SEED PHYSIOLOGY, PRODUCTION & TECHNOLOGY Stratification in Switchgrass Seeds Is Reversed and Hastened by Drying Zheng-Xing Shen, David J. Parrish,* Dale D. Wolf, and Gregory E. Welbaum ABSTRACT 1974), Prunus (Haut, 1932), Polygonum spp. (Justice, 1944; Staniforth and Cavers, 1979), Pyrus spp. (West- Switchgrass (Panicum virgatum L.) seed dormancy is a major wood and Bjornstad, 1968), and Pinus taeda L. (Barnett, obstacle to successful establishment of this multi-purpose species. We have investigated the influences of prolonged stratification, poststrati- 1972). By contrast, partial drying of dormant (freshly fication drying, restratification, and afterripening on germinability of harvested) or partly dormant (stratified for 1 mo) seeds ‘Cave-in-Rock’ switchgrass seeds. Germination can be increased many- of Zizania palustris L. increased subsequent germina- fold to 80% with 14 d of stratification, if the seeds are moved directly tion rate and percentage (Aldridge and Probert, 1992). to germination without drying. However, we have found germinability The secondary dormancy induced by drying seeds (but not viability) may decrease by half or more if the stratified following stratification is herein termed reversion to dis- seeds are first dried and then rehydrated for germination testing. The tinguish it from other types or causes of secondary dor- reappearance of dormancy (secondary dormancy) during poststratifi- mancy. The object of this research is to study the rever- cation drying is herein called reversion. During poststratification drying, sion phenomenology in switchgrass to better understand dormancy reversion increased as the degree of desiccation increased. survival and propagation of this important species under Extended stratification (for 42 d) prevented reversion. Afterripening also reduced the potential for reversion. Stratification and afterripen- both natural and agricultural conditions. ing appeared to work additively to remove revertibility. Restratifying dried seeds showed that, while a drying interruption caused reversion, MATERIALS AND METHODS it also decreased the dormancy variability within a seedlot and short- ened the total stratification time needed to obtain maximum germina- Seedlots tion compared with continuous stratification. Switchgrass seeds can ‘Cave-in-Rock’ switchgrass seeds were used throughout this be moved toward greater germinability by stratification, but drying study. Seedlots 19-92 and 26-92 were purchased in February following insufficient stratification can lead to dormancy reversion. 1993, seedlot 12-93 in November 1993, and seedlot 13-94 in January 1995 from Osenbaugh Grass Seeds (Lucas, IA). Seed- lots 1-94 and 1-98 were hand-collected from plots in Blacks- S witchgrass is a tall-growing, widely adapted, burg, VA, in September of 1994 and 1998, respectively. Seedlot warm-season perennial with uses that include for- 1-92, which was highly germinable, was a composite of 1992- age, soil conservation, wildlife habitat, and biofuel feed- harvested seedlots that had been naturally after-ripened dur- ing storage for nearly 2 yr at ambient (laboratory) tempera- stock (Wolf and Fiske, 1995; Sanderson et al., 1996). tures. All seedlots were cleaned with a seed blower (Ames Establishment of switchgrass, however, can be a major Power Count Co., Brookings, SD) so that average seed weight problem, partly because of seed dormancy. Some culti- was 1.85 mg seed -1 (540,000 seeds kg -1 ). All seedlots (except vars may have up to 95% of their seeds dormant at the 1-92, which was held at ambient temperature) were stored at time of harvest, and those seeds may need as long as 5°C and ≈110 g kg -1 moisture content (MC) until experiments 2 yr of afterripening to become germinable. were conducted a few months after purchasing or harvesting. The efficacy of stratification (periods of wet, cool exposure) in breaking seed dormancy is well docu- Dormancy Reversion as Influenced by Extent mented in many species (Lewak and Rudnicki, 1977; of Poststratification Drying Nikolaeva, 1977; Bewley and Black, 1982; Mayer and The germination of seeds immediately following stratifica- Poljakoff-Mayber, 1989). Dormancy of switchgrass tion was compared with the germination of seeds that were seedlots can largely be overcome by stratification (Zarn- stratified and then partially or fully air-dried to determine if storff et al., 1994; Wolf and Fiske, 1995). However, the dormancy reversion was influenced by the degree of desicca- stratified seeds must be dried before mechanical plant- tion. Twenty-five grams of 13-94 seeds were placed in a cloth ing, and we have observed that drying frequently re- bag, immersed in water for 24 h, drained, sealed in a plastic duces germinability. This decrease in germinability is bag, and stratified at 5°C for 14 d. Seeds were then spread in not a result of lost viability; the seeds that revert to an open, shallow container and dried for 3 d with forced air dormant status will again become germinable if restrati- (except during the first 6 h). The ambient drying air was 22 fied. Secondary dormancy induced by drying after strati- to 25°C with a relative humidity of 30 to 70%. Under these conditions, the seed MC decreased to 150 g kg -1 after 12 h fication has been reported in seeds of Malus (Kaminski, of drying, and was around 90 g kg -1 after 72 h. Seeds were frequently stirred to ensure uniform moisture loss. Subsamples Z.-X. Shen and G.E. Welbaum, Dep. of Horticulture, and D.J. Parrish were periodically tested for MC, which was expressed on a and D.D. Wolf, Dep. of Crop and Soil Environmental Sciences, Vir- dry-weight basis, that is, (Fresh Wt - Dry Wt)/Dry Wt, after ginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061. Re- ceived 26 June 2000. *Corresponding author (dparrish@vt.edu). Abbreviations: MC, moisture content. Published in Crop Sci. 41:1546–1551 (2001). 1546