Equation for Calculating Rate of Transpiration

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Rate of Transpiration Calculator

Distance moved by air bubble in millimeters (mm)
Internal radius of the tube in millimeters (mm)
Duration of observation in minutes (min)
Surface area in square centimeters (cm²)

Results

Water Volume Lost: –
Rate of Transpiration: –
Rate per Unit Area: –

Understanding the Equation for Transpiration

Transpiration is the biological process by which water moves through a plant and evaporates from aerial parts, such as leaves, stems, and flowers. Measuring the rate of transpiration is crucial for understanding plant physiology, water use efficiency, and responses to environmental stress.

The Potometer Method

The most common laboratory method for estimating transpiration rate is using a potometer. This device measures the rate of water uptake by a cut plant shoot. While water uptake is not strictly identical to water loss (transpiration), approximately 99% of the water taken up is lost via transpiration, making this an accurate approximation.

Calculating Water Volume

Before calculating the rate, we must determine the volume of water consumed. Since the water moves through a cylindrical capillary tube, the volume ($V$) is calculated using the geometry of a cylinder:

V = π × r² × d

Where:

  • V = Volume of water ($mm^3$ or $\mu L$)
  • π = Pi (approx. 3.14159)
  • r = Radius of the capillary tube ($mm$)
  • d = Distance the air bubble moved ($mm$)

The Rate Equation

Once the volume is known, the rate of transpiration ($R$) is calculated by dividing the volume by the time elapsed ($t$):

Rate = V / t

The standard unit for this measurement is microliters per minute ($\mu L/min$).

Normalizing for Leaf Area

To compare transpiration rates between different plant species or different sized samples, it is essential to normalize the data by the total leaf surface area. This provides the rate of water loss per unit of surface area:

Rate per Area = Rate / Leaf Area

Factors Affecting Transpiration Rate

  • Light Intensity: Stomata open in light to allow gas exchange for photosynthesis, increasing transpiration.
  • Temperature: Higher temperatures increase the rate of evaporation and the water holding capacity of the air.
  • Humidity: High humidity reduces the water potential gradient between the leaf and the air, slowing down transpiration.
  • Wind Speed: Wind removes the boundary layer of moist air around the leaf, increasing the rate of transpiration.
function calculateTranspiration() { // 1. Get input values var distance = document.getElementById('distanceMoved').value; var radius = document.getElementById('tubeRadius').value; var time = document.getElementById('timeElapsed').value; var leafArea = document.getElementById('leafArea').value; // 2. Validate inputs if (distance === "" || radius === "" || time === "") { alert("Please fill in Distance, Radius, and Time to calculate."); return; } distance = parseFloat(distance); radius = parseFloat(radius); time = parseFloat(time); // Handle Leaf Area (optional) if (leafArea === "") { leafArea = 0; } else { leafArea = parseFloat(leafArea); } // Basic validation for negative numbers or zero time if (distance < 0 || radius < 0 || time <= 0 || leafArea 0) { ratePerArea = rate / leafArea; } // 4. Format and Display Results var resultDiv = document.getElementById('resultsDisplay'); var volumeSpan = document.getElementById('volumeResult'); var rateSpan = document.getElementById('rateResult'); var areaRateSpan = document.getElementById('areaRateResult'); // Display logic volumeSpan.innerHTML = volumeMm3.toFixed(2) + " mm³ (µL)"; rateSpan.innerHTML = rate.toFixed(3) + " µL/min"; if (leafArea > 0) { areaRateSpan.innerHTML = ratePerArea.toFixed(4) + " µL/min/cm²"; } else { areaRateSpan.innerHTML = "Leaf Area not provided"; } // Show the result box resultDiv.style.display = "block"; }

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