Introduction:

The survival of all plants relies on the skill to transport the variety of substances in, through and out of the bodies. At cellular levels, diffusion is generally sufficient for the movement. Evolution of multicellularity rendered diffusion insufficient for moving substances throughout plant as time needed for diffusion is inversely proportional to square distance covered. Plants colonizing land were in the race to intercept light for photosynthesis. This signifies growing taller and exposing large leaves to dry, hostile environment. Algae-like photosynthetic cells in leaves could only function in highly humid environment that was far from soil's water. In ferns and other small plants, roots and root hairs were sufficient to absorb water. As leaves are situated far from leaves just absorbing water was not sufficient. Plants require a system for transporting water from the soil to the leaves. This led to the evolution of Xylem system for long distance water transport.

Moving Water and Mineral in the Xylem:

Water is plentiful compound in plant cells; it accounts for 85% - 95% of weight of most plants, and 5%-10% of weight of seeds. Water is utilized to make organic compounds (like sugars) support plant (via tugor pressure), as the solvent in which significant reaction happens, and as medium in which solvents move. Given critical roles played by water in plants, it appears strange that more than 95% of water collected by the plant evaporates back into atmosphere, frequently within only a few hours after being absorbed. This evaporation of water from shoot of plant is called as Transpiration. The majority transpiration from leaves is by stomata and is the result of leaf architecture.

Leaf Architecture:

Leaves are delicately adapted for photosynthesis. Though, adaptations which improve photosynthesis also improve plants greatest threat, dehydration for instance rate of gas exchange depends among other things on amount of surface area available for exchange and evaporation. Loose internal arrangement of cells in leaves produces the large internal surface area for transpiration.

Internal surface area of leaf is related to atmosphere by intercellular spaces which occupy as much as 70% of volume of leaf. Gates which lock internal surface area with atmosphere are stomata. These gates are abundant; one leaf, for instance, may have more than 80 million stomata. Leaves also have proficient plumbing system of veins for distributing water to the internal evaporative surface: one square centimeter of leaf can contain as many as 6,000 vein endings. Because of the leaf architecture, a well-watered plant can lose great amount of water through transpiration; for instance, a corn plant transpires almost 500 1itres of water during few-month growing season.

Water lost by means of transpiration is replaced by water absorbed from soil by roots. Water moves to leaves as fast as 75cm min-1 that is about speed of tip of second hand sweeping around wall clock. Despite the huge need for water, plants contain no active mechanism for obtaining water when they are stressed.

Structure of the Conducting Cells:

Water should move rapidly though plants to replace water lost by transpiration. Water and dissolved minerals move from roots to leaves by xylary elements.

There are two types of xylary elements in plants: tracheids and vessel elements. Both of these kinds are well developed for conducting water:

They are hollow and dead at maturity, and so contain no cellular organelles to delay water flow. Also, both have thick cell walls and can thus resist changes in pressure related with water flow caused by transpiration. Though, tracheids are generally long (up to 10mm) and thin (10-15mm in diameter), and they overlap each other. Their walls have several thin areas known as bordered pits that are valve like structures which are especially abundant in portions of walls where tracheids overlap. Pits link tracheids in long water-conducting chains and permit slow flow of water by xylem.

Vessel elements are shorter and wider than tracheids ; their diameter are generally between 40 and 80mm, and may be as large as 5000mm. Vessels are stacked end to end and end walls dividing adjacent vessel elements are frequently either whole or partly dissolved as a result, vessel element form cellulose pipes known as vessels ranging in length from centimeter to more than meter. Water can move longer distances in vessels than in tracheids previous to having to travel a pit. Furthermore, their larger diameter and dissolved end walls permit water to move faster than in tracheids.

Water Potential:

Force is liable for water movement. Movement of water though plant is physical process which need no metabolic energy. Rather, water flows passively from one place to another due to difference in potential energy. Potential energy of water in particular system compared to pure water at atmospheric pressure, and at same temperature is named water potential and is abbreviated by Greek letter psi, ψ. Lowering potential energy of water lowers water potential, and increasing potential energy of water increases water potential. Differences in water potential find direction which water moves: water always flow passively from regions of high water potential to regions of lower water potential. Movement of water into, through and out of plants is regulated by water potentials. Any hypothesis for water movement in plants should be based on water moving from regions of high water potential to regions of lower water potential. It should also account for even more obvious requirement:

Movement should reach tops of all trees. Forces concerned in lifting water to treetops are significant. For instance imagine force of water needed to move water to leaves of tall iroko tree. Few hypotheses have been suggested for water movement in plants.

Hypothesis 1:

Water moves up xylary elements using capillarity. Capillarity results from adhesion of water to surfaces of small tubes. This adhesion pulls water up tube, and is visible as curved meniscus atop water column in a glass tube. Though in tubes having diameters of xylary element, capillarity lifts water less than 1m. Thus, capillarity alone can't account for movement of water to top of trees.

Hypothesis 2:

Water is pushed up xylary elements by atmospheric pressure. To understand the hypothesis, imagine filling long hollow tube with closing it at one end, and placing tube open-end down, in tub of water. Movement of water column is balanced by 2 opposing forces: weight of water in tube pulls water column down, whereas atmospheric pressure pushes water up tube. These counteracting pressures reach equilibrium when water column is approx 10.4m high. When length of tube exceeds 10.4m, water column cavitates, means that it forms the partial vacuum filled with water vapor in upper closed end of tube. As atmospheric pressure raises the column of water only approx 10.4m, it can't account for movement of water to tops of tall trees.

