Genotypic variability in cowpea with respect to production of the germinatoin stimulants of the purple witchweed (Striga hermonthica (Del.) Benth.

Abstract

The present investigation was undertaken at the College of Agricultural Studies, Sudan University of Science and Technology to select cowpea genotypes that could be used as mulch, trap crops or intercrops with sorghum or millet to combat the root parasitic weed Striga hermonthica and improve growth and yield of sorghum. Twenty four cowpea genotypes, obtained from the International Institute of Tropical Agriculture (IITA) Ibedan Nigeria, were subjected to series of laboratory, greenhouse and field experiments. The genotypes were grown in a field, harvested, severed into roots, stems and leaves, air dried and powdered and tested for germination inducing activity (GIA) and effects on radicle extension (RE). Three genotypes B301, T100K-901-6 and T198K-317-2 were selected for further studies on GIA of their root exudates and influence on RE. The genotypes B301 and T100K-901-6, selected for prostrate growth habit, were subjected to further laboratory evaluation for GIA, influence on RE and chromatographic behaviour of the stimulatory substances in root exudates as compared to sorghum. Further laboratory studies on effects of pH on GIA were undertaken. Moreover, Based on high GIA TK100-901-6, was selected for studying in-situ GIA and subsequent development of the parasite using the rhizotron technique. Greenhouse experiments and a field trial were, accordingly, conducted. In the field experiment intercropping was integrated with inoculation with Rhizobium leguminosarum TAL1399 and a phosphorus releasing bacterium (Bcillus megatherium var phosphaticum). The laboratory results revealed that the GIA of powder from cowpea, revealed by germination of S. hermonthica, sorghum and millet strains, increased with increasing amount of powder and subsequently declined on raising powder amount. Root powder displayed higher GIA than stem and leaves powders. Powder samples showing GIA > 80% were higher for Striga millet strain than the sorghum congener. Cowpea powder, irrespective of genotype or Striga strain reduced radicle extension in a concentration dependent manner. GIA of roots exudates from hydroponically grown plants, irrespective of genotype, progressively increased with time and was negligible at 72 h, maximal at 216 h and subsequently declined. Moreover, for each sampling date GIA and RE progressively decreassed with increasing volume of root exudates for Striga sorghum strain. For Striga, millet strain, variations in GIA with time V were similar to those for the sorghum strain, however, both GIA and RE increased with exudates volume. The GIA of the root exudates for Striga millet strain for a given sampling date, in contrast to the sorghum congener, increased with exudates volume. Radicle extension of Striga germilings consistently decreased with exudates volume for the Striga sorghum strain, but increased with volume for the millet strain. GIA of root exudates was, invariably, maximal at pH 7. Column chromatography showed polar and non-polar germination stimulants in sorghum and cowpea genotype B301 root exudates, however, for cowpea genotype T100k-901-6 only the polar fractions were active. The results were further confirmed by HPLC. In-situ germination and attachment using cowpea genotype T100K-906-1-6, Striga sorghum strain and employing the rhizotron technique showed high germination (76.6%) and high attachment (47.2%) with no further development. In the greenhouse experiment, irrespective of intercropping, Striga emergence progressively increased with seed bank size. Intercropping with cowpea, significantly, reduced Striga emergence and dry weight. Leaf area, sorghum height and dry weight, invariably, decreased with increasing seed bank size. Intercropped sorghum, invariably, displayed better growth than sole sorghum. In the field experiment intercropping sorghum with cowpea, the seeds of which were previously dressed with R. leguminosarum TAL1399, B. megaterium or their combination delayed and reduced Striga emergence. Furthermore, the treatments improved sorghum growth. Cowpea genotypes dressed with the bacterial combination, invariably achieved the highest reductions in Striga emergence and the highest increase in the measured sorghum growth attributes. In conclusion, cowpea, genotypes B301 and T100K-901-6 could be used as mulch, trap crops or intercrops to combat S. hermonthica in sorghum and increase growth and yield. Similar studies with millet are imperative.