Two commercial fields of Pima Cotton at Regal Farms in Southern Arizona were monitored using the PHYTOGRAM technique. At the end of the season the cotton from individual plants was harvested to form high and low yield groups. The electropotential and field temperature of both groups were found to be positively correlated. The potential of the high yield group had a 50% higher sensitivity to temperature than the low yield group during the main fruiting period. This sensitivity reversed late in the season suggesting the high yield group essentially turned off and the low yield group attempted to "catch up."

Plots of maximum and minimum potentials (PGll Index) indicate lower potentials concomitant with lower canopy temperatures are yield enhancing. Low temperature performance appears more significant than high temperature performance in setting yield.