Deep supercooling was found in the stem tissues of all the Pyrus species studied. There was more than 1 low temperature exotherm resulting from the freezing of supercooled water in stem tissue, and these exotherms were associated with the tissue injury. The supercooled water in the stems of P. nivalis Jacq., P. cordata (Desv.) Schneider and P. elaeagrifolia Pall, was found in both xylem and bark tissues. The supercooling characteristics of vegetative and flower buds are also described. The hardiest and least hardy species found were P. caucasica Fed. and P. pashia D. Don., respectively.
Deep supercooling of stem tissue water was found in all the native species of rose (Rosa spp.) studied. Freezing of this supercooled water was associated with injury to the stems, indicating that maximum cold hardiness of these species is limited to about −40°C. Therefore, these species have some potential for use in breeding to develop cold-hardy cultivated roses, but their hardiness would be limited to −40°C by the supercooling characteristic.
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