Arbuscular mycorrhizal associations and its influence on phenotypic and growth characters of sweet potato genotypes under varied soil phosphorus availability 216

Arbuscular mycorrhizal (AM) associations and thier influence on phenotypic and growth characters of sweet potato (Ipomoea batatas (Lam.) L.) genotypes under varied soil phosphorus (P) availability were examined through a field trial. The study was conducted at the Central Tuber Crops Research Institute, Thiruvananthapuram, India in an acidic laterite (Oxisol) soil. Two fields with and without P fertilization were chosen to yield a wide range of plant characters and AM colonization levels. Sweet potato germplasm comprising 257 indigenous and exotic accessions was evaluated for AM colonization and plant phenotypic/growth characters under two levels of P availability. To study the AM composition and structure in relation to soil P level, the frequency of occurrence (%), relative abundance (%), species richness and diversity index were calculated. Frequency of root colonization by AM, intensity of colonization and AM spore density in soil around roots (rhizosphere) differed between genotypes and within genotypes grown in soils with different levels of available P. Intensity of root colonization by AM increased in parallel with increasing frequency of root colonization irrespective of soil P level. AM spore density in the rhizosphere increased with increasing frequency of root colonization at a level between 26-50% in low P soil, while spore density in high P soils was positively correlated with frequency of colonization between 0-25%. AM communities associated with the sweet potato genotypes consisted of eight species in genera Acaulospora (1), Gigaspora (3)and Glomus (4). Among the various AM fungal isolates, Glomus microcarpum was most abundant in both types of soil. Frequency of occurrence, relative abundance, species richness and diversity index of AM fungi were higher in low P soils than in high P soils. In general a higher frequency of colonization and intensity of colonization showed a positive influence on most plant characters in lower P levels only. In low P soil, there was no relationship between root characters and spore density in the rhizosphere, whereas in high P soil, root growth was positively correlated with higher spore density. Frequency of colonization by AM, intensity of colonization, spore density and community diversity in the rhizosphere of sweet potato genotypes are substantially altered by soil P availability. We conclude that the variation in the magnitude of growth response of the genotypes could be related to the changes in population of specific AM fungi in root and soil.