Effects of arbuscular mycorrhizal inoculation on growth,yield, nitrogen,and phosphorus nutrition of nodulating bean varieties in two soil substrates of contrasting fertility

A pot experiment was conducted to study the response of nodulated bean (Phaseolus vulgaris L.), vars. Ceca and Montalbán, to inoculation with the arbuscular‐mycorrhizae (AM) Glomus spurcum strain IES‐3 and G. mosseae strain IES‐8 in two non‐sterilized non‐amended soil substrates (S‐l and S‐2) of contrasting fertility. S‐l was an acid soil (pH 5.0) with 77% exchangeable aluminum (Al), low available phosphorus (P) (3.5 ug g^‐1 dry soil) and 1.8% organic matter. S‐2 was a soil with pH 7.0, 7.9 μg P g^‐1 dry soil, 9.6% organic matter and no exchangeable Al. Plants growing in S‐1 (S‐1 plants) and S‐2 (S‐2 plants) soils were sampled at the beginning of the flowering stage. S‐1 and S‐2 plants were nodulated by the rhizobial populations native of each soil. S‐1 plants of both varieties did not respond to AM inoculation in term of shoot and root mass, P content, relative abundance of ureides (RAU) and seed yields. Only the total chlorophyll, chlorophyll‐a, chlrophyll‐a/b ratio and nodule mass were significantly enhanced in AM‐inoculated plants. Increased total reducing sugars concentration was detected in roots of S‐1 plants inoculated with G. mosseae. S‐2 plants displayed significantly higher shoot and nodule mass as well as increased total chlorophyll, chlorophyll‐a, chlorophyll‐a/b ratio and P content than S‐1 plants, regardless of the variety and AM inoculation. S‐2 plants of the var Ceca showed a two fold increase in seed yields but similar RAU values (>60%) when compared to S‐1 plants of the same variety. In contrast, the significant increase in RAU detected in S‐2 plants of the var Montalban was not translated into higher seed yields. In S‐2, the productivity of plants of the var Ceca doubled that of the var Montalban. For both bean varieties the highest significant P content and seed yield were observed exclusively in S‐2 plants inoculated with G. mosseae. This Glomelean strain enhanced the sink‐source ratio of the S‐2 plants as evidenced by the higher total reducing sugar concentration in the root mass. Arbuscular‐mycorrhizae inoculation significantly decreased the acid phosphatase activity in the rhizosphere of S‐1 and S‐2 plants, respectively, pointing toward a negative effect of foreign AM on the native microbial biomass. The effectiveness of the rhizobial populations native of each soil type and the weak response elicited by G. mosseae in S‐2 plants do not justify, at present, the inclusion of foreign inocula in the bean crops carried out at S‐1 and S‐2 soils of the Sucre State of Venezuela. Results also indicated the higher adaptability of var Ceca to conditions prevailing in S‐1 and S‐2.