Growth and Mineral Nutrition Responses of Mycorrhizal and Non-mycorrhizal Cowpea, Pigeon Pea and Groundnut to Phosphorus Sources of Different Solubilities

The effects of a superphosphate (SP) and a rock phosphate (RP), with equal total P contents, on the growth and mineral nutrition responses of mycorrhizal and nonmycorrhizal cowpea, pigeon pea and groundnut were investigated in a pot experiment using an air-dry gamma ray-sterilized (1.5Mrad) Andisol subsoil. Adequate amounts of micronutrients were added to the pot soils as supplements. The plants were inoculated with surface-sterilized 100 spores/pot of Glomus etunicatum or none and with Rhizobium strains. At their maturities, dry weights of plant parts and nodule number, root length, arbuscular mycorrhizal fungal (AMF) colonization of roots and AMF spore production were also measured. Shoot nitrogen and phosphorus concentrations were determined by the Micro-Kjeldahl distillation and ammonium-molybdenum blue methods, respectively. Shoot copper and iron concentrations were measured by the atomic absorption spectrophotometry. Mycorrhiza formation was greater with superphosphate than with rock phosphate treatments. Rock phosphate enhanced mycorrhization in cowpea and pigeon pea but it decreased it in groundnut. Superphosphate-treated cowpea and pigeon pea plants were larger than RP-treated plants whether mycorrhizal or not but in groundnut growth increased only when the plant was treated with SP and inoculated with AMF. Mycorrhizal enhancement of shoot growth in superphosphate-fertilized groundnut could be a direct consequence of improved P nutrition resulting from increased hyphal uptake and/ or enlarged absorptive root surface due to increased root fineness. There were similarities in shoot dry matter yields as well as shoot P uptakes of both mycorrhizal and non-mycorrhizal RP-treated plants suggesting that the legumes were so unresponsive to the RP that even mycorrhization could not trigger any response in them. The increased contents of N, Cu and Fe in the shoots of mycorrhizal SP-fertilized plants were observed to be the result of larger shoot sizes except with Fe in groundnut shoot whose enhancement could be related to a good mycorrhiza formation and a concomitant improvement in P nutrition which may further have been influenced by its fine root structure. There was a very close interdependency of fine root structure, AMF colonization, AMFspore production and improved P and Fe nutrition in groundnut as a result of soluble phosphate application.