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PubMed1985

Photosynthesis involvement in the mechanism of action of diphenyl ether herbicides.

Photosynthesis is not required for the toxicity of diphenyl ether herbicides, nor are chloroplast thylakoids the primary site of diphenyl ether herbicide activity. Isolated spinach (Spinacia oleracea L.) chloroplast fragments produced malonyl dialdehyde, indicating lipid peroxidation, when paraquat (1,1'-dimethyl-4,4'-bipyridinium ion) or diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] were added to the medium, but no malonyl dialdehyde was produced when chloroplast fragments were treated with the methyl ester of acifluorfen (methyl 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid), oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene], or MC15608 (methyl 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-chlorobenzoate). In most cases the toxicity of acifluorfen-methyl, oxyfluorfen, or MC15608 to the unicellular green alga Chlamydomonas eugametos (Moewus) did not decrease after simultaneous treatment with diuron. However, diuron significantly reduced cell death after paraquat treatment at all but the highest paraquat concentration tested (0.1 millimolar). These data indicate electron transport of photosynthesis is not serving the same function for diphenyl ether herbicides as for paraquat. Additional evidence for differential action of paraquat was obtained from the superoxide scavenger copper penicillamine (copper complex of 2-amino-3-mercapto-3-methylbutanoic acid). Copper penicillamine eliminated paraquat toxicity in cucumber (Cucumis sativus L.) cotyledons but did not reduce diphenyl ether herbicide toxicity.

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PubMed1985

Oxygen Uptake and Photosynthesis of the Red Macroalga, Chondrus crispus, in Seawater: Effects of Oxygen Concentration.

With an experimental system developed for aquatic plants using the mass spectrometry technique and infrared gas analysis of CO(2), we studied the responses to various O(2) concentrations of gas exchanges with the red macroalga Chondrus crispus S. The results were as follows. (a) Irrespective of the CO(2) concentration, net photosynthesis was O(2) sensitive with a 45 to 70% stimulation at 2% O(2). Even with high CO(2), a significant Warburg effect was detected. (b) Although photosynthesis was CO(2) sensitive, O(2) photoconsumption was only weakly affected by CO(2) even at high CO(2) where it was still photodependent. (c) O(2) photoconsumption was always sensitive to O(2) concentration whatever the CO(2) concentration, but with O(2) exceeding 20% the kinetics disagreed with the Michaelis-Menten model, with saturation being reached more rapidly. With various CO(2) concentrations, the apparent K(m) (O(2)) ranged from 4 to 16% O(2) with a relatively constant V(max) (O(2)) of about one-third the V(max) (CO(2)). (d) Dark respiration seemed to be O(2) insensitive. These results are discussed in relation to the nature of the processes able to consume O(2) in the light, and seem to be consistent with a significant involvement of a Mehlertype reaction.

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PubMed1985

Continuous Measurements of the Free Dissolved CO(2) Concentration during Photosynthesis of Marine Plants: Evidence for HCO(3) Use in Chondrus crispus.

An experimental system consisting of a gas exchange column linked to an assimilation chamber has been developed to record continuously the free dissolved CO(2) concentration in seawater containing marine plants. From experiments performed on the red macroalga Chondrus crispus (Rhodophyta, Gigartinales), this measurement is in agreement with the free CO(2) concentration calculated from the resistance to CO(2) exchanges in a biphasic system (gas and liquid) as earlier reported. The response time of this apparatus is short enough to detect, in conditions of constant pH, a photosynthesis-caused gradient between free CO(2) and HCO(3) (-) pools which half-equilibrates in 25 seconds. Abolished by carbonic anhydrase, the magnitude of this gradient increases with decreasing time of seawater transit from the chamber to the column apparatus. But its maximum magnitude (0.35 micromolar CO(2)) is negligible compared to the difference between air and free CO(2) (11.4 micromolar CO(2)). This illustrates the extent of the physical limiting-step occurring at the air-water interface when inorganic carbon consumption in seawater is balanced by dissolving gaseous CO(2). The direction of this small free CO(2)/HCO(3) (-) gradient indicates that HCO(3) (-) is consumed during photosynthesis.

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PubMed1985

Alterations in Growth, Photosynthesis, and Respiration in a Starchless Mutant of Arabidopsis thaliana (L.) Deficient in Chloroplast Phosphoglucomutase Activity.

