Although results from in vitro ovule culture studies have demonstrated a specific K requirement for fiber growth, a direct association between the K status of the cotton (Gossypium hirsutum L.) plant and fiber quality has not been established under field conditions. To evaluate this relationship, a single cultivar (1985) and two cultivars (1986 and 1987) were grown with 0, 120, 240, or 480 kg K ha‐1 in 10 blocked replicates of each K level on an irrigated, vermiculitic soil. There was a significant seed‐cotton yield response to applied K in each year. Lint yield, however, increased relatively more than seed yield, resulting in a greater lint percentage as plant K supply increased. The greater lint percentage reflected increased fiber length and secondary wall thickness (measured as a micronaire index) obtained from plants that received fertilizer K. For both cultivars, the fiber length, micronaire index, fiber strength and percent elongation, and fiber length uniformity ratio (dependent variables) were each positively related to (i) fiber K concentration at maturity, (ii) leaf K concentration at early bloom, and (iii) an index of soil K availability as independent variables in regression analyses. Comparison of cultivar regressions, however, indicated that fiber quality of ‘Acala GC510’ was higher than that of ‘Acala SJ2’ at low fiber, leaf, or soil K levels. We conclude that K supply to cotton fruit is an important determinant of fiber quality under field conditions, and that the K requirement for producing high lint yield with acceptable quality may differ among genotypes.

The potential for mepiquat chloride (1,1‐dimethylpiperidinium chloride) to control excess vegetative growth and increase yield of cotton (Gossypium hirsutumL.) was evaluated under a wide range of environmental conditions in 35 replicated experiments conducted in the San Joaquin Valley of California from 1979 to 1983. Predominant soils were thermic Xeric Torriorthent's and typic Torriorthents. Yield response to mepiquat chloride (MC) has varied in production fields as well as in these tests. The focus of this study was to identify those variables which have contributed to the variability observed. Plot size among experiments varied from 15 m2to 1.3 ha. Mepiquat chloride was applied at 49 g ha−1when bloom counts reached 0.7 white blooms m−1row and plant height was between 46 to 61 cm. Overall, MC treated plots yielded 1309 compared to 1299 kg ha−1lint for control plots and the yield response varied significantly among experiments. Forty‐six percent of this variability could be accounted for by number of heat units for the growing season and by the final plant height of control plants. Yield increases occurred when the growing season was short or when final height of control plants was either shorter or taller than normal. Yield decreases occurred when heat units ranged between 2600 to 3000 and final control plant height ranged from 90 to 110 cm. Height reduction due to MC treatment varied according to plant height (r=0.57 for n= 17) and was maximal at 105 cm. Mepiquat chloride reduced main stem nodes by 1.0, gin turnout by 0.4%, and lint proportion by 0.3%; increased percent of final yield harvestable on 19 September by 5%, fiber strength by 3 kN m kg−1, micronaire by 0.06 units, and petiole NO3‐N at first open boll by 0.5 g kg−1; and had no effect on fiber length, fiber elongation, petiole PO4‐P and K, or NO3‐N at first bloom. Typical seasonal heat units and control plant heights in the San Joaquin Valley fall within the range where a favorable yield response to MC is not predicted.

Cotton (Gossypium hirsutum L.) cultivars have considerable yield differences on vermiculitic soils where late‐season K deficiency occurs in California. A 2‐yr field study was conducted to evaluate cultivar differences in K use efficiency (defined as higher yield with a limited K supply) in relation to K uptake, K partitioning, and critical internal and external K requirements. The experiment had a split plot design with fertilizer‐K addition levels as mainplots, two cultivar subplots, and 10 blocked replications. Without K addition, yield was 29% (1986) and 35% (1987) greater in the K‐use‐efficient cultivar. Cultivar yield differences reflected greater boll retention at later fruiting positions, but was not related to differences in partitioning of K between vegetative and fruiting structures. When K supply was not limited, cultivar yields were similar. Yield of both cultivars was closely associated with leaf K concentration and soil K availability, but response curves indicated a lower leaf and soil K requirement for the K‐use‐efficient cultivar. The K‐use‐efficient cultivar had a higher K uptake rate during fruit development and greater total K accumulation, particularly at low soil K levels. Defining the physiological bases for such large genetic variation in K use efficiency will help to identify why the cotton plant is more sensitive to K limitation than other crops and will aid breeding efforts to develop germplasm more tolerant of a limited soil K supply.

Recent yield advances in cotton (Gossypium hirsutum L.) hive been associated with early fruiting and high harvest index (hi). This study was conducted to examine the effects of plant density in a narrow‐row (0.76‐m interrow spacing) system on plant morphology, fruiting pattern, and yield of five genotypes that differ in degree of determinacy. ‘Acala SJ‐2’, ‘Acala SJC‐1’, and 2218, 2280, and 2086 (USDA‐ARS Shafter short‐season germplasm) were grown in a factorial design at 5, 10, and 15 plants m‐2 in 1984 and 1985. Growth data were collected four times daring the season and fruiting data obtained from plant mapping at the end of the season. Genotypes did not differ in plant height until after anthesis, when the most indeterminate genotype (Acala SJ‐2) continued vegetative growth longer than the more determinate genotypes 2280 and 2086. Increasing plant density from 10 to 15 plants m‐2 delayed maturity of the more indeterminate genotypes but had no effect on the shorter, more determinate 2280 and 2086. Genotypes 2280 and 2086 were earlier mataring than the other three genotypes regardless of plant density. Earliness was associated with a lower node number of the first fruiting branch, more rapid production of early main‐stem nodes, and increased retention of early fruiting forms. Lint yield decreased 59 kg ha‐1 for each O.l‐m increase in final plant height between 0.77 and 1.36 m when genotypes were grown at 15 plants per m‐2; at densities of 5 and 10 plants m‐2, there was no significant relationship between lint yield and plant height across genotypes and years.

Cumulative effects of annual K additions on cotton (Gossypium hirsutum L.) growth and soil K balance were examined in a field study on an irrigated, vermiculitic soil. In each of 3 consecutive yr, cotton ‘Acala SJ2’ was grown with 0, 120, 240, or 480 kg K ha−1 applied to the same treatment plots in a randomized complete block design with 10 replications. The relationship between seed cotton yield and petiole K concentration was similar across years (r2 = 0.61, P < 0.001), but slopes of the yield response to applied K increased each year; 2.6‐fold more seed cotton was obtained per unit K input in 1987 than in 1985. This cumulative response reflected a yield decline without K addition and increasingly higher yield with each additional input of 480 kg K ha−1. Two processes appeared to control this response. First, without K input, NH4‐extractable K+ in surface soil decreased 20% from August 1985 to August 1987. This decrease greatly exceeded the K removal by cotton, and we surmise that an 11% decrease in soil organic C content contributed to this phenomenon. Loss of cation exchange capacity associated with the loss of soil humus may shift K equilibria towards fixation at interlayer sites in vermiculite at the expense of external K pools extracted by NH+4. Second, with high levels of K input, apparent K uptake efficiency from applied K was 50% greater in 1987 than in 1985. Increased efficiency was attributed to the partial saturation of K fixation sites from previous K additions and thus a larger proportion of newly added K remained in plant‐available K pools. At the highest K addition level, 86% of the 1440 kg K ha−1 applied in the 3‐yr period was fixed beyond extraction by NH+4, and plants remained marginally K deficient. The existence of a cumulative response to applied K on vermiculitic soils would have a marked impact on the economics of K fertilization on cotton.