TRANSPIRATION

Definition

Transpiration is the evaporation of water into the atmosphere from the leaves and stems of plants. Plants absorb soil water through their roots and this water can originate from deep in the soil.

Regulation of Transpiration

Transpiration is extremely costly to the plant, especially when the water supply is limited. A number of special adaptations exist that minimize water loss while optimizing the gain of CO₂.

1. The cuticle and the stomata - leaves are covered by a cuticle that makes the leaf largely impervious both to water and carbon dioxide. By far the largest amount of water transpired by a higher plant is lost through the stomata. Stomatal transpiration involves two processes. The first is evaporation of water from cell wall surfaces bordering the intercellular spaces, or air spaces of the mesophyll tissue. The second is diffusion of water into the atmosphere by way of the stomata. Closing the stomata prevents the loss of water vapor from the leaf and prevents the entry of carbon dioxide into the leaf.
2. Humidity - water is lost much more slowly into air already laden with water vapor.
3. Air currents - a breeze cools your skin on a hot day because it blows away the water vapor that has accumulated near the skin surfaces, and so accelerates the rate of evaporation the same as plants.

Measuring Transpiration

It is often difficult to measure transpiration directly as it usually has to be performed in a controlled setting on individual plants. Measuring transpiration can be accomplish by using a lysimeter or potometer. The potometer is constructed with rubber tubes connecting from a water tank and gauge to a plants root structure. A lysimeters is basically a large tank that holds soil and plants. Water that enters and exits the tank system is carefully measure. This is usually accomplished by weighing the tank several times during the water cycle.

Measures of Soil Moisture

Degree of Saturation: This is the ratio of the volume of water in the soil compared to the volume of air (voids).

Water Content (volumetric): This is the ratio of the volume of water in the soil compared to the total volume of the wet soil.

Water Content (gravimetric): This is the ratio of the mass of the water in the soil compared to the total mass of the wet soil.

Hydraulic Conductivity: This is the rate at which water can travel naturally though a soil.

 

Factors Affecting Rates of Transpiration

Feature	How this affects transpiration
Number of leaves	More leaves (or spines, or other photosynthesizing organs) means a bigger surface area and more stomata for gaseous exchange. This will result in greater water loss.
Number of stomata	More stomata will provide more pores for transpiration.
Size of the leaf	A leaf with a bigger surface area will transpire faster than a leaf with a smaller surface area.
Presence of plant cuticle	A waxy cuticle is relatively impermeable to water and water vapour and reduces evaporation from the plant surface except via the stomata. A reflective cuticle will reduce solar heating and temperature rise of the leaf,helping to reduce the rate of evaporation. Tiny hair-like structures called trichomes on the surface of leaves also can inhibit water loss by creating a high humidity environment at the surface of leaves. These are some examples of the adaptations of plants for conservation of water that may be found on many xerophytes.
Light supply	The rate of transpiration is controlled by stomatal aperture, and these small pores open especially for photosynthesis. While there are exceptions to this (such as night or "CAM photosynthesis"), in general a light supply will encourage open stomata.
Temperature	Temperature affects the rate in two ways: An increased rate of evaporation due to a temperature rise will hasten the loss of water. Decreased relative humidity outside the leaf will increase the water potential gradient.
Relative humidity	Drier surroundings gives a steeper water potential gradient, and so increases the rate of transpiration.
Wind	In still air, water lost due to transpiration can accumulate in the form of vapor close to the leaf surface. This will reduce the rate of water loss, as the water potential gradient from inside to outside of the leaf is then slightly less. Wind blows away much of this water vapor near the leaf surface, making the potential gradient steeper and speeding up the diffusion of water molecules into the surrounding air. Even in wind, though, there is some accumulation of water vapor in a thin boundary layer of slower moving air next to the leaf surface. The stronger the wind, the thinner this layer, and the steeper the water potential gradient. Also, the bigger the leaf, the greater the average thickness of the boundary layer, which means a bigger leaf will have a slightly slower transpiration rate per unit area (although a higher transpiration rate overall).
Water supply	Water stress caused by restricted water supply from the soil may result in stomatal closure and reduce the rates of transpiration.

Estimating Transpiration

Blaney-Criddle Method

   Estimates seasonal AET.

   Can be used for monthly estimates if monthly crop coefficients are locally available.

  Assumes mean monthly air temperature and annual day time hours can be used as a substitute for solar radiation to estimate the energy received by the crop.

   Monthly consumptive factor (f)

Where:  t = the mean monthly air temperature in °F and

             P = the mean monthly percentage of annual daytime hours(Table 4.6).

             U = the seasonal consumptive use in in/season

          K = the seasonal consumptive use coefficient for a crop with a normal growing season (Table 4.7)

Example Calculation

Determine the seasonal consumptive use of tomato crop grown in New Jersey if the mean monthly temperature for May, June, July and August are 61.6. 70.3, 75.1 and 73.4 ^oF, respectively and the percent daylight hours for the given months are 10.02, 10.8, 10.22 and 9.54 as percent of the year, respectively.

Solution:

From table 4.7 the growing season tomatoes is 4 months and the range of the consumptive use coefficient is 0.65 to 0.70. since New Jersey is a humid area, choose the lower value of Ks=0.65.

In term of f calculation: