PHYSIOLOGICAL, STRUCTURAL, AND IMMUNOLOGICAL CHARACTERIZATION OF LEAFAND CHLOROPLAST DEVELOPMENT IN COTTON

Many of the studies of chloroplast ontogeny in higher plants have util ized suboptimal conditions of light and growth to assess development. In this study, we utilized structural, immunological, and physiologica l techniques to examine the development of the chloroplast in fieldgro wn cotton (Gossypium hirsutum cv. ''MD 51 ne''). Our youngest leaf sam ple developmentally was completely folded upon itself and about 0.5 cm in length; leaves of this same plastochron were followed for three we eks to the fully expanded leaf. The chloroplasts at the earliest stage monitored had almost all of the lamellae in small, relatively electro n-opaque grana, with relatively few thylakoids which were not appresse d on at least one surface. During the development of the thylakoids, t he membranes increase in complexity, with considerable stroma lamellae development and an increase in the number of thylakoids per granum. B esides the increase in complexity, both the size and numbers of the ch loroplast increase during the development of the leaf. Developmental c hanges in six thylakoid proteins, five stromal proteins, and one perox isomal protein were monitored by quantitative immunocytochemistry. Eve n at the earliest stages of development, the plastids are equipped wit h the proteins required to carry out both light and dark reactions of photosynthesis. Several of the proteins follow three phases of accumul ation: a relatively high density at early stages, a linear increase to keep step with chloroplast growth, and a final accumulation in the ma ture chloroplast. Photosystem-II(PS II)-related proteins are present a t their highest densities early in development, with an accumulation o f other parts of the photosynthetic apparatus at a latter stage. The e arly accumulation of PS-II-related proteins correlates with the much l ower ratio of chlorophyll a to b in the younger leaves and with the ch anges in fluorescence transients. These data indicate that some of the conclusions on chloroplast development based upon studies of intercal ary meristems of monocots or the greening of etiolated plants may not be adequate to explain development of chloroplasts in leaves from apic al meristems grown under natural conditions.