Cotton,by origin,is a tropical plant and does best under warm temperature conditions during both the vegetative and reproductive growth stages.This includes the germination and emergence phases of plant growth.Cotton germinates and emerges poorly under cool conditions,especially when the soil temperature is below 65 ° Fahrenheit. Cool soil temperatures have a detrimental effect on the germination of cottonseeds in two ways.First,cool soil temperatures at the time of planting and until the seeds are hydrated, can cause imbibitional chilling injury.This occurs when dry seeds imbibe cool water.If this imbibition of cool water occurs too rapidly, the embryo cells may be injured or killed due to membrane stretching and rupture.The second mechanism by which germination and emergence are slowed under cool conditions is through the effect of low temperatures on slowing the metabolic events occurring during this phase.Both of these components,either individually or together,will result in slower emerging seedlings.

The three seedlings below were subjected to chilling temperatures while the three seedlings on the next page were not chilled during the imbibition phase (normal seedlings).During the first six hours of imbibition,the damaged seedlings were exposed to a temperature of 40 ° Fahrenheit. After the chilling period they were moved to a chamber set at 86 ° Fahrenheit for two to four days. The curling,shortening and thickening of the roots are typical of imbibitional chilling injury.The chilling during this phase of imbibition injures and typically kills the root tip meristematic tissue. This results in cessation of normal taproot growth.Subsequently,lateral roots develop to compensate for this loss. Typically these seedlings may survive and produce productive plants if additional stresses such as water deficit or disease are not encountered. The picture to the right is a close-up view of a seedling injured by chilling.The root tip meristematic tissue is dead,which results in the brown color.In many cases,the root tissue behind the dead root tip will be enlarged.Presumably, this results from a buildup of food reserves mobilized to the root meristematic tissue for growth.However, with a dead tip,these materials are not used and,therefore, accumulate. The two seedlings to the left show normal root development. When the two groups are compared it may be noted that seedlings injured by chilling are often short with thickened hypocotyls and radicles, dead root tips,and show some signs of lateral root growth.The six seedlings pictured above depict various morphological responses to chilling injury.