Studies were conducted to investigate some major factors influencing cold hardiness in Hibiscus syriacus in order to provide basic data for future breeding works and safety cultivation of the national flower throughtout this country. The results are summarized as follows:
1. Among the 12 cultivars observed in the field, `Ognyeo´, `Baedal´, and `Ogto´ all of these having single white flower, showed the weakest cold hardiness.
2. `Ognyeo´ exhibited significantly weaker cold hardiness than `Yeonggwang´ under the natural winter conditions. Upper stem, lower stem and roots showed the severe injury symptoms, which coincided with artificially induced low temperature injury.
3. The cold hardiness in mid-axillary shoots was strong in `Saehan´, `Hyangdan´ and `Bicolor´, and weak in `Hitoe´ and `Choongmu´. Among the tested plant parts, the buds located near the terminal shoot exhibited the weakest hardiness whereas those located near the mid-axillary shoot bases showed the strongest cold hardiness.
4. Xylem tissue of upper stem was hardier than that of lower stem and root xylem tissue of the middle stem showed the strongest cold hardiness, even though this tendency differed slightly depending upon the cultivars.
5. The hardy cultivar `Yeonggwang´ showed low temperature injury in lower stem and root at -15℃ and -20℃, and the most in stem parts at -25℃ and -30℃. In contrast, the susceptible cultivar `Ognyeo´ showed very severe injury in root at -15℃ and in lower stem at -20℃.
6. `Yeonggwang´ showed stronger cold hardiness than `Ognyeo´ in most plant parts and tissues. The susceptible tissues in `Yeonggwang´ were xylem and pith followed by buds and cambium. In `Ognyeo´, xylem and buds were the most susceptible followed by cambium and pith.
7. In general, significant difference in cold hardiness was found in cambium tissue of the tested cultivars. The cambium tissue of `Ognyeo´ was quite sensitive to low temperature injury whereas that of `Yeonggwang´ was very hardy.
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