ABSTRACT

Two of the principle components of crop yield are photosynthesis and the proportion of that photosynthate partitioned into reproductive or vegetative parts (harvest index). Improvements in the harvest index, which has a finite limit before leaf area becomes limiting, have contributed more through the years to cotton (Gossypium hirsutum L.) lint yield improvements than photosynthetic improvements. Future cotton lint yield improvements. Future cotton lint yield improvements may likely need a combination of both harvest index and photosynthetic improvements. Thus, the objectives of this study were to determine if CO₂-exchange rate (CER) differences; and determine of gas exchange parameters were related to genotypic lint yield variability.

Field studies were conducted from 1990 to 1992 on eighteen cotton genotypes of varying maturity and regional adaption. Leaf CER, chlorophyll (Chl) concentration, soluble protein concentration, and specific leaf weight (SLW) were periodically determined each year. In 1992, Chl fluorescence measurements were also obtained on the leaves used for the CER measurements. Lint yield and components of yield were determined at the end of each growing season.

Each year the CER of the eighteen genotypes significantly varied about 10% from a low of 28.8 ìmol m^-2s^-1. DES 119 and MD 51 ne consistently had higher CER while the CER of Dixie King and Stoneville 508 were consistently lower. The genotypes differed in leaf photosynthetic correlation was found between leaf photosynthetic performance and leaf Chl concentration (r=0.768**) and SLW (r=0.568*). No difference in the Fv/Fm parameter of Chl fluorescence was found among the genotypes in 1992. A significant positive association was found between lint yield and CER during the boll filling period in 1990 and 1991 (r²=0.2806*). As some breeders bred for higher yielding genotypes, it appears that they unknowingly also selected for increased photosynthesis.