Micronuclei and Radiosensitivity in the Root Meristem of Vicia Faba

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Abstract
Percentages of cells with micronuclei in four regions of the root meristems of Vicia faba are used as measures of sensitivity to acute X-irradiation. There are two peaks in these percentages, occurring at about four and eight days after 360 rads and two and six days after 180 rads. Two peaks exist, probably because the radiation delays cells that were in G1 much more than cells that were in G2 in reaching the first post-irradiation mitosis and consequently in displaying micronuclei in the following interphase. The relative heights of the two peaks therefore reflect the relative numbers of cells in G1 and G2 as well as the relative sensitivity of the two phases to chromosomal damage. The cells of the quiescent centre are injured least by the radiation as they are mostly held at G1. The meristem thus obeys the law of Bergonié and Tribondeau, but differs from that of Zea in that the meristematic cells of the cap initials and stele immediately adjacent to the quiescent centre resemble the quiescent centre much more closely than the stele 250 μ away in the numbers of micronuclei produced. This is consistent with the differences already known between the two species concerning rates of division in the different regions of the meristem and the behaviour of the meristem after severe radiation injury.

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