共查询到20条相似文献,搜索用时 31 毫秒
1.
Plant growth and adaptation to cold and freezing temperatures in a CO2 -enriched atmosphere have received little attention despite their predicted effects on plant distribution and productivity. In this study we looked at the interaction between elevated CO2 and development of freezing tolerance in Norway spruce ( Picea abies (L.) Karst.). First-year seedlings were grown under controlled conditions in an atmosphere enriched in CO2 (70 Pa) for one simulated growth season. We measured shoot growth, registered the timing of growth cessation and bud set, measured needle net photosynthetic rate, and determined needle carbohydrate concentration (fructose+pinitol, glucose, sucrose, inositol, raffinose and starch). Freezing tolerance (LT50 ) was determined after exposing whole seedlings to temperatures ranging from −6.5 to −36.0°C and scoring for visual needle browning. Elevated CO2 did not affect height growth or the timing of growth cessation and bud set. The only statistically significant effects of CO2 treatment were on seedling dry weight, percent dry matter and starch content. During the three weeks after growth cessation and bud set, freezing tolerance increased from −10 to −35°C, and there was a marked increase in all soluble sugars except inositol. However, neither freezing tolerance nor the concentration of soluble sugars was significantly influenced by elevated CO2 . 相似文献
2.
The effect of long-term water stress on photosynthetic carbon metabolism in Casuarina equisetifolia Forst. & Forst. was analysed by measuring CO2 assimilation, stomatal conductance, the quantum yield of photosystem II ( Φ PSII ), enzyme activities, and the levels of photosynthetic intermediates and carbohydrates. CO2 assimilation decreased under water stress while the intercellular CO2 concentration ( C i ) as estimated by gas exchange measurements remained high. However, the estimates of C i from measurements of Φ PSII suggest that the decrease in photosynthesis can be explained in terms of stomatal closure. Water stress decreased total stromal fructose-1,6-bisphosphatase activity and did not alter the activities and activation states of ribulose bisphosphate carboxylase oxygenase and NADP-dependent malate dehydrogenase (NADP-MDH). The concentration of photosynthetic metabolites, glucose, fructose and sucrose decreased, whereas starch concentrations increased under drought conditions. 相似文献
3.
Gas exchange parameters, water relations and carbohydrate partitioning in leaves of field-grown Prunus domestica following fruit removal 总被引:1,自引:0,他引:1
The effect of fruit removal on gas exchange, water relations, chlorophyll and non-structural carbohydrate content of leaves from mature, field-grown plum trees ( Prunus domestica L. cv. Stanley) was determined over 2 consecutive growing seasons. Removal of fruits during stage II of fruit development decreased CO2 assimilation rate within 24 h from 12.6 to 8.5 μmol m-2 s-1 in 1986, and from 12.1 to 10.2 μmol m-2 s-1 in 1987. Depression of net photosynthesis persisted for at least 5 days and was greatest in the early afternoon. Recovery of the CO2 assimilation rate to pretreatment levels coincided in defruited trees with vegetative growth that was more than 5-fold that of fruiting trees in the first 6 weeks after fruit removal in 1986. Estimated photorespiration was similar in both fruiting and defruited trees. The stomatal contribution to the decrease of CO2 assimilation rate, calculated from assimilation/intercellular CO2 curves, ranged from 31 to 46%. Defruiting did not affect leaf water potential, but decreased leaf osmotic potential. Leaf levels of chlorophyll, fructose, glucose, sorbitol and sucrose were not affected by defruiting, whereas starch content increased up to 51% in leaves of defruited trees within 24 h after fruit removal. However, because of the small starch pool present in plum leaves (<1.9% dry weight) it is unlikely that starch accumulation was responsible for the observed decline in CO2 assimilation rate after fruit removal. The decrease of CO2 assimilation rate is discussed in relation to the hypothesis of assimilate demand regulating photosynthesis through a feedback mechanism. 相似文献
4.
