Light-dependent volume changes and reactions in chloroplasts. I. Action of alkenylsuccinic acids and phenylmercuric acetate and possible relation to mechanisms of stomatal control.

Clhloroplasts are known to change their volume in vitro by 3 mechanisms. Light and osmotic pressure appear to be the most significant factors for controlling chloroplast volume. The simplest tvpe of volume change shown by isolated chloroplasts has been described by Nishida (8), who found by absorbancy, gravimetric, and volumetric techniques that chloroplasts change their volume in response to exposure to solutions of different tonicity. Hence, chloroplasts in vitro are osmotically sensitive structures like mitochondria and cells. The 2 other mechanisms for bringing about volume chlanges in chloroplasts require the action of light. One mechanism seems closely geared to the energy transfer reactions that are coupled to electron flow. Packer (12) has shown that suspensions of spinach chloroplasts exlhibit light-induce(d inlcreases of scatteredl light that are rapid and reversible. occurriil in a tillme ilnterval of 20 to 100 seconids. Substances that interact with tlle energy transfer path-way. suclh as amimioniur chloride aind ADP. inhibit liglht-scattering increases (2), while ATP unlder condlitions favorinig its hydrolysis promotes light-scattering increases (14). These observations are consistent with the viewv that volume changes are uncler the control of energy-linked initermediates (2. 14. 15). Itoh et al. (3) have shoNwn that this action of light brings about a low-amplitude shrinkage that results in a 50 to 60 %(decrease in volume of whole chloroplasts, as measured by the volume distribution of chloroplasts in the Coulter counter. Clhloroplasts isolated from Eitglena (1) have also been reported to manifest light-dependent volume changes. Another action of light on chloroplast volume results in high-amplitude swelling (16, 17). Swelling is brought about slowly in the (lark but can be accelerated by light, especially if a cofactor such as phenazine methosulfate has been added to the chloroplasts. Light-dependent, high-amplitude swelling of chloroplasts requires 10 to 90 minutes for comple-