ASCENT OF SAP
Ascent of water and Minerals
The upward movement of water from the root to aerial parts of the plant body is called ascent of sap or often called translocation of water. It is fascinating to understand how water moves in plants to such great heights such as 400 ft. or more. For example, trees like Sequoia semipervians are as tall as 300 to 400 ft. They are the 4000 yrs old and giants among the tree plants. These plants transport water through their stem must be incredible.
Plants like Balsam, which have relatively transparent stem are very good experimental materials for studying the pathway of water. If a Balsam plant is provided with a dilute solution of safranine, the same is absorbed by the root system and transported upwards which can be easily traced because of the red color of the safranine. Another feature of safranine is that safranine has a greater affinity to lignified cell wall material. So wherever or whatever the structures through which the safranine solution is transported, the cell walls will be stained by safranine. So the use of safranine solution is an useful technique to understand the structures involved in the ascent of sap. Anatomical observations of balsam plants which have translocated safranine solution clearly reveal that xylem elements, particularly, tracheids and trachae are the main structures involved in the transportation of water. This experiment can also be used for determining the rate of ascent of sap.
It is very important to note that the water that is transported all along the length of the stem is not just water; it is a solution of water containing various inorganic and organic components. How the solute part of the solution that is absorbed by the root system is a different problem by itself, which is discussed in the earlier chapter, “absorption of minerals”, still the materials transported through xylem is not just water but contains all other inorganic and organic components. That is the reason why the terminology, Ascent of Sap is used instead of Ascent of Water. The sap means water and any other components loaded in to such vessels.
Errect plants with stems of reasonable thickness can be used for this experiment. Using a sharp blade, it is possible to remove only the cortex or cortex with phloem or any other structural parts of the vascular cylinder.
The logic of the experiment is to find out which of the stem structures are involved in the transportation of sap, it may be cortical tissue, phloem or xylem elements, using a sharp blade; it is possible to put an incision in the stem till it reaches the hard part of the stem. Then a ring of tissue can be removed which consists of cortex and phloem, leaving behind the xylem elements intact. This experiment is to determine which of the stem components are used in the ascent of sap. However the removal of cortex and phloem by girdling experiment does not affect the movement of sap. Thus it can be concluded that xylem is alone responsible for the ascent of sap.
At 50 0 C– 58 C0, the solid wax melts into liquid form. If the cut end of stems, with their normal branches containing transpiring surfaces is allowed to take up and transport the liquid wax for sometime, the liquid wax enters into those structures through which it is transported. By lowering the temperature, the wax solidifies and virtually blocks the structures through which the liquid paraffin has transported. Then, if such branches are transferred to water or safranine solution, the branches fail to receive water and the same will wilt. Analytical studies of wax filled structures reveal that xylem elements are the structures responsible for the transportation of water.
The above experiments unequivocally prove that xylem elements are mainly responsible for the movement water upwards. Among the four xylem elements, tracheids and trachae from a kind of continuous tubular system running from the root system to aerial branches. The end to end association of trachae with their disintegrated transverse walls provides tubular and network of channels for the movements of water.
Though trachieds have transverse walls intact, the septal perforations are so large; they provide continuous system and do not afford much resistance for the smooth flow of water in them. The xylem parenchyma among the dead tracheids and tracheary elements provides access for the lateral movement of water from vertical xylem elements towards cortical cells. The analysis of liquid that is exuded through the xylem elements, further show that the liquid is not just pure water, but is contains many inorganic and organic components absorbed by the root system. So the trachea and tracheids with their large lumen and end to end association act as excellent pipelines for the movement of water, minerals and some organic compounds as well.
Using dyes and radioactive isotopes, it has been determined that the average rate of movement of water in xylem is 60-75 cms/minute. This is under normal transpiring conditions. However, this rate is not constant and it varies depending upon the environmental conditions, particularly Relative Humidity (RH) of the atmosphere. Nonetheless, the rate of ascent of sap has been found to be directly proportional to the rate of transpiration. Using the same criteria it is possible to determine the amount of water transported per hour; because the amount of water transpired is more or less directly proportional to the amount transported. Plants like Vicia faba easily transport about 40-50 ml of water per hour. Helianthus annus transports 12-15 liters/hr and silver maple (48 feet) transports about 100-125 liters/hr.
