A reliable means to induce sprouting in dormant seed tubers of yams (Dioscorea species) is required to enhance flexibility in planting date and rate of propagation of the crop. Experiments were conducted to assess the potential of two gibberellin inhibitors, Uniconazole-P and Prohexadione-calcium, to induce sprouting in tubers from three varieties of D. rotundata and four of D. alata. Uniconazole-P and Prohexadionecalcium shortened the period of dormancy in tubers of some varieties. In others, they either had no effect or extended dormancy. The varietal responses were also influenced by whether the tubers were treated at harvest (before shoot senescence) or four weeks after harvest by which time shoots had senesced fully. Tubers stored in darkness at a constant temperature of 30 • C sprouted earlier but lost weight faster than did those stored under natural daylight and ambient temperature. The apparent slow action of Uniconazole-P and Prohexadione-calcium, and their variable effectiveness in relation to variety, would limit their usefulness in seed or breeding programmes, or in determining treatment timing and the best storage environment for the tubers after treatment. I N T RO D U C T I O NTuber dormancy is an important mechanism for adaptation of yams (Dioscorea spp.) to their natural environments (Craufurd et al., 2001) and, because yams are purchased predominantly as fresh tubers for food preparation, long dormancy is very important for shelf life. Hence, long dormancy of tubers is a desirable attribute in yam breeding and selection programmes. It complicates the use of the tuber for propagation, however. Irrespective of when a yam seed tuber is planted, the critical starting point of the growing season is when dormancy ends and sprouts are produced. There is a period of about four months after harvest during which tuber losses are incurred in storage but propagation of the planting material would not be successful. In some environments, early planting could largely obviate losses incurred during seed-yam storage if there was more flexibility in the control of sprouting date through effective means of artificially terminating dormancy. Successful protocols for this would have a tremendous impact on yam cultivation. They could lead to more uniformity in sprouting, off-season propagation of planting materials, multiple cropping cycles of early maturing varieties per year and better control over planting date. Developing such protocols has been rather challenging (Barker et al., 1999a;b;Craufurd et al., 2001).

Seed tuber production of Dioscorea rotundata using the minisett technique was tested on-farm and on-station in different agro-ecological zones in Nigeria. A lower sprouting rate of minisetts was observed in the southern Guinea savanna than in the tropical forest zone. Sprouting and tuber yield in direct field planting of minisetts was strongly influenced by variety of D. rotundata, but D. alata was less affected by sett size. An increase in minisett size in some varieties of D. rotundata would enhance their sprouting potential.

In experiments in 2001 to 2003, 50‐day‐old plants of four varieties of white Guinea yam (D. rotundata) were exposed to 10 h (short‐day) day length for 15 days, 14 h for 30 days or constant 24 h, with natural day length as the control. Under natural day length the male varieties flowered earlier than the female. Short day‐length delayed and reduced the intensity of development of inflorescences, but the response to the 14 h and 24 h treatments differed between the varieties. Copyright © 2005 John Wiley & Sons, Ltd

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To induce sprouting two gibberellin inhibitors, uniconazole-P (UP) and prohexadione-calcium (PC), were applied to foliage of white Guinea yam (Dioscorea rotundata) and water yam (D. alata) 30 days before harvest or to tubers at harvest. The tuber sprouting period was shortened by about 35 days in D. alata TDa 95/00328 in response to UP and PC, and TDa 92-2 in response to PC, but not in others through foliage treatment. Sprouting of tubers was promoted in D. rotundata TDr 99-12 and D. alata TDa 99/00049 and TDa 291 when they were soaked in UP or PC, but the others had a limited response at harvest. Sprouting of TDa 99/00049 tubers started 70 days earlier and the periods of sprouting were 35 to 42 days shorter in response to the UP and PC. Foliar application was more effective in promoting tuber sprouting than tuber soaking at harvest in TDa 92-2 and TDr 93-31, and the reverse was true for TDr 99-12 and TDa 291. The responses of yam varieties to UP and PC seem to vary depending on the stage of dormancy in their tubers.