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1.
Sugar Beets (Beta vulgaris L. cv F58-554H1) were cultured hydroponically in growth chambers. Leaf orthophosphate (Pi) levels were varied nutritionally. The effect of decreased leaf phosphate (low-P) status was determined on the diurnal changes in the pool sizes of leaf ribulose 1,5-bisphosphate (RuBP), 3-phosphoglycerate (PGA), triose phosphate, fructose 1,6-bisphosphate, fructose-6-phosphate, glucose-6-phosphate, adenylates, nicotinamide nucleotides, and Pi. Except for triose phosphate, low-P treatment caused a marked reduction in the levels of leaf sugar phosphates (on a leaf area basis) throughout the diurnal cycle. Low-P treatment decreased the average leaf RuBP levels by 60 to 69% of control values during the light period. Low-P increased NADPH levels and NADPH/NADP+ ratio but decreased ATP; the ATP/ADP ratio was unaffected. Low P treatment caused a marked reduction in RuBP regeneration (RuBP levels were half the RuBP carboxylase binding site concentration) but did not depress PGA reduction to triose phosphate. These results indicate that photosynthesis in low-P leaves was limited by RuBP regeneration and that RuBP formation in low-P leaves was not limited by the supply of ATP and NADPH. We suggest that RuBP regeneration was limited by the supply of fixed carbon, an increased proportion of which was diverted to starch synthesis.  相似文献   

2.
The regulation of ribulose-1,5-bisphosphate (RuBP) carboxylase (Rubisco) activity and pool sizes of RuBP and P-glycerate were examined in the tropical understory species Alocasia macrorrhiza following step changes in photon flux density (PFD). Previous gas exchange analysis of this species following a step increase in PFD from 10 to 500 micromoles quanta per square meter per second suggested that the increase in photosynthetic rate was limited by the rate of increase of Rubisco activity for the first 5 to 10 minutes. We demonstrate here that the increase in photosynthetic rate was correlated with an increase in both the activation state of Rubisco and the total kcat (fully activated specific activity) of the enzyme. Evidence presented here suggests that a change in the pool size of the naturally occurring tight binding inhibitor of Rubisco activity, 2-carboxyarabinitol 1-phosphate, was responsible for the PFD-dependent change in the total kcat of the enzyme. RuBP pool size transiently increased after the increase in PFD, indicating that photosynthesis was limited by the capacity for carboxylation. After 5 to 10 minutes, RuBP pool size was again similar to the pool size at low PFD, presumably because of the increased activity of Rubisco. Following a step decrease in PFD from 500 to 10 micromoles quanta per square meter per second, Rubisco activity declined but at a much slower rate than it had increased in response to a step increase in PFD. This slower rate of activity decline than increase was apparently due to the slower rate of 2-carboxyarabinitol 1-phosphate synthesis than degradation and, to a lesser degree, to slower deactivation than activation. RuBP pool size initially declined following the decrease in PFD, indicating that RuBP regeneration was limiting photosynthesis. As Rubisco activity decreased, RuBP slowly increased to its original level at high PFD. The slow rate of activity loss by Rubisco in this species suggests a biochemical basis for the increased efficiency for CO2 assimilation of successive lightfleck use by species such as A. macrorrhiza.  相似文献   

3.
Cardon ZG  Mott KA 《Plant physiology》1989,89(4):1253-1257
The binding of ribulose 1,5-bisphosphate (RuBP) to inactive (noncarbamylated) sites of the enzyme RuBP carboxylase in vivo was investigated in Spinacia oleracea and Helianthus annuus. The concentrations of RuBP and inactive sites were determined in leaf tissue as a function of time after a change to darkness. RuBP concentrations fell rapidly after the change to darkness and were approximately equal to the concentration of inactive sites after 60 s. Variations in the concentration of inactive sites, which were induced by differences in the light intensity before the light-dark transition, correlated with the concentration of RuBP between 60 and 120 s after the change to darkness. These data are discussed as evidence that RuBP binds to inactive sites of RuBP carboxylase in vivo. After the concentration of RuBP fell below that of inactive sites (at times longer than 60 s of darkness), the decline in RuBP was logarithmic with time. This would be expected if the dissociation of RuBP from inactive sites controlled the decline in RuBP concentration. These data were used to estimate the rate constant for dissociation of RuBP from inactive sites in vivo.  相似文献   

