VERNALIZATION IN WHEAT .1. A MODEL-BASED ON THE INTERCHANGEABILITY OFPLANT-AGE AND VERNALIZATION DURATION

Vernalization treatments of 0 to 70 d initiated when 0 to 8 leaf tips were visible were applied to plants of the winter wheat (Triticum aest ivum L.) cultivars Pioneer 2548 and Augusta. All plants headed irrespe ctive of duration of vernalization. Unvernalized plants of Pioneer 254 8 and Augusta had mean final leaf numbers (FLN) of 20.8 +/- 1.3 and 21 .7 +/- 1.0, respectively. Increased duration of vernalization generall y reduced FLN within an age treatment until an age-dependent point of vernalization insensitivity was reached. Estimates of the minimum days of vernalization required to reach vernalization insensitivity decrea sed in a linear fashion as plant age at the onset of vernalization tre atment increased. The number of leaves appearing after the onset of ve rnalization insensitivity averaged 6.3 +/- 0.5. FLN minus six appears to be a valid estimate in our experimental conditions for the onset of vernalization insensitivity, at least for plants that had six or more leaves appearing after the end of vernalization treatment. Linear reg ressions of FLN minus six against days of vernalization were significa nt for both cultivars (for treatments with six or more leaves emerging after vernalization). The Y-intercepts of the fitted regressions were close to values obtained by subtracting six from FLN of unvernalized plants. Both intercept and slope were controlled genetically. Accumula ted plant age, expressed as leaf stage, enables attainment of vernaliz ation insensitivity, independent of, or in combination with vernalizat ion treatment.