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- Author or Editor: J. R. Havis x
Studies on winter desiccation of broadleaved evergreens have raised the question of low temperature impedance of water movement in roots and stems. Temperature control of a section of stem can be readily accomplished by the use of cooling collars. Johnson (3), Handley (1), and Zimmerman (6) have brought about wilting of the leaves of various tree species by cooling parts of the stem to near or below the tissue freezing point. Presumably, the wilting was caused by restriction of water flow through the cooled section of stems. The wilted foliage recovered after the stem temperatures were allowed to rise, but no attempt has been reported to measure the minimum temperature at which water started to flow through the stem after freezing. To obtain information on this latter question was the primary objective of the study reported here.
Richared Delicious’ apple (Malus pumila, Mill.) fruit tissues exhibited 2 distinct freezing points (exotherms) during freezing. Exotherm 1 occurred at −1.5° to −2.2°C and Exotherm 2 occurred between −4.2° and −7.7°C. The inception temperature of Exotherm 2 varied considerably both within a fruit and among different fruits. During fruit maturation from August to October, no significant change in freezing pattern occurred, although soluble solids increased 55% during the period tested. Inception of Exotherm 1 caused no distinct change in physical properties of cortex tissue. Inception of Exotherm 2 was associated with significant softening and increased ion leakage from the tissue. Just beyond termination of Exotherm 2, the tissue lost its capacity to exhibit multiple exotherms on thawing and refreezing, and underwent massive changes in firmness, ion leakage, and respiration; it was concluded that freezing to this point was lethal to the tissue. The concept that tissue injury is related to specific points on the freezing curve of apples is presented.
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