Phytochrome‐mediated responses in light‐grown corn showing rapid, reverse reciprocity failure^*

Abstract

Three responses (mesocotyl and coleoptile elongation and anthocyanin accumulation in the coleoptile) to end-of-day far-red irradiation in lightgrown corn show rapid failure of the reciprocity law such that short, high ffuence rate irradiations are much more effective than long, low ffuence rate ones of the same ffuence (reverse reciprocity failure). The reciprocity failure cannot be explained by escape from photoreversibility, a change in sensitivity to Pfr, reciprocity failure for photoconversion, or a high irradiance response taking over for long irradiation times. Fluence-response curves measured by varying irradiation time at a low ffuence rate show the threshold ffuence shifted to higher energy in comparison with ffuence-response curves obtained at a high ffuence rate. Red reversal of these responses also shows rapid reciprocity failure in the same direction, a process which can be only partially explained by escape.These responses to end-of-day far-red and red illumination are distinguished from high irradiance reactions by their low ffuence requirements and ready reversibility. These same characteristics are similar to those of classical phytochrome-mediated, inductionreversion responses in etiolated tissue, but it is difficult to explain the rapid, reverse reciprocity failure in terms of standard phytochrome dogma.

Rapid, reverse reciprocity failure for phytochrome control of its own accumulation*

“…To characterize the fluence-response and reciprocity relationships more fully for end-of-day FR stimulation of phytochrome accumulation, we chose to (Gorton & Briggs, 1980, 1981. .…”

“…Bear hybrid WF9 x 38) were imbibed for 3 h in 0.2 mol m"-^ Norflurazon (Sandoz Inc., San Diego, CA), germinated in the dark for one day, then grown for 4 d on an 8 h hght: 16 h dark regime, also in the presence of 0.2 mol m^-^ Norflurazon. Conditions for growth have been described elsewhere, as have the light sources and measurement offluence rates (Gorton & Briggs, 1980,1981. End-ofday irradiations were given after the second, third and fourth white-light periods or after the fourth period only.…”

“…Sample geometry is always the same, and possible scattering differences between samples caused by changes in tissue structure are eliminated. Ptot was measured at 4 C as A(AA) between 660 and 730 nm as described (Gorton & Briggs, 1981). Means of replicates (three to five per treatment) are plotted in the figures, but individual values are shown in Table 1 to give an idea of the variability encountered with this technique.…”

“…Mesocotyl and coleoptile elongation are increased by end-of-day FR and anthocyanin accumulation in the coleoptile is decreased by the same treatment. These responses are saturated by low fluences and are readily reversible (Gorton & Briggs, 1980), but they show a rapid (seconds) failure of the reciprocity law such that short, high fluence rate irradiations are more eflective than long, low fluence rate ones of the same total fluence (Gorton & Briggs, 1981). We have termed this phenomenon 'reverse reciprocity failure' to distinguish it from the reciprocity failure in the opposite direction which is characteristic of the high irradiance reaction (Mancinelli & Rabino, 1978).…”

Physiological Mechanism of the Auxin-Induced Increase in Light Sensitivity of Phytochrome-Mediated Growth Responses in Avena Coleoptile Sections

“…The bulk of these 2-fluence-comparison experiments were performed with a source (source B) described previously (7). Irradiations with VLF red light were given in a 30-s pulse from source B (fluence rate 10 Mmol m-2 s-') to material covered by a 3.5 A filter, as described previously (8). The LF irradiations were given in a 100-s pulse with no filter in place.…”