External calcium requirements for growth and nodulation of six tropical food legumes grown in flowing culture solution

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Abstract
Six tropical food legumes, peanut cv. Red Spanish, pigeonpea cv. Royes, guar cv. Brooks, soybean cv. Fitzroy, and cowpea cv. Vita 4 and CPI 282 15, were grown for 20 days at six constant solution calcium concentrations (2, 12, 50, 100, 500 and 2500 8M ) in flowing solution culture with adequate inorganic nitrogen (500 8M NO3 and with controlled nutrient concentrations. Bradyrhizobium CB756 was added at a rate of approximately 105 cells/ml of nutrient solution. Growth of all genotypes except guar was satisfactory at >12 8M calcium, with 75-100% of maximum root and shoot yield being obtained. Solution calcium concentrations required for maximum top growth were 12 8M for cowpea CPI 28215, 50 8M for peanut, 100 8M for soybean, and 2500 8M for cowpea cv. Vita 4, guar and pigeonpea. Root growth responded to solution calcium concentrations in the same way as top growth, except for cowpea cv. Vita 4 and pigeonpea, which both produced maximum root dry matter at 12 8M calcium. External calcium requirements for unrestricted growth may have been overestimated in guar because phosphorus deficiency appeared to limit growth at <500 8M calcium. Effects of suboptimal calcium concentrations included prevention of nodulation, delays in nodule appearance and a reduction in both nodule numbers and the proportion of plants which nodulated. Guar and pigeonpea formed nodules only at a 50 8M calcium, whereas cowpea and peanut formed nodules at 2 8M calcium. Maximum nodule numbers were recorded at lower (peanut), higher (cowpea cv. Vita 4, CPI 28215 and pigeonpea) or the same (guar) solution calcium concentration as that required for maximum root growth. Nodule formation in peanut was satisfactory at solution calcium concentrations as low as 12 8M. The results are discussed in relation to reports on the adaptation of these legumes to highly weathered soils low in calcium.