January to a maximum 7.4 mg CO(2)/dm(2). hr in June. Individual measurements ranged from 0.02 mg CO(2)/dm(2). hr in February to 12.3 mg CO(2)/dm(2). hr in June. Gross photosynthesis followed a pattern similar to the dark respiratory rate. The mean monthly rate was highest in June (1.65 mg CO(2)/dm(2). hr) and lowest in December (0.02 mg CO(2)/dm(2). hr). Individual measurements ranged from 0.0 mg CO(2)/dm(2). hr in winter to 5.5 mg CO(2)/dm(2). hr in July.Winter studies showed that stem respiration continued down to -11 C, the coldest temperature during this study. Upon warning to -3 C, the dark respiratory rate showed a sudden sharp increase (7- to 12-fold) which required many hours to return to normal levels. No measurable photosynthesis occurred below -3 C. Between -3 and 0 C, the maximal photosynthetic rate was reduced to less than 50% of the respiratory rate, but increased to 89% between 5 to 10 C.On a yearly basis, bark photosynthesis in P. tremuloides reduced the stem respiratory CO(2) loss by 28.7% on a daytime basis and an estimated 16 to 18% on a 24-hour basis.

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PubMed1977

Oxygen Inhibition of Photosynthesis: II. Kinetic Characteristics as Affected by Temperature.

The response of whole leaf photosynthesis of wheat (Triticum aestivum L.) in relation to soluble CO(2) available to the mesophyll cells, under low (1.5%) O(2) at 25, 30, and 35 C, followed Michaelis-Menten kinetics up to saturating CO(2) but deviated at high CO(2) levels where the experimental V(max) is considerably less than the calculated V(max). The affinity of the leaves for CO(2) during photosynthesis was similar from 25 to 35 C with Km (CO(2)) values of approximately 3.5 to 5 muM.In considering the effect of O(2) on photosynthesis at 25, 30, and 35 C where O(2) and CO(2) are expressed on a solubility basis: (a) the effect of O(2) on carboxylation efficiency was similar at the three temperature; (b) increasing temperature caused only a slight increase in kinetic constants Ki(O(2)) and Km(CO(2)), while the ratio of Ki(O(2))/Km(CO(2)) was similar at the three temperatures; and (c) the reciprocal plots of apparent rate of photosynthesis versus (CO(2) - Gamma) at various O(2) levels showed O(2) to be a competitive inhibitor of photosynthesis.A model for separating O(2) inhibition of photosynthesis into two components, direct competitive inhibition and inhibition due to photorespiration, was presented from both simulated and experimental data of photosynthetic response curves to varying CO(2) concentrations at low O(2)versus 21% O(2). The photorespiratory part of O(2) inhibition is considered as a major component at Gamma and increases with increasing temperature and with increase in O(2)/CO(2) solubility ratio. The competitive component of O(2) inhibition is considered as a major component of O(2) inhibition under atmospheric CO(2) levels and is relatively independent of temperature at a given O(2)/CO(2) ratio.

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PubMed1977

Photosynthesis, growth, and the role of chloride.

Previous studies with isolated chloroplasts have indicated that Cl(-) is an essential cofactor for photosynthesis. Considerable support for the postulated Cl(-) requirement in photosynthesis came from the observation that Cl(-) is essential for growth. Data are presented which show that a 60% reduction in growth which occurred in Cl(-) -deficient sugar beet (Beta vulgaris L.) was not due to an effect of Cl(-) on the rate of photosynthesis in vivo (net CO(2) uptake per unit area of attached leaves). The principal effect of Cl(-) deficiency was to lower cell multiplication rates in leaves, thus slowing down their growth and ultimately decreasing their area. The absence of an effect of Cl(-) on photosynthesis in vivo was unlikely to have been due to Cl(-) retention by the chloroplasts because their Cl(-) concentration (measured after nonaqueous isolation) decreased progressively with decrease in leaf Cl(-).An effect of Cl(-) with isolated chloroplasts in vitro, however, was confirmed. Addition of Cl(-) to the reaction medium after washing chloroplasts in EDTA increased the rate of ferricyanide photoreduction 10-fold. This effect of Cl(-) did not appear to be related to the Cl(-) concentration of the chloroplasts since chloroplast Cl(-) was not decreased further by washing in EDTA. It is concluded that Cl(-) has not yet unequivocally been shown to be an essential cofactor for photosynthesis and that the response to Cl(-)in vitro probably does not have a physiological basis.

