Physiological changes during the development of potassium deficiency in cotton

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Growthroom and field studies were used to study the physiology of the onset of K deficiency in cotton (Gossypium hirsutum L.) in order to explain the poor yield response to foliar-applied K fertilizer. This report describes studies conducted in Arkansas on the K nutrition of cotton. Initial studies documented the partitioning of K in plant components during the season. The onset of K deficiency in growth chamber experiments was first detected in roots, followed by stems, petioles and leaves, and then in the fruit. Furthermore, luxury storage of K, prior to peak demand for K by the boll load, could complicate tissue diagnostic recommendations. Visual K deficiencies were first observed in growth chamber experiments 19 days after K was withheld, along with reductions in leaf chlorophyll concentration and significant reductions in leaf photosynthesis. However, leaf ATP and nonstructural carbohydrate concentrations were higher 19 and 26 days after withholding K than in the control, which may have been the result of reduced utilization and translocation of these metabolites. Carbon isotope analyses, and analysis of carbon isotope discrimination, indicated that the most limiting resistance to net photosynthesis was stomatal, but that non-stomatal limitations became more important as the K stress developed. Our studies show that reductions in leaf physiological processes and plant growth did not occur until the petiole K concentration fell below 0.88% on a dry weight basis. Therefore, reductions in lint yield and quality should not develop until this critical petiole level is attained. This information will improve our understanding of tissue diagnosis for a pending K deficiency. Results from these studies were used to explain the inconsistent response of cotton to foliar-applied K fertilizer.