Genetic variation and relationship of traits related to drought tolerance in cocoa (Theobroma cacao L.) under shade and no-shade conditions in Ghana

Drought is the major abiotic stress limiting cocoa seedling survival during establishment. The level of genetic variation and relationship among traits related to drought tolerance were examined in a 6 × 6 incomplete diallel and 8 × 3 incomplete factorial mating designs for parents of diverse genetic groups. Thirty-eight families, 23 from the diallel and 15 from the factorial mating were evaluated in 2012/13 major dry season under shade and no-shade locations. Six-month old seedlings were planted in a randomized complete block design with two replications consisting of 24 plants per plot. Traits related to drought tolerance including stem growth, percentage survival, leaf chlorophyll content and plant vigour were monitored. Analysis of the 38 families showed highly significant (P ≤ 0.01) variation for families, replications, locations and location × family interaction for most of the traits. Preponderance of additive effects was manifested in terms of significant GCA when genetic analysis of the diallel was performed. Heritability was moderate to high (h2b = 0.43–0.63) for broad sense and low to moderate (h2n = 0.25–0.57) for narrow sense. Clones PA 13 and AMAZ 15/15 had positive GCA effects, whereas PA 150 and T60/887 had negative GCA effects for most traits. The highest genotypic correlations were between survival and vigour. Results from the study indicate that there is considerable genetic variation for traits related to drought tolerance in the available germplasm, and the families of some specific clones are able to survive droughts under typical field conditions in Ghana.