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PubMed1980

Effect of irradiance on development of apparent nitrogen fixation and photosynthesis in soybean.

Soybeans grown with 2 millimolar NO(3) (-), which optimized apparent N(2) fixation by Rhizobium symbionts, showed significantly different rates of apparent photosynthesis and C(2)H(2) reduction during seedling development at two irradiances. Those physiological processes were lower for several weeks in plants grown at 1,500 microeinsteins per meter(2) per second than in those exposed to 700 microeinsteins per meter(2) per second. The irradiance-induced retardation was evident in short-term rates of apparent photosynthesis and N(2) fixation, as well as in measures of dry matter and total N accumulation. In spite of their previously inhibited development, plants grown at 1,500 microeinsteins per meter(2) per second were indistinguishable by day 28 from those exposed to 700 microeinsteins per meter(2) per second in terms of whole-shoot CO(2)-exchange rate; by day 35 they were identical in terms of whole-plant C(2)H(2)-reduction rate. On day 38 there was no significant difference in dry weight or N content between treatments. Shifting plants between irradiance treatments on day 21 showed that the higher irradiance also had a short-term inhibitory effect on C(2)H(2) reduction. The fact that 16 millimolar NO(3) (-) prevented the continuous exposure to 1,500 microeinsteins per meter(2) per second from inhibiting apparent photosynthesis suggested that seedlings grown on 2 millimolar NO(3) (-) with Rhizobium were N-limited. Although rates of apparent photosynthesis were similar by day 28, the additional week required to produce equal rates of apparent N(2) fixation between irradiance treatments showed that physiological adaptations of shoots, as well as photosynthesis per se, can affect root nodule activity.

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PubMed1981

Impairment of photosynthesis by chilling-temperatures in tomato.

Chilling of attached tomato leaves (cv. Rutgers) in the dark for 16 hours at 1 C decreased both photosynthesis and transpiration. To separate the effects of chilling on stomatal CO(2) conductance from more direct effects of chilling on the chloroplasts' activities, measurements of photosynthesis and transpiration were made at atmospheric and saturating CO(2) levels. At atmospheric CO(2), the inhibition of photosynthesis was approximately 60%, of which about 35% was attributable to the impairment of chloroplast function and about 25% was attributable to decreased stomatal conductance. However, the affinity of the photosynthetic apparatus for CO(2) was not changed by chilling, since the dependence of the relative rate of photosynthesis on the intercellular CO(2) concentration was unaltered. The apparent quantum requirement for CO(2) reduction also was identical in chilled and unchilled plants. This observation contradicts the widely held notion that the chilling-induced inhibition of photosynthesis is caused by an impairment of the water oxidation mechanism. The impairment of chloroplast activity was not a consequence of an unfavorable water status within the leaf, since chilling caused only a small drop (1 bar) in water potential. A small loss of chlorophyll resulted as a secondary effect of chilling, but this loss of chlorophyll was eliminated as a cause of the inhibition of photosynthesis.No recovery of chloroplast activity occurred during the subsequent light period after chilling. The recovery seemed to be inhibited by light or to require both a light period and a dark period or to occur after a considerable lag period. After a period of both light and dark, restoration of stomatal conductance occurred more slowly than did the recovery of chloroplast activity.

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PubMed1984

Photosynthesis and Ribulose 1,5-Bisphosphate Concentrations in Intact Leaves of Xanthium strumarium L.