The effects of increased atmospheric carbon dioxide and temperature on carbon partitioning, source-sink relations and respiration 总被引:24,自引:8,他引:16
Abstract. Herbaceous C3 plants grown in elevated CO2 show increases in carbon assimilation and carbohydrate accumulation (particularly starch) within source leaves. Although changes in the partitioning of biomass between root and shoot occur, the proportion of this extra assimilate made available for sink growth is not known. Root:shoot ratios tend to increase for CO2 -enriched herbaceous plants and decrease for CO2 -enriched trees. Root:shoot ratios for cereals tend to remain constant. In contrast, elevated temperatures decrease carbohydrate accumulation within source and sink regions of a plant and decrease root:shoot ratios. Allometric analysis of at least two species showing changes in root: shoot ratios due to elevated CO2 show no alteration in the whole-plant partitioning of biomass. Little information is available for interactions between temperature and CO2 . Cold-adapted plants show little response to elevated levels of CO2 , with some species showing a decline in biomass accumulation. In general though, increasing temperature will increase sucrose synthesis, transport and utilization for CO2 -enriched plants and decrease carbohydrate accumulation within the leaf. Literature reports are discussed in relation to the hypothesis that sucrose is a major factor in the control of plant carbon partitioning. A model is presented in support. 相似文献
5.
The growth rate of Lemna giba L. G3 was varied by limiting the supply of nitrogen (N) under otherwise constant condition. Two experimental approaches were used. 1) A series of suboptimally growing cultures were supplid daily with exponentially increasing doses on N. 2) Optimally growing cultures were transferred to a N-fre medium and cultivated in it for 10 days. Lveles of starch, sucrose, glucose, fructose, and systems. At RGR ranging from optimal to 50% of optimal caused decreased levels of soluble sugars, but increased lvels of starch.Starch accumulation showed a strong negative correlation with the CO2 assimilation rate, indicating increased triose phosphate/inorganic phosphate (TP/Pi ) ratio in the chloroplast causing end product inhibition of photosyntheisis. The data indicat the quantitative rlationship betwen the photosynthetic activity and the carbon utilization rate influnces the activity of the sucrose synthesis pathway and thus the rate of the triose hosphate/Pi exchange at the chloroplast membrane, which in turn regulates the activity of starch synthesis and the Calvin cycle. 相似文献
6.
The role of fructose-2,6-bisphosphate (Fru-2,6-P2 ) in regulation of carbon metabolism was investigated in transgenic potato plants ( Solanum tuberosum L. cv Dianella) transformed with a vector containing a cDNA-sequence encoding fructose-6-phosphate,2-kinase (F6P,2-K, EC 2.7.1.105)/fructose-2,6-bisphosphatase (F26BPase, EC 3.1.3.46) in sense or antisense direction behind a CaMV 35S promoter. The activity of F6P,2-K in leaves was reduced to 5% of wild-type (WT) activity, and the level of Fru-2,6-P2 was reduced both in leaves (10% of the WT level) and in tubers (40% of the WT level). Analysis of photosynthetic 14 CO2 metabolism, showed that in plant lines with reduced Fru-2,6-P2 level the carbon partitioning in the leaves was changed in favour of sucrose biosynthesis, and the soluble sugars-to-starch labelling ratio was doubled. The levels of soluble sugars and hexose phosphates also increased in leaves of the transgenic plants. Most notably, the levels of hexoses were four- to six-fold increased in the transgenic plants. In tubers with reduced levels of Fru-2,6-P2 only minor effects on carbohydrate levels were observed. Furthermore, carbon assimilation in tuber discs supplied with [U-14 C]-sucrose showed only a moderate increase in labelling of hexoses and a decreased labelling of starch. Similar results were obtained using [U-14 C]-glucose. No differences in growth of the transgenic lines and the WT were observed. Our data provide evidences that Fru-2,6-P2 is an important factor in the regulation of photosynthetic carbon metabolism in potato leaves, whereas the direct influence of Fru-2,6-P2 on tuber metabolism was limited. 相似文献
7.
