Crop Water Use Efficiency Explained

What is Crop Water Use Efficiency?

Generally, water use efficiency can be defined in many ways: economical, conveyance, or field application. These approaches focus on the management aspects of a crop and are important in farms, as they deal with investment, engineering, and soil characteristics that influence water supply to crops.

More specifically, crop water use efficiency refers to the response of crops to soil water availability.

Crop water use efficiency (WUE) is defined as the amount of carbon–as biomass or grain–that is produced per unit of water used by the crop. Crop water use efficiency can be measured at the leaf, plant, or canopy scale.

It is possible to increase WUE in a field without increasing water supply by altering the mechanisms that control WUE in plants. So, in tandem with engineering and management improvements to increase water use efficiency, scientists are hard at work enhancing plants’ ability to produce more under limited water supplies.

What constitutes yield will differ among crops. It can be seeds, fruits, fiber, or the whole plant.

There is a marked difference between WUE among crop species:

• Cereals produce 2.37 kg of grain per cubic meter (m−3) of water
• Oilseeds produce 0.69 kg m−3 water
• Fiber crops give 0.45 kg m−3 water
• Legumes yield 0.42 kg m−3 water

The effect of external environmental factors like temperature, relative humidity, and soil water availability will affect plant physiology responses. Maize has the highest WUE in irrigated conditions, and sorghum has the best WUE under rain-fed conditions among the cereals.

While developing and discovering new cultivars with improved WUE, the attributes scientists look for can vary.

Transpiration and Photosynthesis Influence Crop WUE

To improve crop WUE, water must be conserved and yield or dry matter production must be increased by reducing transpiration and increasing photosynthesis, respectively. All factors that influence these two processes will also affect WUE.

About 97-99% of the water absorbed by plants is lost due to transpiration, so water use can be defined by the rate of transpiration. Sometimes the rate of evapotranspiration, which is a combination of plant transpiration and evaporation from the soil, is also used to calculate WUE; see Figure 1.

Photosynthesis, on the other hand, is the assimilation of carbon dioxide to form photosynthates that are used to produce energy and all the other bio compounds necessary to build and maintain the structure of the plant and other plant organs. 

Stomatal Conductance is the Gateway for Gases

Connecting transpiration and photosynthesis is stomatal conductance rate, which is measured as the rate of water vapor exiting or CO2 coming in through the stomata. When stomata are open because of higher vapor potential in leaves in comparison to the vapor pressure deficit in air, more water vapor moves out and more CO2 moves in. When stomata are closed due to low plant water potential, loss of water is prevented but carbon dioxide supply to plants is also limited.

Low stomatal conductance, when stomata are closed, results in less photosynthesis and ultimately biomass accumulation.

Different Pathways of Carbon Assimilation

Some plants have developed different types of photosynthesis to get around this problem caused by low stomatal conductance.

There are three types of pathways that end up influencing WUE: C3, C4, and Crassulacean acid metabolism (CAM).

C3 and C4 plants have many similarities. Both get light and absorb CO2 from the air, during the day.

C3 plants use the Calvin Benson cycle during the dark reaction to fix the CO2 in the mesophyll cells of the leaves.  When the stomata are closed, O2 cannot move out and reacts with Rubisco, the enzyme that fixes CO2. So, instead of a 3-carbon sugar, phosphoglyceric acid (PGA), the first product of photosynthesis, a two-carbon compound is formed. This initiates a process called photorespiration, as the two-carbon compound is recycled, producing the enzyme and CO2. Part of the fixed CO2 is released again and energy is wasted in the process; see Figure 2.

Open stomata allow oxygen to move out and prevent photorespiration, but also allow water loss.

Thus, the amount of biomass produced per unit of water is less, as the water conserved by closed stomata lowers the photosynthetic rate. Nearly 85% of plants use the C3 photosynthetic pathway like rice, soybeans, and trees, etc.

Both dry conditions and high temperatures result in more photorespiration when more O2 reacts with Rubisco. At temperatures below 30oC, Rubisco has a greater affinity to CO2.