The interacting effects of the rate of ribulose 1,5-bisphosphate (RuBP) regeneration and the rate of RuBP utilization as influenced by the amount and activation of RuBP carboxylase on photosynthesis and RuBP concentrations were resolved in experiments which examined the kinetics of the response of photosynthesis and RuBP concentrations after step changes from a rate-saturating to a rate-limiting light intensity in Xanthium strumarium. Because RuBP carboxylase requires several minutes to deactivate in vivo, it was possible to observe the effect of reducing the rate of RuBP regeneration on the RuBP concentration at constant enzyme activation state by sampling very soon after reducing the light intensity. Samples taken over longer time periods showed the effect of changes in enzyme activation at constant RuBP regeneration rate on RuBP concentration and photosynthetic rate. Within 15 s of lowering the light intensity from 1500 to 600 microEinsteins per square meter per second the RuBP concentration in the leaves dropped below the enzyme active site concentration, indicating that RuBP regeneration rate was limiting for photosynthesis. After longer intervals of time, the RuBP concentration in the leaf increased as the RuBP carboxylase assumed a new steady state activation level. No change in the rate of photosynthesis was observed during the interval that RuBP concentration increased. It is concluded that the rate of photosynthesis at the lower light intensity was limited by the rate of RuBP regeneration and that parallel changes in the activation of RuBP carboxylase occurred such that concentrations of RuBP at steady state were not altered by changes in light intensity.

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PubMed1989

Photorespiratory ammonia does not inhibit photosynthesis in glutamate synthase mutants of Arabidopsis.

Exposure of ferredoxin-dependent glutamate synthase (EC 1.4.7.1) mutants of Arabidopsis thaliana to photorespiratory conditions resulted in the accumulation of NH(4) (+) and the inhibition of photosynthesis. However, upon transfer from 2% O(2), 350 microliters per liter CO(2), to 21% O(2), 350 microliters per liter CO(2), net photosynthesis declined at a slower rate in methionine sulfoximine treated leaf discs relative to controls. The recovery of photosynthesis was also more rapid in MSO-treated leaf discs although ammonia levels were more than threefold higher. Photosynthesis in leaf discs treated with azaserine was inhibited more than controls when transferred to 21% O(2) and recovered less than controls when returned to 2% O(2) although NH(4) (+) levels were not significantly different. The results obtained are consistent with the view that the rapid inhibition of photosynthesis in the glutamate synthase mutants in photorespiratory conditions is not due to the accumulation of NH(4) (+) but rather to the depletion of amino donors for glyoxylate and the consequent effects of glyoxylate on the lack of return of carbon to the chloroplast.

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PubMed1989

Acclimation of Photosynthesis to Elevated CO(2) in Five C(3) Species.

The effect of long-term (weeks to months) CO(2) enhancement on (a) the gas-exchange characteristics, (b) the content and activation state of ribulose-1,5-bisphosphate carboxylase (rubisco), and (c) leaf nitrogen, chlorophyll, and dry weight per area were studied in five C(3) species (Chenopodium album, Phaseolus vulgaris, Solanum tuberosum, Solanum melongena, and Brassica oleracea) grown at CO(2) partial pressures of 300 or 900 to 1000 microbars. Long-term exposure to elevated CO(2) affected the CO(2) response of photosynthesis in one of three ways: (a) the initial slope of the CO(2) response was unaffected, but the photosynthetic rate at high CO(2) increased (S. tuberosum); (b) the initial slope decreased but the CO(2)-saturated rate of photosynthesis was little affected (C. album, P. vulgaris); (c) both the initial slope and the CO(2)-saturated rate of photosynthesis decreased (B. oleracea, S. melongena). In all five species, growth at high CO(2) increased the extent to which photosynthesis was stimulated following a decrease in the partial pressure of O(2) or an increase in measurement CO(2) above 600 microbars. This stimulation indicates that a limitation on photosynthesis by the capacity to regenerate orthophosphate was reduced or absent after acclimation to high CO(2). Leaf nitrogen per area either increased (S. tuberosum, S. melongena) or was little changed by CO(2) enhancement. The content of rubisco was lower in only two of the five species, yet its activation state was 19% to 48% lower in all five species following long-term exposure to high CO(2). These results indicate that during growth in CO(2)-enriched air, leaf rubisco content remains in excess of that required to support the observed photosynthetic rates.

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PubMed1989

Relative sensitivity of photosynthesis and respiration to freeze-thaw stress in herbaceous species : importance of realistic freeze-thaw protocols.

