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1.
LAWLOR  DAVID W. 《Annals of botany》2002,89(7):871-885
Decreasing relative water content (RWC) of leaves progressivelydecreases stomatal conductance (gs), slowing CO2 assimilation(A) which eventually stops, after which CO2 is evolved. In somestudies, photosynthetic potential (Apot), measured under saturatingCO2, is unaffected by a small loss of RWC but becomes progressivelymore inhibited, and less stimulated by elevated CO2, below athreshold RWC (Type 1 response). In other studies, Apot andthe stimulation of A by elevated CO2 decreases progressivelyas RWC falls (Type 2 response). Decreased Apot is caused byimpaired metabolism. Consequently, as RWC declines, the relativelimitation of A by gs decreases, and metabolic limitation increases.Causes of decreased Apot are considered. Limitation of ribulosebisphosphate (RuBP) synthesis is the likely cause of decreasedApot at low RWC, not inhibition or loss of photosynthetic carbonreduction cycle enzymes, including RuBP carboxylase/oxygenase(Rubisco). Limitation of RuBP synthesis is probably caused byinhibition of ATP synthesis, due to progressive inactivationor loss of Coupling Factor resulting from increasing ionic (Mg2+)concentration, not to reduced capacity for electron or protontransport, or inadequate trans-thylakoid proton gradient (pH).Inhibition of Apot by accumulation of assimilates or inadequateinorganic phosphate is not considered significant. DecreasedATP content and imbalance with reductant status affect cellmetabolism substantially: possible consequences are discussedwith reference to accumulation of amino acids and alterationsin protein complement under water stress.  相似文献   

2.
There is a long-standing controversy as to whether drought limits photosynthetic CO2 assimilation through stomatal closure or by metabolic impairment in C3 plants. Comparing results from different studies is difficult due to interspecific differences in the response of photosynthesis to leaf water potential and/or relative water content (RWC), the most commonly used parameters to assess the severity of drought. Therefore, we have used stomatal conductance (g) as a basis for comparison of metabolic processes in different studies. The logic is that, as there is a strong link between g and photosynthesis (perhaps co-regulation between them), so different relationships between RWC or water potential and photosynthetic rate and changes in metabolism in different species and studies may be 'normalized' by relating them to g. Re-analysing data from the literature using light-saturated g as a parameter indicative of water deficits in plants shows that there is good correspondence between the onset of drought-induced inhibition of different photosynthetic sub-processes and g. Contents of ribulose bisphosphate (RuBP) and adenosine triphosphate (ATP) decrease early in drought development, at still relatively high g (higher than 150 mmol H20 m(-2) s(-1)). This suggests that RuBP regeneration and ATP synthesis are impaired. Decreased photochemistry and Rubisco activity typically occur at lower g (<100 mmol H20 m(-2) s(-1)), whereas permanent photoinhibition is only occasional, occurring at very low g (<50 mmol H20 m(-2) s(-1)). Sub-stomatal CO2 concentration decreases as g becomes smaller, but increases again at small g. The analysis suggests that stomatal closure is the earliest response to drought and the dominant limitation to photosynthesis at mild to moderate drought. However, in parallel, progressive down-regulation or inhibition of metabolic processes leads to decreased RuBP content, which becomes the dominant limitation at severe drought, and thereby inhibits photosynthetic CO2 assimilation.  相似文献   

3.
Rubisco activity decreases under water stress, for reasons as yet unclear. Here, the covariation of stomatal conductance (gs) and relative water content (RWC), often observed during water stress, was impaired to assess the separate effects of these factors on Rubisco activity. Three different treatments were applied to soybean (Glycine max) and tobacco (Nicotiana tabacum): leaf desiccation (LD), in which stomatal closure was accompanied by large decreases of RWC; water stress (WS), in which minor decreases of RWC were observed along with stomatal closure; and exogenous application of abscisic acid (ABA), which triggered stomatal closure without changing RWC. Decreased RWC did not induce decreased initial Rubisco activity, which was impaired only in soybean by 40% when the gs dropped below 50 mmol m(-2) s(-1), regardless of the treatment. The mechanism for decreased activity differed among treatments, owing to decreased activation in LD and to total activity and protein content in WS and ABA. Despite the occurrence of Rubisco regulation, CO2 availability in the chloroplast, not impairment of Rubisco activity, limits photosynthesis during WS.  相似文献   