4.
水稻生育过程中,RuBP羧化酶活性与光合速率、RuBP加氧酶活性与光呼吸速率、RuBP羧化酶活性与加氢酶活性以及光合速率与光呼吸速率之间是相关的。籼型品种与粳型品种间酶活性的高低及光合、光呼吸速率的高低基本一致,籼型三系杂交稻(F1)无明显的光合优势。酶的羧化活性的高低只在一定范围内与光合速率的高低平行。在正常生育条件下,酶蛋白的数量不是水稻光合速率的限制因子。  相似文献   

5.
The rate of photosynthesis under high light (1000 micromole quanta per square meter per second) and at 25°C was measured during development of the third leaf on wheat plants and compared with the activity of several photosynthetic enzymes and the level of metabolites. The rate of photosynthesis reached a maximum the 7th day after leaf emergence and declined thereafter. There was a high and significant correlation between the rate of photosynthesis per leaf area and the activities of the enzymes ribulose 5-phosphate kinase (r = 0.91), ribulose 1,5-bisphosphate (RuBP) carboxylase (r = 0.94), 3-phosphoglycerate (PGA) kinase (r = 0.82), and fructose 1,6-bisphosphatase (r = 0.80) per leaf area. There was not a significant correlation of photosynthesis rate with chlorophyll content. The rate of photosynthesis was strongly correlated with the level of PGA (r = 0.85) and inversely correlated with the level of triose phosphate (dihydroxyacetone phosphate and glyceraldehyde 3-phosphate) (r = 0.92). RuBP levels did not change much during leaf development; therefore photosynthesis rate was not correlated with the level of RuBP. The rate of photosynthesis was at a maximum when the ratio of PGA/triose phosphate was high, and when the ratio of RuBP/PGA was low. Although several enzymes change in parallel with leaf development, the metabolite changes suggest the greatest degree of control may be through RuBP carboxylase. The sucrose content of the leaf was highest under high rates of photosynthesis. There was no evidence that later in leaf development, photosynthesis (measured under high light and at 25°C) was limited by utilization of photosynthate.  相似文献   

6.
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.  相似文献   

7.
John Kobza  Gerald E. Edwards 《Planta》1987,171(4):549-559
The photosynthetic induction response was studied in whole leaves of wheat (Triticum aestivum L.) following 5-min, 30-min and 10-h dark periods. After the 5-min dark treatment there was a rapid burst in the rate of photosynthesis upon illumination (half of maximum after 30s), followed by a slight decrease after 1.5 more min and then a gradual rise to the maximum rate. During this initial burst in photosynthesis, there was a rapid rise in the level of 3-phosphoglycerate (PGA) and a high PGA/triose-phosphate (triose-P) ratio was obtained. In addition, after the 5-min dark treatment, ribulose-1,5-bisphosphate carboxylase (Rubisco, EC 4.1.1.39), ribulose-5-phosphate kinase (EC 2.7.1.19) and chloroplastic fructose-1,6-bisphosphatase (EC 3.1.3.11) maintained a relatively high state of activation, and maximum activation occurred within 1 min of illumination. The results indicate there is a high capacity for CO2 fixation in the cycle upon illumination but attaining maximum rates requires an increase in the ribulose-1,5-bisphosphate (RuBP) pool (adjustment in triose-P utilization for carbohydrate synthesis versus RuBP synthesis). With both the 30-min and 10-h dark pretreatments there was only a slight rise in photosynthesis upon illumination, followed by a lag, then a gradual increase to steady-state (half-maximum rate after 6 min). In contrast to the 5-min dark treatment, the level of PGA was low and actually decreased initially, whereas the level of RuBP increased and was high during induction, indicating that Rubisco is limiting. This regulation via the carboxylase was not reflected in the initial extractable activity, which reached a maximum by 1 min after illumination. The light activation of chloroplastic fructose-1,6-bisphosphatase in leaves darkened for 30 min and 10 h prior to illumination was relatively slow (reaching a maximum after 8 min). However, this was not considered to limit carbon flux through the carbon-fixation cycle during induction since RuBP was not limiting. When photosynthesis approached the maximum steady-state rate, a high PGA/triose-P ratio and a high PGA/RuBP ratio were obtained. This may allow a high rate of photosynthesis by producing a favorable mass-action ratio for the reductive phase (the conversion of PGA to triose phosphate) while stimulating starch and sucrose synthesis.Abbreviations Chl chlorophyll - FBP fructose-1,6-bisphosphate - FBPase fructose-1,6-bisphosphatase - Fru6P fructose-6-phosphate - Glc6P glucose-6-phosphate - PGA 3-phosphoglycerate - Pi inoganic phosphate - Rubisco RuBP carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - Ru5P ribulose-5-phosphate - triose-P triose phosphates (dihydroxyacetone phosphate+glyceraldehyde-3-phosphate)  相似文献   

