The Reversible Inhibition by Red and Far-Red Light of Auxin-Induced Lateral Root Initiation in Isolated Pea Roots

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Abstract
Lateral root initiation was induced by auxin treatment of isolated segments from seedling roots of pea (Pisum sativum) or from roots grown in sterile culture 1 week. Low-intensity red light had no significant effect on root initiation in segments from intact seedling roots but caused marked inhibition of initiation in segments cut from isolated roots grown in culture. The sensitivity to red-light inhibition decreased along the root axis from tip toward the base. A single exposure of 10-20 kergs x cm-2 of incident energy of red light caused a maximum inhibition equal to about 40% of the dark control in a segment 4-10 mm behind the tip. The red light inhibition was reversed completely by far-red irradiation (120 kergs x cm-2). The inhibition by red and reversal by far-red irradiation was achieved repeatedly with successive treatments of the same segments. Blue light at low-intensity was without effect for short periods of time (up to 8 minutes) but continuous exposure to blue light for the duration of the test (7 days) resulted in marked inhibition of lateral root initiation. The evidence for a red-far-red light reversible control of lateral root initiation following auxin stimulation seems clearly established.