A mutant of Arabidopsis thaliana (L.) Heynh. which lacks leaf starch was isolated by screening for plants which did not stain with iodine. The starchless phenotype, confirmed by quantitative enzymic analysis, is caused by a single recessive nuclear mutation which results in a deficiency of the chloroplast isozyme of phosphoglucomutase. When grown in a 12-h photoperiod, leaves of the wild-type accumulated substantial amounts of starch but lower levels of soluble sugars. Under these conditions, the mutant accumulated relatively high levels of soluble sugars. Rates of growth and net photosynthesis of the mutant and wild-type were indistinguishable when the plants were grown in constant illumination. However, in a short photoperiod, the growth of the mutant was severely impaired, the rate of photosynthesis was depressed relative to the wild-type, and the rate of dark respiration, which was high following the onset of darkness, exhibited an uncharacteristic decay throughout the dark period. The altered control of respiration by the mutant, which may be related to the relatively high levels of soluble carbohydrate that accumulate in the leaf and stem tissue, is believed to be partially responsible for the low growth rate of the mutant in short days. The depressed photosynthetic capacity of the mutant may also reflect a metabolic adaptation to the accumulation of high levels of soluble carbohydrate which mimics the effects of alterations in source/sink ratio. The activities of sucrose phosphate synthase and acid invertase are significantly higher in the mutant than in the wild-type whereas ADP-glucose pyrophosphorylase activity is lower. This suggests that the activities of these enzymes may be modulated in response to metabolite concentrations or flux through the pathways.

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PubMed1985

Influence of shoot structure on light interception and photosynthesis in conifers.

The influence of shoot structure on net photosynthesis was evaluated under field conditions for the central Rocky Mountain (United States) conifers Picea engelmannii (Parry ex Engelm.), Abies lasiocarpa ([Hook] Nutt.), and Pinus contorta (Engelm.). In all species, the greater number of needles per unit stem length on sun shoots correlated with a smaller silhouette leaf area to total leaf area ratio (STAR). Decreased STAR was due primarily to greater needle inclination toward the vertical, plus some needle mutual shading. However, photosynthesis expressed on a total leaf area basis did not decrease in sun shoots (lower STAR) but remained nearly constant at approximately 3 micromoles per square meter per second over a wide range of STAR (0.1 to 0.3). Relatively low light saturation levels of 200 to 1400 microeinsteins per square meter per second and diffuse light to 350 microeinsteins per meter per second maintained photosynthetic flux densities in inclined and/or shaded needles at levels comparable to those in unshaded needles oriented perpendicular to the solar beam. As a result, net CO(2) uptake per unit stem length increased as much as 2-fold in sun shoots (low STAR) in direct proportion to increasing needle density.

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PubMed1985

An investigation into the role of photosynthesis in regulating ATP levels and rates of h efflux in isolated meosphyll cells.

Aerated and stirred 10-ml suspensions of mechanically isolated Asparagus sprengeri Regel mesophyll cells were used for simultaneous measurements of net H(+) efflux and steady-state ATP levels.Initial rates of medium acidification indicated values for H(+) efflux in the light and dark of 0.66 and 0.77 nanomoles H(+)/10(6) cells per minute, respectively. When the medium pH was maintained at 6.5, with a pH-stat apparatus, rates of H(+) efflux remained constant. Darkness or DCMU, however, stimulated H(+) efflux by 100% or more. Darkness increased ATP levels by 33% and a switch from dark to light reduced ATP levels by 31%. In the absence of aeration, illumination prevented the accumulation of respiratory CO(2) and the buffering capacity of the medium was about 50% less than that found in the nonilluminated nonaerated medium. As a result, rates of pH decline were similar even though the dark rate of H(+) efflux was approximately 50% greater.Proposals that photosynthesis stimulates H(+) efflux are based on changes in the rate of pH decline. The present data indicate that photosynthesis inhibits H(+) efflux and that changes in rates of pH decline should not be equated with changes in the rate of H(+) efflux.

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PubMed1986

Photosynthesis and Growth of Water Hyacinth under CO(2) Enrichment.