ANJA LOEWE WERNER EINIG LANBO SHI PIERRE DIZENGREMEL & RÜDIGER HAMPP 《The New phytologist》2000,145(3):565-574
The effects of mycorrhiza formation in combination with elevated CO2 concentrations on carbon metabolism of Norway spruce ( Picea abies ) seedlings and aspen ( Populus tremula × Populus tremuloides ) plantlets were analysed. Plants were inoculated for 6 wk with the ectomycorrhizal fungi Amanita muscaria and Paxillus involutus (aspen only) in an axenic Petri-dish culture at 350 and 700 μl l−1 CO2 partial pressure. After mycorrhiza formation, a stimulation of net assimilation rate was accompanied by decreased activities of sucrose synthase, an increased activation state of sucrose-phosphate synthase, decreased fructose-2,6-bisphosphate and starch, and slightly elevated glucose-6-phosphate contents in source leaves of both host species, independent of CO2 concentration. Exposure to elevated CO2 generally resulted in higher net assimilation rates, increased starch as well as decreased fructose-2,6-bisphosphate (aspen only) content in source leaves of both mycorrhizal and nonmycorrhizal plants. Our data indicate only slightly improved carbon utilization by mycorrhizal plants at elevated CO2 . They demonstrate however, that both factors which modulate the sink-source properties of plants increase the capacity for sucrose synthesis in source leaves mainly by allosteric enzyme regulation. 相似文献
8.
Silverleaf whitefly (SLW), Bemisia argentifolii Bellows and Perring, is one of the most noxious pests of numerous field and vegetable crops, causing billions of dollars worth of damage throughout the world. SLW is a phloem feeder whose feeding is likely to interfere with phloem transport. The aim of this study was to test the hypothesis that SLW infestation impairs carbohydrate export from source leaves, and consequently increases their carbohydrate content. The youngest fully expanded leaves of cotton ( Gossypium hirsutum L., cv. Siv'on), grown under SLW-infested and noninfested conditions, were characterized for their diurnal changes in carbohydrate content and photoassimilate export. SLW infestation induced a considerable reduction in net photosynthetic rate (Pn ), coupled with increased sucrose, glucose and fructose and decreased starch concentrations. Export rate was determined after 14 CO2 pulse-labeling both by in situ monitoring of leaf radioactivity and by analyzing the content and radioactivity of the major carbon metabolites. Radioactive counting indicated a lower rate of 14 C efflux for the infested plants. A similar trend was found for the specific activities of sucrose and the three soluble sugars combined (sucrose, glucose and fructose). A single exponential decay function with asymptote was fitted to the above efflux curves. All the calculated exponential coefficients demonstrated lower export rates after SLW injury. These results indicate that SLW impairs photoassimilate export, suggesting possible down-regulation of Pn due to increased foliar soluble sugar contents. 相似文献
9.
L. Blaschke M. Schulte A. Raschi N. Slee H. Rennenberg A. Polle 《Plant biology (Stuttgart, Germany)》2001,3(3):288-298
Abstract: To study physiological responses of mature forest trees to elevated CO2 after lifetime growth under elevated atmospheric CO2 concentrations ( p CO2 ), photosynthesis, Rubisco content, foliar concentrations of soluble sugars and starch, sugar concentrations in transport tissues (phloem and xylem), structural biomass, and lignin in leaves and branches were investigated in 30- to 50-year-old Quercus pubescens and Q. ilex trees grown at two naturally elevated CO2 springs in Italy. Ribulose-1,5-bisphosphate carboxylase/oxygenase content was decreased in Q. pubescens grown under elevated CO2 concentrations, but not in Q. ilex. Photosynthesis was consistently higher in Q. pubescens grown at elevated CO2 as compared with "control" sites, whereas the response in Q. ilex was less pronounced. Stomatal conductance was lower in both species leading to decreased transpiration and increased instantaneous water use efficiency in Q. pubescens. Overall mean sugar + starch concentrations of the leaves were not affected by elevated p CO2 , but phloem exudates contained higher concentrations of soluble sugars. This finding suggests increased transport to sinks. Qualitative changes in major carbon-bearing compounds, such as structural biomass and lignins, were only found in bark but not in other tissues. These results support the concept that the maintenance of increased rates of photosynthesis after long-term acclimation to elevated p CO2 provides a means of optimization of water relations under arid climatic conditions but does not cause an increase in aboveground carbon sequestration per unit of tissue in Mediterranean oak species. 相似文献
10.
