What Is Transpiration Efficiency and Why Does It Matter for Drought-Resistant Farming?

• Transpiration efficiency is emerging as a crucial crop parameter to improve production in water-limited conditions.
• Restricting transpiration by crops is a vital strategy to increase productivity using less water.
• External factors like VPD, soil type, and nutrient status are also necessary to increase transpiration efficiency.

Society must figure out ways to grow more food for an expanding population. However, water resources are limited and growing scarcer due to climate change. The challenge, therefore, is to produce more food for each unit of water used in irrigation. Transpiration efficiency is one of the crop measures that can be used to achieve this goal. In this article, you will discover transpiration efficiency and the factors that control it.

What is Transpiration Efficiency?

Transpiration efficiency (TE) can be estimated at plot, plant, and leaf levels. Transpiration efficiency at the plant level has become a crucial crop characteristic, especially in decreasing water resources for food production. So, the standard transpiration efficiency is described as the net shoot dry matter produced per unit of water transpired by the crop. Some calculations also consider root dry matter, so TE is the net total dry matter produced for water transpired.

The formula to calculate TE is as follows:

TE=0.6C_a(1–C_i/C_a)/(W_i–W_a),

C_a and W_a are ambient CO2 and vapor pressure, and C_i and W_i are the CO2 concentration and vapor pressure in the stomatal chamber.

Transpiration is one of the plant’s vital physiological functions. It cools the plant, pulls water and nutrients into it, and determines the time the stomata are open for photosynthesis. However, too much transpiration results in substantial soil water use and loss, which we need to curtail to produce more food with decreasing amounts of available water by reducing irrigation.

The cumulative transpiration in crops is linearly related to the total dry matter accumulated for a specific site and season. TE depends mainly on the species

The transpiration of some common crops is as follows:

Wheat: 3.1 to 6.7g kg−1

Barley: 3.2 to 5.7 g kg−1

Oats: 2.9 to 4.5 g kg−1

Rice: 2.2 to 5.4 g kg⁻¹

However, several factors will influence TE, root system, ontogeny, and carbon allocation even within a crop. VPD, soil type, and nutrition status are some external factors that can influence TE.

External Factors Affecting Transpiration Efficiency

Comparing TE across species and conditions is challenging, mainly because VPD changes with the environment (see Figure 1).

VPD

VPD is the difference between air’s water vapor and what it can hold when it is saturated. It is measured as kilopascal (kPa). A high VPD of over 1.0 kPa indicates that the air can hold more water as it is dry; plants near the saturation point have a significant water difference to air, resulting in more transpiration and soil moisture loss. At low VPD, the air is nearly saturated, and transpiration is low. Zero VPD is unsuitable for plants as they can’t transpire.

The VPD increases with temperature and will be influenced by climate change (see Figure 1). At high VPD, transpiration rises due to low stomatal conductance up to a certain point, reducing carbon assimilation through photosynthesis in most species. At low VPD, there is more photosynthesis but less water and nutrient absorption due to low transpiration. Hence, a moderate  VPD is better for crop growth and productivity.

Scientists had initially considered that the VPD depends mainly on environmental factors. However, responses to VPD vary with plant types and species. For example, among C4 cereals, pearl millet and sorghum are more resistant to higher temperatures and drought than maize. Also, genotypes across species can restrict transpiration even under high VPD. However, genotypes can vary in stomatal sensitivity within species. So, a genetic component to plant response to VPD can be used in future crop breeding to develop cultivars with efficient TE.

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Figure 1: “Trend in annual vapor pressure deficit (VPD) for the period 1901–2017, estimated using VPD calculated from air temperature and vapor pressure from the Climate Research Unit (CRU) v.TS 3.26 (Harris et al., 2014) (a), and percent change in VPD relative to 1901 average for the regions given here. Bold lines have a 10-yr smoothing function applied (United States (USA), Canada (CAN), Central America (CAM), Northern South America (NSA), Brazil (BRA), Southwest South America (SSA), Europe (EU), Northern Africa (NAF), Equatorial Africa (EQAF), Southern Africa (SAF), Russia (RUS), Central Asia (CAS), Mideast (MIDE), China (CHN), Korea and Japan (KAJ), South Asia (SAS), Southeast Asia (SEAS), Oceania (OCE)) (b),” Grossiord et al. 2020. (Image credits: https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.16485)

Nutrient status

Soil nutrient deficiency can lower TE. The addition of inorganic fertilizers doesn’t seem to have any effect on crop TE, though it does reduce soil water evaporations. Several studies show that adding organic manures to the topsoil or subsoil can increase TE. Similarly, adding beneficial bacteria like Bacillus subtilis or B. licheniformi increases photosynthesis and reduces water stress through partial stomatal closure. Organic amendments improve soil physical and biological properties to enhance crop water use efficiency and boost TE during moderate stress. The change in TE is not connected to more nitrogen absorption or leaf nitrogen status.

