Whole‐plant Responses of Brassica Campestris (Cruciferae) to Altered Sink‐source Relations

Plants may respond to developmentally or environmentally induced changes in the relationship between source (assimilate exporting) and sink (assimilate importing) organs through a number of regulatory mechanisms. The purpose of this work was to describe responses in leaf physiology, organ biomass partitioning, and reproduction of Brassica campestris L. to altered sink‐source relations. Partial defoliation early in ontogeny increased sink‐source ratio after 1 day and was accompanied by a significant increase in area‐based photosynthetic capacity. Assimilate was preferentially partitioned to new leaf tissue at the expense of stem and reproductive tissue so that sink‐source ratio and photosynthesis returned to control levels within a week. Bud excision at flower initiation decreased sink‐source ratio; plants in this treatment responded through altered biomass partitioning, but there were no detected changes in leaf physiology. Plants that were hand‐pollinated had decreased specific leaf weight, and within a week, sink‐source ratio increased. Plants that were pollinated and then completely defoliated produced 40% as many seeds as plants that were not defoliated, presumably through stem and/ or fruit photosynthesis, and at the expense of flower production. Together these results illustrate that regulation of sink‐source relations can occur through complex responses at several levels of organization over a wide range of time scales.