study on freezing tolerance of calendula (calendula officinalis l.) in vegetative and reproductive stages

calendula (calendula officinalis) is relatively cold tolerant plant, but in some years plant seriously injured due to harsh winter. in order to evaluate freezing tolerance of calendula an experiment was carried out at college of agriculture, ferdowsi university of mashhadin a factorial-completely randomized design with three replications and plants of two sowing dates (summer and autumn) were exposed to12 temperatures (0, -2, -4, -6, -8, -10, -12, 14- ,16-, -18, -20 and -22oc). seeds of calendula plants were sown in summer (summer plant) and autumn (autumn plant) in the bed and in six to eight-leaf stages were transplanted to the pots. after the cold acclimation in natural condition, freezing stress was applied with using a thermo gradient freezer. to employ stability of cytoplasmic membrane, percentage of electrolyte leakage (el%) was measured after freezing. also survival percentage (su%) and regrowth of calendula plants determined after three weeks recovery. leaves el% in autumn plants was significantly more than summer plants and autumn plants have higher su%, but plant height, number of lateral branches, numbers of reproductive traits, total dry matter, vegetative and reproductive dry matter in summer plants were more than autumn plants. however, there were no difference between calendula plants for lt50el in both autumn and summer plants, but there was significant difference between them for lt50su and total dry matter, and lt50su and reduced dry matter temperature50 (rdmt50)for summer plants were -18.6 oc and -11.3 oc and for autumn plants were -19.4 oc and -13.7 oc, respectively.