Abstract
The effect of hypocotyl excision on red light-mediated betacyanin induction in cotyledons of Amaranthus caudatus L. was studied using etiolated, 3-day-old half-seedlings and isolated cotyledons. The removal of the hypocotyl promoted betacyanin formation under safelight conditions in a manner competitive with brief (5 mins), but additive with prolonged (6 h) red illumination. If a paper support was provided in order to improve the aeration, betacyanin formation in safelight conditions was further stimulated, reducing the inductive effect of brief, but not changing the action of prolonged red illumination. These results demonstrate that betacyanin photoregulation is restricted to cotyledons of A. caudatus seedlings, with no evidence for transmission of light signals between different organs. Excision and aeration appear to promote selectively a very low fluence response (VLFR) induced by safelight, suggesting different mechanisms of phytochrome phototransduction under VLFR, low fluence response induced by brief saturating red light (LFR) and high irradiance reaction (HIR) occurring under prolonged red illumination.
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