Douglas-fir seedlings 1-12 weeks old were subjected in the laboratory to various intensities of radiation from heat lamps for various periods. As determined from hypocotyl sections, anatomical changes associated with heat damage are, in order of increasing severity, (1) loss of water to intercellular spaces and loss of turgor, (2) progressive collapse of epidermis and parenchyma cells of cortex and pericycle, (3) collapse of phloem and parenchyma cells in the xylem, and (4) collapse of cambium and inner phloem progressing upward from the constriction. Seedlings usually recover and survive if damage has not progressed beyond stage 2. Following formation of a constriction, cells of the pericycle and/or immature xylem elements may give rise to secondary vascular cambiums. These cambiums are primarily responsible for hypocotyl enlargement above the constriction if the seedling remains alive for several weeks following injury. Length of exposure to radiation appears to be as important as temperature. The path of heat conduction appears to be from the epidermis inward to the xylem, then upward due to heating of water in the transpiration stream. Age of seedling and associated increased maturity of the cortex, thickening of the endodermis, or the development of cork, had little effect on heat tolerance.
Document Type: Journal Article
Forest Geneticist, Forest Service, U. S. Dept. Agric., at Oregon State Univ., Corvallis