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Physiology of Water Absorption and Transpiration

Plants and Water

The ancestors of modern day plants evolved in water and it is no surprise that the internal environment of a plant is 80-90% water. The relative humidity inside plants is always higher than 95% and usually higher than 98%. As land plants must remain hydrated to this level if they are to continue growing, they have evolved a number of mechanisms, such as waxy cuticles, to ensure that they do not dry out.

The largest water problem a land plant faces is trying to grow, because this involves the fundamental process on earth, photosynthesis.

 

H2O

+

CO2

+

Radiant Energy

=>

C6H12O6

+

O2

 

Water

 

Carbon Dioxide

 

 

 

Sugar

 

Oxygen

The sugar produced in photosynthesis is the building block for all plant growth and therefore all higher forms of life on earth.

The major water problem for plants is not the water used in photosynthesis, and subsequent growth (less than 2% of total plant water use), but the water that evaporates out of the leaf as it captures CO2 from an atmosphere that only contains 0.035% CO2. The simultaneous movement of CO2 in and out of a leaf occurs by a process known as diffusion, which is the independent movement of individual molecules. This gas exchange in the leaf does not occur all over the leaf, but rather through small adjustable parts called stomata, which usually make up only 1% of the leaf surface area.

The evaporation of water out of the plant to the atmosphere is called transpiration. The relative amounts of photosynthesis (CO2 in) and transpiration (H2O out) occurring at any one time depends on amounts (vapour pressure) of CO2 and H2O in the atmosphere inside and outside the leaf respectively. For example:

 

Flow CO2

=

(CO2 outside)

-

(CO2 inside)

 

 

 

 

=

350

-

200

=

150

 

Flow H2O

=

(H2O inside)

-

(H2O outside)

 

 

 

 

=

20,000

-

10,000

=

10,000

This shows that for every unit of CO2 used in photosynthesis the plant loses about 600 units of H2O). This is known as transpiration ratio or water use efficiency and usually varies between 100 and 1000, depending on the environmental conditions.

The plant can control the loss of water from its leaves by varying the aperture of its stomata (like a tap). However, if a plant restricts the flow of water vapour out of its leaf it automatically restricts the flow of CO2 into the leaf for photosynthesis. This is known as the transpiration compromise or the plant version of 'there are no free lunches'. On average, for each 100 litres of water used by a plant, each process uses:

 

 

Photosynthesis

0.1 litres

 

 

 

Growth (new leaves, roots etc)

1.9 litres

 

 

 

Transpiration

98 litres

 

In short, continued hydration is essential for plant growth, but this is largely influenced by the control of transpiration.

Transpiration

The Pathway

Water flows through plants in well constructed interconnected system of pipes called xylem, vessel elements and tracheids at speeds of one to several meters per hour. Rates are usually faster in woody plants than herbaceous plants, such as grasses, because the xylem vessels are bigger. The important feature of this plumbing system is that it is closed at both ends, the roots and the leaves, by cell walls, cellulose structures like ultra thin tissue paper.

The roots are the water absorbing organs in the soil. The ability of a plant to absorb water from the soil depends on the number, rather than the size of the roots and where they are distributed in the soil. Root densities in soils can be surprisingly high as illustrated below.

A single grass plant in 1 litre of soil:

 

 

Length of roots

=

12 kilometre

 

 

Surface area

=

5 square metres

 

 

Length of roots + root hairs

=

220 kilometre

 

 

Surface area

=

14 square metres

 

 

Degree of soil contact

=

1%

 

 

Maximum distance for H2O to move to a root

=

 10 millimetres

Even root distribution is important for both water and nutrient uptake, because of the relative rates of movement of water and nutrients in the soil and in the plant xylem. Root hairs are more important for nutrient uptake than water because they do not contain xylem. Although water moves in saturated permeable soils at rates of between 0.01 to 0.2 meters per hour, the rate of movement in unsaturated soils (which is most of the time) is 10 to 100 times slower. Remembering that water moves in the xylem of grasses at about 1 meter per hour, we can appreciate that water moves 50 to 1000 times SLOWER in soil than roots.

Useful quantities of water will not move more than a few centimetres in unsaturated soils, therefore if you wish the plant to withdraw H2O from a soil layer it must have roots in it.

