Evidence for an interaction effect during in vitro rooting of oil palm (Elaeis guineensis Jacq.) somatic embryo-drived plantlets

In vitro rooting of oil palm shoots derived from somatic embryos was achieved through a single-phase protocol in which three shoots are cultured in the same culture tube on an [alpha]-naphtaleacetic acid-enriched culture medium. Rooting performance was dependent on both the genetic origin and initial size of the shoot explants. All shoots from a given tube showed a tendency to give roots of the same type, independent of the original size of the explant. Whatever the clonal line, longer-size shoots (L-type: >9 cm) showed higher rooting rates than medium-size (M-type: 7-9 cm) and short-size ones (S-type: 5-7 cm). When groups of three shoots from the same clonal line were rooted together in the same culture tube, the combination of plant size within the group impacted overall quality of rooting. Within triplets of shoots containing more than one short individual, the probability of obtaining adequate rooting was low. Similarly, when more than one long shoot was included in the triplet rooting, quality was also poor. By avoiding such combinations, the rate of well-rooted plantlets increased by 25%, with a maximum of 66% when triplets of S/M/L combination were used. Smaller shoots, which usually showed poor rooting performance, were therefore found to benefit from the presence of their neighbors. This interaction between the sizes of individuals in a given tube was found to be associated with a within-tube correlation effect, a phenomenon previously described as "event coupling," which was estimated using a distorted binomial-type distribution of probabilities. The resulting calculation of a coupling factor (average r=0.60) explains the behavior of shoots within the same culture tube and their average rooting performance. Modeling of the interactions that occurred during in vitro rooting is described here and is recommended for improvement of this critical step in micropropagation. (Résumé d'auteur)