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Potassium, sodium, and chloride content of isolated intact chloroplasts in relation to ionic compartmentation in
Abstract:
Intact chloroplasts were isolated from spinach, sugarbeet, and pea leaves by a rapid procedure which included purification on a two-step Percoll gradient. The content of K+, Na+, and Cl- was determined together with the chloroplast volume from which the apparent concentration of these ions in the chloroplast was calculated. It was demonstrated by successive washing that K+ leaked out of the chloroplasts during isolation whereas Na+ and Cl- did not. For spinach and sugarbeet chloroplasts K+ was the predominant monovalent cation (160-200 mM) with Na+ present in lesser amounts (40-70 mM). Pea chloroplasts contained equal amounts of K+ and Na+ (40 mM). The Cl- concentration in the chloroplast was similar for the three species and was calculated to be near 100 mM. The Cl- concentration in the chloroplast was well above that of the leaf tissue implying accumulation of Cl- in the chloroplast. Qualitatively similar results were obtained for chloroplasts isolated by rapidly fractionating protoplasts of wheat and barley although the concentration of ions was lower than for the above three species. Spinach grown in 200 mM NaCl accumulated high levels of Na+ and Cl- in the leaves but K+ was decreased. The replacement of K+ by Na+ was also observed in the chloroplasts but the level of Cl- in the chloroplast was not greatly increased even at leaf Cl- in excess of 200 mM. Replacement of K+ by Na+ in the nutrient solution also resulted in replacement of K+ by Na+ both in the leaf and in the chloroplast. Spinach grown under Cl- deficiency showed a 70% decrease in growth and a similar decrease in leaf Cl- but the concentration of Cl- in the chloroplast was not significantly decreased. It is concluded that in most instances K+ is the dominant monovalent cation in chloroplasts, although replacement by Na+ can occur to some extent. Chloride is a major anion, being accumulated within the chloroplast at low leaf Cl- and excluded from the chloroplast at high leaf Cl-. Ionic compartmentation within the leaf cell appears to be regulated to maintain a relatively constant level of Cl- in the chloroplast.