Light-induced Structural Changes Research Articles

Light-Induced Changes in the Fluorescence Yield of Chlorophyll a In Vivo: II. Chlorella pyrenoidosa

The long-term fluorescence induction in Chlorella pyrenoidosa consists of a fast rise of the fluorescence yield from the level S (of the first wave transient) to a maximum M, followed by slower decay to a terminal stationary level T. The maximum M is attained within 40 seconds from the onset of illumination while the decay to the terminal level T lasts for several minutes. The fluorescence rise (S --> M) coincides with an increase in the rate of oxygen evolution, which, however, remains constant during the fluorescence decay (M --> T). Poisons of photosynthesis 3, (3,4-dichlorophenyl)-1,1 dimethylurea (DCMU, o-phenathroline) inhibit the fluorescence induction, while uncouplers of photophosphorylation affect the fluorescence time course only when they function at an early stage of the coupling sequence e.g., carbonyl cyanide p-trifluoremethoxy phenylhydrazone, (FCCP, atabrin). Phosphorylation inhibitors affecting only the terminal esterification step (phlorizin) have little effect on the fluorescence kinetics. These results suggest that the fluorescence induction requires the operation of a phosphorylating electron transport and that it is possibly related to the light-induced structural changes which accompany photophosphorylation.

Regulation of light-induced swelling of isolated chloroplasts

A study on light-induced structural changes in isolated chloroplasts has led to some conclusions and assumptions, which could be summarized as follows: 1. Structural changes are associated with a transfer of electrons Izawa, Good 1966; 2. There is a dependence between a high-energy state of chloroplast (X E) and direction (swelling - contraction) of structural changes Hind, Jagendorf 1965;3. Swelling and contraction are connected with the light-induced shift in the internal pH of chloroplast Deamer, Crofts, Packer 1967. The present paper provides some experimental results, which testify a possibility of the key role of ATPase in the changes in structural states of chloroplasts.

Mechanisms of light-induced structural changes in chloroplasts I. Light-scattering increments and ultrastructural changes mediated by proton transport

Chloroplasts suspended in NaCl solutions undergo structural modifications when illuminated by red actinic light in the presence of electron carriers. These changes are manifested by increments in light scattering and volume, and disruptive alterations of grana structure as demonstrated by electron microscopy. Simultaneously with these modifications protons are transported into the chloroplasts in a light-dependent process. It was postulated that the decrease in internal pH which would result from the proton transport can cause many of the changes observed in chloroplasts during illumination. The following experimental results support this argument: 1. (a) The kinetics of proton transport and light-scattering increments resemble each other both in onset and decay. 2. (b) The pH activation curves are similar for both processes, with optima lying between 5.5 and 6.5. 3. (c) Lowering external pH in the dark to levels which reproduce theoretical internal hydrogen ion concentrations causes alterations in light scattering, volume, and ultrastructure similar to those induced by illumination. Hence light-induced modifications of chloroplast structure in NaCl solutions are satisfactorily explained by a decreased internal pH resulting from proton transport into the chloroplast.

Mechanisms of light-induced structural change in chloroplasts II. The role of ion movements in volume changes

1. The effect of certain anions in enhancing light-induced light-scattering increments in isolated spinach chloroplasts has been studied, and the relation of these effects to volume changes and ion movements has been investigated. 2. Changes in transmission or absorbance have been shown to provide a better indication of volume changes of chloroplasts in suspension than do changes in 90° light scattering. Absorbance measurements were therefore employed to study osmotic behavior of chloroplasts suspended in a variety of sodium and ammonium salts. Chloroplasts swell rapidly in the dark in the ammonium salts of weak acids. Since the rate of swelling is greatest with anions whose p K a is highest, it is concluded that chloroplasts are permeable to undissociated acids and ammonia. 3. A mechanism has been proposed to explain the effects of anions in enhancing light-scattering changes in illuminated chloroplasts. It is suggested that on acidification of the chloroplast interior by light-induced hydrogen ion uptake, weakly acidic anions are lost as a result of the displacement of the equilibrium of undissociated acid across the chloroplast membrane, and that the volume changes observed are by osmotic equilibration. The kinetics and pH dependence of the light-induced light-scattering and hydrogen ion changes are consistent with this mechanism. 4. The light-induced movement of cations in suspensions of chloroplasts in acetate and chloride media has also been measured. On illumination in Tris-acetate, the small amount of cation retained by chloroplasts is irreversibly lost but a reversible uptake of cation on illumination of chloroplasts suspended in choline or Tris-chloride is observed. The extent of the light-induced cation movements is too small to account for chloroplast volume changes.