Modificazioni Strutturali Dei Plastidi in Semi Germinanti

Abstract

Structural changes in plastids of germinating seeds. Observations on the Clementine (CITRUS NOBILIS x C. AURANTIUM AMARA PUMILA) cotyledons and the Castor-bean (RICINUS COMMUNIS) endosperm. — The structural changes in the plastids of germinating seeds have been studied with the electron microscope in the cotyledons of the Clementine (Citrus nobilis x C. aurantium amara pumila) and in the endosperm of the Castor-bean (Ricinus communis). In the seeds of the Clementines the chloroplasts are already present in the repening fruit, and contain one or few isolated grana, which are never connected by stroma lamellae. In the endosperm of resting Castor-bean seeds, the plastids, as other cytoplasmic organules, are not easily found, being the cells filled with large and numerous fat bodies or oil vacuoles, crowded and packed together so that some leucoplasts can be found only in the germinating seeds. During seed germination, reserve starch, synthesized from the fat representing the seed storage, is deposited in the Clementine chloroplasts and, in smaller amounts, in the Castor-bean plastids. When the Clementine seeds germinate under light conditions, the chloroplasts develop a larger system of lamellae, but the stroma lamellae are never formed. Instead, if seed germination occurs in the darkness, the lamellar system gradually involves, and a vescicular body is finally formed. The chloroplasts of Clementines show therefore a feature of the greatest interest. Indeed, in the ripening seeds, they become green and form a lamellar system in the darkness, but their lamellar system is however anomalous, lacking of stroma lamellae. During seed germination in the light, the lamellar system, even more developed, retains its anomalous features. On the contrary, when seeds germinate in the darkness, the chloroplasts are unable not only of increasing, but also of preserving their lamellar system, built during fruit ripening, which undergoes an involutive process. In the plastids of both the Castor-bean endosperm and the Clementine cotyledons, rounded organules are formed, at the oldest stages of germination, beetween the two membranes of the plastid. These organules, which can be found also outside the plastid, can be identified with the ones described by Weier and Thompson (1962) as « spherosomes » and by Hohl (1960–1961) as « Einschlusskörpern »; they are here interpreted as an expression of involutive events, consequent to the processes of tissue exhaustion.