The source sink relationship of a cotton plant is critical in understanding its productivity and management during the growing season. Most of the cotton plant’s carbohydrate energy is directed to root growth prior to the time reproductive growth begins. This is a function of carbohydrate source to sink relationships. Carbohydrates are transported from supply areas, called sources, to areas of growth or storage, called sinks. The leaves are the primary source of carbohydrate production during the early vegetative growth of cotton. Carbohydrates are produced through photosynthesis in the leaves and channeled through the phloem to the roots, which act as the main carbohydrate sinks during this phase. The source-to-sink phenomenon also applies to the transport of inorganic nutrients and water. The roots are the source for all inorganic nutrients and water, which are transported through the xylem to sinks throughout the plant. Thus, the root and shoot systems are very interdependent, and damage to either system slows growth and decreases yield.

Figure 1. Source to sink relationships at two stages of cotton growth. During early vegetative growth, most of the carbohydrates produced by the leaves are sent to the root system. Later in the season, how-ever, most of the carbohydrates are sent to the developing bolls, and the root system and shoot growth rate decline.

As bolls begin to develop, they become much stronger carbohydrate sinks than roots and shoots. At this stage, root and shoot growth slow, and boll development dominates plant growth, and the widely established roots continue to supply large quantities of water and nutrients to the shoot.