most responses, saturation occurs when 50$ of the phytochrome is as P„n r K (4,5). Mandoli and Briggs (6,7) however, reported that a low irradiance response in etiolated wheat was saturated when there was less than 3$ This low irradiance response was only observed when plants were grown in total darkness. When plants grown in darkness but exposed to low intensity green ''safe lights” were used they did not show the low intensity response. Rather, these plants showed a second response to red light which saturated when there was auoroximately 50$ P„n (6,7). It is clear therefore that in addition to demonstrating sensitivity to low light intensities (see Figure 1) plants which are grown under strictly dark conditions can also exhibit enhanced sensitivity to PpR. It cannot be overemphasized in the context of this analysis that these etiolated plants do not represent plants which would be found in a normal environment. The kinetics of phytochrome conversion both in vitro and in vivo have been well worked out (Fig. 3) (8). Figure 3. Dose-response curves for phototransformation of oat phytochrome in vitro and pea hook phytochrome in vivo. Actinic beam energy for PR transformation: 8.2 x 10-2 erg cm-2 sec-2; for PFR transformation: 2.9 x 104 erg cm-2 sec-1. From Everett et al. (8).
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