The effects of chronic gamma irradiation on the apical meristem and bud formation of Taxus media

Abstract

Four-year-old plants of Taxus media cv. Hatfieldii were exposed to chronic gamma radiation from Co 60 at daily dose rates of 2·5, 3·75, 7·5, 15 and 30 r/20-hr day. A dose response curve shows that dose rates as low as 3·75 r/day will reduce significantly the frequency of bud formation. At 12·5 r/day bud formation was reduced to 0·5 per cent of the controls. Histological examination reveals that complete disorganization of the shoot apex occurs after an accumulation of 187·5 r at this low dose rate. Evidence for a pattern of differential sensitivity in the stem apex is presented. The decreasing order of radiosensitivity of the apical regions is: the apical initial layer, lateral initial layer, central mother-cell zone, transition zone, axillary cells and the pith meristem region. This pattern of radiation damage can be described as “outer to inner”, i.e. the outer cells are the first and the inmost cells are the last to show radiation damage. This response occurs at all dose rates studied above 2·5 r/day. A recovery mechanism frequently is observed at the lower dose rates (2·5 and 3·75 r/day) which leads to the occasional regeneration of a new operative apex from cells in or near the axillary regions. It is apparent from our data that T. media cv. Hatfieldii is one of the most radiosensitive of the higher plants yet studied.