Photosynthetic rates and ploidy levels among populations of switchgrass

Photosynthetic rates reportedly increase with increasing ploidy levels among lowland and upland ecotypes of switchgrass (Panicum virgatum L.). Such reports are based on data derived from native plant materials and no investigation has tested whether a similar relationship exists among released or experimental populations. A series of studies were conducted to address this question, with rates of photosynthesis measured with an infrared gas analyzer and ploidy levels estimated with flow cytometric analysis of nuclear DNA contents. Analyses of nuclei isolated from young stems and stained with propidium iodide indicated that of the 14 populations examined, eight are tetraploids ('Alamo'(1), 'Kanlow', 'NC-116', 'NC-216', 'PMT-279', 'PMT-785', 'Stuart', and 'Wabasso') and six are hexaploids ('Blackwell', 'Caddo', 'Cave-in-Rock', 'Pathfinder', 'Shelter', and 'Trailblazer'). Twelve of these populations (not including NC-116 and NC-216) were grown in the greenhouse and photosynthetic rates measured on the youngest fully expanded leaf blade 40 to 45 d after germination. Rates of photosynthesis did not differ among populations or between tetraploids and hexaploids. Field studies at Knoxville, TN, Stephenville, TX, and Blacksburg, VA, did indicate, however, that tetraploids often had mean photosynthetic rates 12 to 18% higher than those of hexaploids. These differences were observed at many field sites during May, June, and July, yet based on repeated sampling at Knoxville, rates of leaf photosynthesis measured in September were up to 6.5 mu mol CO2 m(-2) s(-1) higher in hexaploid compared with tetraploid populations. Contrary to earlier reports that photosynthetic rates increase with increasing ploidy level, our results indicate that such a relationship is not consistent for tetraploid and hexaploid populations of switchgrass but rather may change with season or environmental stress.