This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Robinson, S. P. Search for Related Content PubMed PubMed Citation Articles by Robinson, S. P. Agricola Articles by Robinson, S. P.

Articles

Osmotic Adjustment by Intact Isolated Chloroplasts in Response to Osmotic Stress and Its Effect on Photosynthesis and Chloroplast Volume

CSIRO Division of Horticultural Research, G.P.O. Box 350, Adelaide, S.A. 5001, Australia

Spinach leaf chloroplasts isolated in isotonic media (330 millimolar sorbitol, –1.0 megapascals osmotic potential) had optimum rates of photosynthesis when assayed at –1.0 megapascals. When chloroplasts were isolated in hypertonic media (720 millimolar sorbitol, –2.0 megapascals osmotic potential) the optimum osmotic potential for photosynthesis was shifted to –1.8 megapascals and the chloroplasts had higher rates of CO₂-dependent O₂ evolution than chloroplasts isolated in 330 millimolar sorbitol when both were assayed at high solute concentrations.

Transfer of chloroplasts isolated in 330 millimolar sorbitol to 720 millimolar sorbitol resulted in decreased chloroplast volume but this shrinkage was only transient and the chloroplasts subsequently swelled so that within 2 to 3 minutes at 20°C the chloroplast volume had returned to near the original value. Thus, actual steady state chloroplast volume was not decreased in hypertonic media. In isotonic media, there was a slow but significant uptake of sorbitol by chloroplasts (10 to 20 micromoles per milligram chlorophyll per hour at 20°C). Transfer of chloroplasts from 330 millimolar sorbitol to 720 millimolar sorbitol resulted in rapid uptake of sorbitol (up to 280 micromoles per milligram chlorophyll per hour at 20°C) and after 5 minutes the concentration of sorbitol inside the chloroplasts exceeded 500 millimolar. This uptake of sorbitol resulted in a significant underestimation of chloroplast volume unless [¹⁴C]sorbitol was added just prior to centrifuging the chloroplasts through silicone oil. Sudden exposure to osmotic stress apparently induced a transient change in the permeability of the chloroplast envelope since addition of [¹⁴C]sorbitol 3 minutes after transfer to hypertonic media (when chloroplast volume had returned to normal) did not result in rapid uptake of labeled sorbitol.

It is concluded that chloroplasts can osmotically adjust in vitro by uptake of solutes which do not normally penetrate the chloroplast envelope, resulting in a restoration of normal chloroplast volume and partially preventing the inhibition of photosynthesis by high solute concentrations. The results indicate the importance of matching the osmotic potential of isolation media to that of the tissue, particularly in studies of stress physiology.

This article has been cited by other articles:

				K. A. PYKE and C. A. HOWELLS Plastid and Stromule Morphogenesis in Tomato Ann. Bot., November 1, 2002; 90(5): 559 - 566. [Abstract] [Full Text] [PDF]