Breeding for high yield in hot environments (heat tolerance) has tripled the yield of Pirna cotton since 1949. We compared six strains (one primitive non-cultivated line, four cultivars representing advancing stages in the breeding process, and one unreleased advanced line) for their gas exchange properties in the greenhouse. Both photosynthetic rate and stomatal conductance increased with improving genetic yield potential. Photosynthetic rate was enhanced more in the morning than in the afternoon. Stomatal conductance did not limit photosynthesis; rather, the changes resulted from alterations of characteristics of the green mesophyll cells. There is no evidence that increased yield results from the enhanced photosynthetic rates of single leaves. However, the increased stomatal conductance in modem lines was also expressed in the field in 1990, allowing increased transpiration rate and evaporative cooling of leaves. Heat tolerance in Pima cotton may be related to the ability of plants to cool themselves by transpiration.

DICA (3, 4-dichloro-5-isothiazole carboxylic acid) is being used as a chemical hybridizing agent in the cotton breeding program of Chembred Seed Company. This compound produces male sterile flowers. Registration of this compound by EPA requires that a plant residue study be conducted to provide data on the quantitative amounts of residues in F₁ plants and seeds, F₂ seeds and in the soil. This study was carried out in Arizona because the hybrid F₂ cotton seeds will be grown in Arizona.

Three field experiments were conducted in 1990 in Arizona to evaluate cotton crop response to several treatment regimes of multiple applications of PIX (an anti-gibberellic acid plant growth regulator). Treatment regimes used in 1990 employed higher rates of PIX/acre/application and extended times of applications later into the fruiting cycle than earlier experiments in 1988 and 1989. Similar to earlier experiments, results in 1990 demonstrated the ability of some PIX treatments to significantly reduce plant height, relative to the untreated check treatments. The 1990 cotton production season in Arizona consisted of conditions which led to excessive rates of fruit loss and abortion, and somewhat vegetative plants. Final fruit retention levels of 30 to 50% were realized in the three 1990 experiments after a period of fruit loss through July and August. Lint yield results revealed significant differences (P ≤ 0.05) between several selected treatments at only one of the locations in 1990.

Two field studies were conducted in 1991 to further evaluate Upland cotton yield and development responses to PIX application timing as a function cotton growth stage. Treatments imposed in 1991 were intended to further clarify some response trends observed in previous field studies. Treatments in 1991 were all at the maximum label rate of 1.5 pt./acre with application timing the main variable. Timing was based on growth stage and heat unit accumulation since date of planting. The study in Waddell, Az. resulted in no significant yield differences across all treatments. In contrast, the study at the Maricopa Agricultural Center resulted in a statistically significant yield response by approximately 100 lbs. lint /acre for all FIX treatments compared to the untreated check plots. Plant growth and development measurements indicated the height: node ratio counts to be a good reflection of vegetative tendencies under field conditions in the two studies.

Plant Growth Regulators (PGR's) are used in cotton production to reduce excessive vegetative growth. This study was conducted to determine the effect of mepiquat chloride (PIX) on growth, and yield of Pima S7 and Deltapine 90 cotton. Single and multiple applications of PIX were conducted over the growing season. Plant mapping measurements were performed 13 times. Seed cotton yield estimates were obtained with a mechanical picker. Analysis of the mapping data showed that vigor index for control and PIX treated Pima and Deltapine plants was linear for the first 9 weeks, then leveled off as flowering and boll set occurred. PIX treated Pima plants were approximately 5cm shorter than the control after the 9th week. Heights of PIX treated Deltapine were similar to the controls. The number of nodes above white bloom in the FIX treated plants was not modified. PIX treatment of Pima cotton reduced the seed cotton yield compared to controls but yield of PIX treated Deltapine was similar to the controls.

A field study was conducted at the Maricopa Agricultural Center to evaluate the effect of a late - season application of PIX on Upland cotton termination. A 28 August application of PIX or FIX + Prep had only a small effect on late-season growth and no effect on defoliation or yield.

Summaries of cotton crop phenology, as a function of heat units (HU, 86/55 °F limits) have been developed across a wide range of production conditions in Arizona. Optimum ranges of HU accumulations since 1 January are used to describe optimal planting dates for full season varieties. Basic events such as the occurrence of pinhead squares, squares susceptible to pink bollworm, and first bloom are described in terms of HU accumulations since planting. Fruit retention guidelines and height: node ratios measures a crop's vegetative/reproductive balance, are developed as a function of HUAP. The use of the number of nodes above the top white bloom to the terminal (NAWB) is developed as a measure of a crops progression towards cut-out. Also, the expected ranges of HU 's accumulated since planting that are required to accomplish crop cut-out are shown for three general maturity types of Upland cotton.

A field trial was initiated during the 1992 growing season to evaluate the activity of high rate Pix use on upland cotton. The cotton field used for this trial maintained high fruit retention and low to moderate height-to-node ratios throughout the growing season. Fix treatments at label and above label rates both significantly reduced lint yield compared to the non-treated check This reinforces the concept of a feedback approach using crop monitoring (vs scheduled inputs according to calendar dates) to make management decisions on crop inputs.

The commercial product PGRIV was tested in small plots on cotton at the Maricopa Agricultural Center. Multiple foliar applications of this product had no significant effect on lint yield of Pima S-7 and DPL 90 cotton.

A field trial was initiated during the 1992 growing season to evaluate the activity of Cytokin and Piz applied alone or in combination to short staple cotton. The Cytokin treatment significantly increased lint yield over the other treatments. There were no statistically significant differences between the non-treated check and any other treatment. The Cytokin treatment increased lint yield an average of 81 pounds over the check plots. Fruit retention remained high throughout the season, indicating that Pix would not normally have been needed.