The relative effect of a freeze-thaw cycle on photosynthesis, respiration, and ion leakage of potato leaf tissue was examined in two potato species, Solanum acaule Bitt. and Solanum commersonii Dun. Photosynthesis was found to be much more sensitive to freezing stress than was respiration, and demonstrated more than a 60% inhibition before any impairment of respiratory function was observed. Photosynthesis showed a slight to moderate inhibition when only 5 to 10% of the total electrolytes had leaked from the tissue (reversible injury). This was in contrast to respiration which showed no impairment until temperatures at which about 50% ion leakage (irreversible injury) had occurred. The influence of freeze-thaw protocol was further examined in S. acaule and S. commersonii, in order to explore discrepancies in the literature as to the relative sensitivities of photosynthesis and respiration. As bath cooling rates increased from 1 degrees C/hour to about 3 or 6 degrees C/hour, there was a dramatic increase in the level of damage to all measured cellular functions. The initiation of ice formation in deeply supercooled tissue caused even greater damage. As the cooling rates used in stress treatments increased, the differential sensitivity between photosynthesis and respiration nearly disappeared. Examination of agriculturally relevant, climatological data from an 11 year period confirmed that air cooling rates in the freezing range do not exceed 2 degrees C/hour. It was demonstrated, in the studies presented here, that simply increasing the actual cooling rate from 1.0 to 2.9 degrees C/hour, in frozen tissue from paired leaflet halves, meant the difference between cell survival and cell death.

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PubMed1989

Isoprene emission from aspen leaves : influence of environment and relation to photosynthesis and photorespiration.

Isoprene emission rates from quaking aspen (Populus tremuloides Michx.) leaves were measured simultaneously with photosynthesis rate, stomatal conductance, and intercellular CO(2) partial pressure. Isoprene emission required the presence of CO(2) or O(2), but not both. The light response of isoprene emission rate paralleled that of photosynthesis. Isoprene emission was inhibited by decreasing ambient O(2) from 21% to 2%, only when there was oxygen insensitive photosynthesis. Mannose (10 millimolar) fed through cut stems resulted in strong inhibition of isoprene emission rate and is interpreted as evidence that isoprene biosynthesis requires either the export of triose phosphates from the chloroplast, or the continued synthesis of ATP. Light response experiments suggest that photosynthetically generated reductant or ATP is required for isoprene biosynthesis. Isoprene biosynthesis and emission are not directly linked to glycolate production through photorespiration, contrary to previous reports. Isoprene emission rate was inhibited by above-ambient CO(2) partial pressures (640 microbar outside and 425 microbar inside the leaf). The inhibition was not due to stomatal closure. This was established by varying ambient humidity at normal and elevated CO(2) partial pressures to measure isoprene emission rates over a range of stomatal conductances. Isoprene emission rates were inhibited at elevated CO(2) despite no change in stomatal conductance. Addition of abscisic acid to the transpiration stream dramatically inhibited stomatal conductance and photosynthesis rate, with a slight increase in isoprene emission rate. Thus, isoprene emission is independent of stomatal conductance, and may occur through the cuticle. Temperature had an influence on isoprene emission rate, with the Q(10) being 1.8 to 2.4 between 35 and 45 degrees C. At these high temperatures the amount of carbon lost through isoprene emission was between 2.5 and 8% of that assimilated through photosynthesis. This represents a significant carbon cost that should be taken into account in determining midsummer carbon budgets for plants that are isoprene emitters.

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PubMed1989

Mechanism of c(4) photosynthesis: a model describing the inorganic carbon pool in bundle sheath cells.

A theoretical model of the composition of the inorganic carbon pool generated in C(4) leaves during steady-state photosynthesis was derived. This model gives the concentrations of CO(2) and O(2) in the bundle sheath cells for any given net photosynthesis rate and inorganic carbon pool size. The model predicts a bundle sheath CO(2) concentration of 70 micromolar during steady state photosynthesis in a typical C(4) plant, and that about 13% of the inorganic carbon generated in bundle sheath cells would leak back to the mesophyll cells, predominantly as CO(2). Under these circumstances the flux of carbon through the C(4) acid cycle would have to exceed the net rate of CO(2) assimilation by 15.5%. With the calculated O(2) concentration of 0.44 millimolar, the potential photorespiratory CO(2) loss in bundle sheath cells would be about 3% of CO(2) assimilation. Among the factors having a critical influence on the above values are the permeability of bundle sheath chloroplasts to HCO(3) (-), the activity of carbonic anhydrase within these chloroplasts, the assumed stromal volume, and the permeability coefficients for CO(2) and O(2) diffusion across the interface between bundle sheath and mesophyll cells. The model suggests that as the net photosynthesis rate changes in C(4) plants, the level and distribution of the components of the inorganic carbon pool change in such a way that C(4) acid overcycling is maintained in an approximately constant ratio with respect to the net photosynthesis rate.