Janhendrik Krook Dick Vreugdenhil Cor Dijkema Linus H.W. van der Plas 《Physiologia plantarum》2000,108(2):125-133
After a lag phase of 2 days, batch-grown cells of carrot ( Daucus carota L.) cv. Flakkese entered the exponential growth phase and started to accumulate sucrose and hexoses. Short-term feeding 13 C-glucose in this period resulted in only minor labelling of sucrose or fructose. CO2 production from [1-13 C]- and [6-13 C]-glucose revealed, that at least 40% of the added glucose passed through the oxidative pentose phosphate pathway (OPPP), up to 40% through glycolysis leaving only minor 13 C-glucose for incorporation in various cell components in the exponential growth phase. After about 11 days of culture, the medium sugars were exhausted, cells entered the stationary growth phase and consumed stored sugar. Both neutral and acid invertase (EC 3.2.1.26) and sucrose synthase (EC 2.4.1.13) increased 50% from day 0 to days 11–13; thereafter their levels decreased again. Labelling with 13 C-glucose resulted in the accumulation of labelled sucrose and fructose during the stationary growth phase. Sucrose labelling was transient, i.e. after 6 h its level started to decrease again. Labelled fructose, however, evolved slower and increased even after 8 h. In sucrose and fructose up to 20% of the 13 C-label was exchanged from C-1 to C-6 carbons, indicating intensive cycling of at least 40% of the carbon between hexoses and triose phosphates. In the stationary phase only 10% of the labelled glucose passed through the OPPP and about 30% passed through the respiratory pathway; the remaining 60% was incorporated in cell constituents and sugars. Comparing the various cycles revealed that the regulation of the OPPP operated relatively independently from the cytosolic cycling of hexose phosphates through sucrose and from the cycling between hexose phosphates and triose phosphates. 相似文献
11.
D. HEINEKE F. KAUDER W. FROMMER C. KÜHN B. GILLISSEN F. LUDEWIG & U. SONNEWALD 《Plant, cell & environment》1999,22(6):623-628
Acclimation of plants to an increase in atmospheric carbon dioxide concentration is a well described phenomenon. It is characterized by an increase in leaf carbohydrates and a degradation of ribulose 1, 5-bisphosphate carboxylase protein (Rubisco) leading in the long term to a lower rate of CO2 assimilation than expected from the kinetic constants of Rubisco. This article summarizes studies with transgenic plants grown in elevated pCO2 which are modified in their capacity of CO2 fixation, of sucrose and starch synthesis, of triosephosphate and sucrose transport and of sink metabolism of sucrose. These studies show that a feedback accumulation of carbohydrates in leaves play only a minor role in acclimation, because leaf starch synthesis functions as an efficient buffer for photoassimilates. There is some evidence that in elevated pCO2 , plants grow faster and senescence is induced earlier. 相似文献
12.
13.
Single leaf photosynthetic rates and various leaf components of potato ( Solanum tuberosum L.) were studied 1–3 days after reciprocally transferring plants between the ambient and elevated growth CO2 treatments. Plants were raised from individual tuber sections in controlled environment chambers at either ambient (36 Pa) or elevated (72 Pa) CO2 . One half of the plants in each growth CO2 treatment were transferred to the opposite CO2 treatment 34 days after sowing (DAS). Net photosynthesis (Pn ) rates and various leaf components were then measured 34, 35 and 37 DAS at both 36 and 72 Pa CO2 . Three-day means of single leaf Pn rates, leaf starch, glucose, initial and total Rubisco activity, Rubisco protein, chlorophyll ( a + b ), chlorophyll ( a/b ), α -amino N, and nitrate levels differed significantly in the continuous ambient and elevated CO2 treatments. Acclimation of single leaf Pn rates was partially to completely reversed 3 days after elevated CO2 -grown plants were shifted to ambient CO2 , whereas there was little evidence of photosynthetic acclimation 3 days after ambient CO2 -grown plants were shifted to elevated CO2 . In a four-way comparison of the 36, 72, 36 to 72 (shifted up) and 72 to 36 (shifted down) Pa CO2 treatments 37 DAS, leaf starch, soluble carbohydrates, Rubisco protein and nitrate were the only photosynthetic factors that differed significantly. Simple and multiple regression analyses suggested that negative changes of Pn in response to growth CO2 treatment were most closely correlated with increased leaf starch levels. 相似文献
14.
