Developmental Changes in Chloroplast Ultrastructure and Carbon‐Fixation Metabolism of Dendrobium Flowers (Orchidaceae)

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Abstract
The developmental changes in chloroplast ultrastructure and carbon‐fixation metabolism were studied in flowers of Dendrobium Sonia, an orchid with Crassulacean acid metabolism (CAM). The chloroplasts in the flowers were smaller and had fewer thylakoid membranes and grana than in the leaf. During flower development, the chloroplasts degenerate and become less distinct, with dilated thylakoid membranes. The chlorophyll a/b ratio of the flowers and leaf were similar, while total chlorophyll contents were lower than in the leaf. The total chlorophyll content and the RuBPcase activity in the flowers declined as they matured. In contrast, PEPcase activity increased with flower development. At all stages of flower development, PEPcase/RuBPcase ratios were ca. 10–20‐fold higher than in the leaf. As the flower aged, the PEPcase/RuBPcase ratio increased with decreased rate of 14CO2 fixation. The δ13C values of the flowers were less negative than those of the leaves. Following a 14CO2 feed in the light, a considerable amount of 14C‐metabolites appeared in the organic acids (35%–51%) and sugar (30%–50%) fractions in the flowers. The nocturnal malate increase (NMI) in the flower bud was only 2% of the leaf, and this increased to 14% at the fruit capsule stage. Our results indicate that flowers of Dendrobium Sonia had weak CAM activity in the dark and some degree of β‐carboxylation in the presence of C3 metabolism in the light.