Avoiding Photorespiration

Plants that have evolved C4 and CAM pathways manage to provide high concentrations of CO2 to Rubisco and overcome photorespiration.

C4 Plants

The C4 plants, which have independently developed 66 times in various plant lineages, use an additional separate cycle, and photosynthesis is split into two processes that occur in different tissues in the leaves.

The CO2 taken in is converted by phosphoenolpyruvate carboxylase (PEPCase) into oxaloacetate (OAA), which is metabolized to malate, a four-carbon compound, in the mesophyll tissue; see Figure 3. This is called the C4 cycle, as the first product of photosynthesis is a four-carbon compound.

The Rubisco is present in C4 plants in the bundle sheath and not the mesophyll to avoid photorespiration. Malate is moved into the bundle sheath, where it is oxidized to a 3-carbon compound. The free fourth CO2 is fixed, as usual, by the Calvin cycle to form sugars. By avoiding photorespiration, plants use solar energy and water more efficiently, even though a few ATP molecules are used to move malate into the bundle sheath.

C4 plants use half as much water as C3 plants, due to their leaf structure and stomata shape. Moreover, C4 plants maintain a high Co2 gradient between the inside of the leaves (~10ppm CO2) and the outside air (~400ppm CO2). When atmospheric CO2 is higher, the stomata tend to remain closed, thus less water vapor is lost.
C4 plants are more common in warmer climates than cooler regions because the enzyme PEPcase is less active in lower temperatures. Maize, sorghum, and sugarcane are examples of C4 plants. Even though only 3% of plants use the C4 pathway, they produce 20% of global gross production.

CAM Plants

Crassulacean acid metabolism (CAM) occurs in plants found in hot and dry regions, like cacti or pineapple.

These plants have segregated photosynthesis not between sites, but temporally. CO2 fixation accompanied by O2 formation happens at night and the Calvin cycle occurs during the day. During the day, the stomata remain close due to heat and low humidity. Stomata open at night; the CO2 taken in is converted into malic acid by the enzyme PEP carboxylase and stored in the mesophyll.

During the day, CO2 is released from the organic acid and is fixed by the Calvin cycle. Since the stomata are open when humidity is higher and temperatures are lower, there is less water loss through transpiration. Moreover, these plants also have reduced leaf growth or no leaves to lower transpiration losses. This photosynthetic pathway loses the least amount of water vapor and has the highest WUE.

Increasing WUE in Crops

Scientists are trying to increase WUE by choosing genotypes that have the best combination of morphological, physiological, and biochemical attributes to reduce transpiration and photorespiration while increasing photosynthesis.

Improved leaf and plant water use efficiency is due to higher photosynthetic rates per unit leaf area and lower stomatal conductance.

The enzyme rubisco attracts a great deal of scientific effort; however, initial attempts to stop its ability to fix O2 also resulted in its failure to fix CO2. Research on the Rubisco enzyme continues.

Some other promising venues that scientists have tried successfully are to

• increase and improve the distribution of roots to access available water,
• optimize leaf area to balance stomatal conductance and density but maximize photosynthesis,
• choose the PEP carboxykinase (PCK) among the three subtypes found in C4 plants, as it gives the most biomass,
• and recognize that WUE under elevated CO2 conditions depends more on reduced stomatal conductance than raising the photosynthetic rates.

In addition, to enhance crop WUE, external field conditions can also be improved by the following:

●    Crop management through optimal nutrient, irrigation, and tillage practices.
●    Choosing equipment and precision methods that optimize irrigation

Measuring Crop WUE

It is possible to measure leaf and plant scale gas exchanges, which are important for WUE, using small, portable Infrared Gas Exchange Analyzers (IRGAs). For example, the CI-340 Handheld Photosynthesis System, manufactured by CID Bio-Science Inc., can measure photosynthetic, transpiration, stomatal conductance, and internal CO2 concentrations to estimate WUE. Through the use of such tools, scientists are finding ways of improving WUE. To produce enough food for a growing population with limited land, water, and resources, these improvements are becoming more and more vital.

—

Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

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