An irreversible red-light-induced growth response in Avena

The inhibition of the growth rate of the first internode of Avena by red light occurs in three steps. The first step reduces elongation by ca. 15%. It is produced and saturated by 10(-3) to 10(-1) μW sec cm(-2) at λ=660 nm and is irreversible by far-red irradiation. All wavelengths between 400 and 800 nm produce and saturate this step. The second step, produced by red light quantities between ca. 10 and 10(4) μW sec cm(-2) reduces elongation to ca. 50% of the maximal; it is not produced by far-red light but far-red reverses completely this component of the effect of red light. The third step inhibits mesocotyl elongation to ca. 95% of the maximal. The effect of red light in this step depends on the duration of irradiation rather than on the total quantity of energy, and is not reversed by far-red irradiation. The three inhibition steps in the elongation of the mesocotyl are matched by three growth-promotion steps in the growth of the coleoptile, but the extent of the far-red-irreversible first step outweighs in this case by far the extent of the far-red-reversible second step.