The inheritance of bean size, pod size and number of beans per pod in cocoa (Theobroma cacao L.), with a note on bean shape

Abstract

Studies have been made on the F₁ and F₂ generations of crosses between the Amazon clones Nanay 32, Parinari 7, and Parinari 35. There is wide variation in size from pod to pod on a tree, and this is largely paralleled by variation in bean number, bean size remaining relatively constant. However in some trees bean size increases in the largest pods, bean number remaining constant. It appears that if a pod is too small for the inherent number of beans to be present at their inherent size, the number of beans which develop is reduced so that their inherent size is attained; if the pod is larger than is required for the full number of beans to attain their inherent size the beans may increase in size above the inherent value. Thus if the average bean size in a sample of pods taken from a tree indicates the inherent bean size reasonably accurately it is likely that the inherent bean number will be relatively underassessed, and conversely if the observed bean number indicates the inherent number relatively accurately it is likely that the observed bean size will overassess the inherent size. Scatter-diagrams may therefore be more informative than averages.

Bean number and bean size are strongly inherited but the difficulties described above tend to obscure the pattern. In the Pa7 x Na32 F₁ progeny inherent bean sizes are low and probably relatively constant while inherent bean numbers vary widely; in consequence inherent bean numbers are well indicated by samples of pods taken from the trees, and inheritance of bean number in the F₂ progenies is very clear while inheritance of bean size is not. In contrast, in the F₁ of Pa35 x Na32, there is clear segregation in bean size while bean numbers are relatively high and constant; inheritance of bean size in the F₂ progenies is clear while the evidence of inheritance of bean number is weaker.

The average pod weight of a progeny seems to be determined by the inherent bean size and the inherent bean number.

Bean breadth and bean thickness tend to be negatively correlated. It is suggested that where the number of ranks of beans in a pod is high relative to the length of the pod and the inherent bean size, the beans, lacking room for expansion in thickness, tend to increase in breadth so as to attain the inherent size. Short fat pods may therefore tend to contain broad, thin, plate-like beans.