8.
Changes of some photosynthetic properties of high-CO2 grown cells of Chlorella pyrenoidosa during adaptation to low-CO2 conditions have been investigated. The Km value of photosynthesis of the high-CO2 grown cells for dissolved inorganic carbon was 3.3 millimolar and decreased to 25 to 30 micromolar within 4 hours after transferring to air. In the presence of saturating CO2 concentrations the photosynthetic activity of the high-CO2 grown cells was 1.5 times as high as that of the low-CO2 grown cells. There was a significant rise of the photosynthetic activity during adaptation of the high-CO2 grown cells to air, followed by a steady decrease. The activity of ribulose 1,5-bisphosphate carboxylase/oxygenase in both the high- and low-CO2 grown cells was close to the photosynthetic activity of the cells. The concentration of ribulose 1,5-bisphosphate (RuBP) was higher in the low-CO2 adapting and low-CO2 grown cells than in the high-CO2 grown cells regardless of the photosynthetic rate. This seems to be due to an increased RuBP regeneration activity during adaptation followed by maintenance of the new higher concentration. The RuBP level always exceeded the concentration of ribulose 1,5-bisphosphate carboxylase/oxygenase RuBP binding sites in both the high- and low-CO2 grown cells at any dissolved inorganic carbon concentration.  相似文献   

9.
Kinetic parameters of ribulos-1,5-bisphosphate carboxylase/oxygenase (RuBP carboxylase) are usually evaluated in dilute solutions (less than 0.1 mg ml-1). Yet, this enzyme occurs in vivo at 100-200 mg ml-1 and a total protein concentration 300-400 mg ml-1. Enzymes can change their catalytic properties upon 'crowding'. Hence it became of interest to determine whether RuBP carboxylase elicits any properties not observable in dilute solution. Pre-steady state progress curves of fully activated enzyme showed an initial burst followed by a slower rate of product formation. The extent of the burst increased as concentration ratios of RuBP and RuBP carboxylase decreased. The burst corresponds to 1/8 turnover per holoenzyme or 1 turnover per active site. No discontinuity in progress curves was observed with partially activated enzyme.  相似文献   

10.
Rao IM  Terry N 《Plant physiology》1989,90(3):814-819
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.  相似文献   

11.
The effects of ammonium assimilation on photosynthetic carbon fixation and O2 exchange were examined in two species of N-limited green algae, Chlorella pyrenoidosa and Selenastrum minutum. Under light-saturating conditions, ammonium assimilation resulted in a suppression of photosynthetic carbon fixation by S. minutum but not by C. pyrenoidosa. These different responses are due to different relationships between cellular ribulose bisphosphate (RuBP) concentration and the RuBP binding site density of ribulose bisphosphate carboxylase/oxygenase (Rubisco). In both species, ammonium assimilation resulted in a decrease in RuBP concentration. In S. minutum the concentration fell below the RuBP binding site density of Rubisco, indicating RuBP limitation of carboxylation. In contrast, RuBP concentration remained above the binding site density in C. pyrenoidosa. Compromising RuBP regeneration in C. pyrenoidosa with low light resulted in an ammonium-induced decrease in RuBP concentration below the RuBP binding site density of Rubisco. This resulted in a decrease in photosynthetic carbon fixation. In both species, ammonium assimilation resulted in a larger decrease in net O2 evolution than in carbon fixation. Mass spectrometric analysis shows this to be a result of an increase in the rate of mitochondrial respiration in the light.  相似文献   

