Effect of solution pH, external calcium concentration, and aluminium activity on nodulation and early growth of cowpea

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Abstract
Effects of pH, calcium (Ca), and aluminium (Al) on the growth and nodulation of cowpea (Vigna unguiculata) were studied in large-volume, dilute nutrient solutions. Ca concentrations over the range 0.05 mM to 2.5 mM, and pH over the range 4.5 to 5.5, had no significant effect on the growth of the host plant. However, the combination of low pH and low Ca delayed nodulation and strongly depressed nodule number and nodule dry weight. At the highest Ca concentration, effects of pH on nodule number were small and not significant, and effects on nodule weight, although significant, were much smaller than at the lowest Ca concentration. It was concluded that nodulation was more sensitive to low Ca and low pH than was host plant growth. At pH 4.5 and 0.5 mM Ca, A1 delayed and depressed nodulation. An activity of 12.7 8M Al delayed nodulation by four days, and reduced nodule number and nodule dry weight drastically. At higher Al activities, nodulation was reduced to zero. The critical Al activities (10% reduction) were approx. 0.3 and 0.5 ,8M for nodule number and nodule dry weight. The dry weight of tops and roots showed a roughly linear decrease with increasing Al activity over the range 0 to 8 8M, both in inoculated plants supplied with 'starter nitrogen' and in plants supplied continuously with adequate mineral nitrogen. Critical A1 activities for growth were in the range of 4 to 8 PM. It was concluded that nodulation was much more sensitive to Al than was host plant growth. The results are discussed in relation to recent literature on the acid tolerance of cowpea.