INTRASPECIFIC VARIATION IN POLLEN YIELD IN BROMEGRASS (POACEAE: BROMUS)

Variation in pollen production was measured within five hermaphrodite species of bromegrass (Bromus). Anther length is an excellent predictor of pollen production in this genus (R2 = 0.97). Anther length varied considerably within each of the species, both among and within individual plants. Within plants, most of the variation occurred among florets within spikelets; florets in upper spikelet positions were smaller and produced less pollen. In B. inermis, pollen production was decreased by defoliation and increased in shoots that grew on thatching ant (Formica obscuripes) mounds. Whole-shoot pollen yield was determined by spikelet number, number of florets per spikelet, and pollen production per floret. All of these yield components must be considered in attempts to estimate pollen production accurately. IN CONTRAST to the voluminous literature on intraspecific variation in fruit and seed yield, relatively little is known about variation in pollen production (Bertin, 1988). Yet pollen yield can have as much impact on fitness as seed yield, and recent empirical and theoretical studies have emphasized the importance of paternal fitness in the evolution of attractive structures in angiosperms (e.g., Bell, 1985; Charnov and Bull, 1986; Stanton, Snow, and Handel, 1986). As part of a study of the reproductive biology of bromegrass (Poaceae: Bromus L.), I measured intraspecific variation in pollen production in five Bromus species. Lloyd (1980) suggested that a plant's maternal allocation is affected by a series of adjustments through the flowering season; total seed production could be modified by changes in flower number, proportion of pistillate flowers, number of ovules per flower, fruit and seed set, and seed weight. Similarly, serial adjustment of a plant's pollen Received for publication 28 December 1987; revision accepted 16 June 1988. E. J. Cushing, L. F. Delph, D. G. Lloyd, P. A. Morrow, and C. J. Webb made helpful comments on various versions of the manuscript. Financial assistance was provided by several bodies at the University of Minnesota, including the Dayton-Wilkie Fund, the Field Biology Program, the Graduate School (which awarded a Doctoral Dissertation Fellowship and a Doctoral Dissertation Special Grant), and the Department of Ecology and Behavioral Biology. This work was also partially supported by NSF Grant DEB8114302-A02 to D. Tilman and J. Tester for Long-Term Ecological Research at Cedar Creek Natural History Area. Partial support while preparing the manuscript was provided by NSF Grant INT-8603569 and by a grant from the Miss E. L. Hellaby Indigenous Grasslands Research Trust of Dunedin, New Zealand. 2 Current address: Department of Biology, Carleton College, Northfield, MN 55057. production is possible by means of changes in flower number, proportion of staminate flowers, number of stamens per flower, and number of pollen grains per stamen. Bromus flowers are hermaphrodite and always contain three stamens; thus, the remaining options for adjustment of pollen yield are flower number and pollen grains per stamen. I measured these parameters in field populations of five species, and calculated their relative importance in determining pollen yield. MATERIALS AND METHODS-Bromus tectorum L. plants were collected in 1982 from a disturbed site near the Botany Greenhouse of the University of Minnesota, Minneapolis. Bromus inermis Leyss., B. kalmii Gray, and B. ciliatus L. were collected in 1983 from populations in the Cedar Creek Natural History Area, about 50 km north of Minneapolis. The B. latiglumis Shear (Hitchc.) population was sampled in 1982 in Fort Snelling State Park, St. Paul. Voucher specimens of the five species have been placed in the University of Minnesota Herbarium, St. Paul. The five Bromus species differ in a number of reproductive characteristics, including average anther length, pollen size, and sex allocation (McKone, 1985, 1987). The species span a broad range of outcrossing rates (McKone, 1985), from obligate outcrossing in B. inermis, through partial selfing in B. kalmii, B. ciliatus, and B. latiglumis, to nearly complete selfing in B. tectorum. Bromus tectorum is an annual; the other four species are perennial. Pollen production was estimated for individual flowering shoots (culms). I will refer to shoots as plants; except for B. inermis, the species studied are not rhizomatous, and only