The increased supply of photosynthate from maternal tissue is known to promote grain growth in several crop species. However, the effect of increasing photosynthate supply on grain growth receives little attention in rice. This study was aimed at evaluating the effect of increasing photosynthate supply through CO2 enrichment (650 μl I-1 ) on grain growth in three rice cultivars differing in grain size. CO2 enrichment was applied to the pot-grown plants between anthesis and final harvest. The results indicated that high CO2 treatment enhanced the CO2 exchange rate of leaf tissue, and subsequently increased the sucrose level of peduncle exudate, but it did not promote starch accumulation in the developing grains. This phenomenon was linked to the poor CO2 responses for the grain activities of sucrose synthase, UDP-glucose pyrophosphorylase. ADP-glucose pyrophosphorylase, and starch synthases involved in the conversion of sucrose to starch. Significant cultivar differences also existed for the activities of sucrose to starch conversion enzymes with larger grain size cultivars tending to have higher enzymes activities (expressed on a grain basis), resulting in a greater carbohydrate accumulation. 相似文献
15.
Abstract. Transverse slices through developing grains of Triticum aestivum cv. SUN 9E 16 d after anthesis were incubated in simple defined media with various radioactive labels. In some enzymic assays slices were pretreated with 2.5% Triton X-100 or with 5% butanol to remove cellular membranes and endogenous substrates.
Endogenous potassium leaked from endosperm slices into 30mol m−3 sucrose while sucrose was converted partly into starch. Exogenous alkali-ions, except Li+ , stimulated conversion of sucrose to insoluble matter, specifically to starch with K+ . Starch synthetase activity of Triton-pretreated slices was stimulated by K+ at both high and low substrate ADPG concentration, but was not affected by phosphate (25 mol m−3 ).
Phosphate in the medium had no effect on incorporation of sucrose or glucose into alcohol-insoluble material or starch in fresh slices (internal inorganic phosphate (P,) concentration was about 11 mol m−3 ). Three- to four-fold contrasts in internal Pi level, achieved by prolonged preincubations in different media, did not show an inhibition of starch synthesis by Pi . However, phosphate (25mol m−3 ) inhibited starch synthesis, that was mediated by ADPG pyrophosphorylase in butanol-pretreated endosperm slices by 15–18%.
It is concluded that starch synthesis in wheat endosperm is not regulated directly by apoplastic Pi ; level. 相似文献
Endogenous potassium leaked from endosperm slices into 30mol m
Phosphate in the medium had no effect on incorporation of sucrose or glucose into alcohol-insoluble material or starch in fresh slices (internal inorganic phosphate (P,) concentration was about 11 mol m
It is concluded that starch synthesis in wheat endosperm is not regulated directly by apoplastic P
16.
Abstract: The concentration dependency of the impact of elevated atmospheric CO2 concentrations on Arabidopsis thaliana L. was studied. Plants were exposed to nearly ambient (390), 560, 810, 1240 and 1680 μl I-1 CO2 during the vegetative growth phase for 8 days. Shoot biomass production and dry matter content were increased upon exposure to elevated CO2 . Maximal increase in shoot fresh and dry weight was obtained at 560 μl I-1 CU2 , which was due to a transient stimulation of the relative growth rate for up to 3 days. The shoot starch content increased with increasing CO2 concentrations up to two-fold at 1680 μl I-1 CO2 , whereas the contents of soluble sugars and phenolic compounds were hardly affected by elevated CO2 . The chlorophyll and carotenoid contents were not substantially affected at elevated CO2 and the chlorophyll a/b ratio remained unaltered. There was no acclimation of photosynthesis at elevated CO2 ; the photosynthetic capacity of leaves, which had completely developed at elevated CO2 was similar to that of leaves developed in ambient air. The possible consequences of an elevated atmospheric CO2 concentration to Arabidopsis thaliana in its natural habitat is discussed. 相似文献
17.
During starch degradation in intact isolated chloroplasts from Chlamydomonas reinhardtii gas exchange was studied with a mass spectrometer. Oxygen uptake by intact chloroplasts in the dark never exceeded 1.5% of the starch degradation rate [maximum 15 nmol O2 (mg Chl)−1 h−1 consumed. 1 000 nmol glucose (mg Chl)−1 h−1 degraded]. Evolution of CO2 under aerobic conditions [9.8–28 nmol (mg Chl)−1 h−1 ] was stimulated by addition of 0.1–0.5 m M oxaloacetate [393–425 nmol CO2 (mg Chl)−1 h−1 ]. Pyridoxal phosphate (5 m M ) inhibited starch degradation by more than 80%, but had no effect on O2 uptake. Starch degradation rates and CO2 evolution did not differ under acrobic and anaerobic conditions. Increasing Pi in the reaction medium from 0.5 m M to 5.0 m M stimulated starch degradation by 230 and 260% under aerobic and anaerobic conditions, respectively. A rapid autooxidation of reduced ferredoxin was observed in a reconstituted system consisting of purified Chlamydomonas ferredoxin, purified Chlamydomonas NADP-ferredoxin oxidoreductase (EC 1.6.7.1) and NADPH. Addition of isolated thylakoids from C. reinhardtii did not affect the rate of O2 uptake. Our results clearly indicate the absence of any oxygen requirement during starch degradation in isolated chloroplasts. 相似文献
18.
