This study examined pollen morphological variation among Amaranthus species and interspecific hybrids. Ten weedy Amaranthus species, a cultivated grain species, and several putative hybrids resulting from interspecific mating between common waterhemp and Palmer amaranth were grown in a greenhouse. Mature pollen was collected, viewed, and photographed with a scanning electron microscope (SEM). The pollen grains were spherical shaped with polypantoporate, or golf ball-like, aperture arrangement. Differences were observed between the monoecious and dioecious Amaranthus species. Pollen grains of the dioecious species had a greater number of apertures on the visible surface. One exception to these trends was the dioecious species, Palmer amaranth, whose pollen was similar to that of the monoecious species spiny amaranth. However, pollen grain diameters did not differ between the monoecious and dioecious plants. Significant differences also were noted between the pollen from the putative common waterhemp × Palmer amaranth hybrids and the parental-type pollen grains. Pollen of the hybrids was similar in size to the maternal parent but had an aperture number that was intermediate between parents. This indicates that pollen characteristics may be controlled by the female and that hybrids may be more prevalent than originally thought.

Plant model systems have contributed greatly to the dramatic progress in understanding the fundamental aspects of plant biology. Using model weeds will also help facilitate focused funding and research in the weed science community. Criteria for developing model weeds require attention to weedy characteristics that impart economic losses and a wide geographic distribution, attributes that present the potential for political and scientific support. Expressed sequence tag (EST) databases for model weeds are the most practical approach to identifying new genes and obtaining data on the gene expression underlying weedy characteristics. Weeds such as Canada thistle, eastern black nightshade, johnsongrass, jointed goatgrass, leafy spurge, waterhemp, and weedy rice are proposed as model systems.