Plant Physiology - Musienko M. M. 2001

The impact of environmental conditions on plant growth and development. Photomorphogenesis. Phytochrome. Photoperiodism. Vernalization.
The effect of temperature on plant growth and development

Thermoperiodism

Plant growth is possible within a relatively wide Temperature range. Depending on a plant species' individual characteristics and, most importantly, its geographical origin, certain temperature limits define where growth processes can take place. Three cardinal temperature points are distinguished:

·minimum temperature, at which growth just begins;

·optimum, which is most favorable for growth processes;

·maximum, at which growth ceases.

Within the range of 0 to 35 °С, the Effect of temperature on growth obeys the Van 't Hoff rule, although above 35–40 °С growth rate decreases. Depending on their adaptation to the temperature factor, plants are divided into thermophilic (with minimum growth temperatures above 10 °С and optimums from 30 to 40 °С) and cold-resistant (with minimum points from 0 to 5 °С and optimums from 25 to 31 °С). Maximum temperatures for the growth of most plants lie within 35–45 °С. As the temperature rises from the minimum to the optimum, the growth rate increases sharply. For the growth of many plants, daily temperature fluctuations are beneficial—higher during the day and lower at night. F. Went named this phenomenon thermoperiodism.

Vernalization

Seasonal temperature changes, much like changes in day length, have a significant impact on plant development, particularly on the flowering processes of many plant species. This is especially evident in cereals. It is well known that winter cereals transition to flowering only in the following year, following autumn sowing before winter.

The acceleration of winter plant flowering after cold exposure has been known since the middle of the last century, thanks to the work of J. Klippart. In 1915, G. Gassner concluded that winter plant forms have a requirement for cold, without which they cannot transition to flowering. D.N. Maksimov and his coworkers confirmed that the duration of the plant's developmental period up to flowering is determined by temperature influence and photoperiod.

Flowering induction under METABOLISM/18.html">The Influence of low temperatures is called vernalization. Spring cereals require exposure to temperatures from +10 °С to +15 °С for two to three weeks, whereas winter cereals require exposure to lower temperatures (-1, +2°С....+10’С) for two to three months. Only seeds that have passed the imbibition phase are sensitive to vernalization. In winter cereals, the presence of a single embryo is sufficient to perceive The Effect of low temperatures. The organ that perceives vernalization in biennial and perennial plants is the apical meristem of the stem. In 1933, R. Whyte and P. Hudson introduced the term vernalization (from Latin vernalis — spring). Both terms are equivalent.

According to modern data (Fedorov, 1990), there are no obligatory and Sequential Stages of vernalization and light in plant ontogeny. Overwintering plants (winter crops, facultative winter crops) differ from non-overwintering (spring) crops mainly in The rate of passing the tillering phase, which is largely determined by their reaction to light. Different types of development are determined not by vernalization, but by the response to light. Under the influence of light conditions prior to entering winter, overwintering plants delay GROWTH AND DEVELOPMENT, which contributes to their hardening and winter survival.

Facultative winter crops are cultivated in southern regions where autumn arrives with a short day. Under these conditions, they have adapted to inhibit their development, allowing them to prepare for winter. Winter crops sown in more northern regions have adapted to inhibit development not only under short days but also under long days.

Vernalization is a facultative process that occurs at low temperatures characteristic of autumn. Spring-sown crops yield a harvest without undergoing vernalization. The interaction between light reaction and vernalization allows overwintering plants to alter their rate of ontogeny synchronously with seasonal changes in weather conditions.

Plant vernalization occurs at various Selection/3.html">Stages of development. Thus, in wheat, the influence of low temperatures is effective as early as the embryo development period, 5 days after Fertilization. This influence is perceived by the caryopsis embryo in the milk and dough ripeness stages before the caryopsis enters a dormant state. In many plant species, vernalization takes place in swollen caryopses that have emerged from dormancy. There are plants in which both seedlings and seeds are capable of vernalization, for example, Arabidopsis thaliana, which does not flower without vernalization. Apical SHOOT Meristems grown in culture are also capable of vernalization. In vegetable crops, particularly cabbage and celery, low temperatures are perceived only by green plants during the vegetative growth period. Following vernalization, most plants require a long-day photoperiod, although in some it weakens this requirement or eliminates it altogether.

The receptors for low temperature are dividing Cells, such as embryo apices in cereals, or apical bud meristems in vegetative plants. If meristems are vernalized, all Tissues developing from them behave as vernalized. Vernalization processes require sugars; in their absence, the process itself slows down but still finishes at the expense of the embryo's reserves.

It is likely that one of the most essential aspects of vernalization is the alteration of Cell/35.html">Mitochondria under the influence of low temperatures. It has been found that vernalization causes the accumulation of mitochondrial mass in the tissues of wheat, rye, triticale, and the buds of woody species. After vernalization, mitochondria exhibit a greater capacity for Swelling and volume change, through which they accumulate more ions and individual molecules, subsequently releasing them into the external environment.

Given that mitochondria move actively throughout The Cell, their Participation in the redistribution of various substances becomes possible. Mitochondria in the cells of vernalized plants often contain more sugars—the primary products of Photosynthesis—which may be the result of their contacts with Chloroplasts. In non-vernalized cells, contacts between Mitochondria and chloroplasts occur much less frequently. It has been shown that mitochondria retain their newly acquired properties for a sufficiently long period, during which the plants transition to the generative phase.

V.I. Babenko investigated metabolic features in the early phases of cereal ontogeny and proved that metabolic processes in vernalized seeds and vegetative cereal plants differ from those in non-vernalized ones. This is evidenced by his studies on the dynamics of CARBOHYDRATES, organic acids, and PROTEIN AND NUCLEIC complexes. The scientist proved that the photoperiodic response of winter wheat plants and generative Organogenesis depend on different lighting and temperature regimes.

It is hypothesized that The sequence of events during vernalization can be represented as follows:

Low temperatures → Vernalization state → Vernalin → Flowering stimulus (Gibberellin) → Flower Formation.

Perhaps vernalin is an inactive form of Gibberellins, which is later converted in leaves into its active form GA₃. At the same time, species such as chrysanthemums are unable to transmit the flowering stimulus from a vernalized plant to a non-vernalized one.

The Study of the GENETIC ASPECTS OF vernalization has shown that in some plants it is controlled by a single Gene (henbane, rye) or by several genes (Arabidopsis, wheat). The complexity of the vernalization phenomenon is manifested in the possibility of replacing the action of low temperatures with other environmental factors. Therefore, the physiological and Genetic foundations of vernalization should be further studied using the latest scientific achievements. Solving this problem is possible through the harmonious use of research at various levels—population, organismal, cellular, and molecular—and through the integrated application of methodologies from various sciences, primarily plant physiology, genetics, and physicochemical biology. A correct understanding of The Role of vernalization in plant life creates Prerequisites for the development and application of practices aimed at managing the length of the growing season, plant hardiness, and their productivity.