Ricerche Sulla Morfogenesi Dei Plastidi Clorofilliani

Abstract

Studies on chloroplast morphogenesis. The effect of sucrose feeding and light intensity on the plastids of etiolated plants. — The changes in the fine structure of the plastids of etiolated Bean plants, dipped into water or in various sucrose concentrations, for 24, or more, hours, and exposed to conditions of darkness and weak light, were studied at the electron microscope, and protochlorophyll, chlorophyll a and chlorophyll b contents were determined. When the etiolated plants are dipped into water or sucrose solutions, in the dark, rows of tubules and lamellae, often stacked and resembling small grana, are formed in the plastids. These structural changes of plastids of the plants exposed to conditions of darkness, where no protochlorophyll was converted to chlorophyll a, are quite similar to those described by several Authors for plants exposed to light conditions and thought to coincide with the protochlorophyll-chlorophyll transformation. Thus, the preservation of the « Kristallgitterstruktur » of the vesicular centers, or, instead, their transformation into a lapse cluster of tubules, does not seem to be related with protochlorophyll accumulation in them; indeed, an increase of protochlorophyll contents was observed both with the preservation of the crystalline structure, and with its transformation, and low protochlorophyll contents did not always coincide with the transformation of the vesicular centers. In the plants exposed to weak light (1 ft-c) there is chlorophyll a and b accumulation, and a more pronounced tendency toward stacking of tubules and lamellae. In the plants exposed to weak light, dipped into water or sucrose solutions, at the lowest concentrations, and for the shortest periods, the vesicular centers are transformed into clusters of tubules; but with higher sucrose concentrations, or longer dipping periods, their crystalline structure is preserved, just as if their preservation would depend only by an adeguate nutrients supply.

The arrangement of normal lamellae and the formation of grana connected by intergrana lamellae occur, anyhow, only when the etiolated plants are exposed to « high » light (630 ft-c). But chlorophyll accumulation is possible under « weak » light (and a stacking of tubules and lamellae, resembling small grana, also occurs), when sucrose is supplied. The achievement of the « normal », complete structure of the chloroplast is, therefore, here interpreted in the sense that it represents only the functional aspect of its organization, determined by a light intensity favorable to its photosyntetic activity, which is not directly necessary for the synthesis of the contistuents of the lamellar system (chlorophylls, phospholipids).