In plant physiology, measuring the rate of water uptake is a primary method for estimating the rate of transpiration. This is typically achieved using a device called a potometer. Because over 95% of water absorbed by a plant is lost through transpiration, the uptake speed is a highly reliable proxy for water loss.
The Mathematical Formula
To find the rate of water uptake, we first need to calculate the volume of water the plant has "pulled" through the capillary tube. Since the tube is a cylinder, we use the cylinder volume formula:
Volume (V) = π × r² × d
Rate = Volume / Time
Where:
π (Pi): Approximately 3.14159
r: Radius of the capillary tube (half of the diameter)
d: Distance moved by the air bubble
Step-by-Step Calculation Example
Suppose you are conducting an experiment with a leafy shoot:
Measure Distance: The air bubble moves 40 mm.
Check Diameter: Your capillary tube has a diameter of 0.8 mm (radius = 0.4 mm).
Record Time: The bubble moved that distance in 5 minutes.
Calculate Rate: 20.11 mm³ / 5 min = 4.02 mm³ per minute.
Factors Affecting the Rate
When using this calculator for your biology labs, remember that the rate of water uptake will fluctuate based on environmental conditions:
Light Intensity: Higher light usually increases stomatal opening, increasing uptake.
Temperature: Warmer air increases the evaporation rate of water from the leaves.
Humidity: High humidity decreases the concentration gradient, slowing down water uptake.
Wind Speed: Increased air movement removes the water vapor "boundary layer," speeding up transpiration.
Potometer Precautions
For accurate results, ensure the potometer is airtight. The leafy shoot must be cut underwater to prevent air locks (embolisms) in the xylem, which would block water uptake and produce a false reading of zero.
function calculateWaterUptake() {
var distance = parseFloat(document.getElementById('pot-distance').value);
var diameter = parseFloat(document.getElementById('pot-diameter').value);
var time = parseFloat(document.getElementById('pot-time').value);
var resultArea = document.getElementById('uptake-result-area');
var messageBox = document.getElementById('uptake-message');
if (isNaN(distance) || isNaN(diameter) || isNaN(time) || distance <= 0 || diameter <= 0 || time <= 0) {
resultArea.style.display = 'block';
messageBox.className = 'error-box';
messageBox.innerHTML = 'Error: Please enter positive numerical values for all fields.';
return;
}
var radius = diameter / 2;
var volume = Math.PI * Math.pow(radius, 2) * distance;
var rate = volume / time;
resultArea.style.display = 'block';
messageBox.className = 'success-box';
messageBox.innerHTML = 'Total Water Volume Uptake:' +
'' + volume.toFixed(3) + ' mm³' +
'Rate of Water Uptake:' +
'' + rate.toFixed(3) + ' mm³/min';
}