Abstract:Root density distribution of plants is a major indicator of competition between plants and determines resource capture from the soil. This experiment was conducted in 2005 at Anyang, located in the Yellow River region, Henan Province, China. Three cotton (Gossypium hirsutum L.) cultivars were chosen: hybrid Bt-cultivar CRI46, conventional Bt-cultivars CRI44 and CRI45. Six planting densities were designed, ranging from 1.5 to 12.0 plants/m2. Root parameters such as surface area, diameter and length were analyzed by using the DT-SCAN image analysis method. The root length density (RLD), root average diameter and root area index (RAI), root surface area per unit land area, were studied. The results showed that RLD and RAI differed between genotypes; hybrid CRI46 had significantly higher (P < 0.05) RLD and RAI values than conventional cultivars, especially under low planting densities, less than 3.0 plants/m2. The root area index (RAI) of hybrid CRI46 was 61% higher than of CRI44 and CRI45 at the flowering stage. The RLD and RAI were also significantly different (P = 0.000) between planting densities. The depth distribution of RAI showed that at increasing planting densities RAI was increasingly distributed in the soil layers below 50 cm. The RAI of hybrid CRI46 was for all planting densities, obviously higher than other cultivars during the flowering and boll stages. It was concluded that the hybrid had a strong advantage in root maintenance preventing premature senescence of roots. The root diameter of hybrid CRI46 had a genetically higher root diameter at planting densities lower than 6.0 plants/m2. Good associations were found between yield and RAI in different stages. The optimum planting density ranged from 4.50 plants/m2 to 6.75 plants/m2 for conventional cultivars and around 4.0–5.0 plants/m2 for hybrids.(Author for correspondence. Tel +86 (0)10 6273 2850; E-mail: libg@cau.edu.cn)