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PubMed1990

Inheritance of the Reversal of O(2) Response of Photosynthesis in a Flaveria linearis Mutant.

A mutant plant of Flaveria linearis Lag. expresses reversed O(2) response of photosynthesis (i.e. its apparent photosynthesis is stimulated at atmospheric O(2) levels). The objectives of this study were to determine the genetic inheritance of this trait and to investigate the biochemical mechanism for its expression. The mutant plant was crossed reciprocally with a plant of the closely related species Flaveria oppositifolia (DC.) Rydb. and also with another plant of F. linearis. Data on O(2) inhibition of apparent photosynthesis were analyzed on F(2) and F(3) progeny from these F(1) hybrids. In addition, test crosses (mutant x F(1) hybrid) and S(1) progeny from the mutant plant were also analyzed. All F(1) hybrids expressed inhibition of apparent photosynthesis and their progeny segregated in acceptable 3:1 and 13:3 (normal:reversed) ratios. There was little effect of environment on expression of the reversed O(2) response. Selected F(2) plants and the original mutant plant produced progeny in normal:reversed ratios which indicated the trait is controlled by two major genes which show dominant and recessive epistasis. Plants with greater than 20 nanomoles per gram fresh weight per minute of fructose-1, 6-bisphosphatase activity in the cytosol had normal O(2) response of photosynthesis. However, when plants had less than 20 nanomoles per gram fresh weight per minute of this enzyme activity in the cytosol, the O(2) was normal in some and reversed in others. It is proposed that low fructose bisphosphatase activity in the cytosol is controlled by a recessive gene (fbp). A second dominant gene is speculated to be hypostatic to the normal fructose bisphosphatase gene and controls the expression of an unknown factor that determines whether O(2) response of AP is reversed in the presence of fbp (i.e. when fructose bisphosphatase activity is low).

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PubMed1991

Biochemical Correlates of the Circadian Rhythm in Photosynthesis in Phaseolus vulgaris.

A circadian rhythm in photosynthesis occurs in Phaseolus vulgaris after transfer from a natural or artificial light:dark cycle to constant light. The rhythm in photosynthesis persists even when intercellular CO(2) partial pressure is held constant, demonstrating that the rhythm in photosynthesis is not entirely due to stomatal control over the diffusion of CO(2). Experiments were conducted to attempt to elucidate biochemical correlates with the circadian rhythm in photosynthesis. Plants were entrained to a 12-hour-day:12-hour-night light regimen and then monitored or sampled during a subsequent period of constant light. We observed circadian oscillations in ribulose-1,5-bisphosphate (RuBP) levels, and to a lesser extent in phosphoglyceric acid (PGA) levels, that closely paralleled oscillations in photosynthesis. However, the enzyme activity and activation state of the enzyme responsible for the conversion of RuBP to PGA, ribulose-1,5-bisphosphate carboxylase/oxygenase, showed no discernible circadian oscillation. Hence, we examined the possibility of circadian effects on RuBP regeneration. Neither ribulose-5-phosphate kinase activity nor the level of ATP fluctuated in constant light. Oscillations in triose-phosphate levels were out of phase with those observed for RuBP and PGA.

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PubMed1991

Assessing the degree of c(4) photosynthesis in c(3)-c(4) species using an inhibitor of phosphoenolpyruvate carboxylase.

