ISSN:
1432-2048
Keywords:
Cations and photosynthesis
;
Chloroplast (low-salt effects)
;
Light activation (photosynthesis enzymes)
;
Photosynthesis
;
Spinacia
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
Abstract The role of monovalent cations in the photosynthesis of isolated intact spinach chloroplasts was investigated. When intact chloroplasts were assayed in a medium containing only low concentrations of mono- and divalent cations (about 3 mval l-1), CO2-fixation was strongly inhibited although the intactness of chloroplasts remained unchanged. Addition of K+, Rb+, or Na+ (50–100 mM) fully restored photosynthesis. Both the degree of inhibition and restoration varied with the plant material and the storage time of the chloroplasts in “low-salt” medium. In most experiments the various monovalent cations showed a different effectiveness in restoring photosynthesis of low-salt chloroplasts (K+〉Rb+〉Na+). Of the divalent cations tested, Mg2+ also restored photosynthesis, but to a lesser extent than the monovalent cations. In contrast to CO2-fixation, reduction of 3-phosphoglycerate was not ihibited under low-salt conditions. In the dark, CO2-fixation of lysed chloroplasts supplied with ATP, NADPH, and 3-phosphoglycerate strictly required the presence of Mg2+ but was independent of monovalent cations. This finding excludes a direct inactivation of Calvin cycle enzymes as a possible basis for the inhibition of photosynthesis under low-salt conditions. Light-induced alkalization of the stroma and an increase in the concentration of freely exchangeable Mg2+ in the stroma, which can be observed in normal chloroplasts, did not occur under low-salt conditions but were strongly enhanced after addition of monovalent cations (50–100 mM) or Mg2+ (20–50 mM). The relevance of a light-triggered K+/H+ exchange at the chloroplast envelope is discussed with regard to the light-induced increase in the pH and the Mg2+ concentration in the stroma, which are thought to be obligatory for light activation of Calvincycle enzymes.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00380879
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