The second phase of plant development - photo phase - passes in some plants with a shortened day, while in others with a long one. The first p

Photo phase of plant development [Short-day, long-day neutral]

The second phase of plant developmentphoto phase — passes in some plants with a shortened day, while in others with a long one. The first plants were called short-day plants, and second — long-day plants. In addition, there was also detected group of plants, indifferent to the light conditions, they called neutral.

Short-day plants include a species of southern origin (cotton, millet, etc.), and a long-day plants — plants of more northern origin (wheat, barley, etc.). As you know, summer day in the north is much longer than in the south.

It turned out that after the passage of the vernalization phase, winter wheat in short-day conditions does not start to bloom, and in long-day conditions — blooms. While vernalization phase is not passed, photo phase cannot flow. Thus, this experience emphasizes the need of a preliminary passage of vernalization phase, i. e. a sequence of phase changes. Short-day plants need to be put in conditions of shortened the day after passing the vernalization phase. Too many short-day plants may not bloom in the conditions of south part of Canada because of too long duration of a summer day. Such plants include, for example, Jerusalem artichoke, that grows well, but does not bloom at the latitude of southern Canada or northern US.

The photo phase, unlike vernalization, can flow only in green plants as formation of nutrients is necessary in the plant leaf apparatus for occurring changes at the growth point. For example, placing the point of growth of the plant in dark conditions or a short day at the short-day plants, we cannot get the passage of photo phase. To do this, plant leaves should be put in short-day conditions. Only then points of growth will pass the photo phase.

In what conditions the phasic changes occur in short and long-day plants? The experiments showed that the photo phase of short-day plants takes place in the dark. If you illuminate them, at least for a few minutes, interrupting the darkness, the photo phase does not pass. The same occurs when the light is flashing, when in some seconds and minutes the darkness replaces by light. In a long day plant photo phase takes place in the light, and short breaks of illumination with darkness do not interrupt its passage. In this case, it is only necessary that amount of illuminated hours in sum was equal to the duration of illumination in long-day conditions.

Shortening of the day

Shortening of the day is important for the formation of tubers of the potato and for the transition of some tree species in a state of winter dormancy. No matter how the photo stage goes — with a short or a long day, — the transition to rest is associated primarily with the shortening of the day. This question has a very great practical importance. In terms of the long northern days, many crop plants do not fall into a state of rest and freeze completely or very affected by frost.

Photoperiodic reaction and geographical origin

See also Photo phase of plant development and photoperiodism

Between the photoperiodic reaction of plants and their geographical origin soviet researchers V. N. Lyubimenko, O. A. Shcheglova (1927), A. V. Doroshenko and V. I. Razumov (1929) have established a close relationship. Homeland of short-day plants are tropical and subtropical countries. Long-day plants originate in countries that lie to the north and south of the subtropics. The duration of daylight varies at different latitudes. At the equator during whole year a day consist of 12 hours of day and 12 hours of night. As we move away from the equator to the north and south, the length of the day in the summer increases so in polar regions no night in the summer at all, but in the winter continuous night lasts there for several months.

Families that are common in the north (Ranunculaceae, Caryophyllaceae, and Cruciferae) are long-day or neutral. Families Euphorbiaceae, Bromeliaceae are common in the tropics and are presented by only short-day or neutral forms. Crops families Asteraceae, Papilionaceae, Solanaceae are widely distributed in the world in different geographical areas, and accordingly to it, they have species with various photoperiodic responses. On the high mountains, many plants of the north start their growing season late, in the second half of the summer, and also pass their development at a slightly shortened day. Taken from

Neutral plants

Analyzing the issue of neutral plants, V. I. Razumov came to the conclusion that these plants do not represent some kind of sharply distinct group. According to him, some of them can be attributed to the short-day plants with mild expressed properties of short-day plants. Other neutral plants are, on the contrary, the long-day plants, but their long-day ability is not very expressed. For example, Biloxi soybean variety is a typical short-day plant, and Mandarin variety, which is the plant that considered to be neutral, actually speeds up its development in the short day, but relatively insignificant — only on 2-5 days. All northern wheat are long-day. Wheat of Egypt, India, and Arabia are considered to be neutral. If you take a large amount of varieties of one plant species, it is easy to immediately find what photo phase it has. Buckwheat, which is considered to be a neutral plant, belongs to the group of short-day plants, as several varieties of buckwheat accelerate its development in the short day. Pea plant is considered to be neutral, but reveals its long-day nature when a large number of varieties are researched.

Light quality

A lot of researchers’ attention has been paid to the question of the quality of light that is required for the passage of the photo phase. These studies have shown that when a photo phase is being passed, the red light is perceived by the plant as the light, and the blue-violet portion of the spectrum — as the darkness. However, by increasing the intensity of blue light, it also begins to be perceived by plant as a light. New experiments show that if you level the intensity of the light by the number of photons, the red and blue light give the same results.