Avena phototropism

Fig. 3. Comparison of different blue-light action spectra for directional responses (2) positive phototopotaxis of Euglena4 (2) Phototropism of Philobolus121, (3) phototropism of Phycomyces40 (4) second positive curvature of the avena coleoptile58), (5) first positive curvature of the avena coleotpile39)...

The defenders of the carotenoid-photoreceptor-hypothesis have always understood the shape of these action spectra in the blue to mean that the bluelight receptor is a carotenoid. Indeed, in Fig. 6 3 it can be observed, that the three-peak absorption spectrum of trans-0-carotenoid (in hexene) agrees well with the observed action spectrum of the avena coleoptile (Fig. 3 5). However, there remains one loose end which has been the crucial point of controversy in this field, ever since Galston and Baker66 suggested in 1949 that the photoreceptor for phototropism might be a flavin Flavin absorbs in the near UV, /3-carotenoid does not. 

Fig. 5. Absorption and action spectra in the UV-region. Absorption spectra of (2) auxin178), (4) carotene183), (5) flavin165). Action spectra of (1) the negative phototropism of Phycomyces l, (S) second positive curvature of the avena coleotile38)...

The most photosensitive part of the coleoptile with respect to phototropism is the apical 50 qm-zone. The first millimeter of the tip is 160 times more sensitive than the second, and the second millimeter 1,800 times more sensitive than the third 108h In 1937, Bunning29 showed by microchemical method that there is a high concentration of carotenoid (lutein) below the tip of the avena coleoptile, from 250 jum to 2 mm below this point and just in the extreme tip no carotenoid is found. This calls in question whether the enormous local differences in sensitivity can solely be caused by corresponding different absorption gradients. This doubt is further substantiated by the fact, that carotenoid deficient mutants (containing 5 to 10% of the normal siblings) show normal phototropic sensitivity. 

Fig. 15. Specific chemical inhibition of physiological bluelight responses 04) Swimming, inverse phobic response and direct photophobic response of Euglena as a function of iodide concentration118. (B) Phototropic and geotropic curvature of the avena coleoptile as function of azide concentration1S41...

Fig. 22. (A) Comparison of flavin triplet -> triplet absorption spectra (downwards drawn) with bluelight-induced (440 nm) phototropic curvature of aVena coleoptiles as inhibited by strong monochromatic light in the long wave visible region 154). (B) Comparison of the growth response of Phycomyces induced by strong laser light of wavelength longer than 590nm46, with the flavin phosphorescence spectrum los)...

Only for the (extrapolated) limit of zero bending the action spectrum reflects the absorption spectrum of the photoreceptor for phototropism (avena coleoptile). Just for this limit the UV-peak of the action spectrum disappears 162). 

Changes of electric potential have been observed in Avena coleoptiles following phototropic and gravitropic stimuli (e.g., Bose 1907, Brauner 1927, 1959, Backus and Schrank 1952, Grahm and Hertz 1962) and initially were consid-... 

Protocols for the measurement of light-adaptation kinetics that are based on the measurement of phototropic latencies are elaborate. These kinetics are much faster than the corresponding dark-adaptation kinetics. A characteristic feature of the kinetics for large steps up of the fluence rate is the fact that the level of adaptation appears to overshoot the actual fluence rate, a property that holds true for Phycomyces as well as ior Avena coleoptiles. " For oat seedlings, there is evidence that the processes that entail sensory adaptation are related to the phosphorylation of the putative photoreceptor."... 

FIGURE 132.1 Photon fluence-response curves for phototropic bending of Avena coleoptiles. (A) Coleoptiles were irradiated unilaterally with a blue-light pulse (458 nm) of constant duration (30 s) and variable photon-fluence rates. (B) Irradiation with constant photon-fluence rates (0.01,0.1, and 1 pmol s ) and variable pulse duration. Bending angles were measured 2 h after the unilateral light pulse. 

A preirradiation with a strong pulse of blue light desensitizes transiently Avena coleoptiles and causes a shift of the entire bell-shaped fluence-response curve for fPlPP. In Zea coleoptiles, preirradiation reduces mainly the phototropic responsivity, not however, the absolute threshold. After about 120 min in darkness, the responsivity recovers, and the original bell-shaped curve for the fPIPP is reestablished, indicating that the full sensitivity has recovered. "- Earlier authors who worked with a sequence of strong conditioning pulses and subsequent unilateral pulses of lesser irradiance similarly found evidence for... 

Blaauw, O.H. and Blaauw-Jansen, G., The phototropic responses of Avena coleoptiles, Acta Bot. Neerl, 19, 755, 1970. 

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