Pollen of Syzygium (Myrtaceae) from SE Asia, especially Thailand

Pollen of 57 species of Syzygium from SE Asia were surveyed by SEM. The grains are all ± triangular in polar view, uniformly small (mean diameter 10.4 μm) with few surface sculpturing features. All species examined exhibit a distinct apocolpium and, usually, apocolpial field. The presence of an apocolpial field cannot be linked to any known taxonomic grouping within the genus. It does not appear possible to distinguish the species of Syzygium on the basis of features of the pollen visible under SEM, though section Jambosa tends to have larger pollen with a proportionally larger apocolpium than species not in that section. Estimates for the number of pollen grains per stamen in species with larger pollen grains (± section Jambosa) are 5× as great as for other species in the genus. TEM preparations showed the pollen surface to bear small supratectal processes and to be covered with pollenkit. The foot layer is well developed and more or less of the same thickness as the tectum. The columellae are of variable width and the endexine c. 0.05 μm thick. Various features of flower size are found to be positively correlated to pollen size. As large pollen grains are linked to large staminal numbers and large stamens (long filaments and large anthers) it is suggested that investigation of the breeding biology of section Jambosa is required.