After pollination occurs the cotton boll begins to develop. Under optimum conditions it requires approximately 50 days for a boll to “open” after pollination occurs. Boll development can be characterized by three phases: enlargement, filling, and maturation.

The enlargement phase of boll development lasts approximately 3 weeks. During this time the fibers produced on the seed are elongating and the maximum volume of the boll and seeds is attained. Also during this time, the fiber is basically a thin walled tubular structure, similar to a straw. Each fiber develops from a single epidermal cell on the seed coat. During the boll enlargement and fiber elongation phase, the development of the fiber is very sensitive to adverse environmental conditions. Low water availability, extremes in temperature and nutrient deficiencies (especially potassium) can reduce the final fiber length.

The filling phase of boll development begins during the fourth week after flowering. At this time, fiber elongation ceases and secondary wall formation of the fiber begins. This process is also known as fiber filling, or deposition. Cellulose is deposited inside the elongated fiber every 24 hours, filling the void space of the elongated fiber. The deposition of cellulose into the fiber cell is also sensitive to environmental conditions. Water, temperature and nutrients (especially potassium) are the primary environmental factors that influence this stage of boll development. The filling phase of boll development continues into the sixth week after pollination.

The boll maturation phase begins as the boll reaches its full size and maximum weight. During this phase, fiber and seed maturation take place and boll dehiscence occurs. The capsule walls of the boll dry, causing the cells adjacent to the dorsal suture to shrink unevenly. This shrinking causes the suture between the carpel walls to split, and the boll opens.

Figure 1. Development of the cotton boll. Undeveloped cotyledons have a jelly-like consistency. As the cotyledons develop, they become firm and leaf-like, and a black layer that will become the seed coat begins to form around the seed. As the black layer matures and the outside of the boll dries, the dorsal suture splits, allowing the cotton boll to open.