Crop yield response to water - Cra

temperatures clearly influence reproductive performance disproportionately more thanvegetative performance, it is not known how temperature stress reduces pod formation.Whether the maintenance of leaf production and expansion at high temperatures may be acause or a consequence of poor pod formation, it results in the major proportion of biomassbeing allocated to vegetative development. The reduction in pollen viability due to heat stressmay be associated with poor pod set.Bambara groundnut is a short-day species. While flowering is generally set by thermal time,very unusually, the onset of pod growth is affected by photoperiod (Harris and Azam-Ali,1993). At daylengths longer than 12 hours, the crop will take longer to initiate pod filling,delaying maturity.Irrigation practiceGenerally, bambara groundnut is grown as a rainfed crop in semi-arid Africa. Where available,irrigation practices enhance the total productivity of the crop. However, even when water isnot limiting yield is still subjected to the daylength control of pod growth. This has importantimplications both for rainfed and irrigated production as sowing date needs to be set by theoptimal daylength for pod filling and seasonal rainfall and/or irrigation need to fit withinthis production interval. When water resources are limited and daylength is not a constraint,any available water should be applied during the late vegetative period (up to flowering) toensure that the maximum numbers of pods are set, provided that water is not so restricted asto shorten the crop life cycle markedly.YieldYields under favourable conditions may range from 3.0 to 3.8 tonne/ha (Collinson et al., 2000).However, as a result of genetic variability and because the environments in which bambaragroundnut is grown are characterized by various biotic and abiotic stresses, typical yields areextremely low and vary between 0.65 and 0.85 tonne/ha for most of the semi-arid tropics(Hampson et al., 2000). In practice, there are large differences in reported yields between andwithin countries, with yields as low as 0.06 to 0.11 tonne/ha (Zambia), 0.05 to 0.66 tonne/ha(Swaziland), and 0.07 to 0.86 tonne/ha (Zimbabwe). At harvest, the ratio of pods to total drymatter (Harvest Index, HI) is in the range of 0.30 to 0.45. The HI decreases under drought,temperature stress and longer day lengths, and is maximal when applied water is sufficientfor maximum canopy cover to be achieved and daylength and temperatures are optimal.The nutritional composition of bambara groundnut (protein content is 16-25 percent) iscomparable or superior to other legumes (Linnemann and Azam-Ali, 1993), providing animportant supplement to cereal-based diets.ReferencesCollinson, S. T., Sibuga, K. P., Tarimo, A.J.P. & Azam-Ali, S.N. 2000. Influence of sowing date on the growth and yieldof bambara groundnut landraces in Tanzania. Experimental Agriculture, 36, 1-13.Edje, O.T., Mavimbela, E.K. & Sesay, A. 2003. Effect of seed source on growth and yield of three landraces ofbambara groundnut (Vigna subterranea) in Swaziland. In: Proceedings of the International Symposium on BambaraGroundnut, Botswana College of Agriculture, 8-12 September, 2003.BAMBARA GROUNDNUT 225