The Role of Transpiration in Climate Change Adaptation Strategies

• Transpiration is a key physiological process maintaining plant form, water balance, temperature, and nutrient absorption.
• Climate change-driven transpiration alterations occur due to increased CO2 levels, temperature, water deficit, and precipitation patterns.
• Adaptation strategies in agriculture, which will be the worst hit sector by climate change, target transpiration rate and its changes to improve productivity in future scenarios.

Formulating adaptation strategies requires knowledge of climate change patterns and how and when their effects will occur. Plant productivity and growth rates will change due to alterations in plant physiological processes, moisture availability, CO2 fertilization, and rising temperatures. This article will focus on climate change’s influence on transpiration in driving adaptation strategies.

Transpiration Changes

Agriculture is the industrial sector that is worst affected by climate change. It impacts plant functions like photosynthesis, transpiration, respiration rate, water use efficiency, growth rate, and productivity.

Transpiration or loss of water vapor from plants to the atmosphere occurs mainly through leaf stomata, pores formed by guard cells. Stomatal conductance regulates all gas exchange between plants and the atmosphere. Plant processes affecting the movement of other gases like CO2 or O2 will affect the transpiration rate.

Atmospheric conditions like higher CO2 levels, temperature, soil water deficit, and vapor pressure deficit (VPD) decrease transpiration. Global increases in evapotranspiration are also expected.

Figure 1: Gas exchanges occurring through the stomata, NASA. (Image credits: https://www.upsbatterycenter.com/blog/climate-change-plant-transpiration/)

Strategy 1: Reducing Photorespiration to Benefit from CO2 Fertilization

Higher carbon dioxide (CO2) levels will result in CO2 fertilization and increase the photosynthetic rate. As the carbon assimilation rate rises, stomata close as the need to let in CO2 reduces. As a result, transpiration is expected to be reduced by 30% in crop plants.

Transpiration has a cooling effect on plants and their surroundings. Any reduction in transpiration increases leaf temperature causing stomata to close to save water absorbed from soils.

Closing of the stomata reduces CO2 levels in the leaves, and at higher leaf temperatures, CO2 becomes less soluble in chloroplasts and less available for photosynthesis. The enzyme Rubisco, which fixes CO2, starts reacting with oxygen, which is also sensitive. The process is energy intensive and breaks down ATPs to release CO2, called photorespiration. Since fixed carbon is released, this process can reduce yield by 20-40% in wheat and soybean.

CO2 fertilization occurs by ensuring there is more CO2 available for reaction with Rubisco, but increased temperatures impose a penalty through higher photorespiration. For example, rice yields rise by 0.5 t/ha for every 75 ppm increase in CO2 levels, but yield decrease by 0.6 t/ha for every 1 °C increase. Thus, any reduction in photorespiration will improve yield.

C3 plants, like rice, wheat, soybean, etc., will suffer from photorespiration, but not C4 plants, which have evolved to avoid photorespiration. Around 80% of crop plants have C3 photosynthesis and are expected to improve productivity from CO2 fertilization. However, since they also suffer from photorespiration, they will benefit from any reduction strategies. Therefore reducing photorespiration in C3 plants is one of the top priorities of climate change adaptation.

Strategy 2: Irrigation that Exploits Improvement in Water Use Efficiency

Reduction in transpiration by closing stomata due to higher CO2 levels and temperatures saves water absorbed by plants and makes it available for other functions in the plant, like maintaining water balance and turgor pressure, and use in photosynthesis.

Reduction in transpiration can reduce 5-20 percent of a plant’s water loss.

C4 plants respond to more CO2 in the air by partially closing their stomata and reducing transpiration without impacting photosynthesis. Hence, their water use efficiency also increases, and they need less carbon-fixed water consumption per unit.

C3 plants show higher water use efficiency since photosynthesis is increasing and water absorption from the soil is decreasing. However, there is a difference of 43 percent in transpiration rates based on crop age. At early stages, plants transpired less than later in the crop cycle for similar leaf area index. Recognizing these patterns could improve farm management by suggesting suitable irrigation based on crop stages.

Strategy 3: Crop Breeding to Choose Transpiration Limiting Traits

Direct reduction in transpiration also occurs due to water deficit in aerial and soil environments. Transpiration occurs when there is a vapor pressure deficit (VPD) or a gradient between drier outside environments and wetter interior leaf conditions. A higher VPD indicates hotter and drier conditions; a lower VPD means more humid and cooler outer conditions. A high VPD can result from dry air or soil water limitation.