12.
Rates of photosynthesis, sucrose synthesis, starch accumulation and degradation were measured in sugar beet (Beta vulgaris L.) and bean (Phaseolus vulgaris L.) plants under a square-wave light regime and under a sinusoidal regime that simulated the natural daylight period. Photosynthesis rate increased in a measured manner in direct proportion to the increasing light level. In contrast to this close correspondence between photosynthesis and light, a lag in photosynthesis rate was seen during the initial hour under square-wave illumination. The leaf appeared to be responding to limits set by carbon metabolism rather than by gas exchange or light reactions. Under the sinusoidal regime starch degradation occurred during the first and last 2 hours of the photoperiod, likely in response to photosynthesis rate rather than directly to light level. Starch broke down when photosynthesis was below a threshold rate and accumulated above this rate. Under square-wave illumination, accumulation of starch did not begin until irradiance was at full level for an hour or more and photosynthesis was at or near its maximum. Under a sinusoidal light regime, sucrose synthesis rate comprised carbon that was newly fixed throughout the day plus that from starch degradation at the beginning and end of the day. Synthesis of sucrose from recently fixed carbon increased with increasing net carbon fixation rate while its formation from degradation of starch decreased correspondingly. The complementary sources of carbon maintained a relatively steady rate of sucrose synthesis under the changing daytime irradiance.  相似文献   

13.
Restoration of Photosynthesis in Pot-Bound Tobacco Plants   总被引:5,自引:1,他引:4  
Premature senescence is observed in pot-bound tobacco plants(Nicotiana tabacum L. cv. xanthii) and this is shown to be accompaniedby gross accumulation of leaf starch and a marked decline inphotosynthesis, ribulose- 1, 5-bisphosphate (RuBP) carboxylaseactivity, and soluble protein content. Starch content, uptakeof CO2 by leaf discs, and the activity of RuBP carboxylase inwhole leaf extracts were measured daily following transfer ofrestricted plants to larger pots. Starch declined rapidly, whilstphotosynthetic activity was fully restored within a period ofseveral days, and frequently exceeded the maximum rates measuredin non-restricted plants. This overcompensation, which couldbe related to availability of carbohydrates accumulated as starch,is discussed in relation to the well-established horticulturalpractice of ‘potting on’.  相似文献   

14.
The relationship between loss of ribulose-1,5-bisphosphate carboxylase (RuBPCase) and the decline in photosynthesis during the senescence of barley primary leaves was assessed. Loss of RuBPCase accounted for about 85% of the decrease in soluble protein. RuBPCase was highly correlated with in vitro RuBPCase activity (r = 0.95) and gross photosynthesis (r = 0.96). However, the rate of photosynthesis per milligram RuBPCase increased during the early stages of leaf senescence. The concentration of nonreducing sugars was negatively correlated (1% level) with photosynthesis. Free α-amino N, in contrast to nonreducing sugars, declined markedly during senescence. A decrease in chlorophyll and an increase in in vitro protease activity was observed, but these changes did not appear to be closely related to the decline in photosynthesis and RuBPCase. Mesophyll resistance increased at the same rate that photosynthesis and RuBPCase declined. Stomatal resistance increased more rapidly than mesophyll resistance and accounted for about 24% of the total increase in resistance to CO2 diffusion. The concentration of CO2 in the intercellular air spaces decreased during the last stage of senescence. Although loss of RuBPCase probably is the primary event responsible for the decline in photosynthesis during leaf senescence, other factors such as in vivo regulation and stomatal aperture must also be considered.  相似文献   

