A new plant growth regulator PGR-IV is made of a fermentation broth of soil bacteria, yeast, and fungi that contains gibberellic acid and indolebutyric acid in a nutrient solution blend. This growth regulator has been tested in cotton over the last 10 years and has been reported to enhance the capacity of the plant to sustain growth under suboptimal conditions for growth. Although research has been conducted to evaluate its effects on root growth, nutrient uptake, boll retention, earliness, leaf photosynthesis, and yield, no studies have been reported on the whole-plant carbon balance and transpiration responses of cotton at suboptimal conditions for growth. The objectives of these experiments were to determine the effects of PGR-IV on the carbon and water economies of cotton plants grown in whole-plant test chambers under water stress.

Under water stress conditions the overall carbon fluxes and growth characteristics of cotton plants were severely affected, but applications of PGR-IV caused no effects on the overall carbon and water fluxes of the cotton plant. In a 25-d experimental period without irrigation, cotton plants showed a 71% reduction in the total number of leaves and a 90% reduction in the plant leaf area. The reductions in leaf area caused concomitant reductions in the daily rates of gross carbon uptake by 87%, carbon losses through respiration by 82%, net carbon gains by 92%, and transpiration by 95%. These reductions in the carbon and water fluxes under nonirrigated conditions caused a 60% reduction in the total plant dry weight, most of which was caused by reductions in the dry weight of leaves, petioles, stems, fruits, and branches. Dry matter allocation in roots was not severely affected by the water stress regime.

Application of PGR-IV did not affect the overall carbon and water fluxes of the cotton plant, only numerical tendencies were observed. PGR-IV application caused small reductions in the total plant dry weight (2%), but at the same time induced a greater allocation toward reproductive structures. Application of PGR-IV caused the development of longer fruiting branches and greater number of fruits. Internode length and total plant height were not affected by PGR-IV under either irrigated or nonirrigated conditions. According to these results we conclude that PGR-IV did not alleviate the detrimental effects of water stress on the cotton plant, but may induce the development of more productive plants.