USE OF DEGREE-DAYS IN MULTIPLE-TEMPERATURE EXPERIMENTS

This research compared results from germination and growth when the ex periment duration was chronologically set or based on degree-days, See ds of smooth brome (Bromus inermis Leyss.), plains rough fescue (Festu ca altaica Trin, subsp, hallii (Vasey) Harms), prairie coneflower (Rat ibida columnifera (Nutt.) Woot. and Standl.), and silver sagebruh (Art emisia cana Pursh.) cvere germinated at 5, 10, 15, 20, and 25 degrees C for 28 days or 400 degree-days (Base temperature = 0 degrees C). Roo t and shoot weights of seedlings of these species were compared at 5, 10, 15, 20, 25 and 30 degrees C after growing them 20 days or 200 degr ee-days, With the exception of prairie coneflower, optimal temperature s for germination were 2 to 4 degrees C lon er when incubated 400 degr ee-days compared to 28 days, Total germination for prairie coneflower was not significantly different (P =0.454) at 28 days or 400 degree-da ys, Interacting effects of the duration of experiments and temperature significantly (P less than or equal to 0.001) influenced root and sho ot weight of all species, Except for shoot weight of smooth brome, pre dicted optimum temperatures for root and shoot growth were 7 to 21 deg rees C lower at 200 degree-days than 20 days. These experiments illust rate that results from germination and growth studies can vary substan tially depending on whether chronological time or degree-days are used as the end point, Thus, ecological interpretations or management reco mmendations can be quite different, Degree-days may be more meaningful than chronological units for germination and growth studies because t hey integrate time and temperature, The use of degree-days as an end p oint for experiments rather than chronological time deserves further c onsideration by researchers.