Successful manipulation of the growth cycle of yam (Dioscorea spp.) for year-round production for food security and climate change

Yams are important items of diet throughout the Caribbean Region and, among them, the cultivars of the species, Dioscorea alata L. and D. esculenta (Lour.) Burkill are particularly important in the Eastern and Southern Caribbean, although the latter is so to a lesser extent. With a growing period of approximately nine months and a corresponding dormancy period, these cultivars are produced on an annual basis like clockwork, providing food yams for approximately three months of the year, the period of availability coinciding with the period of dormancy as the latter marks the natural period of storage of the tuber. Consequently, the breakage of dormancy marks the beginning of the end of natural storage as the emerging sprouts signal the onset of the next growing period. This coincides with the start of the rainy season and the yam is adapted to existing climatic conditions that provide the ideal environment for both plant establishment and tuber growth. Cultivars of the two species bear no reproductive seed and therefore, are completely dependent on vegetative means for perpetuation. Any adverse development during storage or early growth that prevents sprouting or tuber initiation and growth, results in the permanent loss of the affected material and potentially threatens loss of the specific cultivar. Hence, climate change events such as shifts in the onset of the rainy season, extension of the dry period and incidences of severe weather associated with excessive flooding that can result in death of roots and developing tubers, pose real risks to the persistence of the cultivars of D. alata and D. esculenta that are produced in the Caribbean Region. In order to counteract these effects, what is needed is the manipulation of the growth cycle to allow for year-round production so that the changes that have been observed with respect to climatic and weather patterns will not result in a loss of planting material and, by extension, loss of germplasm. It is well-established that the growth cycle is under hormonal control and that the key to year-round production is the breaking of the physiological lock on the growth cycle that is responsible for the seasonal production by which these species are characterized. There have been many attempts by researchers in the past to achieve this, but without success until relatively recently. Most attempts have explored the use of natural and synthetic plant growth regulators. Further, past attempts have focussed on the breakage of natural dormancy and induction of early sprouting, but instead, the success reported here, was achieved by delaying sprouting to beyond the period of natural dormancy in a novel approach that eliminates the use of growth regulators and other chemicals. This paper reports how reduced temperature storage has been used successfully to break the physiologically controlled growth cycle in cultivars of D. alata and D. esculenta, producing yams all year long by delaying the breakage of dormancy, ensuring not only an extension of the availability of yam tubers for food but also availability of viable planting material throughout the year. It was found that tubers can be stored successfully for an entire cycle, so that tubers from the previous year can meet tubers of the current year’s harvest providing a buffer against crop loss from any adverse biotic or climatic event. The effects on sprouting patterns are described. The use of reduced temperature storage for this purpose has not been reported and this account serves to fill an important gap in documented yam dormancy research.