Flowering Behavior in Prairie Cordgrass and Switchgrass in Response to Variation in Ambient Daylength.

Climate change, in particular warmer winters and springs, may impact native warm-season grasses by causing them to break winter dormancy earlier than normal. However, reproductive development in these grasses is strongly influenced by photoperiod. Therefore, the objective of this study was to determine the growth and flowering behavior of prairie cordgrass (Spartina pectinata L.) and switchgrass (Panicum virgatum L.) under increasing and decreasing ambient daylengths outside the range of photoperiods they would normally encounter during the normal growing season in their natural habitats. The experiment was conducted in greenhouses located at Urbana, IL (40^ON) and Brookings, SD (44 ^ON), which represent a latitudinal difference in photoperiod length. Two populations of prairie cordgrass and one upland cultivar, Cave-In-Rock, of switchgrass were used at Urbana and two populations of prairie cordgrass and two upland cultivars, Summer and Sunburst, of switchgrass were used at Brookings. Ramets of each plant from the fields were transplanted into pots placed in greenhouses with ambient light conditions on the 21^st of each month from Oct. to Mar.. Plants were observed for dormancy breaking patterns and vegetative and reproductive growth under decreasing (Oct. 21^st through Dec. 21^st) and increasing (Dec. 22^nd through Jun. 21^st) photoperiods. Breaking dormancy and growth patterns were similar at both locations. Both switchgrass and prairie cordgrass broke dormancy and resumed growth within two weeks after moving from the field to greenhouses for each of the 6 collection dates. However, all prairie cordgrass collected from Oct. to Jan. ceased growth and showed signs of dormancy within three weeks, after which plants were completely dormant. However, in general, those field-collected plants broke dormancy and resumed normal growth again during late Feb. in the greenhouse. Prairie cordgrass collected in Feb. and Mar. displayed normal growth at both locations and flowered at IL. Switchgrass collected during Oct. to Jan showed signs of dormancy, but not as complete as for cordgrass, about 3 to 4 weeks after emergence; and the first flush of tillers did not flower. However, a second flush of tillers appeared during Apr., and many of those tillers flowered. Our results indicated that early dormancy breaking by warm spring weather could cause significant changes in growth and flowering of these two native warm-season grasses.