After vernalization, which rate of passage is not affected by a light regime, comes the next phase of plant development - a photo phase . To pass th

Photo phase of plant development and photoperiodism [stage]

After vernalization, which rate of passage is not affected by a light regime, comes the next phase of plant development — a photo phase. To pass this stage, some plants need a certain period of stay in the light, as in them the light accelerates the onset of fruiting, while other plants need a certain period of stay in the dark, as in them the light retards fruiting. The first plants are called the plants of a long day; their development on photo phase goes, most probably, is under continuous light. In short-day plants those phasic changes that are caused by the passage of the photo stage, mostly occur in the dark. But since in the continuous dark plants cannot exist as the photosynthesis process terminates, these plants need a periodic change of light and darkness and during light period of the day goes their nutrition processes, and during the dark period — phasic changes.

Photoperiodism in photo phase

Works of V. I. Razumov showed that the plant needs not a photoperiodism per se, but conditions for photo phase passage. It turned out that even the short-day plants can be grown under continuous light, as long as the light was low enough tension and could be perceived as dark. In the Razumov’s experiments the Bukhara, Pamir and Irkutsk panicgrasses were grown first at the short day and the remainder of the day — when illuminated at 10 lux. All of these varieties did not occur in the delay in paniculation at this light. In Voronezh and Bezenchuksky panicgrasses only additional illumination of 0.5 lux did not cause the delay. And finally, Mongolian, Korean and Chinese panicgrasses gave the whisk only in a short day. Illumination of 0.5 lux has caused a delay in panicle ejection.

From the above it is clear that the doctrine of photoperiodism cannot be regarded as a special theory of development, and is just one part of this theory. Taken from http://worldofschool.org

Photoperiodic aftereffect

Especially clear and convincing fact proving phasic nature of photoperiodism is the so-called photoperiodic aftereffect, opened by Russian scientists Egiz and studied in more detail by Razumov. It turns out that to speed up the fruit-bearing there is no need to act on the plants with corresponding photoperiod throughout his life. It's enough to expose it to this effect within a relatively short time period prior to the formation of flower organs.

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