Water hyacinth (Eichhornia crassipes [Mart.] Solms) plants were grown in environmental chambers at ambient and enriched CO(2) levels (330 and 600 microliters CO(2) per liter). Daughter plants (ramets) produced in the enriched CO(2) gained 39% greater dry weight than those at ambient CO(2), but the original mother plants did not. The CO(2) enrichment increased the number of leaves per ramet and leaf area index, but did not significantly increase leaf size or the number of ramets formed. Flower production was increased 147%. The elevated CO(2) increased the net photosynthetic rate of the mother plants by 40%, but this was not maintained as the plants acclimated to the higher CO(2) level. After 14 days at the elevated CO(2), leaf resistance increased and transpiration decreased, especially from the adaxial leaf surface. After 4 weeks in elevated as compared to ambient CO(2), ribulose bisphosphate carboxylase activity was 40% less, soluble protein content 49% less, and chlorophyll content 26% less; whereas starch content was 40% greater. Although at a given CO(2) level the enriched CO(2) plants had only half the net photosynthetic rate of their counterparts grown at ambient CO(2), they showed similar internal CO(2) concentrations. This suggested that the decreased supply of CO(2) to the mesophyll, as a result of the increased stomatal resistance, was counterbalanced by a decreased utilization of CO(2). Photorespiration and dark respiration were lower, such that the CO(2) compensation point was not altered. The photosynthetic light and CO(2) saturation points were not greatly changed, nor was the O(2) inhibition of photosynthesis (measured at 330 microliters CO(2) per liter). It appears that with CO(2) enrichment the temporary increase in net photosynthesis produced larger ramets. After acclimation, the greater total ramet leaf area more than compensated for the lower net photosynthetic rate on a unit leaf area basis, and resulted in a sustained improvement in dry weight gain.

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PubMed1986

Contribution of Metabolites of Photosynthesis to Postillumination CO(2) Assimilation in Response to Lightflects.

In the shade plant Alocasia macrorrhiza grown in low light, photosynthetic CO(2) assimilation during a 5 second lightfleck plus postillumination CO(2) assimilation can allow up to 60% more photosynthesis than that which occurs during 5 seconds of steady state light of the same intensity (RL Chazdon, RW Pearcy 1986 Oecologia. 69: 524-531). Metabolites of photosynthesis were measured to determine if the pool of ribulose 1,5-bisphosphate (RuBP) could account for all of the postillumination CO(2) assimilation following a lightfleck in Alocasia. It was found that the pool of triose-P was much larger than that of RuBP and could account for five times more postillumination CO(2) assimilation than could RuBP. The same trend was seen in the sun plant Phaseolus vulgaris when it was grown in the shade. In contrast, sun-grown Alocasia and Phasiolus did not have a large pool of triose-P relative to RuBP following a lightfleck. In sun plants, carbon may rapidly be converted to RuBP in the light whereas in shade plants there may be a restriction in the path between the triose-P and RuBP pools. It is hypothesized that in shade plants the buildup of triose-P rather than RuBP during the lightfleck prevents inhibition of electron transport which may otherwise occur because of competition for ATP between the two kinases of the photosynthetic carbon reduction cycle. Utilization of the triose-P for postillumination CO(2) fixation would require the capacity for significant postillumination ATP synthesis. The extensive grana stacking and large intrathylakoid space which accompanies the high level of chlorophyll in low-light-grown Alocasia could be an important contributing factor to postillumination ATP formation.

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PubMed1987

Photosynthesis and Activity of Ribulose Bisphosphate Carboxylase of Wheat and Maize Seedlings during and following Exposure to O(2)-Low, CO(2)-Free N(2).

Seven day old wheat and maize seedlings were exposed to 1300 or 2000 microeinsteins per square meter per second photosynthetically active radiation in CO(2)-free air for 3 hours with either 1% O(2) in N(2) or N(2)-only and then returned to normal air of 340 microliters per liter CO(2), 21% O(2) in N(2). Activity of the ribulose bisphosphate carboxylase and amount of the substrate, ribulose 1,5-bisphosphate, were measured during and following the CO(2)-free treatments as was photosynthetic CO(2) fixation. Photoinhibition of photosynthesis was observed only with wheat seedlings following the N(2) only treatment. During the CO(2)-free treatments, the levels of RuBP rose during all experiments except when wheat was photoinhibited. The activity of the ribulose bisphophate carboxylase, measured directly upon grinding the leaves, declined during the CO(2)-free conditions. The carboxylase total activity increased in minutes in the leaf during and following the CO(2)-free treatments. The specific activities of the wheat carboxylase went from 0.16 to 1.06 micromoles CO(2) fixed per milligram protein per minute while the maize carboxylase varied from 0.05 to 0.36 micromole CO(2) fixed per millogram protein per minute. This suggests that in these seedlings considerable inactive carboxylase must be stored in a form not activatable in extracts by CO(2) and Mg(2+). Possible mechanisms of regulation of photosynthesis by the ribulose bisphosphate carboxylase must consider not only the amount of active enzyme, but the amount of enzyme which the plant can make activatable upon demand.

