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Studies on the effects of ionizing radiation on some western coniferous species El-Lakany, Mohamed Hosny Hassan

Abstract

The primary objective of this study was to investigate the radiosensitivity of Pseudotsuga menziesli (Mirb.) Franco, Douglas-fir, from two different provenances representing the coastal and interior regions in British Columbia, Picea sitchensls (Bong.) Carr., Sitka spruce, and Tsuga heterophylla (Raf.) Sarg., Western hemlock. A secondary objective was to correlate the radiosensitivity with some cytogenetical and biochemical characteristics. Induction of mutations and radiostimulation of seed germination and seedling growth were also sought. Filled seeds of the above mentioned species were given five different dosages of gamma-irradiation from a cobalt-60-source control, 500, 2,000, 5,000, and 10,000 R. Stratification for 14 days at 0°-2°C as post-irradiation treatment was tested. Germination values were evaluated and germinants were transplanted. Survival and growth under controlled environmental conditions were recorded for 182 days. The species exhibited differential responses to seed irradiation. All showed drastic reduction in germination and survival at the higher dosages, (5,000-10,000 R). There was some stimulation of Interior Douglas-fir seed germination and seedling survival at 500 and 2,000 R irradiation dosages over the control. The same exposures stimulated the height growth of Coastal Douglas-Fir. Stratification after irradiation reduced seed germination and seedling growth and survival in all the species. The tolerance to gamma-irradiation decreased in the following Order: Interior Douglas-fir, Coastal Douglas-fir, Western hemlock and Sitka spruce. Sitka spruce had a significantly larger nuclear volume than Western hemlock and Douglas-fir. No correlation was found between nuclear volume, or interphase chromosome volume, and LD₅₀ (germination). The amount of DNA per cell and per chromosome differed significantly among the three species with Sitka spruce having the highest DNA content followed by Western hemlock, Coastal Douglas-fir and Interior Douglas-fir. Significant negative correlations were found between DNA content per cell and per chromosome, and LD₅₀ (germination). This indicated that DNA content plays a more important role than nuclear volume in determining the radiosensitivity of the species. The differences in radiosensitivity, nuclear volume and DNA content between the Coastal and Interior forms of Douglas-fir are discussed in relation to their ecogeographical distribution and taxonomy. Chromosome breaks, micro-nuclei and chromosome erosion were detected in irradiated seeds of Douglas-fir. Intraspecific hybridization was carried out in Douglas-fir using gamma-irradiated pollen grains. Pollen irradiation up to 5,000 R increased the number of filled seeds/cone. Seedlings from pollen irradiated at 500 R, exhibited some increase in height growth. Similar effects were obtained for pollen germination in vitro. The possibilities of utilizing seed and pollen irradiation in forest tree improvement are discussed' and recommendations are made for future mutation breeding work.

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