Osmoregulation in Cotton Fiber: Accumulation of Potassium and Malate during Growth.

Kinetics and osmoregulation of cotton (Gossypium hirsutum L.) fiber growth (primarily extension) have been studied. Growth is dependent on turgor pressure in the fiber. It is inhibited when a decrease in the water potential of the culture medium due to an addition of Carbowax 6000, equals the turgor pressure of the fiber. Potassium and malate accumulate in the fiber and reach peak levels when the growth rate is highest. Maximum concentrations of potassium and malate reached in the fiber can account for over 50% of the osmotic potential of the fiber. As growth slows down, levels of potassium and malate decrease and turgor pressure declines. Cotton ovules are capable of fixing H(14)CO(3) (-) in the dark, predominantly into malate. Fiber growth is inhibited by the absence of potassium and/or atmospheric CO(2). We suggest that potassium and malate act as osmoregulatory solutes and that malate, at least in part, arises from dark CO(2) fixation reactions.