LEAF GAS-EXCHANGE PARAMETERS VARY AMONG COTTON GENOTYPES

In trying to increase the yield of upland cotton (Gossypium hirsutum L.), breeders have indirectly selected more for increases in harvest index than in photosynthesis. Future cotton yield advances may need increases in both harvest index and photosynthesis. The objectives of this study were to determine if differences in leaf CO2-exchange rate (CER) existed among cotton genotypes, investigate physiological mechanisms for the gas exchange differences, and determine if gas exchange parameters were related to genotypic lint yield variability. Field studies were conducted from 1990 to 1992 on 18 cotton genotypes. Leaf CER, chlorophyll (Chl) concentration, soluble protein concentration, specific leaf weight (SLW), lint yield, and yield components were determined each year. In addition, Chl fluorescence measurements were made in 1992. The CER of the 18 genotypes significantly varied about 10% from a low of 26.0 mu mol m(-2) s(-1) to a high of 28.8 mu mol m(-2) s(-1). DES 119 and MD 51 ne had higher CER, whereas the CER of Dixie King and Stoneville 508 were lower. The genotypes differed in leaf Chl and soluble protein concentration in both 1991 and 1992. The CER was positively correlated with leaf Chl concentration (r = 0.768*) and SLW (r = 0.568*) in 1991. No difference in the Fv/Fm parameter of Chl fluorescence was found among the genotypes in 1992. A significant positive association (r(2) = 0.205**) was found between lint yield and CER during the bell filling period in 1990 and 1991. CER was positively associated with fiber micronaire (r = 0.772*) and fiber maturity (r = 0.797*) in 1992. As some cotton breeders bred for higher yielding genotypes, they may have inadvertently selected for increased photosynthesis.