Genetic variation for root-shoot relationships among cotton germplasm

Twenty-five cotton (Gossypium spp.) genotypes were used to evaluate the genetic variability in partitioning of biomass into roots and shoots when plants were grown under conditions of declining soil water and different atmospheric evaporative demands. The entries ranged from primitive race stocks to modern cultivars and were selected on field observations of growth under water stress conditions in the field. Seeds of each genotype were planted in soil (56 kg) in large containers in the greenhouse. Water was added to the soil to bring the water content to field capacity and the plants were allowed to grow with no additional water until they reached the permanent wilting point. Large fans were utilized in the second experiment to reduce the leaf boundary layer resistance and increase the evaporative demand. When the plants of each entry had reached the permanent wilting point, the plants were harvested, the roots washed free of the soil, and the dry weights of both roots and shoots were determined. Information on total water used and days to permanent wilting were also collected for each genotype. Differences were observed in partitioning of total biomass between roots and shoots between experiments and genotypes. There was no significant interaction, however, between entries and experiments. Root-shoot ratios increased in plants grown in the more stressful environment resulting from a significant increase in root dry weights with little change in shoot dry weights. The distribution for root-shoot ratios coincided in general with the distribution for root weights.among the entries, with a 59% decrease from the highest to lowest value. The exotic strains also in general had higher root-shoot ratios than the commercial varieties, the herbaceum species and the experimental strain (Lubbock dwarf). There was no direct relationship between shoot weights and root weights among the genotypes for either environment. Those plants that grew large tops did not necessarily grow correspondingly large root systems (e.g. T141 has a large root system with a very small shoot compared to the herbaceum species, which has a relative large shoot and a small root system). The lack of a correlation between shoot and root growth along with genotypic differences in changes in root-shoot ratios in response to environmental demand may provide an opportunity to exploit the observed variability to improve production for a wide range of growth conditions by altering the root development and function independent of shoot development.