Comparative Growth of Obsolete and Modern Cotton Cultivars. II. Reproductive Dry Matter Partitioning1

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Abstract
Although cotton (Gossypium hirsutum L.) lint yield increases have been realized as a result of plant breeding efforts, there is little documentation concerning alterations induced in reproductive growth patterns by selection processes. This study was designed to give insight into those facets of reproductive dry matter partitioning which have been altered via breeding programs based on the selection for yield. Twelve cultivars, six each from both the Stoneville and Deltapine backgrounds, were planted on either 26 Apr. or 12 May 1982. Reproductive growth was monitored at various times during the season by both numbers and dry weights of squares, immature bolls, and mature bolls. In addition, flowers were counted on a row of each plot every week during July and August. There were no significant planting date interactions regarding cultivar differences. The modern cultivars produced a greater proportion of their squares and flowers earlier than the obsolete cultivars. There were significant correlation coefficients for the relationship of cultivar release year to total flowers occurring before 90 days after planting (DAP) (r = 0.61 and 0.57 for the April and May plantings respectively). The trend of greater production of early squares and flowers results in larger amounts of early bolls in the newer cultivars. The percentage of total bolls lost due to abscission between 117 and 142 DAP is similar for most cultivars. Therefore, the greater number of bolls produced by the new cultivars at 142 DAP is attributable to the quantity present prior to shedding. The modern cultivars exhibited greater reproductive to vegetative ratios with ‘STV 213’, ‘STV 825’, ‘DPL 41’, and ‘DPL 16’ having 0.95, 0.86, 0.84, and 0.71 kg reproductive dry weight/kg vegetative dry weight at 117 DAP, respectively. The modern cultivars appear to produce a larger lint yield by two major processes. The first is a greater partitioning of dry matter to reproductive organs. The second is an increased amount of reproductive development occurring when maximal leaf mass and area are present.