1. Live pump theories: Plant physiologists of 1880’s like Westermeir, Godlewski, Sir J. C. Bose (1920s), Priestly and others opined that the ascent of sap or water is due to pump like activity of living cells found among the dead xylem elements. The theories, proposed by the above mentioned authories, are basically revolve round the activity of living cells, which acted as living pumps and dead xylem elements as reservoirs. Living cells, by rhythmic pulsatory activity are believed to pump water upwards, just like the heart pumping blood. In support of this theory, J.C. Bose designed an apparatus called dendrometer and demonstrated the rhythmic expansion and contraction of cells in the stem. Some physiologists who supported the views of J.C. Bose used cardiac stimulants like adrenaline and camphor etc. and claimed that the pulsatory activity increases by 1000 fold. Similarly anesthetics and respiratory inhibitors like DNP and KCN have been found to affect the rate of ascent of sap but they did not inhibit the process. In spite of overwhelming support by many contemporary scientists, Strasburger and Overtan thought otherwise and to prove their claim they showed that even after killing the cells of seventy years old 122 ft Oak tree by picric acid or fuchsin solution, still water reached the top of the tree. Thus they concluded that living cells are not essential for the ascent of sap. Nevertheless, it is difficult to discount the role of living cells in such an important process because if the stem of a tall tree is cut and than kept in a defined nutrient solution with adequate supply of water, the plant does not survive for a long time and the plant ultimately dies. So it is clear that without the activity of living cells, plants cannot maintain the transport of water for a long time.
1. Root Pressure Theory:
Protagonists of this theory believed that all plant roots absorb excess of water by an active process and builds up a hydrostatic pressure within the root system, called root pressure. This activity pushes the water upwards all along the length of the stem. It is true, that some plants develop root pressure and when conditions are favorable, but the magnitude of root pressure developed is hardly 2 atm. Very rarely it reaches 4-6 atms. The root pressure of one atms can push the water to a height of 10-15 ft; that is the maximum. So the root pressure of 3-6 atms can just support the transportation of water to the height of 45-75 ft. But there are plants which have grown to the height of 200-400 ft. In spite of it, the water is transported to their tops. In fact, it has been found that many tall plants belonging to gymnosperms and angiosperms do not exhibit any root pressure. Added to this root pressure that develops in certain species it is possible only when there is excess of water and the RH of the atmosphere is very high. From this, it is clear, that root pressure is not the force that is responsible for the transportation of water upwards, and it must be something else.
In contrast to vital forces that are supposed operate many experimental findings show that the ascent of sap is mainly due to passive forces that develop within the plant, due to certain environmental factors that act upon the plant. Historically speaking, different physiologists proposed different theories, like capillary force theory, atmospheric force theory, imbibitional force theory, transpiration pull theory, etc. Except for the transpiration pull suction force theory, none of the other theories can explain the mechanism of the ascent of sap. Still the atmospheric pressure capillary forces, imbibitional forces operate in plants; but their contribution to ascent of sap is not that signification.
This theory was proposed by Dixon and Jolley, but in later years, the theory has been expanded and modified to explain certain observations that were not explained before. Among the notable scientists who supported this concept are Curtis and Clark (1951), Krammar, Hamel and Levit. This theory is also called as Suction force theory.
The most important factor that triggers this process is the moisture content of the surrounding air. The negative pressure in the atmosphere at 50% (Relative Humidity) RH is around 1000 bars, which generates a steep DPD gradient and it acts as a powerful force for the rapid and enmass movement of water from the leaves into the atmosphere.
Such outward movement of water, from millions of leaves containing billions of stomata into the atmosphere, creates a kind of unidirectional pull which operates upon the water containing xylem columns. The pulling force that operates on such water columns is called Transpiration Pull or Suction Pressure.
