Stem and root carbohydrate dynamics in modern vs. obsolete cotton cultivars

Starch reserves in the lower stem and root are important sources of photoassimilates for completion of reproductive development in cotton. The objective of this research was to determine if carbohydrate levels in the lower stem and roots have been altered because of more than 100 years of breeding efforts. In 2001 and 2002, 33 cultivars released from 1900 to 2000 were evaluated. In addition, two elite lines were included in 2002. Plants were sampled at first bloom and cutout. Tissues were analyzed for starch content and concentration by using a colorimetric technique. Analysis of variance by year revealed cultivar differences for starch concentration at both sample times, but starch content was only significant at first bloom in 2002. These differences, though, did not show a strong relationship with release date, indicating that a century of breeding efforts had not altered stem and root starch dynamics. Orthogonal contrast did reveal differences in starch content and concentration between obsolete, modern, and elite lines. However, these differences were not consistent across experimental years. At first bloom in 2001, modern cultivars had lower root and stem starch concentrations, 69.8 and 63.6 mg g(-1), compared to 94.5 and 84.8 mg g(-1) for the obsolete lines. Modern cultivars also had lower root starch content, 537.5 mg, compared to 784.4 mg for the obsolete lines. At cutout in 2001, modern cultivars had lower root and stem starch concentrations, 20.6 and 20.9 mg g(-1), compared 39.7 and 32.4 mg g(-1) for the obsolete lines. Modern cultivars also had lower stem starch content, 349.0 mg, compared to 518.4 mg for the obsolete lines. These same trends were not seen in 2002. In 2002, modern cultivars did not differ from obsolete cultivars for all parameters. Elite lines, though, had higher stem starch concentrations at cutout, 56.9 mg g(-1) compared to 39.5 and 46.6 mg g(-1) for the obsolete and modern lines, respectively. Elite lines also had higher stem starch contents at cutout, 318.6 mg compared to 181.7 and 195.7 mg for the obsolete and modern lines, respectively. This higher stem starch concentration and content for elite lines may indicate higher photosynthetic rates. Environmental conditions seem to affect starch dynamics more than genetics when one considers the high significance of year on most data in this study.