cardinal temperatures, response to temperature and range of thermal tolerance for seed germination in wheat (triticum aestivum l.) cultivars

understanding the seed germination response of cultivated genotypes to temperature is important, agronomically. the genotypes that germinate in lower temperatures could be useful for situations (such as late winter plantings) where temperatures are low in germination stage. therefore, this experiment was conducted to study the cardinal temperatures, response to sub- or supra-optimal temperatures and thermal tolerance of seed germination in 12 wheat cultivars from the north of iran. the effects of seven constant temperatures between 5 and 34 °c on the seed germination characteristics were also evaluated. the base (tb), optimum (to) and maximum or ceiling (tc) temperatures for germination changed from 2.04 to 2.9°c, 31.81 to 32.42°c and 38.08 to 42.08°c, respectively. the analysis of variance revealed no significant differences between cultivars for cardinal temperatures. the effect of cultivar on thermal tolerance was not also significant, but differences between cultivars for thermal tolerance in supra-optimal temperatures were more than sub-optimal temperatures. generally, all cultivars were more responsive to supra-optimal temperatures than sub-optimal temperatures. in the other hand, the slope of regression line of germination rate (gr) versus supra-optimal temperatures was more than that of sub-optimal temperatures. the difference between cultivars was significant in response to sub-optimal temperatures, only. the effect of cultivar, temperature and their interactions on germination characteristics including maximum germination (gmax), time to 10% germination (d10), germination uniformity (gu) and germination rate were significant (p= 0.01), statistically. the evaluated cultivars showed the greatest gmax in 13 or 20, but the greatest gr in 32°c. also, d10 and gu decreased as temperature increased to 25 or 32 and 32 or 34°c, respectively. the results showed that the difference between 5°c and higher temperatures for gmax was very less than gr, d10 and gu. this means high gmax can be obtained in temperatures near to tb under without soilborn pests conditions.