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ature (T b ) for the particular growth phase. Several studies, including Mullahey et al. (1990), Sanderson and Initial screening of warm-season grasses for cultivation in cool, Wolf (1995b), Mitchell et al. (1997, and Sanderson and short season growing areas has been focused on frost and chilling tolerance. Adoption of warm-season grasses in these areas has resulted Moore (1999), have included development of growth in an increase in degree-day modeling of their growth. These predictive and management models. However, these models utimodels are dependent on accurate determination of the basal temperalized blanket base temperatures for many cultivars and tures for growth. In this study, base temperatures for seedling growth across all growth stages. For pearl millet {Pennisetum were estimated for switchgrass (Panicum virgatum L.), big bluestem typhoides (Burm. f.) Stapf & C.E. Hubb. [ϭ P. glaucum (Andropogon gerardii Vitman), Indian grass (Sorghastrum nutans L. (L.) R. Br.] (Ong, 1983)} and groundnut [Arachis hypo-Nash), and prairie sandreed [Calamovilfa longifolia (Hook) Scribn.]. gaea L. (Leong and Ong, 1983)], there were only small Seedlings at the two-leaf stage were grown at 4, 8, 12, 16, and 24؇C variations in T b for different growth and developmental in growth chambers for 4 wk with representative harvests every week. phases. However, for wheat (Triticum aestivum L.), val-Relative growth rates were calculated for each species at each temperues of T b have been reported to vary depending on plant ature and these were used, in conjunction with regression techniques, to estimate base temperatures for growth. The base temperatures age (Angus et al., 1981) and phenological stage (Slafer were then correlated with chilling sensitivity of the plants, estimated and Savin, 1991). using visual scores, chlorophyll fluorescence, and electrolyte leakage. Warm-season plant growth at suboptimal tempera-The estimated base temperatures ranged from 2.6 to 7.3؇C. There 'Cave-in-Rock' (CIR, originally developed from southern Illi-

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The effect of preplant conditioning on germination of three flower seeds, Bupleurum griffithii (Tourn.) L. (thorough-wax), Ammi majus L. (greater ammi), and Cirsium japonicum DC. Per. (Japanese thistle), were studied. Seeds were osmoconditioned with -1.2 MPa polyethylene glycol 8000 (PEG) solution and matriconditioned with moist Micro-Cel E (ratio of 2 seed: 0.6 carrier: 3 water by weight for Bupleurum and Cirsium; for Ammi the ratio was 2:1.4:6) and moist expanded vermiculite #5 (the ratio was of 2 seed: 0.6 carrier: 2 water for Bupleurum). In some treatments, water in the matriconditioning mixture was replaced with 1 mm gibberellin A4+7 (GA) or 0.2 % KNO3. In Bupleurum, matriconditioning with Micro-Cel E was generally superior to matriconditioning with vermiculite or osmoconditioning with PEG. A 4-day matriconditioning with Micro-Cel E and germination in the dark reduced the period required for 50% (T50 of final germination by 4 days and improved the percentage germination at 20C (73 % vs. 95%), compared to nonconditioned seeds germinated in the dark. The treatment also improved the percentage of germination at 15C (68% vs. 95%) and effectively removed the thermoinhibition of germination at 25 and 30C. Germination was inhibited to a greater extent for seeds kept in the light during matriconditioning and germination than for seeds conditioned in darkness and germinated in light or conditioned in light and germinated in darkness. Nitrate added during conditioning in light prevented inhibition of germination, provided seeds were kept in darkness during germination. In A. majus, germination in light after 4-day matriconditioning reduced the T50 by ≈2 days, but had little effect on percentage germination. Both GA and irradiance equally promoted germination when added during osmoconditioning, with nitrate having no effect. In C. japonicum, a 4-day matriconditioning or a 7-day osmoconditioning reduced the T50 of germination by -2 days and improved the percentage germination to some extent. Neither irradiance nor nitrate had any significant effect.