Inhibition of Ferricyanide Reduction in Chloroplasts Prepared from Water‐stressed Cotton Leaves1

Abstract

Water stress was applied to cotton (Gossypium hirsutum L.) chloroplasts and leaves by the following treatments: (i) prepared chloroplasts were. placed in assay media different osmotic potentials; (ii) cut shoots were allowed to transpire under illumination; (iii) whole potted plants were held for 5 days without irrigation; and (iv) cell sap was expressed from individual leaves via the xylem in a pressure bomb. These treatments caused a decrease in ferricyanide reduction when isolated chloroplasts were assayed potentiometrically for the Hill reaction. The above treatments caused (i) 1.7%, (ii) 1.4%, (iii) 1.8%, and (iv) 1.9% decrease of initial activity rcspectively per bar of water potential decline. This decrease in activity, which was nearly linear with the degree of stress, occurred from —3 to —30 bars leaf water potential in treatments ], 3, and 4, and from —15 to —30 bars in treatment 2. These results indicate that water stress in the leaf can directly influence the photochemical activity of the chloroplasts.