An analog of phosphoenolpyruvate, 3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate (DCDP), was used to inhibit phosphoenolpyruvate carboxylase and, therefore, assess the contribution of the C(4) pathway to photosynthesis in detached leaves of several C(3)-C(4) intermediate species. There was no effect of 4 millimolar DCDP on apparent photosynthesis or on inhibition of apparent photosynthesis by 210 milliliters per liter of O(2) for the C(3)-C(4) species Panicum milioides, Moricandia arvensis, and Neurachne minor or the C(3) species Flaveria pringlei. However, when leaves of Flaveria linearis (C(3)-C(4)), Flaveria brownii (C(4)-like), and Flaveria trinervia (C(4)) were fed 4 millimolar DCDP, photosynthesis was reduced 32, 60, and 90%, respectively. Photosynthetic inhibition by 210 milliliter per liter of O(2) was also significantly increased in 4 millimolar DCDP-fed leaves of F. linearis and F. brownii but not in F. trinervia when compared with control values. These results with DCDP clearly demarcate C(3)-C(4) species into species including Panicum, Moricandia, and Neurachne whose reduced photorespiration occurs without any C(4) photosynthetic involvement and species of Flaveria in which C(4) photosynthesis contributes to CO(2) assimilation.

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PubMed1992

Regulation of Photosynthesis in Triazine-Resistant and -Susceptible Brassica napus.

The response of photosynthetic carbon assimilation and chlorophyll fluorescence quenching to changes in intercellular CO(2) partial pressure (C(i)), O(2) partial pressure, and leaf temperature (15-35 degrees C) in triazine-resistant and -susceptible biotypes of Brassica napus were examined to determine the effects of the changes in the resistant biotype on the overall process of photosynthesis in intact leaves. Three categories of photosynthetic regulation were observed. The first category of photosynthetic response, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)-limited photosynthesis, was observed at 15, 25, and 35 degrees C leaf temperatures with low C(i). When the carbon assimilation rate was Rubisco-limited, there was little difference between the resistant and susceptible biotypes, and Rubisco activity parameters were similar between the two biotypes. A second category, called feedback-limited photosynthesis, was evident at 15 and 25 degrees C above 300 microbars C(i). The third category, photosynthetic electron transport-limited photosynthesis, was evident at 25 and 35 degrees C at moderate to high CO(2). At low temperature, when the response curves of carbon assimilation to C(i) indicated little or no electron transport limitation, the carbon assimilation rate was similar in the resistant and susceptible biotypes. With increasing temperature, more electron transport-limited carbon assimilation was observed, and a greater difference between resistant and susceptible biotypes was observed. These observations reveal the increasing importance of photosynthetic electron transport in controlling the overall rate of photosynthesis in the resistant biotype as temperature increases. Photochemical quenching of chlorophyll fluorescence (q(P)) in the resistant biotype never exceeded 60%, and triazine resistance effects were more evident when the susceptible biotype had greater than 60% q(P), but not when it had less than 60% q(P).

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PubMed2004

Genetic modification of photosynthesis with E. coli genes for trehalose synthesis.

Improvement in photosynthesis per unit leaf area has been difficult to alter by breeding or genetic modification. We report large changes in photosynthesis in Nicotiana tabacum transformed with E. coli genes for the trehalose pathway. Significantly, photosynthetic capacity (CO2 assimilation at varying light and CO2, and quantum yield of PSII electron transport) per unit leaf area and per leaf dry weight were increased in lines of N. tabacum transformed with the E. coli gene otsA, which encodes trehalose phosphate synthase. In contrast, transformation with otsB, which encodes trehalose phosphate phosphatase or Trec, encoding trehalose phosphate hydrolase, produced the opposite effect. Changes in CO2 assimilation per unit leaf area were closely related to the amount and activity of Rubisco, but not to the maximum activities of other Calvin cycle enzymes. Alterations in photosynthesis were associated with trehalose 6-phosphate content rather than trehalose. When growth parameters were determined, a greater photosynthetic capacity did not translate into greater relative growth rate or biomass. This was because photosynthetic capacity was negatively related to leaf area and leaf area ratio. In contrast, relative growth rate and biomass were positively related to leaf area. These results demonstrate a novel means of modifying Rubisco content and photosynthesis, and the complexities of regulation of photosynthesis at the whole plant level, with potential benefits to biomass production through improved leaf area.

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