Journal of Agricultural Science, Cambridge (1993), 120, 75-78, © 1993 Cambridge University Press 75 Implications of daylength sensitivity in bambara groundnut (Vigna subterranea) for production in Botswana D. HARRIS 1 AND S. N. AZAM-ALI 2 * 1 SADCC/ODA Land & Water Management Project, Private Bag 00108, Gaborone, Botswana 2 Department of Agriculture and Horticulture, Faculty of Agricultural and Food Sciences, University of Nottingham, Sutton Bonington Campus, Loughborough LE12 5RD, UK (Revised MS received 20 May 1992) SUMMARY Bambara groundnut (Vigna subterranea (L.) Verde.) is grown throughout southern and western Africa, primarily as a subsistence crop, but agronomic information is scarce. The rates of emergence, flowering and pod production were assessed for 20 weekly sowings between 16 October 1990 and 26 February 1991 at Sebele, Gaborone, Botswana. Daylength during this period changed from 12-7 h to 13-7 h and back to 12-6 h. Mean time to 50% emergence was 111 days while mean time to 50% flowering was 475 days. The thermal time to first pod production varied with daylength, decreasing as daylength decreased, to an approximately constant value when daylength after flowering was less than about 12 h. Consequently, the plant size at which pods were produced also varied. The practical implications for date of sowing and plant spacing are discussed. INTRODUCTION so " ^ ecomes t0 ° dry to support germination. Periods of heavy rainfall are infrequent during a season and Interest is growing in improving the productivity of are unpredictable in any practical sense. Nevertheless, bambara groundnut (Vigna subterranea (L.) Verde.) sowing is possible at any time from October to and its contribution to rural economies. Recent February, although crops planted after February are reviews (Linnemann 1987; Linnemann & Azam-Ali unlikely to mature because of rapidly falling tempera- 1992) have emphasised both the lack of detailed tures late in the season. Within this wide range of information and the enormous unfulfilled potential of planting dates, daylength changes markedly (Fig. 1). this crop. Its tolerance to drought (Nyamudeza 1989; Recent work in controlled environments by Lin- Lewitt 1990) and poor soils (National Academy of nemann (1991a, b) has shown that fruit set in some Sciences 1979) makes it ideally suited to production in genotypes of V. subterranea is delayed or even marginal areas where low-input agriculture is the prevented by long photoperiods. Thus it is possible norm. Minimal input arable cropping is predominant that sowing date might influence yield in Botswana in semi-arid Botswana (L&WMP 1988) and many through the effect of daylength on development. This subsistence farmers try to grow Vigna subterranea if paper examines the relation between sowing date (and possible. However, there is no reliable source of seed daylength) and development of V. subterranea under and agronomic information concerning the effect of irrigation in Botswana, planting date, sowing rates etc. is not available. Observations suggest that yields are highly variable MATERIALS AND METHODS and crop failures quite common. The rainy season in south-eastern Botswana extends The Department of Agricultural Research in Bot- from October to May, although occasional storms swana holds some accessions of V. subterranea occur outside these summer months. There is no well- although none has been screened for daylength defined start to the season and, although the long- sensitivity. Consequently an unnamed seed lot was term mean annual rainfall is 525 mm, dry periods of obtained from neighbouring Zimbabwe for which 20-30 days are not uncommon (Jones 1987). Es- preliminary observations suggested some daylength tablishment of rainfed crops is dependent on planting sensitivity. Seeds were sown every week for 20 weeks opportunities following heavy rainfall and before the from 16 October 1990 to 26 February 1991. If no rain * To whom correspondence should be addressed.