Intermittent pollen-tube growth in Fagales, especially in alders(Alms sieboldiana)

Abstract

Summary: In Fagales, the ovary is still immature when pollen is delivered to the stigma and the fertilization occurs more than one month after pollination. Developmental study of the pollen-tube growth in the pistil of three species(Casuarina equisetifolia, Alnus sieboldiana and Myrica rubra)in Fagales showed that the pollen tube grows intermittently in close association with the development of the ovary and ovules. At the stage when the ovary develops an ovule primordium, many pollen tubes germinated on the stigma and reached the style, where they remain for several weeks in Casuarina and Alnus. The pollen-tube growth to the style proceeds irrespective of ovules and embryo sac. Thereafter, the pollen tube resumes growing to the ovary with an immature ovule(s)(megaspore tetrad stage in Casuarina and Alnus and megaspore mother cell stage in Myrica). In these species, the pollen tube reaches the ovule with immature embryo sac. Except in the last step, where the tube grows from the tissue of ovule(the chalaza or the nucellus)to an embryo sac, a mature embryo sac is not necessary for pollen-tube guidance in the pistil. While the intermittent pollen-tube growth appears to play an important role in the selection of a single pollen tube from many and one ovule from two(in Casuarina and Alnus), its detection provides insight into a study of the mechanism of pollen-tube guidance.