Physiology of flowering and grain filling in faba bean

The development of flowers and then that of seeds are key processes in the formation of yield in faba bean (Vicia faba L), as in other grain legumes. Winter faba bean generally has a quantitative vernalization requirement, allowing flowering to occur at a lower node than in unvernalized plants. Some germplasm is day-neutral, other germplasm is long-day with a critical daylength between 9.5 and 12 h. Progress toward flowering follows a conventional thermal-time model. with 830-1000 degrees C-d above 0 degrees C required for the onset of flowering and an optimum temperature of 22-23 degrees C. Flowers may abscise from the crop because of lack of pollination, because proximal flowers on the same raceme are fertilized, because of vegetative-reproductive competition for assimilate, or because of stresses such as drought. The large seed size of faba bean has enabled this species to be a model for studies of the molecular physiology of seed development. Seed filling in the retained pods proceeds through well defined pre-storage and storage phases. During the pre-storage phase, cell expansion occurs mostly in the endosperm and seed coat while the embryo is in a cell division phase. Extracellular invertase from the inner cell layers of the seed coat acts on sucrose unloaded from the phloem, ensuring that the rapidly dividing embryo cells are bathed in hexose-rich fluid. With further development of the embryo, endosperm sugar levels become depleted and the embryo relies more directly on nutrients released by the seed coat. In the transition to the storage phase, the cotyledon cells expand, synthesize storage proteins and starch, and undergo endopolyploidization. Thin-walled parenchyma cells in the seed coats differentiate into transfer cells and the enhanced area of plasma membrane results in increased nutrient flow to the rapidly growing embryo. Release of sucrose and potassium into the seed apoplasm is energy-coupled through a plasma membrane H+-ATPase and a sucrose/H+-antiport. Subsequent radial transfer of nutrients to the storage parenchyma cells of the cotyledons follows a symplastic pathway through numerous plasmodesmata. Cotyledon cell expansion stops when the mechanical restraints of the seed coat and space within the pod cavity are met. It is now possible to identify genes for manipulation that may make seed setting and final seed size less susceptible to environmental stresses. (C) 2009 Elsevier B.V. All rights reserved