Internode elongation was measured in plants of Phaseolus vulgaris and Glycine max grown under 8 h photoperiods at 25 W m−2 in white fluorescent light, followed by light-extensions varying in quality, irradiance and duration. Two distinct responses to light were observed under these conditions.
A reduction in PFR/P increased elongation, but elongation was also modified by a second reaction in which internode length increased with increase in the duration and irradiance of the day-extension. This light-promoted response occurred in both red and blue light. In the PFR-inhibition response, light acted directly on the expanding internode. The light-promoted response, in contrast, required irradiation of the leaf.
The response to a short end-of-day exposure to far-red light progressively diminished as successive internodes expanded under the treatment, whereas the light-promoted response increased. The two processes appeared to interact and, in the later-expanding internodes, the effect of a reduction in PFR was greater under long day-extensions with mixed red and far-red light than in the end-of-day treatments
