Cotyledonal chloroplasts in the hypogeal seeds of clementine

Abstract

Clementine (Citrus nobilisxCitrus aurantium amara pumila) is a chloroembryophyte with green quiescent embryos and hypogeal germination. The cotyledonal chloroplasts have been studied during germination in the dark and under two different irradiances 120 and 240 μmol·m^-2·s^-1 throughout a period of three weeks. The plastids of the outer adaxial and inner regions develop differently. In the light, the former differentiate a photosynthetically active thylakoid system with an ultrastructural organization and a polypeptide composition resembling that of leaf chloroplasts. The “inner” chloroplasts maintain an organization reminiscent of chloroplasts of the quiescent embryo and never get beyond the photosynthesis/respiration compensation point; their differentiation pattern appears essentially the same under the two different irradiances. These observations and the germination in the dark indicate that the above differentiation is not strictly photodependent. The greening ability of the cotyledons provides, on occasion, an additional photosynthetic supply to this plant.