Breeding for higher yields, dwarfing, taste, shallow tuber formation, mechanization-ready varieties, climate change, nutrient-use efficiency, disease, resistance, drought tolerance, and early harvest is a vital strategy for biodiversity, and thereby sustainable yam cultivation. Unfortunately, yam suffers from low multiplication ratios, dormancy, and long growth periods, resulting in a painstakingly long breeding cycle, with reports of new varieties of water and white yam requiring 18-22 years.
An adaptive mechanism which enhances yam’s drought tolerance, dormancy is marked by starch reduction and minimum metabolic activity including low respirations rates and activities of amylase, phosphorylase and G-6-PD. Typically, it lasts between 30-50 days, dependent on species, or until sprouting begins—anywhere from 28 to 120 days after sprouting. On one hand dormancy represents a tremendous disadvantage, only allowing one generation to be bred annually; however, on the other hand, dormancy enabling long-term storage of yams (see research on Storage/Shelf-life). Understanding the mechanisms and building the capacity to control (i.e. either prolong or break) dormancy, therefore, is a significant area of research.
Thus far, it is evident that dormancy is largely endogenous, under tight genetic control and location specific. For example, planting of D. alata>/i> over a period of seven months in Guadeloupe resulted in sprouting only between March and April. While bulbil-producing species D. opposita has induced sprouting via light and/or heat and correlated batatasin concentration to dormancy duration, this has not been replicated in more tropical species. Instead, most work on D. alata, D. rotundata, D. esculenta, and other tropical species has focused on prolonging shelf-life, for example with giberellic acid (GA). In contrast, in vitro cultures, GA hurries initiation of microtubers (176 to 93 days), but they are small and shriveled. Some preliminary work has demonstrated the effectiveness of ethylene or its precursors (i.e. ethrel, 2-chloroethanol, and ethylene chlorohydrin) in breaking dormancy. Hamina et. al. (2013) also curbed dormancy with 30uM and 100 uM fluoridine treatments of long dormancy varieties, D. rotundata var TDr 131 and D. alata var TDa 98/01166 , in coco coir medium. [Craufurd]
Most yams are cultivated from ‘seed’ yams, realistically small yam tubers or cuttings from larger (1kg+) ware (for consumption) yams. This represents a significant trade-off, however, requiring farmers to sacrifice 10-30% of yields for subsequent planting—not to mention diminishing production of ware yams, which is a status symbol. Lack of synchronicity between fertility of male and female plants and a higher ratio of male to female flowers contributes to the lack of true seed breeding. However, manual pollination, though labor intensive, is possible and can be further enabled by in vitro pollination such that the 10-day ‘window’ of stigma receptivity can be prolonged and within 10-12 weeks, a seed matures. Kashihara et. al. (2013) demonstrated effectiveness of pollen cryopreservation at -20°C after one year in four species and Daniel et. al. (2002) for 2 years at -80°C.
As long as breeding efficiency is limited by dormancy to one generation per year, cultivation of and research on yam is limited principally by multiplication ratios. The traditional planting method of milking, whereby an intended ware yam is harvested prematurely with the head being replanted to produce seed yam for the following year, provides 5-10 ware yams over the two-year (i.e year 1 for seed, year 2 to convert seed to ware) production. The Yam Minisett Technique (YMT) increased this multiplication ratio significantly to 20-40 ware yams annually, but was not readily adopted since its introduction in the 1970s due to poor outreach programs and added requirements of labor and infrastructure. A more recent YIIFSWA initiative to address these obstacles to adoption created the Adopted Yam Minisett Technique (AYTM), which has been well-received to date. However, its advantages to adoption are offset by sacrifices in multiplication such that it produces 12-20 ware yams. The next evolution in macropropagation, vine-cutting provides up to a 30-fold increase in ware yam compared to YMT, in turn, reducing the time to rear a new variety from 10 years to 3.5 years (Lopez-Montes). Pelemo et. al. further detailed optimal vine cutting, stipulating that 11-15g minitubers were sufficient to produce ware yams (thus a multiplication ratio of 1:66-100) and that 5-11g vine cuttings could produce 100-300 seed tubers (thus an annual multiplication ratio of 1:50-150). Tissue culture, particularly with induction of somatic embryogenesis, is also an avenue for increased multiplication ratios with the added benefit of definitively providing clean seed. See pages of Micropropagationand Macropropagation.