Since the length of dry periods and the number of drought events will rise due to climate change, this reduction in the transpiration rate in rainfed agriculture in arid areas will impact the photosynthesis rate by limiting CO2 entry into leaves. The plant has less transpiration in the early stages of growth to save soil water for later use during the crucial reproductive phase. This strategy limits crop growth, and ultimately, crop productivity falls.

Many crops like maize, soybean, sorghum, peanuts, chickpea, cowpea, and pearl millet close their stomata during high VPD in the afternoon to reduce the transpiration rate. Species of crops will also show other strategies to minimize transpiration in dry conditions, like leaf area reduction.

For example, in an experiment, C4 crops, maize, pearl millet, and sorghum, reduced transpiration rate at high VPD. Pearl millet and sorghum also reduced leaf area, but this strategy was missing in maize, which has higher transpiration rates than the other two crops.

A great genotypic variation in transpiration-limiting traits has been reported in several crops, such as maize, sorghum, soybean, peanut, pearl millet, cotton, etc. Breeding programs can take advantage of this variation to find drought-proof cultivars with traits best suited for current and future regional conditions.

Strategy 4: Narrow Leaves to Keep Trees Cool

Figure 2: “Predicted leaf-level transpiration response for five genotypes to variations in leaf width (Lw). Results were generated using individual genotype-specific physiology parameters at constant environmental conditions (air temperature = 25 °C, relative humidity = 60%, photosynthetic photon flux = 1500 μmol·m−2·s−1, and wind speed = 5 m·s−1). Symbol locations depict the genotype transpiration at inherent Lw (reported in legend) and physiology. The non-linear lines that intersect each symbol illustrate the predicted genotype transpiration response to alterations in Lw. Vertical bars bracket the range of Lw observed across the genotypes in this study. Red maple genotype symbols are as follows: ‘Summer Red’ (○), ‘Autumn Flame’ (▽), ‘Red Sunset’ (△), ‘October Glory’ (□), and ‘Autumn Blaze’ (⋄),” Bauerle et al. (2011). (Image credits: HortScience horts 46, 2; 10.21273/HORTSCI.46.2.163)

Transpiration can be affected by a combination of the plant’s physiology and morphology. Occasionally, morphology can be the more critical factor overriding physiological processes. Also, in certain conditions,  plants must adapt to evapotranspiration.

While limiting transpiration is the aim for annual crops, trees exposed to light, as in orchards, benefit from more transpiration.

A model showed that those with narrower leaves had more transpiration for different genotypes of the same species with varying leaf widths. There can be a 25 percent difference in transpiration rate based on leaf size. When the leaves area is less and wind speed is less, leaf width significantly affects transpiration rates. The narrow leaves increase the leaf boundary, boosting transpiration and the subsequent cooling effect.

In a warmer climate, higher transpiration is desirable when soil water is not limiting, as it cools the leaves and prevents them from reaching harmful levels.

Smaller leaves are an easy trait to use when breeding for more productivity in tree crops in the future.

Strategy 5: Changing Cropping Calenders

In rainfed agriculture, crops must contend with untypical dryness, excess rain, and temperature variations in the growing season. Future rainfed agriculture in drylands will use adaptation strategies that increase water use efficiency by adjusting cropping calendars. Changing sowing dates to take advantage of more soil water will improve water use efficiency and yield without increasing water use for agriculture. Sowing date changes will shift vapor loss from evaporation (or non-productive water loss) to transpiration to enable crop growth.

Climate Change Effects on Plants

Scientists must be able to measure transpiration accurately, rapidly, and non-destructively in the myriad of experiments they conduct to find adaptation strategies. CID Bio-Science Inc’s CI-340-Handheld-Photosynthesis-System is a handy portable device that they can use in laboratories, greenhouses, fields, and forests.

Anthropogenic activities have increased CO2 levels from pre-industrial 280 ppm to 395 ppm. Other greenhouse gases in the atmosphere have also increased, notably methane from 715 ppb to 1882 ppb and nitrous oxide from 227 ppb to 323 ppb. Temperatures have already increased by 0.74°C, and depending on people’s adaptation and mitigation efforts, the expected temperature increase will be anywhere from 1.4 to 5.8°C.

Climate change has already resulted in a 0.74°C temperature rise. We must adapt to these changes to improve food production for future and larger populations.

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