As a result of transpiration pull, the water column found in xylem elements is virtually pulled upwards and outwards from one end, but the same water column is also subjected to another opposing force called gravitational pull. Because of this the water column becomes narrow because of tension. As the outer region of the water column is still adhered strongly to the hydrophilic xylem wall materials the xylem cells are also subjected to tension, hence they become narrow. Whether or not, the water columns are subjected to such tensions under rapid transpiration conditions can be demonstrated by cutting the twig. When such stems are cut, the water snaps back at the cut ends which clearly indicate that the water columns in plants under favorable transpiring conditions exist in a state of tension.
When such fine water columns are subjected to a transpiration pull as great as 1000 bars or more, from one end and a gravitational force of 980 cm / sec.2 or less at the other end, the water column is likely to snap, but it does not because inter molecular forces that hold the water molecules together are greater than the opposing forces.
Such forces are called cohesive forces; they are nothing but hydrogen bonds between the water molecules. The overall strength of water column in such narrow xylem elements has been estimated to be many folds higher than the transpiration pull and the gravitational pull put together.
As the water column is strong enough to withstand the opposing pulls, the direction of the movement of water depends upon the strength of the opposing forces. It has been found that the transpiration pull is greater than its counter force. So water is pulled upwards. It has been estimated that the transpiration pull of one atmospheric pressure can pull the water up to 15-20 feet in height. With the cell walls, and perforated transverse septa acting as resistant forces, still one atmospheric pressure can pull the water up at least to the height of 10 feet. The tallest plants known to mankind are about 400 ft. in height. And the forces required are about 40 atms or 40 bars. Even an ordinary plant like tomato or helianthus is known to develop a transpiration pull to the extent of 100-200 atmospheric pressures; which is more than enough to pull the water to the height of 1000-2000 ft.
EVIDENCE IN SUPPORT OF COHESION TRANSPIRATION PULL THEORY
1. Transpiration pull developed in the aerial regions at 50% RH in the air is more than 1000 bars.
2. Cohesive force that holds the water molecules in a column as narrow as xylem vessels is very strong and they withstand the opposing transpiration and gravitational pulls.
3. Because of the opposing forces, tension develops in the water column. As a result, xylem elements become slightly narrow. This results in the contraction and expansion of the stem which has been demonstrated by using dendrometer. The diurnal behavior of rhythmic contraction and expansion is a good evidence for the water column to be in tension which the transpiration is rapid or not.
4. The operational mechanism and success of this theory has been demonstrated by and simple experiment, where a narrow column of glass tube filled with water is immersed in mercury at one end and the other end is tightly threaded into a porous plaster of paris container filled with water. If air is blown over the porous pot, water vapors escape from the porous pot rapidly and then water is pulled upwards. The water column doesn’t break. In fact, the upward pull developed is so strong, that mercury is also pulled upwards to great heights, more than what atmospheric pressure can support.
5. The forces that operate cohesion transpiration pull are just passive forces and no metabolic energy is involved in this phenomenon, because present the movement of water upwards.
Almost every vascular plant consists of tracheids of large diameters; particularly angiosperms contain additional structures called tracheae. Such structures are present in large numbers and they are always filled with water where the water is continuously transported upwards as a column of water day and night. Due to fluctuation in temperature and metabolic process such as respiration by parenchymatous tissues, there is a greater possibility of gasses to diffuse in and to accumulate in the xylem elements and cause cavitations. Many people discern that bubbles weaken the cohesive strength of water column and tend to break the column. The objection is valid in the sense, cavitations do develop. But they develop only in a few xylem elements and not in all xylem columns. In fact, such air bubbles escape into others and thus they have bypass. Added to this, even in the presence of bubbles, the water column does not break instead the bubble moves along the column and escapes wherever an outlet is found. The claim that disturbance of plant by shaking, breaks the water column; but such breaks are only in a few columns and even such columns regain the continuity very quickly .
While transpiration pull is responsible for the upward movement of water, any mineral salts that are absorbed will be loaded into the water column, thus even mineral nutrients move upwards and also laterally for the nonliving xylem elements are in association with living parenchymatous cells, thus mineral nutrients are also transported.