Although the detrimental effects of cool temperatures on cotton fiber growth and development have been well documented, the mechanisms underlying these responses are not well understood. Using cotton from ovule cultures (Gossypium hirsutum cv. Acala SJ-1) as a model, we are attempting to identify steps in fiber development that are adversely affected by cool temperatures. The effects of cool temperatures on cellulose synthesis were investigated by feeding 21d old cultures (14)C-glucose after cycling from 34 C to either 25 , 28 , 31 , 34 , 37 or 40 C. After 4 hours, ovules were harvested, freeze-dried and weighed. Crystalline cellulose, isolated by extraction in acetic-nitric reagent, was quantified by scintillation counting. These experiments showed that incorporation of (14)C-glucose into cellulose (expressed as cpm per mg dry weight) increased with temperature until it reached a plateau around 28 . The rate was roughly constant from 28 to 34 . Above this temperature, incorporation began to decrease. Approximately 80-90% of the acetic-nitric insoluble cellulose was found in the fibers and 10-20% in the ovule. Respiration was also monitored in ovule cultures incubated at various temperatures by trapping evolved (14)CO₂. In contrast to cellulose synthesis, the rate of respiration continued to rise until about 36 C. The uncoupling of cellulose synthesis and respirating indicates that above about 28 , energy that might otherwise be available for cell wall synthesis is "wasted". This behavior may reflect the presence of a "thermal kinetic window" proposed to exist in cotton and other plants.

The effects of temperature on ovule metabolism were also illustrated by the uptake of glucose from the culture medium. In cv. Acala SJ1, glucose uptake of ovules increased as temperature increased from 15 to 22 to 28 . Glucose uptake at 34 was very similar to 28 . Some varietal differences in this response have been noted. In contrast to SJ1, glucose uptake by a cool tolerant Yugoslavian cotton cultivar showed a quite similar, while uptake at 34 was slightly reduced. Experiments are in progress to determine which step(s) in the biosynthetic pathways of the cells are adversely affected by cool temperatures.