Measuring Tomato Plant Transpiration: Finding the Right Tools to Estimate Greenhouse Ventilation
Ahmad Tusi, Ph.D.
Gifu, Japan - Greenhouses are evolving fast, and understanding how air moves inside them is more important than ever. Whether you're growing tomatoes in Japan, Melon in Indonesia, or cucumbers in The Netherlands, having precise control over the microclimate is key to boosting productivity. A recent study by Ahmad Tusi (Lampung University, Indonesia) and Teruaki Shimazu (Gifu University, Japan) dives deep into how we can measure transpiration in real-time to improve greenhouse ventilation estimates using something called the Water Vapor Balance (WVB) method.
This blog takes you through their findings in a more relaxed and digestible way, showing how different tools stack up when it comes to monitoring how much water plants lose through transpiration—and why that matters.
Why Measure Transpiration in the First Place?
Plants "breathe" by losing water through their leaves—a process called transpiration. Inside a greenhouse, this water vapor adds up and influences humidity, temperature, and, more importantly, ventilation needs. If we can measure transpiration rates accurately, we can estimate how much fresh air the greenhouse needs to bring in. This is crucial for plant health, disease prevention, and optimizing CO2 levels for photosynthesis.
One way to do this is through the Water Vapor Balance (WVB) method, where scientists calculate the difference in water vapor inputs and outputs to figure out how much air is moving in and out of the greenhouse.
The Tools Put to the Test
In the experiment, conducted at Gifu University, Japan, the researchers tested four different ways to measure transpiration on tomato plants:
Electronic Weighing Device (Control) – Think of this like a high-precision scale that detects even tiny changes in plant weight.
Sap Flow Sensor (SF) – This tool wraps around the plant stem and measures water movement inside the plant.
Water Level Sensor (WL) – Monitors changes in water level inside the plant pot.
Water Flow Meter (WF) – Measures how much water flows to the plants in the irrigation system.
Each method was tested over different seasons and plant setups, and data were recorded every minute or every hour depending on the tool.
What They Found
1. Sap Flow Sensor vs. Electronic Weighing
The sap flow sensor had a strong correlation with the electronic scale (r = 0.89), meaning it did a pretty good job. It was slightly lower in readings due to minimal water loss from soil evaporation (about 6.3%), but overall, it was reliable for short-term, real-time monitoring.
Bonus: The sap flow method is less sensitive to external disturbances like people walking around.
2. Water Level Sensor: Somewhat Accurate, But Sensitive
This method showed a moderate correlation (r = 0.73) to the control. While it captured daily trends, it had scattered data and was highly sensitive to vibrations (like people working in the greenhouse). Plus, it had time lag issues for short-interval measurements.
Verdict: Good for long-term trends, not so great for minute-by-minute accuracy.
3. Water Flow Meter: Reliable for Total Water Use, Not Real-Time
This method tracked how much water was delivered to the plant beds. It worked well in measuring total daily transpiration, especially under sunny conditions. However, its correlation with the sap flow sensor was lower (r = 0.59), mostly because it struggled to respond quickly to short-term changes.
On cloudy days, the water flow meter had delays in matching the actual transpiration pattern.
 |
| Sap Flow Performance |
Daily vs. Real-Time: Why It Matters
One of the main challenges in greenhouse management is balancing short-term responsiveness (like sudden heat spikes) with daily averages (for long-term planning). Tools like the sap flow sensor and electronic weighing device allow growers to detect changes every minute, making them perfect for active climate control.
Meanwhile, water level and flow sensors are more suited to checking overall water consumption, which can still be useful but may lag behind in rapidly changing conditions.
Pros and Cons at a Glance
| Method | Accuracy | Time Resolution | Strengths | Weaknesses |
|---|---|---|---|---|
| Electronic Scale | ★★★★ | Minute-level | High accuracy, industry standard | Sensitive to external movements |
| Sap Flow Sensor | ★★★★ | Minute-level | Real-time data, stem-level measurement | Needs regular calibration |
| Water Level | ★★☆☆ | Hourly | Simple setup, tank-based | Sensitive, less accurate in short-term |
| Water Flow Meter | ★★☆☆ | Daily | Good for total water use | Time lag, not real-time |
⭐ Legend: ★ = Fair, ★★ = Moderate, ★★★ = Good, ★★★★ = Excellent
So, Which One Should You Use?
If you're managing a high-tech greenhouse and need real-time ventilation control, go with the sap flow sensor or electronic weighing device. They provide the detail and speed needed to adjust conditions dynamically.
If budget is a concern, and you're more interested in daily irrigation planning, then the water flow meter might be enough. But be aware of its limitations during periods of sudden weather changes.
Final Thoughts: Smarter Ventilation Starts with Smarter Measurement
This study reinforces one key point: not all transpiration measurement tools are created equal. Depending on your goals, budget, and the level of precision required, different tools may serve you better.
With the increasing availability of affordable sensors and data loggers, integrating these technologies into greenhouse systems has never been easier. In the end, whether you're a researcher or a commercial grower, understanding how plants lose water is a crucial step to managing the environment they grow in.
Because when it comes to greenhouse farming, every drop (and every gram of water vapor) counts.
Based on the research: "Real-Time Comparison of Several Transpiration Methods for Estimating Greenhouse Ventilation Rate via Water Vapor Balance Method" by Ahmad Tusi & Teruaki Shimazu, INMATEH Agricultural Engineering Journal, Vol. 63(3), 2021. link of paper: https://doi.org/10.35633/inmateh-63-09
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