evaluation of freezing tolerance of three ajowan (trachyspermum ammi (linn). sprague) ecotypes in controlled conditions

ajowan is one of the endemic plants in khorasan province, and there is a little information on its tolerance to cold stress. in order to study freezing tolerance of ajowan, an experiment was conducted in faculty of agriculture, ferdowsi university of mashhad, based on factorial-completely randomized design with three replications and three ecotypes of ajowan (neishabour, birjand and torbat-e-heidarieh) were imposed on eight freezing temperatures (0 (control), -1.5, -3, -4.5, -6,-7.5, -9 and -10.5 °c). plants were grown in natural environment till 4-5 leaf stage, then for freezing treatments transferred to thermo-gradient freezer. the cell membrane stability was evaluated by electrolyte leakage index (el) and temperature for killing 50% of samples according to the electrolyte leakage (lt50el) was determined. furthermore, survival percentage, leaf number and dry weight, temperature for killing 50% of samples according to survival (lt50su) and reduced dry matter temperature 50 (rdmt50) were determined after three weeks recovery in the glasshouse. response of ajowan ecotypes for electrolyte leakage was different and birjand ecotype had the lowest %el, whereas the slope of %el in mentioned ecotype was lower than two other ecotypes. however there were no significant differences among ajowan ecotypes on lt50su. decreasing temperature to -7.5 °c reduced survival percentage of neishabour and torbat-e-heidarieh ecotypes to lower than 20 percent, whiles in this temperature birjand’s survival percentage was about 60 percent. it seems that birjand ecotype with the lowest electrolyte leakage, the highest survival and dry matter and the lowest lt50su was more tolerant than two other ecotypes.