Fruit Set in Lupinus angustifolius Cv. Unicrop. II. Assimilate Flow During Flowering and Early Fruiting

Abstract

Assimilate distribution in Lupinus angustifolius was studied during 7 weeks after emergence of the main inflorescence by feeding [¹⁴C]urea to a single leaf or group of leaves on a plant and measuring ¹⁴C in plant parts 24 h after feeding. Nine times of feeding were involved, each with ¹⁴C treatments encompassing leaves of the main stem and first-order laterals. The inflorescence was a minor sink for assimilates compared with root, main stem and developing lateral shoots. During the first 4 weeks the inflorescence relied on main stem leaves, especially the upper leaves, but in the fifth week leaves of upper laterals became principal sources of assimilates. The transition occurred after four to six basal fruits had set, and just as upper flowers were commencing to abscise. Estimates were made of transfer of photosynthetically fixed carbon from the uppermost main stem leaf to the inflorescence, using data on carbon dioxide exchange and changes in carbon content of this leaf and its translocatory commitment to the inflorescence as determined by ¹⁴C feeding. Assimilate flow from leaves to flowers was confined largely to organs of the same or adjacent orthostichies (2/5 phyllotaxis), but these affinities became less definite during fruiting. Import of ¹⁴C by reproductive units was related to phenology, position on an inflorescence, and accumulation of dry matter. All flowers attracted assimilates strongly at the bud stage but lost sink strength progressively after opening. Upper flowers destined to abscise failed to import assimilates or to bleed from phloem for several days before being shed. Lower flowers which set fruits showed a rapid resurgence of sink strength once their corollas had senesced and the young fruits had commenced to elongate.