How Improving Transpiration Efficiency in Crops Boosts Yield

• Transpiration is essential for crop survival, growth, and productivity, but high rates cause significant water losses.
• The effect of environment and genetics on transpiration is used to dodge adverse effects on yield.
• Mechanisms and traits like antitranspirants, limited transpiration, and transpiration efficiency can improve yields with less water use.

Agriculture uses nearly 75% of global water resources. Producing more food with less water is urgent, as water resources are scarce and demand from other economic sectors is high. Plants lose water through transpiration, so this process is in focus, especially since water-use-efficient cultivars are lacking for several crops and regions. Find out how transpiration influences yield and how scientists are trying to limit its effects on crop productivity.

Importance of Transpiration

Transpiration is the loss of plant water vapor, mainly through leaf stomata. Plants can lose up to 97-99% of the water they absorb through the roots.

Transpiration influences crops survival, growth, and productivity because of its following roles :

• It creates a negative water pressure in the leaves, pulling water and nutrients from the soil.
• Transpiration cools the plant, helping it survive heat and drought.
• It is necessary for cell expansion and vertical plant growth.
• Transpiration regulates the carbon dioxide (CO2) available for photosynthesis, as gas exchange occurs through the stomata.

When the stomata close during water stress, photosynthetic rate and growth are reduced, and survival is affected when cooling doesn’t occur in heat stress. The trait transpiration efficiency is used to estimate the influence of transpiration on crop yield; see Figure 1.

Figure 1: “The relationship between crop dry matter production or grain yield (Y axis) and Crop ET or evapotranspiration (X axis) can be represented as in (A), with the slope of the line representing transpiration efficiency,” Unkovich et al. 2018. (Image credits: http://dx.doi.org/10.1016/j.agwat.2018.04.016)

Transpiration efficiency in Crops

Transpiration efficiency (TE), which measures transpiration in relation to crop productivity, can be estimated at various scales as follows:

• At the plot level, it is described as water-use efficiency (WUE), equal to grain yield/water received through rainfall or irrigation. It is also expressed as total biomass/evapotranspiration.
• At the plant level, TE = biomass/water transpired. Some authors also define plant TE as net shoot dry matter gain/water transpired or net shoot dry matter gain/total water use.
• At the leaf level, TE equals immediate CO2 assimilation (A)/ transpiration (T) or A/T.

A comparison of cultivars used in 1860 and 1986 showed no difference in TE between older and modern cultivars. However, the new cultivars have a shorter lifespan. Faster phenological development and a higher shoot-to-root ratio reduce total crop water use and produce a higher harvest index.

Due to the increasing population and water scarcity, the emphasis is currently on improving the TE, which depends on environmental and plant genetic factors.

Influence of Environment

External factors like temperature, humidity, light intensity, and wind speed will regulate transpiration rate and control yield.

Vapor pressure deficit (VPD), the difference in water vapor pressure in the atmosphere and leaves, is a significant driver of transpiration. High temperature, wind speed, light intensity, and low humidity raise VPD and increase transpiration. A low VPD of less than 0.3 kPa provides less water and nutrients for plants and is suitable only for cuttings’ propagation. A VPD between 0.5 and 1.5 kPa provides the best transpiration to maintain plant functions and crop productivity without excessive water loss. VPD over 1.5 kPa results in high transpiration, which is good only if adequate soil water content is used to compensate for the heavy water loss.

CO2 levels and exchange with plants are stable. However, the ratio of CO2 to water vapor changes when humidity changes. Hence, the relative rate of CO2 and H2O exchanged between plants and air will change. Also, more CO2 in the air will close stomata and reduce transpiration.

High VPD causes heavy water losses per unit of CO2 fixed, leading to stomatal closure in all plants, which results in carbon starvation and hydraulic failure.

Table 1: “Mean plot TE or transpiration efficiency of some crop species (kg shoot dry matter/ha/mm water transpired),” Unkovich et al. 2018. (Credits: http://dx.doi.org/10.1016/j.agwat.2018.04.016)

Influence of Crop Species

Transpiration efficiency differences among crop species are chiefly between C4 and C3 plants. Crops with a C3 photosynthetic pathway have TE that is 30% lower than C4 plants. The reason is the high photosynthetic efficiency and lower respiratory losses of C4 crops like maize, millets, sorghum, and sugarcane, with a TE of 73–105 kg/ha/mm. The C3 plants with lower photosynthetic rates and dry matter accumulation have a TE of only 34–55 kg/ha/mm. However, the TE difference between C3 and C4 plants will be moderated by the region in which they are grown and its environments.

