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Fruiting of Cotton. III. Nutritional Stress and Cutout¹

Decreases in growth, flowering, and fruit (boll) retention of cotton (Gossypium hirsutum L.) occur during the season in many modern cultivars even though it is an indeterminate plant. if they are pronounced, these decreases are commonly referred to as cutout. If cutout occurs too early it may decrease yield. Conversely, complete and permanent cutout at the end of the season would facilitate the control of insect pests. Cutout is strongly affected by boll load and may occur because of competition for photosynthate, a change in hormonal status, or both. Previous results indicated that changes in free abscisic acid (ABA) did not correlate with the decreases in growth, flowering, and boll retention that occurred as boll load increased during the season. Therefore, experiments were conducted in the field to test the hypothesis that cutout occurs when the demand for photosynthate exceeds the supply. The supply of photosynthate per plant was manipulated by thinning the plant population at different times to permit better light penetration (irradiance) into the canopy and thus increase total photosynthesis per plant. The demand for photosynthate was manipulated by partial defruiting. All flowers were removed during the first 3 weeks of flowering. Thinning at the start of the season caused individual plants to produce more flowers and bolls, but did not alter the timing of changes in flower production and boll retention rates during the season. Carrying capacity per plant varied inversely with plant population. Thinning later in the season caused an immediate increase in boll retention and a subsequent increase in flowering rate. Decreasing the demand by removing flowers early in the season caused subsequent and rather prolonged increases in flowering and boll retention rates. Early defruiting also moderated the decline in growth rate late in the season. Decreases and subsequent increases in flowering rate lagged behind the changes in growth and boll retention because flowering rate was determined by prior events (production and retention of flower buds). The results are consistent with the hypothesis that growth, flowering, and boll retention decrease when the demand for photosynthate increases and exceeds the supply. Therefore, an increase in photosynthesis should permit more bolls to be set before cutout.

Key Words: Abscission • Active boll load • Boll retention • Competition • Flowering • Growth • Organic nutrients • Photosynthesis • Plant population

² Plant physiologist, Western Cotton Research Laboratory, 4135 E. Broadway Road, Phoenix, AZ 85040.

Received for publication June 28, 1984.

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