Plant growth regulators (PGR's) encompass a broad category of compounds that in minute amount promote, inhibit or otherwise modify plant physiological or morphological processes. Some PGR's are plant hormones or their analogues, others are simply enzyme regulators. In the last two decades, many compounds have been developed and tested on cotton (Gossypium hirsutum L.) often with variable results due to the lack of knowledge on the mode of action and the optimum application conditions. Several promising new compounds have emerged in the past few years as enhancers of nutrient uptake, photosynthesis, carbohydrate translocation, root extension, fruit retention, and crop yield. A series of studies have been conducted at the University of Arkansas for the last few years to evaluate the effect of these compounds on growth, development and some physiological parameters of cotton plants grown under both controlled environment and field conditions.

The tests conducted ranged from basic to applied, and from growth chamber to greenhouse to field conditions. These include studies of 1) Nutrient uptake; 2) Membrane leakage; 3) Carbohydrate metabolism; 4) Photosynthesis; 5) Water relations; 6) In-furrow application; 7) Rate and timing, including yield and yield components.

The following provides a brief summary of some of the results. More details can be obtained from Guo et al., 1993; Oosterhuis and Janes, 1994. PHCA (Microflo Company, Lakeland, FL), when applied at 2 or 4 pt./A, significantly increased the uptake of K, Ca and Mg by cotton plants from the nutrient solution in which plant roots were growing, while Cytokin (PBT Inc., Corrales, NM) was more effective when applied in combination with Microplex or Calcium Chelate (both from Miller Chemical & Fertilizer Corporation, Hanover, PA). As long as membrane leakage is concerned, all 7 PGR's studied in 1993 (PGR-IV, PHCA, Atonik, Cytokin, Pix, Crop(+) and Roc-X) decreased the ion leakage out of leaf discs in comparison with the control, indicating an increased membrane integrity. Sucrose translocation from cotton leaves to stems was promoted by PHCA, while the leaf photosynthetic rate was enhanced by most of the PGR's, though statistically nonsignificant. Under severe water deficit, PGR-IV decreased the stomatal resistance, and increased the transpiration rate before the stomates were closed completely, accompanied by a higher photosynthetic rate. In 1992, PGR-IV out-yielded the control by 18%, while PHCA increased the lint yield by 18.9% in 1993.

PGR evaluation is an on-going program at the University of Arkansas. The field performance and mode of action of new compounds as well as old ones will be continuously investigated over different environments and seasonal conditions. Certain tests and measurements have been used in PGR evaluation, and more studies need to be conducted to elucidate the mode of action of a specific PGR in order to find out the optimum application conditions.