Response of cowpea (Vigna ungiculata (L.) Walp.) cultivars to vesicular-arbuscular mycorrhizal symbiosis

Typescript (photocopy).The host plant and soil P level are two of the major factors that determine the benefits derived from vesicular-arbuscular mycorrhizal (VAM) associations. This investigation was conducted to maximize the benefits of the cowpea-VAM symbiosis through identification of highly responsive cultivars and the optimal growth medium P level. A greenhouse experiment was conducted using four cowpea cultivars at four initial P levels, 13, 23, 33, and 43 ppm. Percent root colonization by VAM fungi was negatively correlated with P content of the growth medium and shoot P concentration. The range of soil P required in the growth medium in order to benefit from Glomus fasciculatum was identified for these cultivars based on growth response. Cultivar variability for response to VAM fungi was also evaluated in two field experiments conducted in the United States and Sri Lanka. In the US experiment, 19 cowpea cultivars were screened under 3 soil treatments, including fumigation, inoculation with G. fasciculatum after fumigation, and natural soil. Cowpea cultivars responded differently to soil treatments as reflected by shoot dry weight and leaf area. More cultivars showed increased shoot dry weight in the natural soil treatment than in the inoculation with G. fasciculatum after fumigation treatment. These cultivars were identified. Soil fumigation reduced growth, nodulation and nitrogenase activity. Inoculation with G. fasciculatum helped to alleviate most of these negative effects. The same experiment was conducted in Sri Lanka, with and without rhizobial inoculation. In many cultivars, rhizobial inoculation increased the benefits derived from VAM fungi. Cultivars showed differential response to rhizobial inoculation. Unlike in the US experiment, soil fumigation reduced only the nitrogen fixation parameters. Inoculation with Glomus fasciculatum did not lead to recovery of reduced nitrogen fixation. These two field experiments demonstrated the variation among cowpea cultivars for response to inoculation with both VAM fungi and Rhizobium. Since cultivars did not differ significantly in percent root colonization, this parameter did not explain cultivar difference in response to VAM fungi. A detailed study of cultivar root systems revealed that average length of fine roots was negatively correlated with mycorrhizal dependency of cowpea cultivars