Source / Sink Relations and Pathways for Carbohydrate Transport and Targeting in Cotton

Sugar transport from photosynthetic source leaves to heterotrophic sink organs is a fundamental aspect of plant growth and development, and consequently of yield in crop plants.  The pathway from source to sink is not direct, in part because what constitutes a source and what constitutes a sink changes as the plant matures.  After germination and exhaustion of seed storage reserves, the primary source organs are the photosynthetic leaves or, in some species, photosynthetic stems.  The sink organs include the growing shoot and root apices, as well as the cambial tissues for lateral expansion, but also include storage tissue.  Assimilated carbon and nutrients in these reservoirs may be mobilized during seasonal growth, as exemplified by perennials, or when demand out paces photosynthetic output, such that these tissue can transition between source and sink relatively easily.  Gossypium species are perennials and store starch in the stems and roots.  Cultivated cotton is grown as an annual, and indications are that stored starch is utilized to accommodate the demands of boll maturation, with developing fiber and the growing constituting the principal and competing sinks.  The molecular and biochemical events occurring in various source and sink organs of cotton, and how they change during plant maturation, is poorly understood.  In this presentation, the primary paths of photoassimilate transport and source / sink relations will be discussed with the goal of stimulating discussion on research priorities and targets for yield improvements.