15.
Young bean plants (Phaseolus vulgaris L. cv Seafarer) grew faster in air enriched with CO2 (1200 microliters per liter) than in ambient CO2 (330 microliters per liter). However, by 7 days when increases in overall growth (dry weight, leaf area) were visible, there was a significant decline (about 25%) in the leaf mineral content (N, P, K, Ca, Mg) and a drop in the activity of two enzymes of carbon fixation, carbonic anhydrase and ribulose 1,5-bisphosphate (RuBP) carboxylase under high CO2. Although the activity of neither enzyme was altered in young, expanding leaves during the acclimation period, in mature leaves the activity of carbonic anhydrase was reduced 95% compared with a decline of 50% in ambient CO2. The drop in RuBP carboxylase was less extreme with 40% of the initial activity retained in the high CO2 compared with 50% in the ambient atmosphere. While CO2 enrichment might alter the flow of carbon into the glycolate pathway by modifying the activities of carbonic anhydrase or RuBP carboxylase, there is no early change in the ability of photosynthetic tissue to oxidize glycolate to CO2.  相似文献   

16.
Activity of ribulose 1,5-bisphosphate (RuBP) carboxylase in leaf extracts of the constitutive Crassulacean acid metabolism (CAM) plant Kalanchoe pinnata (Lam.) Pers. decreased with increasing leaf age, whereas the activity of phosphoenolpyruvate (PEP) carboxylase increased. Changes in enzyme activities were associated with changes in the amount of enzyme proteins as determined by immunochemical analysis, sucrose density gradient centrifugation, and SDS gel electrophoresis of leaf extracts. Young developing leaves of plants which received high amounts of NO 3 - during growth contained about 30% of the total soluble protein in the form of RuBP carboxylase; this value declined to about 17% in mature leaves. The level of PEP carboxylase in young leaves of plants at high NO 3 - was an estimated 1% of the total soluble protein and increased to approximately 10% in mature leaves, which showed maximum capacity for dark CO2 fixation. The growth of plants at low levels of NO 3 - decreased the content of soluble protein per unit leaf area as well as the extractable activity and the percentage contribution of both RUBP carboxylase and PEP carboxylase to total soluble leaf protein. There was no definite change in the ratio of RuBP carboxylase to PEP carboxylase activity with a varying supply of NO 3 - during growth. It has been suggested (e.g., Planta 144, 143–151, 1978) that a rhythmic pattern of synthesis and degradation of PEP carboxylase protein is involved in the regulation of -carboxylation during a day/night cycle in CAM. No such changes in the quantity of PEP carboxylase protein were observed in the leaves of Kalanchoe pinnata (Lam.) Pers. or in the leaves of the inducible CAM plant Mesembryanthemum crystallinum L.Abbreviations CAM Crassulacean acid metabolism - RuBP ribulose 1,5-bisphosphate - PEP phosphoenolpyruvate - G-6-P glucose-6-phosphate  相似文献   

17.
Responses of foliar and isolated intact chloroplast photosynthetic carbon metabolism observed in spinach (Spinacia oleracea cv Wisconsin Bloomsdale) plants exposed to a shortened photosynthetic period (7-hour light/17-hour dark cycle), were used as probes to examine in vivo metabolic factors that exerted rate determination on photosynthesis (PS) and on starch synthesis. Compared with control plants propagated continuously on a 12-hour light/12-hour dark cycle, 14 to 15 days were required, subsequent to a shift from 12 to 7 hours daylength, for 7-hour plants to begin to grow at rates comparable to those of 12-hour daylength plants. Because of shorter daily durations of PS, daily demand for photosynthate by growth processes appeared to be greater in the 7-hour than in the 12-hour plants. The result was that 7-hour plants established a 1.5- to 2.0-fold higher total PS rate than 12-hour plants.

Intact chloroplasts isolated from the leaves of 7-hour plants (7-h PLD) displayed 1.5- to 2.0-fold higher PS rates than plastids isolated from 12-hour plants (12-h PLD). Plastid lamellae prepared from 7- and 12-h PLD isolates displayed equivalent rates of ferredoxin-dependent ATP and NADPH photoformation indicating that electron transport processes were not factors in the establishment of higher 7-h PLD PS rates. Analyses, both in leaves as well as intact PLD isolates, of dark to light transitional increases in Calvin cycle intermediates, e.g., ribulose-1,5-bisphosphate (RuBP) and 3-phosphoglycerate (3-PGA), as well as estimations of activities of RuBP carboxylase and fructose-1,6-bisphosphate phosphatase, indicated that 7-hour plant leaves displayed higher PS rates (than 12-hour plants), because there was a higher magnitude of activity of the Calvin cycle.

