Photoassimilate Translocation

Abstract

During evolution, early land plants were challenged by serious environmental pressures for their survival. The main challenge was the absorption and retention of water. This pressure for survival led to the differentiation of roots for absorption of water and inorganic nutrients and leaves for light absorption, photosynthesis, and gaseous exchange. Development of leaves rendered plants capable of carrying out photosynthesis. Xylem strands are responsible for transport of water and minerals from roots to the aerial parts of the plants, while translocation of photosynthetic products (photoassimilates) is facilitated by phloem elements. It is estimated that as much as 80% of the photosynthetically fixed carbon can be exported out of mature leaves. Storage or photosynthesizing organs, which have surplus sugars, can either metabolize or export them. These are known as source. On the contrary, actively metabolizing organs or the ones which store carbohydrates need to import them. These plant parts are known as sinks. Life cycle of a plant is characterized by source-sink transitions due to changes in sink strength. A plant consists of series of sources and sinks, with many sinks competing for sugars exported by the source organs. Phloem plays a major role in connecting source and sink. In the early developmental stage of a plant, roots and shoots majorly compete for receiving photoassimilates, and later on many other organs become effective sinks. These include reproductive structures, buds and flowers, and developing grains or the underground storage organs, such as tubers. Sink strength or sink dominance refers to the capacity of sink organs to acquire sugars from the transporting vascular strands. Distribution of sugars in the sink is the key factor in determining the harvest index (HI), which refers to the ratio of dry weight of harvestable part (economically important) of the plant to the total dry weight of the plant. The higher the ratio (high value of HI), the higher the plant productivity. Thus, transport of photoassimilates is targeted as the key factor determining plant productivity. Various abiotic and biotic factors adversely affect translocation of sugars. Accumulation of sugars in the cytosol of mesophyll cells at the source is the key factor for inhibiting photosynthesis. Studying phloem structure is important since it would facilitate in learning about the molecular mechanism involved in sugar translocation. Mineral deficiency may result in increase in the ratio of roots to shoots. Various sucrose transporter proteins (SUTs), facilitating intracellular sugar transport between various subcellular compartments as well as from cell to cell, have potential roles in controlling sucrose movement to the desired sinks. The chapter deals with the source and sink concept, pathways involved in photoassimilate transport, and unique features of sugar translocation. Later part of the chapter shall explore the mechanisms of phloem translocation, including phloem loading and unloading of photoassimilates and distribution of photoassimilates that encompasses allocation and partitioning.