Self-incompatibility in the nitrogen-fixing tree, Acacia retinodes: quantitative cytology of pollen tube growth.

A 10×10 diallel of crosses involving trees in two natural populations of A. retinodes (Leguminosae: Mimosoideae) showed a high level of self-incompatibility, with ISI (Index of Self-Incompatibility) of 0.08. In both self- and cross pollinations, pollen tubes grew through the style to the ovules within 11 h. Tube growth rate is approximately similar in both types of mating: 3.3 to 4.5 μm/min for the first 6 h, and 1.2 to 2.0 μm/min for the succeeding 5 h. Three potential explanations for self incompatibility are proposed: (1) an S-gene controlled system in which pollen tubes are arrested in the nucellus of the ovule; (2) sperm: egg interactions at fertilization and (3) action of post-zygotic lethal genes.