Figure 2: “Mean transpiration efficiency (TE) for maize, sorghum, and pearl millet grown in different soil types in lysimeters in the field under well-watered conditions. (A) Experiment 3 with relatively high vapor pressure deficit (VPD) and (B) Experiment 4 with relatively low VPD,” Vadez et al. 2021. (Image credits: https://academic.oup.com/jxb/article/72/14/5221/6291390)

Soil type

Soils have varying hydraulic capacities and different proportions of sand, loam, and clay. Hence, each soil has a different water-holding capacity. Sandy soils have a high matrix potential, allowing faster water movement in response to high VPD and producing a low TE. In contrast, soils with high clay content have lower matrix potential, restricting water movement and transpiration to give high TE. This effect on soils depends on crop species. Maize performs differently under various soils, but soil type does not affect pearl millet.

Under low VPD, soil type is not relevant.

The external factors do not work independently on TE but can be understood only in conjunction with various crop traits. For example, soil influence on TE is strongly connected with the type of crop root systems.

Internal Factors Affecting Transpiration Efficiency

Both root and shoot system traits are crucial for improving TE, as productivity depends on the optimum functioning of the whole plant.

Roots systems

Root systems’ contribution to soil type is vital to explaining differences in TE in varying soils. Maize doesn’t perform well in sandy soils, but pearl millet has better hydraulic conductance and can grow well in all soil types. However, scientists don’t know enough about this connection to explain the difference in TE in different soils.

The roots connect to the soil in the rhizosphere. The rhizosphere’s thin layer can break the water flow from soil to roots, causing the stomata to close and restrict transpiration. The possible root traits that are part of the hydraulic system in soils are the presence of root hairs and mucilage. Barley mutants without root hairs were more able to respond to drought than those with root hairs. Root mucilage produced in response to drought helps maintain root and soil connection. Root density, distribution, conductivity, anatomy, and root-to-shoot ratios could also influence hydraulic conductance.

Crop ontogeny

Ontogeny or growth stages can influence TE depending on the species. Sorghum plants have a constant TE from emergency to physiological maturity under controlled/irrigated conditions. However, 50% of the water supply is needed during grain filling, which accounts for one-third of the total crop cycle. Hence, dryland sorghum with less soil water during grain filling produces less yield, reducing the harvest index below a typical 0.50.

In corn, yield and TE were highest at the mid-grain filling stage and reduced thereafter. Therefore, farmers with limited water should prioritize irrigation before and during grain filling, similar to sorghum.

Source-sink allocation

Crops exposed to drought alter their carbon allocation depending on the presence or absence of harvestable products.

When the sinks or cobs are removed from maize in irrigated conditions, TE is substantially reduced, but this has less effect on sorghum and none on pearl millet. In sorghum, the root: shoot ratio of 0.3 in crops with grain increased to 0.7 in plants with no seeds.

Removing grains or the sink reduces biomass accumulation in high VPD conditions without affecting transpiration rates. Similarly, de-fruiting apples in summer increases water use efficiency (WUE) by closing the stomata, but it decreases WUE when de-fruiting is done at maturity later in the season. In summer, the biomass is diverted to other organs, and the stomata close to slow photosynthesis when excess glucose levels in the plants are sensed. The photosynthesis in aging leaves also decreases while transpiration rates are still high, leading to less WUE in later crop stages.

Crops like maize that have had more breeding efforts to strengthen sink effects show more TE variation than pearl millet with less research for cultivar development.

More Research on Shoot and Root Systems

More research is needed to understand the mechanisms driving TE in various crops. Both roots and shoots must be investigated, and precision tools will be required to help scientists collect and analyze data in the field in real time. CID Bio-Science Inc. supplies instruments relevant to TE research, like the CI-340 Handheld Photosynthesis System, which simultaneously measures transpiration, stomatal conductance, and photosynthesis, and scanners for use in minirhizotron systems, like the CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager.

Learn more about how CID Bio-Science Inc. can help in your crop TE research and improve food production.

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Grossiord, C., Buckley, T. N., Cernusak, L. A., Novick, K. A., Poulter, B., Siegwolf, R. T., … & McDowell, N. G. (2020). Plant responses to rising vapor pressure deficit. New phytologist, 226(6), 1550-1566.

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