Germination of warm-season grasses under constant and dynamic temperatures

Abstract

Fifteen collections of 10 native and exotic grasses were germinated at constant 25 degrees C, and at gradual and abruptly alternating temperature regimes characteristic of wet seed bed temperatures in the southwest desert grassland in summer, winter, and spring. All species but bristle grass (Setaria macrostachya H.B.K. and S. leucopila Schum.) had high total germination under summer and spring temperatures (mean- 60% and 67%, respectively) and all but bristlegrass and bush muhly (Muhlenbergia porteri Scribn.) had relatively high total germination under winter temperatures (mean-53%). In general, total germination percentage was similar for gradual and abruptly alternating temperature regimes within a season. Constant 25 degrees C and abruptly alternating temperature germination percentages were similar enough to those at more realistic gradually alternating temperatures for most species to permit use of these tests to calculate estimates of summer bulk seeding rates. Time to 50% germination (D50) was slightly less for gradual than abruptly alternating summer temperatures, but was generally similar for these regimes under winter and spring temperatures. To determine if germination responses to constant temperatures could be used to estimate responses to dynamic temperatures, 12 collections of 8 species were tested for germination at constant temperatures of 5.4,10, 20, 25, 30, 35, 40, and 45 degrees C. Total germination and rate of germination (1/D50) increased and decreased as third-order polynomial functions of temperature. Polynomial regression estimates of 1/D50 were used to calculate estimates of D50 for diurnally alternating temperature that were within an average of 0.6 and 13 days of measured values for summer and spring temperatures respectively. Linear regression estimates of 1/D50 for suboptimal to optimal temperatures were similarly used to estimate D50 for dynamic winter temperature regimes that averaged within 3.6 days of measured values for most of the species. Differences between estimated and measured D50 for bush muhly under winter temperatures, and for Lehmann lovegrass (Eragrostis lehmanniana Nees) collections under winter and spring temperatures, indicate sensitivity of these species to extreme temperature in addition to accumulated heat. Similar measured and estimated D50 for most of the collections for summer, winter, and spring temperatures indicates that these species are primarily responding to cumulative heat effects. Even though most of the species have high germination percentages for winter or spring temperatures, field seedling emergence is much less likely in winter and Possibly less likely in spring than in summer. Slower germination rates during these cooler seasons would require long periods of soil water availability at the surface to allow germination.