The distance the meniscus moved along the capillary tube.
The internal radius of the potometer tubing (usually 0.5mm – 1mm).
Enter this to calculate the rate per unit area.
Volume of Water Lost:0 µL
Transpiration Rate (Absolute):0 µL/min
Transpiration Rate (Per Leaf Area):0 µL/min/cm²
How to Calculate Rate of Transpiration in Plants
Transpiration is the biological process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Measuring the rate of transpiration is a fundamental experiment in plant biology, often conducted using a device called a potometer.
The Potometer Calculation Formula
The most common laboratory method assumes that the rate of water uptake (measured by the movement of an air bubble in a capillary tube) is approximately equal to the rate of transpiration. The calculation involves finding the volume of the cylinder of water absorbed.
1. Calculate the Volume of Water Lost:
The volume is calculated using the formula for the volume of a cylinder:
Volume (V) = π × r² × d Where:
π (Pi) is approximately 3.14159
r is the radius of the capillary tube (in mm)
d is the distance the bubble moved (in mm)
The result gives the volume in cubic millimeters (mm³), which is equivalent to microliters (µL).
2. Calculate the Rate:
The rate is simply the volume divided by the time taken for the movement.
Rate = Volume / Time Common units are µL/min or mm³/min.
3. Normalizing for Leaf Area:
To compare transpiration rates between different plants, it is crucial to account for the total surface area of the leaves. A larger plant will naturally lose more water than a smaller one, even if their stomatal efficiency is the same.
Normalized Rate = Rate / Total Leaf Area This results in units such as µL/min/cm².
Factors Affecting Transpiration Rate
Several environmental factors can influence how fast a plant loses water:
Light Intensity: Stomata open in the light to allow gas exchange for photosynthesis, increasing transpiration.
Temperature: Higher temperatures increase evaporation rates and the kinetic energy of water molecules.
Humidity: High humidity reduces the concentration gradient between the inside of the leaf and the outside air, slowing down transpiration.
Wind Speed: Wind blows away water vapor accumulating around the leaf surface, maintaining a steep concentration gradient and increasing transpiration.
Why Measure Transpiration?
Understanding transpiration rates is vital for agriculture and ecology. It helps scientists determine crop water requirements, design irrigation systems, and understand how plants adapt to drought conditions (xerophytes). Calculating the rate allows for quantitative comparisons under different environmental stresses.
function calculateTranspiration() {
// Get input values
var distanceInput = document.getElementById('distanceMoved');
var radiusInput = document.getElementById('tubeRadius');
var timeInput = document.getElementById('timeTaken');
var areaInput = document.getElementById('leafArea');
var errorDiv = document.getElementById('errorDisplay');
var resultDiv = document.getElementById('resultDisplay');
var normRow = document.getElementById('normalizedRow');
// Parse values
var distance = parseFloat(distanceInput.value);
var radius = parseFloat(radiusInput.value);
var time = parseFloat(timeInput.value);
var area = parseFloat(areaInput.value);
// Reset display
errorDiv.style.display = 'none';
resultDiv.style.display = 'none';
// Validation logic
if (isNaN(distance) || isNaN(radius) || isNaN(time)) {
errorDiv.innerHTML = "Please enter valid numeric values for Distance, Radius, and Time.";
errorDiv.style.display = 'block';
return;
}
if (time <= 0 || radius 0) {
var normRate = rate / area;
document.getElementById('resRateNorm').innerHTML = normRate.toFixed(4) + " µL/min/cm²";
normRow.style.display = 'flex';
} else {
normRow.style.display = 'none';
}
// Show results
resultDiv.style.display = 'block';
}