Under the stress of long-term exposure to gamma radiation at dose rates ranging from 1.83 to 62.48ensp;mGy/h, planted saplings of black ash (Fraxinus nigraMarsh.) responded in a variety of ways. These were compared with naturally growing black ash trees and saplings and with green ash (F.pennsylvanicaMarsh.). With increasing dose, development time shortened, the number of leaflets decreased, and phyllotaxy changed from opposite to helical. The LD50(lethal dose to 50 of exposed plants) (acute, seed germination) was 98 forF.nigraand 174 Gy forF.pennsylvanica. In black ash, the long-term LR50(lethal dose rate to 50 of exposed plants, in milligrays per hour) has leveled off at approximately 14ensp;mGy/h, while the accumulative LD50(total accumulated lethal dose to 50 of exposed plants, in grays) continued to increase with additional irradiation, and after 11 years of irradiation was over 900 Gy. Plantation black ash is surviving, and even thriving, at a dose rate of 15.6ensp;mGy/h, which may be a stable survival dose rate. Leaves in the two species are similar, with more leaflets per leaf on later developed nodes. Leaf size increased with total accumulated dose. Green ash is surviving at the highest dose rate at which it has been found (5.93ensp;mGy/h). The frequency of morphological changes, most of which are teratological (e.g., apparent fused leaves, alternate or missing leaflets, chlorophyll mottling and sectoring, abnormal venation, irregular leaves), increased with radiation dose. In locations with severe winters, stresses during the winter dormant period are very important to survival. It is proposed that some of the features that enable ash to survive in a "harsh" environment may help it to tolerate the stress of gamma irradiation better than other species.
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