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Short tracheids were found close to the wound tissue surface. Their ability to adsorb nutrients and water is discussed. 相似文献
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PGA-supported O2 evolution by pea chloroplasts was not inhibited immediately by ADP; the rate of O2 evolution slowed as time passed, corresponding to the effect of ADP on CO2 assimilation, and indicating that glycerate 3-phosphate kinase was a site of inhibition. Likewise, upon the addition of AMP, inhibition of PGA-dependent O2 evolution became more severe with time. This did not mirror CO2 assimilation, which was inhibited immediately by AMP. In Sedum chloroplasts, PGA-dependent O2 evolution was not inhibited by ADP and AMP. In chloroplasts from peas and Sedum, the magnitude of MgADP and MgATP stimulation of PGA-dependent O2 evolution was not much larger than that given by ATP, and it was much smaller than MgATP stimulation of CO2 assimilation. Analysis of stromal metabolite levels by anion exchange chromatography indicated that ribulose 1,5-bisphosphate carboxylase was inhibited by ADP and stimulated by MgADP in Sedum chloroplasts.
The appearance of label in the medium was measured when [U-14C] ADP-loaded Sedum chloroplasts were challenged with ATP, ADP, or AMP and their Mg2+ complexes. The rate of back exchange was stimulated by the presence of Mg2+. This suggests that ATP, ADP, and AMP penetrate the chloroplast slower than their Mg2+ complexes. A portion of the CO2 assimilation and O2 evolution data could be explained by differential penetration rates, and other proposals were made to explain the remainder of the observations.
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Labeled metabolite analyses indicated that the osmotic stress treatment caused the substrate for fructose 1,6-bisphosphatase (FBPase) to build up in the absence of NH4Cl, and the substrate for phosphoribulokinase to increase in the presence of NH4Cl. These data were interpreted as indicating that the most severe effect of osmotic stress on photosynthesis is at the site of FBPase, and that this inhibition is mediated by osmotically induced stromal acidification. Phosphoribulokinase activity inhibition at the low osmotic potential treatment was apparently less severe and not mediated by stromal acidification. A third site of osmotic inhibition, which was reversed by NH4Cl, and therefore was assumed to be mediated by stromal acidification, was at the step of ribulose 1,5-bisphosphate carboxylase.
Additions of NH4Cl also enhanced the activity of the pH-insensitive phase of the photosynthetic carbon reduction cycle, 3-phosphoglyceric acid reduction, at the stress treatment. This effect was thought to be mediated by the removal of the block at FBPase. A model was proposed to outline the relative severity of osmotic stress effects at various sites of the photosynthetic carbon reduction cycle.
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