Inhibited light activation of fructose and sedoheptulose bisphosphatase in spinach chloroplasts exposed to osmotic stress

The relationship between leaf K' concentration, in vitro dehydration, and nonstomatal-controlled photosynthesis was investigated using leaf slices that were vacuum infiltrated with media containing varying sorbitol concentrations. Ammonium ions, which facilitate stromal alkalinization, reversed the increased sensitivity of KV-deficient leaf slice photosynthesis to cell dehydration. However, NH4+ had no effect on photosynthesis of K+-deficient leaf slices under nonhypertonic conditions. These data suggest that endogenous extra-chloroplastic K may modulate dehydration inhibition of photosynthesis, possibly by facilitating stromal alkalinization.

mentioning

confidence: 96%

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confidence: 99%

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confidence: 99%

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Leaf K⁺ Interaction with Water Stress Inhibition of Nonstomatal-Controlled Photosynthesis

Berkowitz¹,

Whalen²

1985

“…Severe osmotic stress of chloroplasts and cells can inhibit FBPase2 activity by inhibition of light activation (3) (24).…”

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confidence: 99%

Mild Water Stress Effects on Carbon-Reduction-Cycle Intermediates, Ribulose Bisphosphate Carboxylase Activity, and Spatial Homogeneity of Photosynthesis in Intact Leaves

Sharkey

Seemann²

1989

216

122

We have examined the effect of mild water stress on photosynthetic chloroplast reactions of intact Phaseolus vulgaris leaves by measuring two parameters of ribulose bisphosphate (RuBP) carboxylase activity and the pool sizes of RuBP, 3-phosphoglycerate (PGA), triose phosphates, hexose monophosphates, and ATP. We also tested for patchy stomatal closure by feeding 14CO2. (24).Much of the evidence for direct effects of mild water stress on photosynthesis is based on estimates of the partial pressure of CO2 inside the leaf, which is an attempt to remove the complication of stomatal closure by calculation. In several studies, no effect of mild water stress on CO2 limited photosynthesis was found (20,24,29,32). However, the rate of photosynthesis at moderately high CO2 was reduced and was not increased by increasing CO2 or decreasing 02. This behavior has been interpreted as an effect on the capacity for starch and sucrose synthesis (22).Many of the effects of abscisic acid and water stress are similar (5,14). It has recently been demonstrated that the presumed effects of abscisic acid on the photosynthetic apparatus (15,19) were caused by closure of stomata in patches of the leaf (7,28). It is possible that mild water-stress effects on the biochemistry of photosynthesis are an artifact caused by patchy stomatal closure.The reduction in the rate of photosynthesis induced by water stress must be reflected in a reduction in the rate of the reaction catalyzed by RuBPCase. This can be caused by reduced activity of RuBPCase, or by changes in the availability of either RuBP or CO2. We have examined the effect of mild water stress on two parameters of RuBPCase activity and the pool sizes of RuBP, PGA, triose phosphates, hexose monophosphates, and ATP. We also tested for patchy stomatal closure to assess the availability of CO2. A preliminary report of some of these data has appeared (23).

“…However, inhibition of electron transport to the level of the primary reductant of PSI, and inhibition of ATP synthesis, are probably not significant enough to account for the observed decreases in CO2 fixation. Among the Calvin cycle enzymes, fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, and the enzymes between ribose-5-P and glycerate-3-P seem to be those most affected by water stress (5,7). Three mechanisms of water stress inhibition of CO2 fixation have been hypothesized.…”

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confidence: 99%

The Effect of Low Osmotic Potential on Nitrite Reduction in Intact Spinach Chloroplasts

Behrens¹,

Xu²,

Werner³

et al. 1985

The effect of water stress (reduced osmotic potential) on photosynthetic nitrite reduction was investigated using intact, isolated spinach (Spinacia okracea) chloroplasts. Nitrite-dependent 02 evolution was inhibited 39% at -29.5 bars osmotic potential, relative to a control at -I1 bars. In the presence of an uncoupler of photophosphorylation this inhibition was not seen. Reduced osmotic potential did not inhibit either methyl viologen reduction or photosynthetic 02 reduction. These results indicate that an inhibition of electron transport to ferredoxin cannot account for the observed inhibition of nitrite-dependent 02 evolution. In vitro assay of nitrite reductase activity showed that the interaction of the enzyme with nitrite was not affected by changes in the concentrations of ions or molecules that might be caused by water stress conditions. These results indicate that the most likely site for the effect of water stress on chloroplastic nitrite reduction is the interaction of ferredoxin with nitrite reductase. cycle have been shown to be affected (4, 5, 24). However, inhibition of electron transport to the level of the primary reductant of PSI, and inhibition of ATP synthesis, are probably not significant enough to account for the observed decreases in CO2 fixation. Among the Calvin cycle enzymes, fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, and the enzymes between ribose-5-P and glycerate-3-P seem to be those most affected by water stress (5, 7). Three mechanisms of water stress inhibition of CO2 fixation have been hypothesized. First, Berkowitz and Gibbs (6) have proposed that water stress prevents light-dependent alkalization of the stroma, and the resulting acid condition is a mediating mechanism of water stress inhibition of CO2 fixation. In support of this hypothesis, they found that treatments that alkalated the stroma were successful in preventing water stress inhibition. However, Boag and Portis (7) were unable to demonstrate stromal acidification under similar conditions with their preparations. They provide evidence consistent with inhibition of fructose-1 ,6-bisphosphatase and sedoheptulose-l ,7-bisphosphatase due to an inhibition of both the rate and magnitude of light activation of these enzymes. Finally, Kaiser and Heber (13) suggest that increased stromal solute concentrations due to shrinkage of the stromal volume inhibits the activity of certain Calvin cycle enzymes. They showed that stromal extracts subjected to moderate water stress with KCI were significantly inhibited in their ability to reduce glycerate 3-P and ribose 5-P. In contrast, KCl (10-30 mM) incubated with intact chloroplasts has been shown by Berkowitz and Gibbs (6) to prevent partially the inhibition of CO2 fixation.While CO2 fixation is the major function of chloroplasts, several other light-dependent reactions critical to plant metabolism occur there as well, and may be affected by water stress. These include 02, sulfite, and nitrite reduction. The reduction of nitrite to ammonia, in particular...