Characterization of New Populations in Sunn Hemp (Crotalaria juncea L.) and Relationship of Their Traits to Yield

Abstract

Sunn hemp (Crotalaria juncea L.) is a short-day, erect shrubby annual, generally 1 to 4 m in height. It is a candidate species for crop rotation schemes as a means of weed suppression and as a green manure crop to improve soils and reduce root-knot nematode (Meloidogyne spp.) infestations. An ongoing breeding program has resulted in a locally adapted population, ‘Selection PBU,’ that varies significantly from the non-adapted ones. In 2007 a field study was undertaken, using two cycles of selection, in order to determine the correlation between juvenile and mature-plant traits and to evaluate the effect of young plant selection on forage yield and other mature-plant traits. Furthermore, several cycles of breeding populations, their parental material and the tropical cultivar ‘Tropic Sun’, for a total of eight populations, were tested in Alabama in 2008 and 2009. Their morphological characteristics were evaluated to identify how the changes in photoperiodicity requirements affected the more recent selection cycles compared to their parental populations. Additionally, germination and growth chamber studies were conducted to determine the effect of temperature and genotype on seedling emergence and early plant growth. Our results indicated that selecting for traits, such as height and number of leaves at four weeks can significantly improve biomass, both at flowering and harvesting time, as well as seed yield. There is potential for future selections to reach higher uniformity, as shown by the higher heritability values for the latest selection cycle compared to the more recent one. Applying equal selection intensity as an even split of 31.6% among, and 31.6% within, family selection, produces the largest genetic gain. Selection PBU was the best or among the best populations for morphological characteristics important for the release of a cultivar in a temperate environment. The most important characteristics were number of days until flowering, days until harvesting, biomass and seed weight, as well as seed quality. The variety ‘‘Tropic Sun’’ was the poorest performer since it produced excessive vegetative matter, very few flowers and no seed. The breeding program has been successful in producing plants that are not photoperiodic and they have become adapted to the sub-tropical conditions of the Southeast. In the process, the plants have become more compact, and they flower and are ready for harvest much earlier than their parental populations. This has allowed producing sufficient amounts of good quality seed to allow for seed-increase and for the continuation of the breeding cycles.