Post-Anthesis Economy of Carbon in a Cultivar of Cowpea

Abstract

Budgets for transfer of carbon from individual leaves and other source organs to fruits and nodulated roots were constructed for stages of the post-flowering development of symbiotically-dependent cowpea (Vigna unguiculata L. Walp. cv. Vita 3-Rhizobium strain CB756). Exportable surpluses of carbon from sources, assessed from net exchanges of CO₂ and changes in carbon content, were allocated to sink organs in proportion to carbon consumption (growth and respiration) and the ability of each sink organ to attract assimilates from the sources, as demonstrated by ¹⁴C-feeding. The first 10 d after flowering showed high sink activity by roots, stem and petioles, low consumption by fruits, with the upper three trifoliate blossom leaves providing the bulk of the required assimilates. The next 10 d showed a sharp decline in photosynthesis of the leaf subtending the oldest fruit followed by similar declines in leaves at the other fruiting nodes. All leaflets at fruiting nodes abscised during the final 10 d period, while the two lower leaves, not subtending fruits, remained green and supplied most of the carbon required by developing fruits and roots. Throughout fruiting all currently-active sources supplied all sinks, with only slight evidence of blossom leaves specializing in nourishing their subtended fruits. Of the carbon translocated from leaves during fruiting 32% came from the topmost leaf, 28% from the leaf below this, 16% from the next leaf, and the remaining 24% from the lowest three leaves. Some 80% of the fruit's total intake of carbon came from leaves, the rest from mobilization of stored carbon (partly sugars and starch)from other vegetative parts.