Graduate Thesis Or Dissertation
 

Freezing resistance of tissues in the twig of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)

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  • The relative freezing resistance of tissues in the stem, foliage and buds of terminal twigs from Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) was investigated at about ten-day intervals from summer until the following spring. Tissues from growing twigs collected before development of dormancy had no freezing resistance and were killed immediately after subcooling. Tissues at the base of inner bud scales and in the pith of stems at the base of buds were among the last to harden in autumn. Pith parenchyma and pith rays of the interfascicular region of the bud trace and the transfusion tissue in needles were most susceptible to injury after development of freezing resistance. Abaxial mesophyll was more susceptible to injury than adaxial mesophyll of hardened needles. The cortex was more resistant than other tissues during development of freezing resistance but parenchyma of the pith lost freezing resistance more slowly and was more resistant after growth processes resumed in the spring. The phloem-cambium region of the stem lost freezing resistance earlier and became more susceptible to injury than other tissues of the twig after midwinter0 Survival of the vegetative apex depended on the ability of its water to subcool. Heat of fusion of water on freezing produced a sharp exotherm that was a precise index of the temperature at death and freezing resistance of the vegetative apex. Prolonged subfreezing temperatures after development of freezing resistance in late autumn increased hardiness of twigs beyond that induced by the natural environment6 Both formation of ice in the pith of stems enclosed by coriaceous bud scales, and artificial dehydration reduced water content and promoted subcooling and freezing resistance of vegetative apices. Moisture content was reduced from 200 to 70 percent on a dry weight basis after three days at _90 C in early February and freezing resistance of apices increased 0 0 0 from -16 C to -20 C when cooled at 1 C per minute0 These findings suggested that diffusion of water from the protoplasm to sites of extracellular ice, and not low temperature, was the direct cause of hardening at subfreezing temperatures0 The basic freezing curve for Douglas-fir is characteristic of plant tissues in general but was modified by exotherms associated with freezing and death of vegetative apices from November until April. A decline n length of the plateau after the second subcooling temperature during hardening, and increase in length of the plateau during loss of hardiness reflected probable seasonal change in quantity of intracellular water. Unhardened tissues survived freezing of extracellular water but were injured before intracellular water was completely frozen. Hardened tissues, on the other hand, were not injured until after most of the intracellular water was frozen and the cooling rate of the stem returned to normal. Observations suggested a definite relationship between time of acquisition and loss of freezing resistance and number of days to bud burst after March 31. Trees which developed freezing resistance earliest in fall broke buds earliest in spring. Trees which broke buds latest in spring were also those which were the last to deharden, Ready availability of potassium appears to enhance early development and loss of freezing resistance. Trees with the highest content of potassium in the current season's foliage burst buds earliest and those with the lowest content of potassium were the last to burst buds. Night temperatures of 20 C delayed but did not prevent loss of hardiness when day temperatures were 250 C in March. However, continuous temperature of 2° C failed to reharden twigs that had lost freezing resistance at 25° C. Injury to all tissues, except the vegetative apex, declined at 250 C in March when moisture stress exceeded a critical level. These observations suggest that environ mental conditions unfavorable for growth enable Douglas-fir to maintain resistance to injury from spring frosts.
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