Calcium and phytochrome control of leaf unrolling in dark-grown barley seedlings.

The red light-stimulated component of unrolling in sections from 7-d-old dark-grown barley (Hordeum vulgare L.) leaves is inhibited by ethyleneglycol-bis-(β-aminoethyl ether)-N,N,N',N'-tetracetic acid (EGTA). A free-Ca(2+) activity of less than 40 μM restores the ability to respond to red light, but only if supplied within 1 h of red light. Magnesium ions are an ineffective substitute. At least two processes in unrolling appear to be Ca(2+)-sensitive.Fluence-response measurements indicate that the levels of the far-red-absorbing from of phytochrome (Pfr) still present 4 h after red-light treatment should be above saturation for the unrolling response; consequently, loss of Pfr does not explain the loss in effectiveness of Ca(2+) during prolonged EGTA treatment. However, if a further red-light treatment is given simultaneously with Ca(2+) addition 4 h after the initial light stimulus, then full unrolling occurs in EGTA-treated sections. These data indicate that, under normal circumstances, a functional change in the properties of Pfr must occur, uncoupling it from the transduction chain.