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The following points highlight the ten important factors that affecting transpiration. The factors are: 1. Relative Humidity 2. Atmospheric Temperature 3. Light 4. Air Movements 5. Atmospheric Pressure 6. Availability of Water 7. Leaf Area 8. Leaf Structure 9. Root/Shoot Ratio 10. Mucilage and Solutes.
Factor # 1. Relative Humidity:
Relative humidity is the percentage of water vapour present in the air at a given time and temperature relative to the amount required to be present to make the air saturated at that temperature. The rate of transpiration is inversely proportional to the relative humidity, i.e., the rate of transpiration is higher when the relative humidity is lower and lower when the relative humidity is higher.
It is because the leaf interior has a nearly saturated air in its intercellular spaces. Relative humidity of the atmospheric air governs its vapour pressure deficit or DPD or water potential. Since DPD of atmospheric air is higher at low relative humidity, more of water vapours will diffuse out of the leaf interior as compared to high RH when DPD is lower.
Factor # 2. Atmospheric Temperature:
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A high temperature opens stomata even in darkness. Besides producing a heating effect, it lowers the relative humidity of the air and increases vapour pressure inside transpiring organ. For 10°C rise in atmospheric temperature vapour pressure inside leaves doubles while relative humidity decreases by 50%. Consequently, rate of transpiration increases.
However, leaf temperature does not rise corresponding to rise in atmospheric temperature due to cooling effect of transpiration. Very high temperature may cause desiccation and closure of stomata. Very low temperature also closes the stomata and hence decreases the rate of transpiration.
Factor # 3. Light:
In the majority of plants stomata open in the presence of light and close in darkness. A strong light further increases transpiration probably due to its heating effect. Because most of the transpiration occurs through stomata, the rate of transpiration is quite high in light. It falls down appreciably in the darkness.
Factor # 4. Air Movements (Wind):
Transpiration is lower in the still air because water vapours accumulate around the transpiring organs and reduce the DPD of the air.
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The movement of the air increases the rate of transpiration by removing the saturated air around the leaves. Up to 20-30 km/hr the rate of transpiration increases with the wind velocity. A wind velocity of 40-50 km/hr decreases transpiration by closing the stomata due to mechanical effect, drying and cooling of the transpiring organs.
Factor # 5. Atmospheric Pressure:
Low atmospheric pressure enhances evaporation, produces air currents and increases the rate of transpiration.
Factor # 6. Availability of Water:
The rate of transpiration depends upon the rate of absorption of soil water by roots. This is further influenced by a number of soil factors like soil water, soil particles, soil temperature, soil air, etc. A decrease in water uptake by the root causes partial dehydration of the leaf cells resulting in closure of stomata and wilting.
Wilting:
It is the loss of turgidity of leaves and other soft aerial parts of a plant causing their drooping, folding and rolling. The symptoms of wilting are not shown by thick-walled tissues. Therefore, they are less conspicuous in sclerophyllous plants.
Wilting is of three types:
(i) Incipient Wilting:
There are no external symptoms of wilting but the mesophyll cells have lost sufficient water due to transpiration being higher than the availability of water. It occurs during midday for a brief period in almost all plants even when sufficient water is present in the soil.
(ii) Temporary Wilting (Transient Wilting):
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It is the temporary drooping down of leaves and young shoots due to loss of turgidity during noon. At this time the rate of transpiration is maximum.
The rate of water absorption is less due to shrinkage of roots and depletion of water around the root hairs. Lower leaves show wilting earlier than the upper ones. Temporary wilting is corrected only after the rate of transpiration decreases in the afternoon accompanied by replenishment of water around the root hairs.
(iii) Permanent Wilting:
A permanent wilting is that state in the loss of turgidity of leaves when they do not regain their turgidity even on being placed in a saturated atmosphere. It occurs when the soil is unable to meet the requirement of plant for transpiration.
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Water is present in the soil largely in unavailable form (echard). At permanent wilting percentage (PWP) or coefficient (PWC) the soil contains 10-15% water depending upon its texture (about 10% in loam soil). After permanent wilting the plant dies.
Factor # 7. Leaf Area (Transpiring Area):
A plant with large leaf area will show more transpiration than another plant with less leaf area. However, the rate of transpiration per unit leaf area decreases in a canopy due to density of foliage, shading effect and decrease of air movement inside the canopy.
Factor # 8. Leaf Structure:
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(a) Thickness of Cuticle:
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Cuticular transpiration decreases with the thickness of cuticle and cutinisation of epidermal walls.
(b) Number and Position of Stomata:
Because most of the transpiration takes place through the stomata, their number influences the rate of transpiration. Most dicots have stomata restricted to lower surface while the isobilateral monocot leaves possess equal number of stomata on both the surfaces.
(c) Sunken Stomata:
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The sunken stomata are device to reduce the rate of transpiration by providing an area where little air movement occurs.
(d) Stationary Layer and Hair:
The hair insulates the surface of the leaf from air currents and air temperature. They hold a stationary layer of air (also called boundary layer). The thicker the boundary or stationary layer, the lower is the rate of transpiration. It is because the leaf will first lose water to stationary layer and from there it would travel to the outside.
(e) Mesophyll:
Compact mesophyll (as having more of palisade tissue and fewer intercellular spaces) reduces transpiration while a loose mesophyll (having more of spongy tissue and larger intercellular spaces) increases transpiration.
(f) Leaf Modifications:
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Formation of prickles, leaf spines, scaly leaves, phyllodes, phylloclades (instead of leaves), are all modifications found in xerophytes to reduce transpiration. In xerophytes the leaves are also smaller (to reduce the effect of heating) and leathery (to prevent wilting).
Factor # 9. Root/Shoot Ratio:
A low root/shoot ratio decreases the rate of transpiration while a high ratio increases the rate of transpiration. The latter is due to the fact that an extensive root system is more efficient in water uptake from soil. Increased availability of water also increases transpiration.
Factor # 10. Mucilage and Solutes:
They decrease the rate of transpiration by holding water tenaciously.
