studying the impacts of cold temperature on morphological and phonological development of poa pratensis and poa trivialis regarding gdd

poa genus has a high value for grazing and forage conservation concerning thelivestock. in order to study the impacts of cold temperature and growing degree days(gdd) on seedling morphological trait and phonological development of two speciesinvolving poa pratensis and poa trivialis, this research was conducted in laboratory andgreenhouse and outdoor of research institute of forest and rangeland in tehran, iran. theseed samples were moist chilled at 4 °c for two weeks as compared to the control treatment.samples of both treatments (chilling and control) were transferred into a germinator at20±4°c for 15 days. all the pots were placed in a normal glasshouse at 20±10°c for onemonth. the 30th day age pots have been transferred into the cold rooms at 4°c for two weeks incomparison with control treatment. the seedling vegetative traits were measured same as thelaboratory. for outdoor experiment, half of pots of greenhouse were transferred outdooruntil their flowering and growth stages and gdd were recorded. in the flowering stage,morphological characteristics including stem number, stem length, peduncle length,panicle length, plants‟ fresh and dry weight of each ecotype were recorded. the collecteddata of each experiment were separately analyzed using factorial experiment based oncompletely randomized design. the results of laboratory experiment showed thesignificant effects of chilling (4 °c) treatment on germination percent, speed of germinationin both species and vigor index for p. pratensis. regarding p. pratensis in vegetative stage,the mean values of all of traits were higher in control treatment than those of coldtreatment. similarly, in outdoor experiment, the mean values of reproductive traits in thecontrol treatment were higher than those for cold treatment concerning both species. p.pratensis had higher mean values for all of reproductive traits as compared to p. trivialis.results also showed that gdd of plants which were subjected to cold treatment werelower than those for control treatment for both vegetative and flowering stages. it has beenconcluded that cold treatment reduced flowering dates in reproductive stage.