Vernalisation Or Yarovization

There are  plants,  for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature. This phenomenon is known as vernalization. Vernalization is a means of preventing precocious reproductive development late in the growing season, ensuring instead that seed production does not begin until the beginning of the next growing season so that the seed will have sufficient time to reach maturity

Vernalization refers specifically to the promotion of flowering by a period of low-temperature and should not be confused with other miscellaneous effects of low-temperature on plant development. The term itself is a translation of the Russian yarovizatsya; both words combining the root for spring (Russian, yarov; Latin, ver) with a suffix meaning ‘‘to make’’ or ‘‘become.’’ Coined by the Russian T. D. Lysenko in the 1920s, vernalization reflects the ability of a cold treatment to make a winter cereal mimic the behavior of a spring cereal with respect to its flowering behavior. VERNALIZATION OCCURS MOST COMMONLY IN WINTER ANNUALS AND BIENNIALS. Typical winter annuals are the so-called ‘‘winter’’ cereals (wheat, barley, rye). ‘‘Spring’’ cereals are normally daylength insensitive. They are planted in the spring and come to flower and produce grain before the end of the growing season. Winter strains, however, if planted in the spring would normally fail to flower or produce mature grain within the span of a normal growing season. Winter cereals are instead planted in the fall. They germinate and over-winter as small seedlings, resume growth in the spring, and are harvested usually about midsummer. The over-wintering cold treatment, or vernalization, renders the plants sensitive to long days.

OBJECTIVE QUESTIONS

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Question-1 When does a spring wheat produce flower ?

1. In spring
2. In summer
3. In winter
4. In both spring and winter

Question-2 Flowering Hormone Produced in some plants as a result of low temperature treatment is called-

1. Abscisin
2. Florigen
3. Vernalin
4. Dormin

Root Pressure

What is Root Pressure ?

Root pressure has its basis in the structure of roots and the active uptake of mineral salts from the soil. The xylem vessels are located in the central core of a root, the region known as the stele. Surrounding the stele is a layer of cells known as the endodermis. In most roots, the radial and transverse walls of the endodermal cells develop characteristic thickenings called the Casparian bands (Figure ). This Casparian bands presents an effective barrier to the movement of water through the apoplastic space of the endodermis. The result is that water can move into or out of the stele only by first passing through the membranes of the endodermal cells and then through the plasmodesmatal connections.As roots take up mineral ions from the soil, the ions are transported into the stele where they are actively deposited in the xylem vessels. The accumulation of ions in the xylem lowers the osmotic potential and, consequently, the water potential of the xylem sap. In response to the lowered water potential, water follows, also passing from the cortical cells into the stele through the membranes of the endodermal cells. Since the Casparian band prevents the free return of water to the cortex, a positive hydrostatic pressure is established in the xylem vessels. This Positive Pressure is Known as Root Pressure

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How Can We Proof or See that Root Pressure Is Exists ?

If the stem of a well-watered herbaceous plant is cut off above the soil line, xylem  sap will exude from the cut surface. Exudation of sap, which may persist for several hours, indicates the presence of a positive pressure in the xylem. The magnitude of this pressure can be measured by attaching a manometer to the cut surface (Figure 2.9). This pressure is known as root pressure because the forces that give rise to the exudation originate in the root.

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Can Root Pressure Is the principle force which account for Rise of  Sap* in a tree ?

The answer is probably no, for several reasons

Various Reason Are ...

1. Xylem sap is not as a rule very concentrated and measured root pressures are relatively low. Values in the range of 0.1 to 0.5 MPa are common, which are no more than 16 percent of that required to move water to the top of the tallest trees.
2. In addition, root pressure has not been detected in all species.
3. Finally, it has been clearly established that during periods of active transpiration, when water movement through the xylem would be expected to be most rapid, the xylem is under tension (i.e., negative pressure*)