Intrinsic factors controlling growth under sink limited conditions

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Growth in plants is either limited by source activity (carbon fixation) or by the capacity of the sinks to use or store carbon. We still do not know exactly what limits growth in major sinks such as storage organs. In spring ephemerals, storage organ growth is mostly determined by cell size rather than by cell number, and these organs get larger when plants are grown at lower temperatures. We want to test the role plays by endoreduplication in determining cell size in these organs. The level of endoreduplication can be monitored by quantifying cell DNA content using flow cytometry. We will determine when endoreduplication takes place and how it is influenced by growth temperatures in different spring ephemerals and a few temperate storage crops such as beet and potato. We will then try to modulate the number of endoreduplication cycles by applying metabolites and phytohormones known to influence the cell cycle.**** Source and sink activities are continuously modulated to avoid disequilibrium. For example, under sink limitation, photosynthesis is downregulated, which can even lead to leaf senescence. However, so far no signaling pathway from sink to source has been conclusively confirmed. We have recently identified metabolites that could be involved in inducing leaf senescence under sink limited conditions in Erythronium americanum. We will now confirm their role as signaling molecules, using different approaches. First, we will prolong sink growth by delaying leaf senescence using phytohormones. This approach should allow us to separate the sink limitation signal from that of leaf senescence and identify metabolites specific to each process, using metabolomics. Second, we will compare leaf and bulb metabolite profiles at the onset of leaf senescence in both source and sink limited plants of the same species. We will use wild leek of different ages, as we have recently found that growth is source limited in seedlings, and gradually becomes sink limited as plant ages. Third, we will compare metabolite profiles of spring ephemeral species from different taxonomic families to determine how widespread these sink limited signaling molecules are.**** This study will help us understand why plants have evolved to senesce their leaves under sink limited conditions, even though this reduces total plant growth capacity. Identifying the signaling molecules involved is an important step toward answering this question, but also if we attempt to delay leaf senescence and improve growth in sink limited species. This study will also characterize the role played by endoreduplication in determining final cell size in storage organs of many temperate species which will improve our comprehension of the factors that limit sink growth and thus yield. Two Ph.D., one master's, and 5 B.Sc. students will gain expertise in whole-plant source-sink studies and in functional metabolomics during the 5 years of the project.***