Phenolic acids have been implicated in phytohormone modulation. Several phenolic acids have been reported to affect indole-3-acetic acid (IAA) degradation via IAA oxidase. IAA was promoted by p-coumaric and p-hydroxybenzoic acid, while ferulic, caffeic and protocatechuic acid inhibited IAA oxidase. Other phenolic acids have manifested a marked abscisic acid (ABA) antagonism. Applications of exogenous salicylic or gallic acids, at typical metabolic concentrations, were found to counteract the abscission effected by endogenous ABA. IAA and ABA play a significant role in the abscission process during cotton fruit development, however, their respective actions may be 'fine-tuned' by the phenolic acids. Our study, therefore, addressed the variability in phenolic acids during fruit development of field-grown cotton (Gossypium hirsutum cv. Stoneville-506).

Concentrations of various phenolic acids were determined by reverse phase-HPLC over the period immediately preceding and succeeding anthesis. This period has been shown to exhibit dramatic changes in phytohormone concentration in cotton fruit. Contemporaneously, the phenolic acid profile displayed great temporal variability. The concentration of p-coumaric acid decreased significantly at the white flower stage (day of anthesis) and again at the boll stage (ca. 6-7 days post-anthesis). These changes constituted a 31 and 52 percent decrease, respectively, relative to the previous phenological stage. Conversely, ferulic acid increased to a maximum concentration at red flower, then decreased gradually to a minimum at the boll stage. Generally, p-coumaric acid concentrations were relatively high, while concentrations of ferulic acid tended to be low at phenological ages at which the fruit has a high probability of abscission. The concentration of ferulic acid, however, was found to be sensitive to plant water status. Ferulic acid concentration was correlated negatively to stomatal diffusive resistance. In 1988, plants were subjected to shading treatments of 0, 37 and 63 percent interception of incident solar radiation. With the concomitant reduction in photosynthetic rate, plant exhibited a significant increase in abscission. An increase in p-coumaric or decrease in ferulic was not associated with the measured decrease in photosynthesis nor the increased boll abscission rate. There was, however, a significant decrease in the ABA antagonists, salicylic and gallic acids. A better understanding of the temporal variability of the phenolic acids may lead to a greater insight into cotton fruit development and abscission and the sensitivity of these processes to environmental stress.