Although both the foliar level of starch and sucrose, as well as starch synthesis rate, often was higher in 7-hour compared with 12-hour plant foliage, the higher 7-hour plant total PS rates indicated that maximal sucrose and starch levels did not mediate any `feedback' inhibition of PS. The higher 7-hour plant foliar and PLD PS rates resulted in higher glucose-1-P levels as well as a higher ratio of 3-PGA:Pi, both factors of which would enhance the activity of chloroplast ADP-glucose pyrophosphorylase, and which were attributed to be causal to the higher starch synthesis rates observed in 7-hour plant foliage and PLD isolates.

  相似文献   

18.
The rate of CO2 fixation by ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) following addition of ribulose 1,5-bisphosphate (RuBP) to fully activated enzyme, declined with first-order kinetics, resulting in 50% loss of rubisco activity after 10 to 12 minutes. This in vitro decline in rubisco activity, termed fall-over, was prevented if purified rubisco activase protein and ATP were added, allowing linear rates of CO2 fixation for up to 20 minutes. Rubisco activase could also stimulate rubisco activity if added after fallover had occurred. Gel filtration of the RuBP-rubisco complex to remove unbound RuBP allowed full activation of the enzyme, but the inhibition of activated rubisco during fallover was only partially reversed by gel filtration. Addition of alkaline phosphatase completely restored rubisco activity following fallover. The results suggest that fallover is not caused by binding of RuBP to decarbamylated enzyme, but results from binding of a phosphorylated inhibitor to the active site of rubisco. The inhibitor may be a contaminant in preparations of RuBP or may be formed on the active site but is apparently removed from the enzyme in the presence of the rubisco activase protein.  相似文献   

19.
Heat stress in leaves under natural conditions is characterized by rapid fluctuations in temperature. These fluctuations can be on the order of 10 degrees C in 7 s. By using a specially modified gas-exchange chamber, these conditions were mimicked in the laboratory to analyse the biochemical response to heat spikes. The decline in ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) activity during prolonged heat stress is generally associated with an increase in ribulose 1,5-bisphosphate (RuBP) levels. However, rapid heating caused an initial decline in RuBP which was subsequently followed by a small decline in Rubisco carbamylation. The ratio of RuBP to Rubisco sites declined from a saturating concentration to a sub-saturating concentration, providing a possible mechanism for the decarbamylation of Rubisco. If RuBP is saturating (>1.8 RuBP Rubisco site(-1)), it acts as a cap on the catalytic site and keeps Rubisco activated. Measurements of triose-phosphate levels and NADP-malate dehydrogenase activation (a stromal redox proxy) indicated that the regeneration of RuBP by the Calvin cycle was limited by the availability of redox power.  相似文献   

20.
We developed a continuous-addition method for maintaining subsaturating concentrations of ribulose-1,5-bisphosphate (RuBP) for several minutes, while simultaneously monitoring its consumption by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). This method enabled us to observe the effects of subsaturating RuBP and CO2 concentrations on the activity of Rubisco during much longer periods than previously studied. At saturating CO2, the activity of the enzyme declined faster when RuBP was maintained at concentrations near its Km value than when RuBP was saturating. At saturating RuBP, activity declined faster at limiting than at saturating CO2, in accordance with previous observations. The most rapid decline in activity occurred when both CO2 and RuBP concentrations were subsaturating. The activity loss was accompanied by decarbamylation of the enzyme, even though the enzyme was maintained at the same CO2 concentration before and after exposure to RuBP. Rubisco activase ameliorated the decline in activity at subsaturating CO2 and RuBP concentrations. The results are consistent with a proposed mechanism for regulating the carbamylation of Rubisco, which postulates that Rubisco activase counteracts Rubisco's unfavorable carbamylation equilibrium in the presence of RuBP by accelerating, in an ATP-dependent manner, the release of RuBP from its complex with uncarbamylated sites.  相似文献   

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