INFLUENCES OF GENOTYPES, ROCK PHOSPHATE AND PLANT DENSITIES ON MYCORRHIZAL DEVELOPMENT AND THE GROWTH-RESPONSES OF 5 DIFFERENT CROPS

Establishing endomycorrhizal associations, attributable to Glomus sp., increased the benefits obtained when alfalfa and maize were grown in increasing volumes of substrate. This suggests that in the field, optimum spacings may be influenced by the presence or absence of endomycorrhizal associations which enable large volumes of soil to be exploited for nutrients and water. Increasing plant densities decreased dry matter production per seedling and nullified the benefits to be gained from endomycorrhizal associations. Thus, when there were five maize seedlings per unit volume of substrate, inoculation with G. mosseae increased dry weights from 20 to 41 g per plant whereas with 37 or more seedlings there were no discernible effects of endomycorrhizas. Adding increasing amounts of Ca3(PO4)2 decreased the proportion of roots colonised by Glomus sp. and lessened the benefits gained. The responses of five chickpea genotypes to inoculations with Rhizobium and Glomus sp., singly and in combination, differed markedly when grown in sand with Kodjari rock phosphate. Only nodulation with Rhizobium sp. increased the growth of genotypes Annigeri and G 130; with K 850, L 550 and Robut, nodulation produced a greater response than inoculation with Glomus sp. However, while the responses of K 850 and L 550 to dual inoculation were additive, that of Robut appeared to be interactive. Like those of chickpea, the responses of different peanut genotypes (all of which were nodulated), to inoculation with G. epigaeum differed appreciably. The shoot dry weights increased by factors ranging from 40 to 360%. While inoculation had little effect on the rank order of nine genotypes, the tenth genotype TMV 2 changed from being the smallest in the absence of endomycorrhizas to being the second largest after inoculation. Consistently, the responses of a range of peanut cultivars to granular forms of rock phosphate were greater than those to powdered froms. In three sets of paired comparisons, shoot dry weights were 2.03 g with granular formulations and 1.75 g with powdered forms. Additionally, nodulated plants with mycorrhizas grew better than nodulated plants without mycorrhizas. Crops grown in semi-arid areas in subsistence agricultural regimes are usually widely spaced. The results reported in this paper help to explain this long established practice. They also indicate that the selection of productive genotypes, responsive to nodulation and the formation of endomycorrhizas is likely to prove rewarding particularly if their use is linked with the application of inexpensive rock phosphate.