Rootstock influences on growth and bearing precocity in juvenile cocoa

Within the cocoa germplasm used for variety development in West Africa, knowledge of varieties that are suitable as rootstocks to promote growth and precocity of clones is lacking. Six cocoa varieties were used to generate 36 clones for evaluation. For each of the six scions selected as ortets from previous progeny trials, the corresponding seed-derived family was regenerated through manual pollinations. The scions were grafted in a full diallel fashion on each of the six seed-derived family, and field evaluated in four replications following a randomized complete block design. Also, seed-derived varieties were evaluated in the nursery for vigour characteristics, and under similar conditions as the 36 clones in the field. Genotype heterozygosity at 72 SNP loci was significantly correlated with vigour characteristics of the six varieties. Seed-derived varieties of high vigour benefited clone performance for vigour, and dry bean yield when used as rootstocks but not as scion. The maximum photochemical efficiency of photosystem II (Fv/Fm) was different among seed-derived varieties and clones in the dry season but not the rainy season. Among clones, higher Fv/Fm values were associated with higher-vigour rootstock varieties. Diallel grafting model enabled partitioning of total variance into several sub-components. The variance due to specific compatibility ability was not significant for any of the measured traits. Decomposition of the reciprocal variance revealed that significant reciprocal effects were mainly due to significant maternal (rootstock) effects. The key finding is that, for any scion, the most precocious clone is obtained by grafting on high vigour rootstock varieties.