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.