Proteins, non-structural carbohydrates and organic acids in the flowers of sweet orange (Citrus sinensis (L.) osbeck)

The concentration of soluble proteins in open flowers of sweet orange (Citrus sinensis (L.) Osbeck) was about 35% of the protein content found in green leaves, while ovary and stigma, with attached style, contained up to 85%. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) indicates that flowers and leaves contained ribulose bisphosphate carboxylase-oxygenase (Rubisco). At least 12 polypeptides, ranging from 22 to 100 kDa, were more distinct in flowers than in leaves. The polypeptide band at about 100 kDa in the flower extract suggests the presence of phosphoenolpyruvate carboxylase (PEPCase) in citrus flowers. Activity of Rubisco and PEPCase, expressed in μmol g tissue fresh weight⁻¹ hr⁻¹, averaged 14.2 and 64.6 for intact open flowers, and 547.9 and 63.3 for the leaves, respectively. Starch and reducing sugars were 1.2- and 8.4-fold higher, respectively, and dark respiration rates were about five-fold greater for open flowers than fully expanded, sun leaves. Quinic and malic acids made up about 96% of the organic acids found in flowers and leaves. Ascorbic, citric, fumaric, and shikimic acids were in small or trace amounts. The large accumulation of starch and soluble sugars, in addition to the presence and activity of Rubisco and PEPCase, indicates that the flowers of sweet orange would have some capability to perform photosynthetic CO₂ assimilation, the metabolites of which might play some important role in flower development and fruit setting.