Regulation of growth by light. Influence of a long irradiation by light sources with a wide spectrum

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Naturally plants usually are not exposed to influence of near or distant red light - on them the sunlight with a wide spectrum falls. In these conditions approximately half of phytochrome is presented by the form of Fdk. As Fk and Fdk have wide and blocked spectra of absorption, they are constantly transformed in both directions so the same molecules at a long irradiation do not remain in the form of Fdk. it is probable, any separate molecule continuously passes from one form in another - a phenomenon known under the name ' krugoobrashchenija ' (cycling) of a pigment. Speed krugoobrashchenija increases with increase in intensity of light and at bright solar illumination is very great.

At krugoobrashchenii between Fk and Fdk phytochrome does not turn directly from one form to another, and passes through a number of intermediate forms (fig. 11.10). For example, when Fk absorbs a photon, it quickly turns to the form with an absorption maximum at 692 nanometers. This form exists very not for long. After less 1 ms it spontaneously passes in other intermediate form with an absorption maximum at some bolshej to length of a wave. Three or four such intermediate forms quickly appear and then disappear, before rather stable form of Fdk is formed. Intermediate forms arise and at transformation of Fdk into Fk. As all of them have very short period of a semilife in comparison with Fk and Fdk, at short-term influence of light they are of little importance. However at a long irradiation light of high intensity of a molecule of a pigment quickly pass cycles of Fkch=Fdk, intermediate forms are continuously formed and there is a tendency to accumulation of forms with the greatest period of a semilife. At a bright sunlight about 20% of phytochrome can be in intermediate conditions. It is obvious that at the analysis of action of phytochrome naturally it is necessary to consider and intermediate forms.

Intermediate forms between Fk and Fdk could play also a role in a number of reactions on high obluchennost. Here concern such morfogeieticheskie the phenomena, as vytjagivanie gipokotilja, formation antotsianinov in sprouts and flowering of some plants of long day. These processes are promoted by a long irradiation red, distant red and dark blue light of high intensity, instead of influence of weaker light influencing transformation of phytochrome. Spectra of action for these reactions have peaks in the field of 420-480 nanometers and 710 720 nanometers (fig. 11.13). Some of these morfogeneticheskih changes can be caused a short irradiation at 660 nanometers and to prevent the subsequent so short irradiation at 730 nanometers, but the long irradiation is always more effective. it is visible, these reactions demand low level of Fdk for a long time, than high level during the short period more likely; In such result can result influence of dark blue light or red light with length of a wave 710 720 nanometers. However the reasons of requirement for an intensive irradiation are not clear. The important role could play krugoobrashchenie a pigment and its formation promezhutochnyh.form as these processes depend on intensity of light. However other explanations have been offered also, and more many work is required to understand reactions on high obluchennost. It is not known, for example, whether influence of dark blue light by its absorption fitohromami by Fk and Fdk is caused or the pigment participates in it like what regulate phototropism.