Variation in Responses of Bambara Groundnut (Vigna Subterranea (L) Verdc.) Landraces to Sowing Date, Heat, Photoperiod and Drought Stresses

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Bambara groundnut is an underutilized and until lately, under researched crop. Its ability to produce some yield where other crops such as groundnut fails has been established. Variation in yields of bambara groundnut have been reported by several workers. These differences have been attributed to variations in photoperiod at the different sowing dates within environments and between environments. The balanced nutritional quality of the crop coupled with its resistance to drought makes it a crop of choice to achieve food security, especially, in the dry areas of Africa. A total of 11 experiments were conducted over a period of three years. The objective of the experiments were to determine variation in performance of bambara groundnut landraces to sowing dates, heat, drought and photoperiod stresses. Nine field experiments were conducted in Ghana (Tono near Navrongo, Wenchi and Kumasi). Two controlled environment trials were conducted at the University of Guelph, Guelph-Ontario, Canada. Seven bambara groundnut landraces were studied in Ghana and 13 landraces were studied in Guelph. Four experiments were conducted in 2007 in three agro-ecological zones of Ghana namely; Wenchi (Transition) Tono (Guinea Savannah), and Fumesua-Kumasi (Forest) to determine the effect of sowing date and heat on the yield of five bambara groundnut landraces namely; Burkina, NAV 4, NAV Red, Black eye and Tom. Thirteen bambara groundnut landraces were evaluated for photoperiod response at 12 h:12 h and 14 h:10 h light:darkness at the Crop Science Department, University of Guelph, Canada. Four hundred grams of Sunshine Mix LA4 was put in 78-4 gallons pot per growth chamber. 4 g of 20:20:20 N:P:K was put into each pot and thoroughly mixed with the soil. One walk- 5 in growth chamber was used for each treatment. Each landrace was replicated in six pots. Growth chamber temperatures were maintained at 30 oC in the day and 25oC in the night at a relative humidity of 60%. Growth chamber PAR ranged between 250-300 umol m-2 s-1. A second controlled environment study was conducted in Guelph to evaluate the response of 13 bambara groundnut landrace to drought. 78-4 gallons pots were filled with sandy loam soil per growth chamber. Plants were irrigated till 30 DAS after which irrigation ceased in the drought treatments. Photoperiod was maintained at 12 h:12 h in both the irrigated and drought treatment. The control was irrigated every three days. Pod yields ranged between 600 kg/ ha to 5.5 t/ha and seed yields ranged between 420 kg/ha to 3.8 t/ha respectively for the various sowing dates. Pod yield of over 5 t/ha was produced by Burkina and Black eye. Harvest indices ranged from 0.12-0.53. Burkina landrace proved to be highly drought and heat tolerant. It also proved to be an early maturing landrace. Leaf area index was highest for all the landraces at 105 DAS. Tom obtained a significantly highest leaf area index of 6.5 and Burkina the least of 4.2. Tom was a highly vegetative landrace. Pod yield was highest in the Transition than the Guinea savannah and Forest agroecologies. Where irrigation is available, sowing bambara groundnut just before the rains in February and March in the Transition and Forest agroecologies of Ghana produced high pod yields. Pod yield were low in the late April and June sowings when the heavy rains had established. Though a drought tolerant crop high temperatures beyond 38oC and low relative humidity negatively affected bambara groundnut pod yield apparently through the drying of pollen tube even in the presence of irrigation. Days to seedling emergence was significantly less under 12 h photoperiod than 14 h photoperiod (p=0.02). Days to flowering was significantly higher 6 under 14 h than 12 h photoperiod (p<0.0001). Pod numbers and pod dry weights were significantly higher under 12 h than 14 h photoperiod. Five out of the 13 landraces; Burkina, Mottled Cream, Zebra coloured, Tan One and Tan Two (both from Tanzania) podded under 12 and 14 h photoperiod though pod numbers and pod dry weights were higher under 12h than 14 h photoperiod. All but two of the landraces (Tom and Black seed) podded under 12 h photoperiod. Leaf area per plant, shoot dry weight per plant and root dry weight per plant were all higher under 14 h than 12 h photoperiod. Genotypes that produced pods under 14 h photoperiod were observed to be early maturing relative to the others. Morphologically, three plants of Tan One produced three, four and five leaflets on different petioles of the same plant instead of the well known trifoliate character of the crop. Landraces were identified for cultivation in the lower and higher latitudes. Tan One, Tan Two, Burkina and Black eye were identified as relatively highly drought tolerant. Leaflets of these landraces showed spindle shape, erect orientation and reduced canopy size in response to drought.
Thesis submitted to the Department of Crop and Soil Sciences, Faculty of Agriculture, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana in partial fulfillment of the requirement for the degree of DOCTOR OF PHILOSOPHY in Crop Physiology