The xylem becomes of necessarily larger diameter, as it collects water from smaller roots to larger roots, than the stem. The reverse occurs in the leaves where successively smaller xylem elements distribute water to the leaves, then the veins within the leaves, finally ending in small groups of cells surrounding the stomata. The end of each small xylem element is closed by a cell wall which is mainly cellulose and acts like an ultra think tissue paper. The water then seeps along the cell walls to the sub-stomata cavity, where it evaporates and moves through the stomata to the outside atmosphere. The important point about the xylem pathway is that is is a continuous column of water, not broken by bubbles, and contained in a cellulose pipe whose walls are porous in places.

Figure_1.png

Figure 1. Plants have continuous columns of xylem in vascular bundles, from the smallest root to the youngest leaf.

Forces Allow Water Through Plants

The forces that move water from the roots to the top of the tallest tree, without a mechanical pump like a heart, have fascinated plant physiologists for centuries. We now know there are two forces involved, with the major one being atmospheric demand. Rather than being pumped up to the leaves from the roots, water is actually pulled up to the leaves by the evaporation of water on the cell walls of the sub stomatal cavity within the leaf. The method of 'water pulling' depends on having continuous columns of water in pipes that are closed at ends to prevent bubbles entering (xylem). It is called the 'cohesion theory' of water movement because it depends on columns of water not breaking.

As the rate of water movement through the plant depends on the rate of evaporation from the leaf, it is strongly influenced by environmental conditions:

1.   Air temperature

2.   Relative humidity

3.   Wind speed

4.   Radiation intensity

The second mechanism that contributes to water movement is an osmotic pumping mechanism that operates in the roots. In this mechanism, nutrients organic and inorganic, in the cells of the root attract water to move into the root, in the same way that salt in your salt shaker attracts water. This causes pressure to build up in the root, forcing water up the xylem. Although this osmotic pumping mechanism operates continually, it does not have the capacity to supply the flow of water needed for transpiration during daylight hours. However, it is the major mechanism operating at night and the one responsible for developing the root pressure that results in the small droplets of water we often see on the edges of leaves in the morning.

These two mechanisms of water transport work in plants, with the evaporative mechanism being dominant in the transpiration process and the osmotic pump taking over at night.

Control of Transpiration

The plant can exert strong control over the rate of water loss through varying stomatal apertures.

Stomates respond to environmental factors such as light, temperature and relative humidity to ensure that the plant is making the best use of its resources to photosynthesize and grow. This usually means that under well watered conditions stomates respond to light, being open during the day and closed at night (Figure 2).

Figure_2.png

 

 

1

no water limitation

stomates open

 

 

2

some water limitation

midday stomatal regulation

 

 

3

severe water limitation

midday stomatal closure

 

 

4

soil dry

complete stomatal closure

Figure 2. Influences of stomata regulation on the daily pattern of transpiration of plants growing at different levels of soil water availability.

When plant roots can no longer absorb enough water to satisfy transpiration from its leaves, it begins to dehydrate. At this point the stomates can over-ride photosynthesis and close down for certain parts of the day to avoid further water loss (Figure 2).

Under extensive water stress where the plant is unable to rehydrate at night, the stomates will remain closed all day. While this is very effective in preventing water loss, it prevents the absorption of CO2 and therefore photosynthesis. The plant cannot live indefinitely on its internal reserves.

Consequences of Water Stress

When transpiration exceeds water absorption by the roots, the plant dehydrates. This usually happens each day with the plant rehydrating again each night. As the soil dries out this rehydration is not complete, resulting in the plant becoming water stressed (Figure 3).

Figure_3.png

Figure 3. Daily cycles of plant hydration during soil drying cycle following an irrigation. The relative sensitivities of important plant functions, leaf growth, photosynthesis and transpiration are illustrated.

Water stress affects different plant processes such as growth, photosynthesis and transpiration in different ways. The growth of leaves and stems is much more sensitive to water stress than photosynthesis and transpiration (Figure 3). Therefore it is possible to restrict the growth of a plant by water stressing it, without underlying affecting its photosynthesis and survival.

Through an understanding of the water availability in the soil and the atmospheric demand, water can be used as a potent management variable to manipulate the growth and maintenance of turf grasses.