Hypothesis 3:

Water is pumped up xylary elements. Water in xylem moves in xylary elements that are dead. Also there are no pumping cells in xylem. Thus water is not actively pumped by xylem.

Hypothesis 4:

Water is pushed up thrugh root pressure. On several mornings leaves have water droplets at their edges. This loss of water from leaves of intact plants is known as guttation and is common in herbaceous plants growing in moist soil on cool damp morning. Guttation is caused by root pressure which is produced as follows:

i) Minerals actively absorbed at night are pumped into apoplant surrounding xylary element

ii) Influx of solutes decreases water potential of xylary element, thus causing water to move into it from surrounding cells.

iii) As there is only negligible transpiration at night, pressure in xylem increases as high as +0.2 Mpa.

iv) Finally this pressure forces liquid water out of leave by hydathodes.

Guttation carries on as long as plant is kept under conditions favoring rapid absorption of minerals and minimum transpiration, like in wet soils at night. Though most pressure can push water numerous meters up a plant, it can't push water to tree tops.

Hypothesis 5:

Water is pulled up plants by evaporation. This hypothesis was prepared more than century ago, and today is referred to as transpiration-cohesion hypothesis for water movement. Process is as follows:

i) Solar-powered transpiration of water dries cell wall of mesophyll cells.

ii) This loss of water from wall lowers water potential of cell, thus causing it to take up water from neighboring cells which have higher water potential as they are farther away from air space.

iii) Cells farther away from site of evaporation have even larger water potentials, therefore causing water to move from cell to cell toward air spaces.

iv) Cells bordering tracheids replace the water with water from xylem. This loss of water with water from xylary elements develops negative pressure thus lifting water column up plant.

v) Negative pressure decreases water potential all way down to tips of roots, even in tallest trees. Tension lowers water potential in root xylem so much that water flows passively from soil, across root cortex and into stele. Water in stele is then pulled up xylem to leaves to replace water lost through transpiration.

Factors Affecting Transpiration:

Environmental Factors:

1) Atmospheric Humidity:

Transpiration happens as long as water potential of atmosphere is less (that is more negative) than water potential of leaf. Dry air increases gradient and thus increases transpiration. Likewise, transpiration normally illustrates in humid air.

2) Internal concentration of CO₂:

Concentration of CO2 in atmosphere hardly ever deviates much from O.O3%. Though CO2 concentration in leaves changes significantly, particularly when stomata close and photosynthesis removes CO2 from intercellular spaces of leaf. Low concentration of CO2 in leaves cause stomata to open, while high concentrations cause them to close. Therefore reduced supply of CO2 for photosynthesis (that is low internal concentration of CO2) opens stomata and increases transpiration.

3) Wind:

Thin moist layer of air adjacent to transpiring leaf is known as boundary.  Thick boundary layer decreases diffusion gradient and thus decreases transpiration. Wind generally replaces boundary layer with drier air, thus increasing water-potential gradient and increasing transpiration. Leaves of several grasses can temporarily decrease transpiration. Upper epidermis of leaves of several grasses has vacuolated cells known as bulliform cells, that are sensitive to water loss. These cell shrink when they dry out, thus rolling leaf into cylinder. This shape increases leaf's boundary layer and decreases amount of light which reaches leaf, thus decreasing transpiration.

4) Air Temperature:

In direct sunlight, temperature of leaf may exceed that of air by as much as 10°C. Increasing leaf temperature increases water vapor pressure in leaf that in turn increases water potential and leads to faster rates of transpiration. Transpiration is most rapid at 20°C-30°C.

5) Soil:

Any factor which affects water availability also affects transpiration; thus transpiration is affected water contents of soil. Plants can absorb water from soil as long as the vapor potential is less than that of soil. Plants functions as wicks which evaporate sub-surface water from soil that describes why soils covered by plants lose water faster than does bare soil. Approximately all water lost below 15cm in soil is lost via transpiration. Weeds thus compete with crop plants, not only for light and nutrients, but also decrease availability of water to soil.

6) Light intensity:

Light frequently causes stomata to open and thus increases transpiration. Though stomata usually open at sunrise and close at sunset, these are not all-or-nothing effects; instead stomata open slowly in morning over the period of about one hour and slowly close throughout afternoon. Effect of light on stomata opening is indirect. Light helps stomata opening through stimulating photosynthesis that decreases internal concentration of CO2 in leaf. Regulation of transpiration by light is significant, as it prevents plants from unnecessarily losing water when it is very dark for photosynthesis.

Structural Adaptations:

Apart from environmental factors listed there are other structural adaptations which influence transpiration by decreasing it these are:

Cuticle: Retention of water and survival would be approximately not possible for plants without cuticle. Cuticle is the effective means of conserving water: less than 5% of water lost by plant evaporates through cuticle. Generally, thicker cuticle gives more protection from desiccation than these ones. The desert plants typically have thick cuticles, while those of aquatic plants are thin.

Trichomes: Though trichomes increase thickness of boundary layer overlying leaf main means by which trichomes reduce transpiration i.e. by reflecting light and therefore decreasing temperature of leaf.

Sunken Stomata:

Sunken Stomata increase boundary layer surrounding guard cells. Thus, plants with sunken stomata normally transpire less than do plants with raised stomata.

Decreased Leaf Area:

Several desert plants have really reduced leaves, thus decreasing their evaporative surface. In the plants, succulent stems which store large amounts of water replace leaves as main photosynthetic organs.

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