Calibration of the maize yg² assay using gamma radiation and ethylmethanesulfonate

A standard protocol for the yellow‐green‐2 (yg2) forward mutation assay in Zea mays is proposed. A detailed calibration of the assay using ¹³⁷Cs gamma rays and ethylmethanesulfonate (EMS) was conducted. Gamma ray‐induced mutant sectors in leaves 4 and 5 exhibited one‐hit kinetics. The radiation doses ranged from 25 to 500 rads. The mean induced mutation rate per rad of gamma radiation was 4.54 × 10⁻⁶. This value was constant for the primordial cells of leaves 4 or 5. The induction of forward mutation by EMS also exhibited one‐hit kinetics in the concentration range 0.25–20 mM (0.33–23.54 μmol EMS/kernel). The mean induced mutation rate per mM EMS was 1.79 × 10⁻⁴, and the mean induced mutation rate per μmol of EMS per kernel was 1.52 × 10⁻⁴. Using the induced mutation rates for gamma radiation and EMS, the rad equivalent was calculated. One rad of gamma radiation is equivalent to the exposure of a 2.53 × 10⁻⁵ M EMS solution or to 2.99 × 10⁻⁸ mol of EMS per kernel.