Afield study was conducted in Yuma County to evaluate the relative effects of a plant growth regulator application and several defoliation treatments on Pima cotton. There were no statistically significant effects recorded with regard to the plant growth regulator application. There was a significant difference among defoliation treatments by analysis of percent leaf drop estimates. Promising results were recorded for DROPP as a defoliant material for Pima cotton under the given test conditions.

Four short staple cotton varieties were grown with and without an application of PIX to see its affect on their growth, maturity and yield. PIX is a plant growth regulator thatmodifies plant architecture, Two of the tall -statured varieties, Delta Pine 90 and Acala 1517-75, showed increases in lint yield of 5.8 and 13.7%, respectively, coupled with a hastening of their maturity. Stoneville 506, a short- statured, medium- maturing variety was unaffected by the plant growth regulator. A tall, gangly variety, Germains GC 365, was shortened in height and in maturity, but exhibited a small decrease in yield.

Hormone analyses were conducted to determine why large squares seldom shed while young bolls do. Large squares contained five times as much free auxin as flowers, and they contained 16 times as much bound auxin. The high auxin content of large squares is probably a major reason that they almost never shed unless injured (for example, by insects). Free and bound auxin both decreased to very low levels at flowering and remained low for four days thereafter. This low concentration of auxin at, and just after, flowering is probably a major reason that bolls are likely to shed during the week after flowering. Both free and bound auxin increased rapidly between 7 and 9 days after flowering, possibly accounting for the decrease in boll shedding rate at this stage of development. Amide-linked IAA was the major form of auxin in squares, whereas ester IAA (presumably bound to sugars) was the major form of auxin in bolls.

In 1984,1985, and 1986, leaf water potentials of irrigated cotton declined markedly as the crop approached cutout. Midday transpiration rates also declined, indicating a water-stressed condition. The water stress occurred despite the fact that measurements were made only when the soil was fully charged with water. In 1986, plant hydraulic conductances were estimated. The conductance was high early in the season, declined to a low value during cutout, and increased during regrowth to the high value of the early season. It is suggested that root length and efficiency of water uptake, or both, decrease during boll filling as most assimilates are partitioned into the bolls. Daily drip irrigation prevented this susceptibility to stress during boll filling. In 1984 and 1986, drip irrigation decreased the length and severity of cutout, resulting in an increased boll load at the end of the season.

The plant regulators XE-1019 and PIX were tested at the Maricopa and Marana Agricultural Centers. Few yield responses were found in 1987, apparently due to the short stature of the crop at both locations.

Two new Dropp formulations show promise as cotton defoliants. In addition to excellent leaf drop, most squares, flowers, and young bolls were also shed, removing the food source for late season insects.

Ethephon was applied to Deltapine 55 cotton with 35% of the bolls open. Ethephon significantly increased the percent first pick yield of the cotton. There was no significant difference in the total yield.

PIX was applied to long and short staple cotton in 5, 10 and 20 gallons of water in an incomplete factorial design involving 0.5, 1 and 2 pints of the product per acre. Plant heights were significantly shortened and the percent of lint obtained in the first picking was significantly increased when Piz was applied on the short staple cotton. No statistically significant yield differences were observed between the volumes of dilution or the rates of application for either long or short staple cotton. A factor underlying the experiment was that the monsoon rains. They kept the surface of the ground moist and the plants looking good; however, the subsurface moisture had apparently been depleted, and the plants were under some stress. This stressed condition offset what good the FIX might have done for yield.