Phytochrome responses are those which are controlled reversibly by red & far-red light. These can be broadly categorized as:
Fast responses those which occur within a time span of seconds to minutes. It was found that when mung bean root tips were kept in a specific solution (containing ATP, IAA, Ascorbic acid, Mncl2, KCI) the root tips adhered to a glass beaker when irradiated with red light. This effect was reversed by the far-red light within 30 seconds.
Slow responses those that take hours to days to manifest themselves. Slow responses like seed germination, hypocotyl elongation, leaf expansion, abscission, flowering, fruiting have been shown to be under phytochrome control. Some of the slow responses controlled by phytochrome are
(i) reversal of red light stimulated seed germination by far-red light
(ii) demonstration of control of internodal lengthening by phytochrome, the effects were seen after 8 days. The plants were kept in light-dark cycle received either R or FR light etc.