The planting of fennel (Foeniculum vulgare L.) in the fall is important in temperate areas where this leads to more efficient use of limited available water, although winter mortality is a serious barrier for employing a fall planting system for this plant. Improved cold tolerance through selection of frost-resistant ecotypes could reduce winter losses or acclimate the plants to cold temperatures. To determine whether cold acclimation (AC) could reveal potential frost tolerance, two local ecotypes of fennel (Gonabad and Kerman) were exposed to low temperatures under controlled conditions to reveal any potential frost tolerance. Compared with non-acclimated control plants, AC increased survival at a lethal temperature (LT50), the temperature at which plants showed 50% less dry matter (DMT50) and the number of nodes per main stem in both ecotypes at 3 weeks after applying a freezing treatment, whereas plant height and plant dry weight were less in AC treatment compared with non-acclimated (NAC) plants. Percentage of survival and the number of nodes per main stem did not show significant reduction up to -6°C but at a lower temperature was reduced significantly (p £ 0.01). Increasing freezing intensity beyond -3°C reduced the plant height as compared with controls, and plant DMT production was also less in all treatments as compared with 0°C. Plant height, number of nodes per main stem, and plant DMT in the Gonabad ecotype were 73%, 68%, and 146%, respectively, more

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