Porogamy and Chalazogamy in Walnut (Juglans regia L.)

Abstract

Pollen tube growth in Juglans regia L. has been widely studied, with entry to the ovule variously reported as being porogamous or chalazogamous. We examined the processes involved by using fluorescence and bright field microscopy of sectioned material and whole mounts and squashes of flower parts and ovules. Results indicate that there is a single, consistent path for pollen tube growth from the stigma to the ovary, but within the ovary, pollen tube growth varies. Entry to the ovule is either chalazogamous or porogamous depending on the extent of development of the single, orthotropous ovule. Pollen germination is delayed in flowers pollinated early in the receptive period. These flowers have less well developed ovules with a gap between the base of the stylar canal and the ovule. Pollen tubes arriving at the ovule at this stage are apparently unable to traverse this open space and they bypass the micropyle. Instead, they grow along the surface of winged outgrowths that arise from the base of the ovule. These structures serve an obturator-like role in directing pollen tube growth in the ovary but, unlike a true obturator, they direct the pollen tubes to the base of the ovule where they penetrate the funiculus and enter the ovule through the chalaza. Pollination late in the anthesis period produces tubes that grow into the ovary after development of the ovule has progressed to the point that the integument is closely appressed to the base of the style. These tubes grow directly into the micropyle.