Abstract: The objective of this study was to analysis K-efficiency and root morphology in two kinds of K-efficiency cotton genotypes. The results showed that K-efficiency of genotype 103 was 160.2 g g^-1 and 47.5 g g^-1 at low (4 mg L^–1) and high (40 mg L^–1) K levels while genotype 122 was 133.8 g g^-1 and 37.9 g g^-1. In addition, genotype103 had higher ability to transfer K to leaves than genotype 122. Under two K levels, Leaves K accumulation of genotype 103 was 84.6% and 62.6% of the whole plant, while that of genotype 122 was 63.4% and 56.0%. High K-efficiency genotype 103 had higher total root length, surface area and volume than low K-efficiency genotype 122 regardless of the K concentration treated. Especially when the concentration of K in the environment was poor, genotype103 still had better root system than genotype122. When K was insufficient, the total root length and total root surface area of genotype 103 increased by 46.2% and 13.9%, while genotype 122 showed decreases in root parameters. Root system was classified into fine roots, middle roots and coarse roots according to root diameter. Low K condition limited the development of coarse roots. In genotype 103, coarse root length, surface area and root volume decreased by 54.0%, 62.8%, and 75.2% respectively, meanwhile, genotype 122 also had a reducing coarse root parameters. Effect of low K condition on fine roots varied in different cotton genotypes. The fine root length, surface area and volume of genotype 103 increased by 69.0%, 77.0%, and 80.4%, compared with these in appropriate K treatment, and were 1.9, 2.3, and 2.6 times higher than these of genotype 122, respectively.

Key words: Cotton, Genotype, K-efficiency, Root morphology