Proline Accumulation in Water Stressed Cotton Leaves1

Greenhouse‐grown cotton (Gossypium hirsutum L.) plants were subjected to a drying cycle when the plants had four true leaves and the proline content of the leaf tissue was determined. Free proline accumulated to about 100 times the concentration of the well‐watered controls as the stress increased. The threshold of the predawn leaf water potential for proline accumulation was about —15 to —17 bars. Total proline content also increased, which suggested that there was some proline synthesis. Free glutamic acid concomitantly decreased. Because proline did not accumulate until the water deficits were quite severe concentration of free proline is not a good indicator of the onset of plant water stress.