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PubMed1987

Photosynthesis of Euglena gracilis under Cobalamin-Sufficient and -Limited Growing Conditions.

Cobalamin is essential for growth of Euglena gracilis and photosynthesis. Methylcobalamin in Euglena chloroplasts (Y Isegawa, Y Nakano, S Kitaoka, 1984 Plant Physiol 76: 814-818) functions as a coenzyme of methionine synthetase. The requirement of cobalamin for photosynthesis appeared remarkably high in Euglena grown under the dark-precultured condition. The required amount of cobalamin for normal photosynthetic activity was 7.4 x 10(-11) molar, while 7.4 x 10(-10) molar cobalamin was required for normal growth. The lowered photosynthetic activity in cobalamin-limited cells was restored 20 hours after feeding cyanocobalamin or methionine to cobalamin-limited cells. Lowering of photosynthetic activity was due to loss of photosystem I activity. This photosynthetic activity was recovered after supplementation by methionine or cobalamin. The results suggest that methionine serves for the stabilization of photosystem I. This paper is the first report of the physiological function of cobalamin in chloroplasts of photosynthetic eukaryotes.

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PubMed1987

Effects of CO(2) and O(2) on Photosynthesis and Growth of Autotrophic Tobacco Callus.

Mean inhibition of net photosynthesis in autotrophic tobacco callus by 21 and 40% O(2) was 30 and 47%, respectively, similar to intact leaves. Increasing CO(2) concentrations (500-2000 microliters per liter) produced a steady decline in percent inhibition at both O(2) concentrations, indicating that O(2) inhibition resulted primarily from photorespiration. Net photosynthetic rate was plotted as a function of CO(2) concentration at 1, 21, and 40% O(2) for calculation of kinetic constants. Values for V(max) were similar at all O(2) concentrations (x = 5.27 mumol CO(2) per gram fresh weight per hour), indicating that O(2) inhibition of net photosynthesis was fully reversible by CO(2). To determine whether CO(2) and O(2) produced similar effects on growth, autotrophic callus was incubated for three weeks in atmospheres of normal air, high CO(2), high O(2) and high CO(2)/high O(2). Growth in high CO(2) was nearly double that in normal air. High O(2) decreased growth significantly relative to air, but growth in high CO(2)/high O(2) was similar to that in air. Lack of CO(2) reversal of growth inhibition by O(2) indicates that prolonged exposure to high O(2) results in toxicity arising from a nonphotorespiratory source. Growth of heterotrophic callus (2% sucrose), however, was not inhibited by 40 or 60% O(2), suggesting that O(2) toxicity in autotrophic callus results primarily from disruption of photosynthetic functions.

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PubMed1987

Photosynthesis in Flaveria brownii A.M. Powell : A C(4)-Like C(3)-C(4) Intermediate.

Leaves of Flaveria brownii exhibited slightly higher amounts of oxygen inhibition of photosynthesis than the C(4) species, Flaveria trinervia, but considerably less than the C(3) species, Flaveria cronquistii. The photosynthetic responses to intercellular CO(2), light and leaf temperature were much more C(4)-like than C(3)-like, although 21% oxygen inhibited the photosynthetic rate, depending on conditions, up to 17% of the photosynthesis rate observed in 2% O(2). The quantum yield for CO(2) uptake in F. brownii was slightly higher than that for the C(4) species F. trinervia in 2% O(2), but not significantly different in 21% O(2). The quantum yield was inhibited 10% in the presence of 21% O(2) in F. brownii, yet no significant inhibition was observed in F. trinervia. An inhibition of 27% was observed for the quantum yield of F. cronquistii in the presence of 21% O(2). The photosynthetic response to very low intercellular CO(2) partial pressures exhibited a unique pattern in F. brownii, with a break in the linear slope observed at intercellular CO(2) partial pressure values between 15 and 20 mubar when analyzed in 21% O(2). No significant break was observed when analyzed in 2% O(2). When taken collectively, the gas-exchange results reported here are consistent with previous biochemical studies that report incomplete intercellular compartmentation of the C(3) and C(4) enzymes in this species, and suggest that F. brownii is an advanced, C(4)-like C(3)-C(4) intermediate.

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PubMed1987

Mechanism of c(4) photosynthesis: the size and composition of the inorganic carbon pool in bundle sheath cells.

