Cotton yield is a function of the number of floral buds produced, how many of the buds result in fruit and the final size of the fruit form. These yield related processes are all subject to genetic aid environmental controls. On the Texas High Plains, cotton production is limited by both temperature and water availability. The growing season is rather short and is plagued by cool temperatures in the spring and fall arid extremely hot temperatures during the summer. The rainfall is usually inadequate to meet demand throughout much of the growing season such that moderate to severe water stress is common.

This study was undertaken to develop an understanding of the influence of temperature, water stress and genetic factors on fruit site production, reten tion of fruiting forms and fruit growth rates under field conditions. The temperature effects were related to the available energy using a "heat unit" model. The experimental design was a split-split plot randomized block with planting dates (3) as the main Plots and irrigation levels (2) as sub-plots. Three cultivars differing in degree of indeterminacy were used and included Coker 5110, Tamcot 788, and Paymaster Dwarf. The study was conducted at the Texas Tech Research Farm at Lubbock, Texas during the 1980 and 1981 growing seasons. The physical environment was constantly monitored using an automated data acquisition system. Heat unit models were derived from the temperature data collected at three hour intervals. Growth analysis was conducted at 14 day intervals from emergence through the growing season.

The results of the study indicated that under nonlimiting soil water conditions both fruit numbers and fruit mass per unit ground area (m2) were highly cor related with the available heat using a base temperature of 15C. When water stress was a limiting factor, neither time (days) nor heat were highly related to fruit production. Partitioning of dry matter into continued sympodial development was negatively correlated with heat accumulation. The negative correlation was more pronounced under water stress conditions. Determinate cultivars partitioned a greater amount of dry matter into fruit development and attained "cutout" at an earlier time than did indeterminate cultivars. The genetic variation was related to a lower node of first fruit formation and a faster rate of sympodial and floral bud development prior to the onset of flowering.

The results of this study indicate that the number of floral buds, the retention of fruit and the growth rate of the fruit are dependent upon the available energy (heat units).