A detailed restriction fragment length polymorphism map was used to determine the chromosomal locations and subgenomic distributions of quantitative trait loci (QTLs) segregating in a cross between cultivars of allotetraploid (AADD) Gossypium hirsutum (''Upland'' cotton) and Gossypium barbadense (''Sea Island,'' ''Pima,'' or ''Egyptian'' cotton) that differ markedly in the quality and quantity of seed epidermal fibers. Most QTLs inf luencing fiber quality and yield are located on the ''D'' subgenome, derived from an ancestor that does not produce spinnable fibers. D subgenome QTLs may partly account for the fact that domestication and breeding of tetraploid cottons has resulted in fiber yield and quality levels superior to those achieved by parallel improvement of ''A'' genome diploid cottons. The merger of two genomes with different evolutionary histories in a common nucleus appears to offer unique avenues for phenotypic response to selection. This may partly compensate for reduction in quantitative variation associated with polyploid formation and be one basis for the prominence of polyploids among extant angiosperms. These findings impel molecular dissection of the roles of divergent subgenomes in quantitative inheritance in many other polyploids and further exploration of both ''synthetic'' polyploids and exotic diploid genotypes for agriculturally useful variation.
The behavior of water stressed cotton (Gossypium hirsutum L.) is well documented, but our knowledge of traits which can be genetically manipulated to improve drought tolerance is incomplete. This study was conducted to determine which morphological and physiological factors lessen the effects of water stress on the yield of two short-season cultivars [i.e., TAMCOT HQg5 CHQg5) and G&P 74 ÷ (GP74)] with a common parent in ancestry. Plants were grown in a rain shelterlysimeter facility containing a Pedernales fine sandy loam soil (fine, mixed, thermic Udic Paleustalf) in 1990 and 1991 at Temple, TX. Water stress was imposed by replenishing a fraction of the water lost to evapotranspiration, beginning about 78 d after emergence. HQ95 and GP74 did not differ in leaf area index (LA]) or in the rate of leaf area development before water stress was imposed. The rate of LAI decline and average LAI were similar between cultivars when water stress was imposed. HQg5 used more water and used it at a faster rate than GP74 when water stressed. HQg5 produced more bolls and had higher yield under both well-watered and water stressed conditions than GP74 in each of the 2 yr. The largest difference in boll load between cultivars occurred on sympodia branches in the lower canopy, where HQ95 had 37, 60, and 182% more bolls than GP74 when plants received 0, 50 or 75, and 100% of the depleted soil water. Whether well watered or water stressed, individual boll weight did not differ between the two cultivars. However, the harvest index and the production efficiency of bolls (i.e., bossl pr unit leaf area) of HQ95 was consistently higher than GP74 for all water regimes. On average, HQ95 allocated 21% more dry matter to yield and produced 32% more bolls per square meter than GP74. While differences in yield between the cultivars mirror harvest index, large differences in boll production efficiency suggested that the intrinsic photosynthetic capacity of HQ95 may be greater than GP74. C OTa~ON is grown under a wide range of climatesfrom the humid sub-tropics of South Carolina to the semi-arid desert of California. Yet, water supply remains a critical limitation of yield. Genetic variation in yield and water-use efficiency has been reported for cotton subjected to water deficits (Quisenberry and McMichael, 1991; Cook and EI-Zik, 1993). Also, the effects of water deficits and relationships between cotton growth and water requirements are well documented (Grimes and EI-Zik, 1982; Morrow and Krieg, 1990). However, a fundamental understanding of the intrinsic factors which could improve the ability of cotton to withstand water deficits remains to be developed. We know that cotton fruit production and retention
Volume, 36, no. 4, p. 925: The units of y-axis of Fig. 3 were incorrectly printed as g[p.mol C0d(m2s)]; they should have been g [mol COd(m2s)]. The corrected figure is printed below.
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