Gaucher C Costanzo N Afif D Mauffette Y Chevrier N Dizengremel P 《Physiologia plantarum》2003,117(3):392-402
The effects of high O3 (200 nl l−1 during the light period) and high CO2 (650 μl l−1 CO2 , 24 h a day) alone and in combination were studied on 45-day-old sugar maple ( Acer saccharum Marsh.) seedlings for 61 days in growth chambers. After 2 months of treatment under the environmental conditions of the experiment, sugar maple seedlings did not show a marked response to the elevated CO2 treatment: the effect of high CO2 on biomass was only detected in the leaves which developed during the treatment, and assimilation rate was not increased. Under high O3 at ambient CO2 , assimilation rate at days 41 and 55 and Rubisco content at day 61 decreased in the first pair of leaves; total biomass was reduced by 43%. In these seedlings large increases (more than 2-fold) in glucose 6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) activity and in anaplerotic CO2 fixation by phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) were observed, suggesting that an enhanced reducing power and carbon skeleton production was needed for detoxification and repair of oxidative damage. Under high O3 at elevated CO2 , a stimulation of net CO2 assimilation was observed after 41 days but was no longer observed at day 55. However, at day 61, the total biomass was only reduced by 21% and stimulation of G6PDH and PEPC was less pronounced than under high O3 at ambient CO2 . This suggests that high CO2 concentration protects, to some extent, against O3 by providing additional carbon and energy through increased net assimilation. 相似文献
19.
Canopy development of a model herbaceous community exposed to elevated atmospheric CO2 and soil nutrients 总被引:1,自引:0,他引:1
To test the prediction that elevated CO2 increases the maximum leaf area index (LAI) through a stimulation of photosynthesis, we exposed model herbaceous communities to two levels of CO2 crossed with two levels of soil fertility. Elevated CO2 stimulated the initial rate of canopy development and increased cumulative LAI integrated over the growth period, but it had no effect on the maximum LAI. In contrast to CO2 , increased soil nutrient availability caused a substantial increase in maximum LAI. Elevated CO2 caused a slight increase in leaf area and nitrogen allocated to upper canopy layers and may have stimulated leaf turnover deep in the canopy. Gas exchange measurements of intact communities made near the time of maximum LAI indicated that soil nutrient availability, but not CO2 enrichment, caused a substantial stimulation of net ecosystem carbon exchange. These data do not support our prediction of a higher maximum LAI by elevated CO2 because the initial stimulation of LAI diminished by the end of the growth period. However, early in development, leaf area and carbon assimilation of communities may have been greatly enhanced. These results suggest that the rate of canopy development in annual communities may be accelerated with future increases in atmospheric CO2 but that maximum LAI is set by soil fertility. 相似文献
20.
The effects of the apoplastic, i.e. external, concentration of sucrose (0–30 m M ) on O2 evolution, O2 consumption, starch, sucrose, glucose and fructose content, and uptake and efflux of sucrose in mesophyll protoplasts of Pisum sativum L. cv. Fenomen were studied. Neither photosynthesis, dark respiration, sucrose nor starch content change with increased apoplastic sucrose concentration. The contents of glucose and fructose in the protoplasts increase with increased apoplastic sucrose concentration. The sucrose efflux increases with increased external sucrose concentrations between 1 and 5 m M , but above this range the efflux decreases with increased external sucrose concentrations. These findings indicate that although external sucrose does not enter the protoplasts, there is a relationship between the external sucrose pool and the internal pools of sugars in the mesophyll protoplasts. The results suggest an active sucrose efflux from the protoplasts at physiological concentrations of apoplastic sucrose (max 5 m M ) and a simple diffusion mechanism at higher concentrations. 相似文献