Controlling Transpiration

The relationships between VPD, genetics, and transpiration are manipulated while producing new cultivars so that CO2 fixation is maximized and transpiration is lowered. Different methods can achieve these results. Some chief methods used are discussed below.

1. Antitranspirants

A standard trait is using antitranspirants to control transpiration and improve plant water status and crop productivity. The three main types of antitranspirants are -metabolic, reflective, and films. Antitranspirants have been used successfully to ameliorate drought and improve rapeseed, soybean, sugarbeet, and wheat yields. However, the exact plant mechanisms that reduce water-loss responses through antitranspirant use are still unknown.

2. Genetically Improving TE

Significant genetic variations that exist to restrict transpiration have been observed within crop species like soybean, chickpea, cowpea, peanut, sorghum, pearl millet, and wheat. It provides a venue to reduce transpiration and water losses even during high VPD. As shown in Figure 2, drought-resistant cultivars reduce transpiration compared to drought-sensitive ones.

When transpiration is measured throughout the crop cycle, it is clear that timing is crucial. The difference in water use and TE arises due to the restriction of water loss later in the crop cycle to ensure water availability during the critical stages of grain-filling. Hence, the focus is on reducing transpiration at essential stages for efficient water use.

Figure 2.: “Transpiration response to a ladder of increasing VPD conditions, in a terminal-drought-sensitive (H77/833-2, open circles) and a terminal-drought-tolerant (PRLT-2/89-33, closed circles) pearl millet genotype,” Vadez et al. 2014. (Image credits: https://doi.org/10.1093/jxb/eru040)

3. Limited transpiration (LT)

Limited transpiration (LT) is another concept that scientists are trying out. Hypothetically, it is a trait that restricts water use during high VPD, which occurs commonly around midday. It also seeks to reduce water use during the vegetative stages to ensure more soil water for later use during grain-filling. It is expected to help plants in drought conditions only and not in ideal situations, like well-irrigated fields or high rainfall, as stomata closure will reduce CO2 capture during this time.

Crop model simulations for rainfed sorghum, soybean, lentil, maize, chickpea, and wheat have shown that this trait could improve yields by 6-10% in severe drought but less than 5% in irrigated conditions.

Measuring Transpiration Efficiency in Crops

Given the increasing importance of transpiration due to climate change in current and future situations, it is crucial to have reliable estimation methods. The CI-340 Handheld Photosynthesis System, produced by CID Bio-Science Inc., offers a rapid, accurate, and non-destructive method of estimating transpiration and photosynthesis at the plant level. Instruments like this can be an asset for scientists in the field to improve crops and food production.

Sources

Mphande, W., Farrell, A. D., Vickers, L. H., Grove, I. G., & Kettlewell, P. S. (2024). Yield improvement with antitranspirant application in droughted wheat associated with both reduced transpiration and reduced abscisic acid. The Journal of Agricultural Science, 162(1), 33–45. doi:10.1017/S0021859624000157

Rubí, R., Greg, M., Sarah, S-B., Alexander, E. L , & Geoffrey, P. M. (2024). Crop modeling suggests limited transpiration would increase yield of sorghum across drought-prone regions of the United States. Frontiers in Plant Science, 14. DOI=10.3389/fpls.2023.1283339

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Unkovich, M., Baldock, J., & Farquharson, R. (2018). Field measurements of bare soil evaporation and crop transpiration, and transpiration efficiency, for rainfed grain crops in Australia–A review. Agricultural water management, 205, 72-80.

Vadez, V., Kholova, J., Medina, S., Kakkera, A., & Anderberg, H. (2014). Transpiration efficiency: new insights into an old story. Journal of Experimental Botany, 65(21), 6141-6153.

van Herwaarden, A. F., & Passioura, J. B. (2001). Using harvest index to diagnose poor water use efficiency. Australian Grain, 11(5), 3-6.

Ward P. R., Hall D. J. M., Micin S. F., Whisson K., Willis T. M., Treble K., Tennant D. (2007) Water use by annual crops. 1. Role of dry matter production. Australian Journal of Agricultural Research 58, 1159-1166. https://doi.org/10.1071/AR07076