Fiber growth on cultured ovules is initiated by phytohormone and inhibited by the absence of atmospheric CO₂. Variation in response of ovules to culture was observed by Beasley. In this study we examined the differential response of ovules to culture and to CO₂ among 15 cotton Gossypium hirsutum L. genotypes. Unfertilized ovules were cultured at 30 C for 14d in an elevated CO₂ atmosphere, with sister ovules from the same ovary cultured in ambient CO₂ as controls. The response of ovules to culture was greatly influenced by genotype, CO₂, and year or environment. There were significant genotypic differences in the responses to culture and to CO₂ between years. In 1987, ovules from field plants of half of the genotypes cultured on BTN medium tended to produce callus in ambient CO₂, while these ovules showed normal fiber development in 5% CO₂; the differential response of genotypes to culture was diminished by enriched CO₂. The number of fibered ovules was increased by CO₂, as percentage of the control, from 0 to 227% depending on genotype. The low percentage increases resulted because some genotypes grew well in ambient CO₂.

In 1988, 6 genotypes were selected as representative of the range of responses to culture (normal ovule or fiber growth, intermediate, and only callus or few fibers in ambient CO₂). Ovules from greenhouse plants of the 6 genotypes cultured on BT medium all grew normal fibers with or without enriched-CO₂ except GSC 25, whose ovules produced only callus in ambient atmosphere.

GSC 25 ovules tended to brown and die when cultured in ambient CO₂, but remained white and expanded in 5% CO₂ when cultured on BT medium without exogenous hormones; with the addition of 5 uM of IAA alone the ovules produced callus in ambient CO₂ but grew normal fibers in 5% CO₂ enrichment.

When ovules from field plants of the 6 genotypes were cultured at the end of growing season (Oct. 11) , they grew callus or few fibers in ambient CO₂ while producing normal fiber in CO₂ enrichment; the CO₂ effect on fiber growth was markedly increased. Therefore, both culture medium and parental plants altered the response to culture and to CO₂. BT medium and greenhouse plants supported better fiber growth in ambient atmosphere. The environment in which the parental plants were growing apparently shifted the response of the ovules to culture.

The results indicate that CO₂ conditionally affects the response of ovules to culture, i.e., when ovules grow normally the CO₂ effect is minimal; when ovules grow abnormally, CO₂ can prevent callusing and promote normal ovule and fiber growth. CO₂ could moderate adverse conditions for normal fiber development on ovules subjected to certain stresses, although the external factors and the mechanism of CO₂ action in buffering against the stresses are unclear. The phytohormones are known to play a key role in fiber development in vitro. The results show that CO₂ promotes normal ovule and fiber development. It may interact with phytohormones in this respect, but further study is needed to determine its exact function.