Switchgrass (Panicum virgatum L.) is a widely adapted warm‐season perennial that has considerable potential as a biofuel crop. Evolutionary processes and environmental factors have combined to create considerable ecotypic differentiation in switchgrass. The objective of this study was to determine the nature of population × location interaction for switchgrass, quantifying potential differences in latitudinal adaptation of switchgrass populations. Twenty populations were evaluated for biofuel and agronomic traits for 2 yr at five locations ranging from 36 to 46° N lat. Biomass yield, survival, and plant height had considerable population × location interaction, much of which (53–65%) could be attributed to the linear effect of latitude and to germplasm groups (Northern Upland, Southern Upland, Northern Lowland, and Southern Lowland). Differences among populations were consistent across locations for maturity, dry matter, and lodging. Increasingly later maturity and the more rapid stem elongation rate of more southern‐origin ecotypes (mainly lowland cytotypes) resulted in high biomass yield potential, reduced dry matter concentration, and longer retention of photosynthetically active tissue at more southern locations. Conversely, increasing cold tolerance of more northern‐origin ecotypes (mainly upland cytotypes) resulted in higher survival, stand longevity, and sustained biomass yields at more northern locations, allowing switchgrass to thrive at cold, northern latitudes. Although cytotype explained much of the variation among populations and the population × location interaction, ecotypic differentiation within cytotypes accounted for considerable variation in adaption of switchgrass populations.

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Switchgrass (Panicum virgatum L.) is a warm‐season native grass, used for livestock feed, bioenergy, soil and wildlife conservation, and prairie restoration in a large portion of the USA. The objective of this research was to quantify the relative importance of latitude and longitude for adaptation and agronomic performance of a diverse group of switchgrass populations. Six populations, chosen to represent remnant prairie populations on two north–south transects, were evaluated for agronomic traits at 12 locations ranging from 36 to 47°N latitude and 88 to 101°W longitude. Although the population × location interactions accounted for only 10 to 31% of the variance among population means, many significant changes in ranking and adaptive responses were observed. Ground cover was greater for northern‐origin populations evaluated in hardiness zones 3 and 4 and for southern‐origin populations evaluated in hardiness zones 5 and 6. There were no adaptive responses related to longitude (ecoregion). Switchgrass populations for use in biomass production, conservation, or restoration should not be moved more than one hardiness zone north or south from their origin, but some can be moved east or west of their original ecoregion, if results from field tests support broad longitudinal adaptation.

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chromosome numbers ranging from 2n ϭ 2x ϭ 18 to 2n ϭ 12x ϭ 108 (Nielson, 1944;McMillan and Weiler, Greater knowledge of the magnitude of genetic variability for bio- Henry and Taylor, 1989). Switchgrass is classified mass yield and yield components in switchgrass, Panicum virgatum L., and relationships among the biomass yield component traits would into upland and lowland ecotypes based on morphology facilitate the breeding improvement of the species. Accordingly, we and habitat preference (Porter, 1966). All confirmed conducted two replicated experiments to assess genetic variation for lowland ecotypes have been tetraploids and most upland biomass yield and yield components and quantify relationships among ecotypes are octoploids (Hopkins et al., 1996). those traits in different switchgrass populations. In Exp. 1, 228 half-sib Information on the variability and associations of biofamilies from SU C 3 and 261 from NU C 3 populations were evaluated at mass yield and yield components in switchgrass is lim-Perkins, OK, while 278 half-sib families from the SL C 0 population ited. Talbert et al. (1983) reported narrow-sense heriwere evaluated at Stillwater, OK. Exp. 2 comprised 11 lowland switchtability estimates of 0.25 and 0.59 based on individual grass populations tested at Chickasha and Perkins, OK, in 1998. Sub-half-sib progeny and half-sib progeny means, respecstantial differences (P Ͻ 0.01) in biomass yield per plant existed among half-sib families within the respective SU C 3 , NU C 3 , and SL C 0 popula-