We sought to characterize the inorganic carbon pool (CO(2) plus HCO(3) (-)) formed in the leaves of C(4) plants when C(4) acids derived from CO(2) assimilation in mesophyll cells are decarboxylated in bundle sheath cells. The size and kinetics of labeling of this pool was determined in six species representative of the three metabolic subgroups of C(4) plants. The kinetics of labeling of the inorganic carbon pool of leaves photosynthesizing under steady state conditions in (14)CO(2) closely paralleled those for the C-4 carboxyl of C(4) acids for all species tested. The inorganic carbon pool size, determined from its (14)C content at radioactivity saturation, ranged between 15 and 97 nanomoles per milligram of leaf chlorophyll, giving estimated concentrations in bundle sheath cells of between 160 and 990 micromolar. The size of the pool decreased, together with photosynthesis, as light was reduced from 900 to 95 microeinsteins per square meter per second or as external CO(2) was reduced from 400 to 98 microliters per liter. A model is developed which suggests that the inorganic carbon pool existing in the bundle sheath cells of C(4) plants during steady state photosynthesis will comprise largely of CO(2); that is, CO(2) will only partially equlibrate with bicarbonate. This predominance of CO(2) is believed to be vital for the proper functioning of the C(4) pathway.

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PubMed1988

Early Inhibition of Photosynthesis during Development of Mn Toxicity in Tobacco.

Early physiological effects of developing Mn toxicity in young leaves of burley tobacco (Nicotiana tabacum L. cv KY 14) were examined in glass-house/water cultured plants grown at high (summer) and low (winter) photon flux. Following transfer of plants to solutions containing 1 millimolar Mn(2+), sequential samplings were made at various times for the following 9 days, during which Mn accumulation by leaves increased rapidly from approximately 70 on day 0 to approximately 1700 and approximately 5000 microgram per gram dry matter after 1 and 9 days, respectively. In plants grown at high photon flux, net photosynthesis declined by approximately 20 and approximately 60% after 1 and 9 days, respectively, and the onset of this decline preceded appearance (after 3 to 4 days) of visible foliar symptoms of Mn toxicity. Intercellular CO(2) concentrations and rates of transpiration were not significantly affected; moreover, the activity of the Hill and photosystem I and II partial reactions of chloroplasts remained constant despite ultimate development of severe necrosis. Though the activity of latent or activated polyphenol oxidase increased in parallel with Mn accumulation, neither leaf respiration nor the activity of catalase [EC 1.11.1.6] and peroxidase [EC 1.10.1.7] were greatly affected. These effects from Mn toxicity could not be explained by any changes in protein or chlorophyll abundance. Additionally, they were not a consequence of Mn induced Fe deficiency. Therefore, inhibition of net photosynthesis and enhancement of polyphenol oxidase activity are early indicators of excess Mn accumulation in tobacco leaves. These changes, as well as leaf visual symptoms of Mn toxicity, were less severe in plants cultured and treated at low photon flux even though the rates of leaf Mn accumulation at high and low photon flux were essentially equivalent.

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PubMed1988

Stimulation of CAM Photosynthesis in Kalanchoë blossfeldiana by Transferring to Nitrogen-Deficient Conditions.

Kalanchoë blossfeldiana Poelln. cv Hikan plants were grown hydroponically with nutrient solution containing 5 millimolar NO(3) (-) (or NH(4) (+)) for 1 to 2 months and then transferred to nutrient solution containing no nitrogen. CO(2) uptake at night, nocturnal increase in titratable acidity, and activity of phosphoenolpyruvate carboxylase increased after the transfer. Thus, transfer to nitrogen-deficient conditions stimulates Crassulacean acid metabolism (CAM photosynthesis) in K. blossfeldiana. The importance of the plant nitrogen status (nitrogen-withdrawal status) for induction and stimulation of CAM photosynthesis is discussed.

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PubMed1988

Photosynthesis in Flaveria brownii, a C(4)-Like Species: Leaf Anatomy, Characteristics of CO(2) Exchange, Compartmentation of Photosynthetic Enzymes, and Metabolism of CO(2).

