14.4.1.1—
Induction-Reversion Responses
These are the classical phytochrome responses (Borthwick, 1972). A change in the biological parameter being monitored is induced by a brief irradiation of low intensity red light and reversed by a subsequent far-red pulse. The accumulation of anthocyanin in Sinapis illustrates this point (Fig. 14.6). Another well known example is lettuce seed germination. This is repeatedly photoreversible for up to 100 alternate red and far-red irradiations (Borthwick, 1972).
Figure 14.6
Accumulation of anthocyanin in Sinapis in the dark following irradiation
treatments at time zero with 5 min red (o), 5 min far-red (Ñ ), or 5 min
red followed immediately by 5 min far-red (
Mohr et al., 1971).
This simple red/far-red photoreversibility forms the basis of the concept that Pfr is the biologically active form of the pigment whereas Pr is inactive. Attempts to quantify the relationship between the number of Pfr molecules formed and the magnitude of the induced response have been both indirect and direct.
Indirect correlations are based on the premise that observed increases in the magnitude of the response with increasing light dose are a function of the degree of photoconversion of Pr to Pfr, i.e. the more quanta, the more Pr is converted to Pfr and therefore the greater is the response. The increase in anthocyanin in response to increasing doses of red light (Table 14.1) illustrates this point (Lange et al., 1971). The so-called law of reciprocity (irradiance × time = constant) must hold for the light doses used for this interpretation to be valid (see 14.3.2). This establishes that the magnitude of the response is directly proportional to the total number of incident quanta regardless of the time or irradiance of the
irradiation providing those quanta (Table 14.1). The effects of irradiance level during the brief irradiations used in induction-reversion experiments are thus attributed entirely to the degree of photoconversion.
Biological action spectra are an extension of this principle (Fig. 14.7). The magnitude of the response at different wave-lengths is interpreted to be a function of the relative effectiveness of the quanta at those wavelengths in the phytochrome photoconversion process. The close agreement between the action spectra of several biological responses on the one hand (Fig. 14.7) and those of the phototransformation reactions of the isolated pigment on the other (Fig. 14.3) lends strong support to this notion (Borthwick, 1972; Shropshire, 1972). In these cases a seemingly good correlation exists between Pfr level and response magnitude.
Figure 14.7
Action spectra for induction and reversion of plumular hook opening in bean seedlings
(after Withrow et al., 1957).
In contrast, however, the majority of rigorous attempts to demonstrate a direct quantitative correlation between the photometrically detectable Pfr level and the relevant biological response in the same system have been unsuccessful (Hillman, 1972). The reasons for this are not understood. An apparent exception is lipoxygenase levels in Sinapis (Oelze-Karrow & Mohr, 1973).
Increases in response with increasing doses of quanta must eventually saturate. If the level of Pfr is rate-limiting the photoresponse will saturate when the photoconversion process is saturated i.e. when photoequilibrium is reached. If the response system itself is rate-limiting the response may saturate well before photoequilibrium. Examples of both extremes have been observed (Hillman, 1967). Light doses which saturate the photoconversion of Pr to Pfr do not appear to saturate the inhibition of mesocotyl lengthening in Avena (Loecher, 1966), nor the accumulation of anthocyanin in Sinapis (Drumm & Mohr, 1974). In contrast, inhibition of lipoxygenase accumulation is saturated by very low (< 3%) Pfr levels (Mohr, 1972). Other photoresponses fall between these extremes with many being saturated at less than 80% Pfr (Hillman, 1967). In addition, whereas many parameters, such as anthocyanin formation, show a graded response, others, such as lipoxygenase accumulation, respond in an all-or-none fashion to changes in Pfr level, suggesting some form of cooperative, threshold mechanism (Oelze-Karrow & Mohr, 1973).
Implicit in the far-red reversibility of an induced response is that Pfr can act in the dark. Light is strictly a trigger. The magnitude and multiplicity of the responses indicate an extensive amplification mechanism. Unlike photosynthesis where light energy is converted stoichiometrically with quantum yields of less than 1.0, the low irradiances which actuate these phytochrome responses lead to final quantum yields well in excess of unity (Galston, 1974).