Abstract
Wintering Sasa senanensis, dwarf bamboo, is known to employ deep supercooling as the mechanism of cold hardiness in most of its tissues from leaves to rhizomes. The breakdown of supercooling in leaf blades has been shown to proceed in a random and scattered manner with a small piece of tissue surrounded by longitudinal and transverse veins serving as the unit of freezing. The unique cold hardiness mechanism of this plant was further characterized using current year leaf blades. Cold hardiness levels (LT20: the lethal temperature at which 20% of the leaf blades are injured) seasonally increased from August (−11°C) to December (−20°C). This coincided with the increases in supercooling capability of the leaf blades as expressed by the initiation temperature of low temperature exotherms (LTE) detected in differential thermal analyses (DTA). When leaf blades were stored at −5°C for 1–14 days, there was no nucleation of the supercooled tissue units either in summer or winter. However, only summer leaf blades suffered significant injury after prolonged supercooling of the tissue units. This may be a novel type of low temperature-induced injury in supercooled state at subfreezing temperatures. When winter leaf blades were maintained at the threshold temperature (−20°C), a longer storage period (1–7 days) increased lethal freezing of the supercooled tissue units. Within a wintering shoot, the second or third leaf blade from the top was most cold hardy and leaf blades at lower positions tended to suffer more injury due to lethal freezing of the supercooled units. LTE were shifted to higher temperatures (2–5°C) after a lethal freeze-thaw cycle. The results demonstrate that the tissue unit compartmentalized with longitudinal and transverse veins serves as the unit of supercooling and temperature- and time-dependent freezing of the units is lethal both in laboratory freeze tests and in the field. To establish such supercooling in the unit, structural ice barriers such as development of sclerenchyma and biochemical mechanisms to increase the stability of supercooling are considered important. These mechanisms are discussed in regard to ecological and physiological significance in winter survival.
Highlights
Dwarf woody bamboos are the dominant component of forests and grasslands in Japan (Numata, 1979)
The cold hardiness level in mid-winter expressed as the lowest temperature that can be tolerated without any injury is about −15◦C and ranks the third after Sasa kurilensis and Sasamorpha borealis among the major dwarf bamboo species that occur in the cool temperate region in Japan (Ishikawa, unpublished; Konno and Sakai, 1975)
DEEP SUPERCOOLING OF SASA LEAF BLADES IN REGARD TO TISSUE UNITS AND THEIR FREEZING BEHAVIOR AT NON-LETHAL AND LETHAL TEMPERATURES The longevity of leaf blades of S. senanensis is usually 2 years and the leaf blades that newly appear in June ∼ July are important for photosynthesis during the rest of the year and the following year except for the period of snow cover (Lei and Koike, 1998)
Summary
Dwarf woody bamboos are the dominant component of forests and grasslands in Japan (Numata, 1979). Is one such evergreen woody species belonging to Bambusodae supertribe, Poaceae It occurs mainly on the Japan sea side of Japan from southern Honshu to Hokkaido and reaches a mean height of 1.5–2.0 m in the forest understory (Ohshima, 1962; Lei and Koike, 1998). Cold hardy species (Ishikawa, unpublished) among the woody bamboo species that distribute worldwide from tropical to cool temperate regions or high elevations (Soderstrom, 1981). The cold hardiness level in mid-winter expressed as the lowest temperature that can be tolerated without any injury is about −15◦C and ranks the third after Sasa kurilensis and Sasamorpha borealis among the major dwarf bamboo species that occur in the cool temperate region in Japan (Ishikawa, unpublished; Konno and Sakai, 1975)
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