Light microscopic examination of leaf cross-sections showed that Flaveria brownii A. M. Powell exhibits Kranz anatomy, in which distinct, chloroplast-containing bundle sheath cells are surrounded by two types of mesophyll cells. Smaller mesophyll cells containing many chloroplasts are arranged around the bundle sheath cells. Larger, spongy mesophyll cells, having fewer chloroplasts, are located between the smaller mesophyll cells and the epidermis. F. brownii has very low CO(2) compensation points at different O(2) levels, which is typical of C(4) plants, yet it does show about 4% inhibition of net photosynthesis by 21% O(2) at 30 degrees C. Protoplasts of the three photosynthetic leaf cell types were isolated according to relative differences in their buoyant densities. On a chlorophyll basis, the activities of phosphoenolpyruvate carboxylase and pyruvate, Pi dikinase (carboxylation phase of C(4) pathway) were highest in the larger mesophyll protoplasts, intermediate in the smaller mesophyll protoplasts, and lowest, but still present, in the bundle sheath protoplasts. In contrast, activities of ribulose 1,5-bisphosphate carboxylase, other C(3) cycle enzymes, and NADP-malic enzyme showed a reverse gradation, although there were significant activities of these enzymes in mesophyll cells. As indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the banding pattern of certain polypeptides of the total soluble proteins from the three cell types also supported the distribution pattern obtained by activity assays of these enzymes. Analysis of initial (14)C products in whole leaves and extrapolation of pulse-labeling curves to zero time indicated that about 80% of the CO(2) is fixed into C(4) acids (malate and aspartate), whereas about 20% of the CO(2) directly enters the C(3) cycle. This is consistent with the high activity of enzymes for CO(2) fixation by the C(4) pathway and the substantial activity of enzymes of the C(3) cycle in the mesophyll cells. Therefore, F. brownii appears to have some capacity for C(3) photosynthesis in the mesophyll cells and should be considered a C(4)-like species.

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PubMed1988

Chloroplast osmotic adjustment and water stress effects on photosynthesis.

Previous studies have suggested that chloroplast stromal volume reduction may mediate the inhibition of photosynthesis under water stress. In this study, the effects of spinach (Spinacia oleracea, var ;Winter Bloomsdale') plant water deficits on chloroplast photosynthetic capacity, solute concentrations in chloroplasts, and chloroplast volume were studied. In situ (gas exchange) and in vitro measurements indicated that chloroplast photosynthetic capacity was maintained during initial leaf water potential (Psi(w)) and relative water content (RWC) decline. During the latter part of the stress period, photosynthesis dropped precipitously. Chloroplast stromal volume apparently remained constant during the initial period of decline in RWC, but as leaf Psi(w) reached -1.2 megapascals, stromal volume began to decline. The apparent maintenance of stromal volume over the initial RWC decline during a stress cycle suggested that chloroplasts are capable of osmotic adjustment in response to leaf water deficits. This hypothesis was confirmed by measuring chloroplast solute levels, which increased during stress. The results of these experiments suggest that stromal volume reduction in situ may be associated with loss of photosynthetic capacity and that one mechanism of photosynthetic acclimation to low Psi(w) may involve stromal volume maintenance.

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PubMed1989

Leaf phosphate status, photosynthesis, and carbon partitioning in sugar beet: I. Changes in growth, gas exchange, and calvin cycle enzymes.

Sugar beets (Beta vulgaris L. cv F58-554H1) were cultured hydroponically for 2 weeks in growth chambers with two levels of orthophosphate (Pi) supplied in half strength Hoagland solution. Low-P plants were supplied with 1/20th of the Pi supplied to control plants. With low-P treatment, the acid soluble leaf phosphate and total leaf P decreased by about 88%. Low-P treatment had a much greater effect on leaf area than on photosynthesis. Low-P decreased total leaf area by 76%, dry weight per plant by 60%, and the rate of photosynthesis per area at light saturation by 35%. Low-P treatment significantly decreased the total extractable activity of phosphoglycerate kinase (by 18%) and NADP-glyceraldehyde-3-phosphate dehydrogenase (by 16%), but did not decrease the total activities of ribulose-1,5-bisphosphate (RuBP) carboxylase (RuBPCase) and ribulose-5-phosphate kinase. Low-P treatment decreased the initial activities of three rate-limiting Calvin cycle enzymes, but had no effect on the initial activity of RuBPCase. Furthermore, low-P treatment significantly increased the total extractable activities of fructose-1,6-bisphosphatase (by 61%), fructose-1,6-bisphosphate aldolase (by 53%), and transketolase (by 46%). The results suggest that low-P treatment affected photosynthetic rate through an effect on RuBP regeneration rather than through RuBPCase activity and that the changes in Calvin cycle enzymes with low-P resulted in an increased flow of carbon to starch.

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