1. What is WUE?
Water Use Efficiency (WUE) is a critical parameter in agronomy, plant physiology, and environmental sciences, representing the ratio of biomass produced (or yield) to the amount of water used during the process. It measures how effectively a plant uses water for growth and development. WUE is typically expressed in units such as grams of biomass per kilogram of water or yield per unit of water consumed.
High WUE indicates that a plant can produce more biomass or yield with less water, which is particularly important in arid and semi-arid regions where water resources are limited. Enhancing WUE is a key objective in crop breeding and agricultural management to ensure sustainable food production under water-scarce conditions.
2. How to Measure WUE?
Several methods can be employed to measure or calculate WUE, each with its own advantages and limitations:
Agronomic Gravimetric Method:
Description:
This method involves measuring the plant's or yield's weight and the total irrigation amount delivered to the field/plant.
Calculation:
WUE = Total Biomass (g) / Total Water Used (kg).
Procedure:
The total water supplied to the plants is recorded.
At the end of the experimental period, the total biomass or yield is harvested and weighed.
Advantages:
Simple, direct measurement.
Disadvantages:
Subject to overestimation of irrigation data, because it accounts for the water supplied to the plant rather than the water actually used by the plant.
Labor-intensive
Not suitable for dynamic or real-time measurements.
Gas Exchange Measurements:
Description:
This method involves measuring the rate of photosynthesis and transpiration using gas exchange analyzers.
Calculation:
Instantaneous WUE = A / E.
Procedure:
Leaf-level measurements are taken using a portable gas exchange system.
Transpiration rate (E) and photosynthetic rate (A) are recorded.
Advantages:
Provides real-time data and can be used to study the physiological responses of plants to environmental changes.
Give the molecular ratio between water and CO2.
Disadvantages:
Limited to leaf-level measurements and may not represent whole-plant WUE.
Give only momentarily measurement.
Measure only one plant at a time
Isotopic Method:
Description:
This method involves analyzing the carbon isotope composition (δ13C) of plant tissues.
Calculation:
δ13C values are correlated with WUE based on known relationships.
Procedure:
Plant samples are collected and analyzed using mass spectrometry.
The ratio of 13C to 12C is used to infer WUE.
Advantages:
Provides integrated WUE over the plant's lifetime and can be used for field-grown plants.
Disadvantages:
Requires specialized equipment and expertise.
Measure only one plant at a time
3. How Plantarray Improves the Accuracy of WUE Measurement
Plantarray improves WUE measurement by refining the gravimetric approach and adjusting the two key factors in the WUE calculation: plant weight and water usage. First, the plant’s weight is measured at the end of the experimental phase, either as fresh weight using the Plantarray system or manually as dry weight (oven dried), if needed. This weight is then divided by the total cumulative transpiration of the experiment, providing the plant transpiration-water use efficiency. Thus, this method accurately reflects the water actually used by the plant for growth, as opposed to the classic agronomic gravimetric method which overestimated the irrigation data.
After retrieving the WUE values, they can be inserted back into the SPAC-Analytics to gain information on the plant's weight during the experiment.
4. How to Measure Plantarray's Dynamic WUE
Description:
Plants excels in adapting their performance according to the constantly changing environment, therefore a single averaged WUE value for the entire growth period, is probably not representing the actual WUE during all the growth stages of a crop, the following method is necessary to accurately measure dynamic WUE.
Calculation:
SPAC-Analytics defines WUE as the ratio between the accumulated plant biomass gain and accumulated water lost by the plant through transpiration every day, over a selected period of time.
WUE = i=1daysPlant Growthi i=1daysDaily Transpirationi
The WUE is calculated as the slope of the linear regression from the start to the end of the accumulation time
Procedure:
For this part a daily accurate measurement of plant's net weight or plant growth is crucial, as well as daily transpiration of the plants.
The process to get a valid WUE values and accurately measures the accumulated plant growth, requires several consecutive days of proper night irrigation. This involves at least 2-3 pulses of irrigation per night with ensured drainage between pulses. The weight at dawn (after irrigations are completed and before sunrise) should not decrease between two consecutive days.
For example, if WUE is calculated over a period of 5 days, plant growth and daily transpiration must have positive values each day, as can be seen in the graph below.
Figure 1 - a 5 days period of accumulated plant growth vs accumulated daily transpiration downloaded from SPAC-Analytics. The linear regression result shows the slope of the fitted line, as the WUE value (0.048) with high R2 (0.997), indicating a strong correlation between plant growth and transpiration
Advantages:
Real-time Data: Provides continuous monitoring and real-time analysis of WUE.
Comprehensive Analysis: Integrates multiple physiological parameters for a holistic view of plant performance.
Dynamic Measurement: Allows for the study of WUE under varying environmental conditions and throughout the growth period.
Automation: Reduces the need for manual measurements and minimizes human error.
Disadvantages:
Drought Measurements: During drought phases, the plant's gross weight typically decreases due to water shortage. This reduction in gross weight makes it impossible to calculate WUE, as the decrease in weight does not necessarily reflect a decrease in biomass but rather a decrease in the plant's soil water content. Consequently, the system may not provide reliable data on the plant's water use efficiency under drought conditions.
Growing Medium Sensitivity: The accuracy of the system depends on the type of growing medium and its ability to return to the pot's water capacity accurately. This means the drain needs to be fast, and water pore variations should be low. It works well under high hydraulic conductivity substrates like sandy media, where water drains easily, whereas, it is challenging to achieve in organic substrates such as peat based potting soil due to its ability to constantly increase its volume meaning increase its water capacity.
Root-Soil Water Capacity: A given soil will retain more water without a plant than with a plant. This is because plant roots occupy some of the soil volume, thereby reducing the soil's water-holding capacity. This phenomenon can be referred to as " over water capacity”. Thus, the root expansion reduces the pot's overall water capacity over time, and consequently, while the total gross weight of the pot decrease, the plants themselves are actually developing and increasing in weight. This shift makes it more challenging to accurately calculate the plant's net weight, as the reduction in gross weight can be misinterpreted without considering the growing root system's impact on water availability. Important to note, that this issue occurred less when using inert, low-porosity mineral substrate like sand, as well as plants that established their root system in the pot
One solution to minimize over water capacity is to start measurements when the plant’s roots are well established in the pot and have increased their transpiration.
Alternative Method:
Independent Plant Growth tracking, is an alternative method to measure plant net weight, where additional plants, grow alongside the Plantarray experiment. And destructive canopy weight measurement, are taken daily (or every few days) to understand the canopy growth patterns. Eventually, this method is utilized the plant’s transpiration read by the system, and normalize them to those destructive measurements.
Advantages:
High accuracy: Due to the simple manner of the independent weight measurement technique
Detailed Growth Trajectory: Provides comprehensive data on canopy growth patterns.
Pot and Medium Independence: Does not require special considerations for the pot’s drainage or the specific medium's performance regarding water capacity and hydraulic capability, simplifying the Plantarray's experimental procedure.
Drought Measurements: Independently tracking the plant's net weight, makes it possible to measure changes in biomass even when the gross weight decreases due to water stress. This allows for a precise calculation of WUE under drought conditions, providing valuable insights into how plants manage water use efficiency during periods of limited water availability.
Disadvantages:
Labor-Intensive: Requires significant manual effort and time for daily or frequent destructive measurements.
Resource-Demanding: Requires additional plants and space