4.
Ramanjulu  S.  Sreenivasalu  N.  Giridhara Kumar  S.  Sudhakar  C. 《Photosynthetica》1998,35(2):259-263
Three-month-old plants of mulberry (Morus alba L. cv. Kanva-2) were subjected to a drought stress by withholding water supply. As the leaf water potential (W) was dropping progressively with the severity of treatment and increasing stress duration, the values of leaf area, dry mass accumulation, total chlorophyll (Chl) content, net photosynthetic rate (PN), stomatal conductance (gs), and transpiration rate (E) were declined. The photosystem 2 (PS2) photochemical efficiency significantly decreased only at a severe stress treatment. The intercellular CO2 concentration (Ci) remained unaltered during a mild stress, yet it increased under moderate and severe stresses. The Ci/gs ratio reflected the mesophyll efficiency during water stress. Rewatering of the plants led to an almost complete recovery of PN, E, and gs, indicating that a short-term stress brings about reversible effects only.  相似文献   

5.
The response of net photosynthetic CO(2) uptake (A) to increasing leaf intercellular CO(2) concentration (c(i)) was determined in antisense Nicotiana tabacum plants, derived from six independent transformation lines, displaying a range of sedoheptulose-1, 7-bisphosphatase (SBPase) activities. The maximum in vivo ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation (V(c,max)) and RuBP regeneration (J(max)) rates were calculated from the steady-state measurements of the A to c(i) response curves. In plants with reductions in SBPase activity of between 9% and 60%, maximum RuBP regeneration capacity declined linearly (r(2)=0.79) and no significant change in apparent in vivo Rubisco activity (V(c,max)) was observed in these plants. No correlation between V(c,max) and a decrease in capacity for RuBP regeneration was observed (r(2)=0.14) in the SBPase antisense plants. These data demonstrate that small decreases in SBPase activity limit photosynthetic carbon assimilation by reducing the capacity for RuBP regeneration.  相似文献   

6.
Central paradigms of ecophysiology are that there are recognizable and even explicit and predictable patterns among species, genera, and life forms in the economics of water and nitrogen use in photosynthesis and in carbon isotope discrimination (delta). However most previous examinations have implicitly assumed an infinite internal conductance (gi) and/or that internal conductance scales with the biochemical capacity for photosynthesis. Examination of published data for 54 species and a detailed examination for three well-characterized species--Eucalyptus globulus, Pseudotsuga menziesii and Phaseolus vulgaris--show these assumptions to be incorrect. The reduction in concentration of CO2 between the substomatal cavity (Ci) and the site of carbon fixation (Cc) varies greatly among species. Photosynthesis does not scale perfectly with gi and there is a general trend for plants with low gi to have a larger draw-down from Ci to Cc, further confounding efforts to scale photosynthesis and other attributes with gi. Variation in the gi-photosynthesis relationship contributes to variation in photosynthetic 'use' efficiency of N (PNUE) and water (WUE). Delta is an information-rich signal, but for many species only about two-thirds of this information relates to A/gs with the remaining one-third related to A/gi. Using data for three well-studied species we demonstrate that at common WUE, delta may vary by up to 3 per thousand. This is as large or larger than is commonly reported in many interspecific comparisons of delta, and adds to previous warnings about simplistic interpretations of WUE based on delta. A priority for future research should be elucidation of relationships between gi and gs and how these vary in response to environmental conditions (e.g. soil water, leaf-to-air vapour pressure deficit, temperature) and among species.  相似文献   

7.
The Chlamydomonas reinhardtii cia3 mutant has a phenotype indicating that it requires high-CO(2) levels for effective photosynthesis and growth. It was initially proposed that this mutant was defective in a carbonic anhydrase (CA) that was a key component of the photosynthetic CO(2)-concentrating mechanism (CCM). However, more recent identification of the genetic lesion as a defect in a lumenal CA associated with photosystem II (PSII) has raised questions about the role of this CA in either the CCM or PSII function. To resolve the role of this lumenal CA, we re-examined the physiology of the cia3 mutant. We confirmed and extended previous gas exchange analyses by using membrane-inlet mass spectrometry to monitor(16)O(2),(18)O(2), and CO(2) fluxes in vivo. The results demonstrate that PSII electron transport is not limited in the cia3 mutant at low inorganic carbon (Ci). We also measured metabolite pools sizes and showed that the RuBP pool does not fall to abnormally low levels at low Ci as might be expected by a photosynthetic electron transport or ATP generation limitation. Overall, the results demonstrate that under low Ci conditions, the mutant lacks the ability to supply Rubisco with adequate CO(2) for effective CO(2) fixation and is not limited directly by any aspect of PSII function. We conclude that the thylakoid CA is primarily required for the proper functioning of the CCM at low Ci by providing an ample supply of CO(2) for Rubisco.  相似文献   

