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

Abstract

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...

Effects of water deficit on photosynthetic capacity

“…Ferredoxin-depeadent nitrite reduction by isolated intact chloroplasts was inhibited in sorbitoi media only at osmolarities exceeding 1 osmol kg ' (Kaiser et al1981b, Behrens et al 1985. Inhibition of ferredoxin-mediated reactions might also prevent NADP*-reduction as well as reductive activation of certain Calvin cycle enzymes such as fructose and sedoheptulose bisphosphatases (EC 3.1.3.11, EC 3.1.3.37) orphosphoribulokinase (EC 2,7.1.19).…”