8.
Laisk A  Sumberg A 《Plant physiology》1994,106(2):689-695
Photorespiration was calculated from chlorophyll fluorescence and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetics and compared with CO2 evolution rate in the light, measured by three gas-exchange methods in mature sunflower (Helianthus annuus L.) leaves. The gas-exchange methods were (a) postillumination CO2 burst at unchanged CO2 concentration, (b) postillumination CO2 burst with simultaneous transfer into CO2-free air, and (c) extrapolation of the CO2 uptake to zero CO2 concentration at Rubisco active sites. The steady-state CO2 compensation point was proportional to O2 concentration, revealing the Rubisco specificity coefficient (Ksp) of 86. Electron transport rate (ETR) was calculated from fluorescence, and photorespiration rate was calculated from ETR using CO2 and O2 concentrations, Ksp, and diffusion resistances. The values of the best-fit mesophyll diffusion resistance for CO2 ranged between 0.3 and 0.8 s cm-1. Comparison of the gas-exchange and fluorescence data showed that only ribulose-1,5-bisphosphate (RuBP) carboxylation and photorespiratory CO2 evolution were present at limiting CO2 concentrations. Carboxylation of a substrate other than RuBP, in addition to RuBP carboxylation, was detected at high CO2 concentrations. A simultaneous decarboxylation process not related to RuBP oxygenation was also detected at high CO2 concentrations in the light. We propose that these processes reflect carboxylation of phosphoenolpyruvate, formed from phosphoglyceric acid and the subsequent decarboxylation of malate.  相似文献   

9.
The effects of short-term (minutes) variations of CO2 concentration on mesophyll conductance to CO2 (gm) were evaluated in six different C3 species by simultaneous measurements of gas exchange, chlorophyll fluorescence, online carbon isotope discrimination and a novel curve-fitting method. Depending on the species, gm varied from five- to ninefold, along the range of sub-stomatal CO2 concentrations typically used in photosynthesis CO2-response curves (AN)-Ci curves; where AN is the net photosynthetic flux and Ci is the CO2 concentrations in the sub-stomatal cavity), that is, 50 to 1500 micromol CO2 mol(-1) air. Although the pattern was species-dependent, gm strongly declined at high Ci, where photosynthesis was not limited by CO2, but by regeneration of ribulose-1,5-bisphosphate or triose phosphate utilization. Moreover, these changes on gm were found to be totally independent of the velocity and direction of the Ci changes. The response of gm to Ci resembled that of stomatal conductance (gs), but kinetic experiments suggested that the response of gm was actually faster than that of gs. Transgenic tobacco plants differing in the amounts of aquaporin NtAQP1 showed different slopes of the gm-Ci response, suggesting a possible role for aquaporins in mediating CO2 responsiveness of gm. The importance of these findings is discussed in terms of their effects on parameterization of AN-Ci curves.  相似文献   

10.
The principles, equipment and procedures for measuring leaf and canopy gas exchange have been described previously as has chlorophyll fluorescence. Simultaneous measurement of the responses of leaf gas exchange and modulated chlorophyll fluorescence to light and CO2 concentration now provide a means to determine a wide range of key biochemical and biophysical limitations on photo synthesis in vivo. Here the mathematical frameworks and practical procedures for determining these parameters in vivo are consolidated. Leaf CO2 uptake (A) versus intercellular CO2 concentration (Ci) curves may now be routinely obtained from commercial gas exchange systems. The potential pitfalls, and means to avoid these, are examined. Calculation of in vivo maximum rates of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) carboxylation (Vc,max), electron transport driving regeneration of RuBP (Jmax), and triose-phosphate utilization (VTPU) are explained; these three parameters are now widely assumed to represent the major limitations to light-saturated photosynthesis. Precision in determining these in intact leaves is improved by the simultaneous measurement of electron transport via modulated chlorophyll fluorescence. The A/Ci response also provides a simple practical method for quantifying the limitation that stomata impose on CO2 assimilation. Determining the rate of photorespiratory release of oxygen (Rl) has previously only been possible by isotopic methods, now, by combining gas exchange and fluorescence measurements, Rl may be determined simply and routinely in the field. The physical diffusion of CO2 from the intercellular air space to the site of Rubisco in C3 leaves has long been suspected of being a limitation on photosynthesis, but it has commonly been ignored because of the lack of a practical method for its determination. Again combining gas exchange and fluorescence provides a means to determine mesophyll conductance. This method is described and provides insights into the magnitude and basis of this limitation.  相似文献   

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