Can Boosting Photosynthesis Efficiency Improve Crop Yields?

• Photosynthesis efficiency is very low, averaging 1-3.5% in the fields, and is the next bottleneck for increasing grain production.
• Photosynthesis has not been a primary target of improvement in crop breeding programs aimed at increasing yields.
• Scientists estimate that photosynthesis efficiency must be increased by 50% to meet future global food security requirements.

The rate of grain yield increase seen in the second half of the twentieth century is plateauing, even as the population continues to grow. Agricultural land is lost due to urbanization and soil degradation, and more grains are diverted for meat and bioenergy production. Therefore, photosynthesis, the process central to producing biomass, is under focus to increase grain yield.

Yield Traits

Crop yield depends on more than photosynthesis and relies on several other plant traits and processes. Though photosynthesis fixes carbon and provides energy, yield is influenced by plant development, crop canopy structure and architecture, leaf area index, source-sink relations, and sink strength.

In the last six decades, no yield increment has come from increasing photosynthesis. Photosynthetic rate per unit leaf area is the driving force for plant growth, but dry matter accumulation is determined by other processes not linearly related to yield. The dry matter production and yield have increased by expanding the total photosynthetic area and adjusting carbon allocation. The grain yields were increased by improving the harvest index through breeding, nutrition management, irrigation management, and disease control, among other factors. These yield traits that drove increases have little potential for further development.

Photosynthesis was not considered a limiting yield variable, so it was not included in crop breeding programs. In many cases, crop breeding has reduced the efficiency of photosynthesis. Additionally, photosynthesis is less sensitive to environmental changes than leaf area. So, everyday agriculture practices were also not enough to change photosynthesis and drive dry matter accumulation and yield.

Addressing social and environmental challenges by increasing yield requires quick solutions that can become functional in a matter of years. Many scientists believe that photosynthesis is the next bottleneck they need to address.

Photosynthesis is well studied, and so are its limitations. The question that scientists are now addressing is if and how photosynthesis can be improved to increase yield. Improving yield through photosynthesis is possible through longer crop seasons or increased efficiency.

Natural Photosynthesis is an Inefficient Process

Photosynthesis efficiency is the fraction of solar light energy that is converted into chemical energy (or biomass) through the process of photosynthesis.

Photosynthesis is very inefficient, so there is room for improvement. Most crops in field conditions convert an average of only 1% of the solar radiation they intercept into dry matter. In some crops, especially C4 plants such as wheat, rice, and soybeans, the maximum photosynthesis efficiency in the field is approximately 4%.

To understand the efficiency of photosynthesis and where energy losses occur during photosynthesis and dry matter production, we must consider the yield potential (Yp) and primary productivity (Pn) of plants.

Understanding the Theory Underlying Energy Conversion

According to Long et al. (2006), primary production (Pn) is the total biomass a plant accumulates in the growing season. It includes carbon fixed through photosynthesis and lost due to respiration, structure growth, and maintenance. The formula calculates it:

P n = St·ɛi·ɛc/k                                            (1)

Where,

• St is the total annual incident solar radiation
• ɛi is the efficiency of radiation interception by plants. It depends on canopy architecture, development, closure, extent, and longevity.
• ɛc is the efficiency of the conversion of intercepted radiation into biomass. It depends on the total photosynthetic rate of all leaves in the canopy minus respiratory losses.
• k is the energy of the plant mass and doesn’t differ between vegetative organs.

The yield potential (Yp) is the yield of a variety grown in conditions optimized for it through nutrients, water, pests, weeds, and disease management. For example, maize yield has increased over the last 50 years, with 50% of the improvement attributed to crop breeding and 50% to optimized management of growing conditions. The equation for its calculations is given below:

Y p = η·Pn                                             (2)

Where,

• Pn is the primary productivity described in equation (1)
• η is the harvest index, the biomass allocated for producing harvestable products like seeds, fruits, etc.

These two equations demonstrate that yield depends on vegetative (canopy and leaf) traits, light interception, and photosynthesis, or energy conversion into biomass.

Table 1: “Efficiency of the transduction of intercepted solar radiation into plant carbohydrate through photosynthesis of crop leaf canopies,” Long e. al. 2006. (Credit: https://doi.org/10.1111/j.1365-3040.2005.01493.x)

Table 1 illustrates the steps between light interception and yield, where energy is lost rather than being converted into biomass.

Radiation interception efficiency: The ɛi is much less than St because not all light wavelengths are absorbed adequately by leaves. Some of the losses that determine ɛi are as follows:

• Light interception: Approximately 50% of the solar light falls within the near-infrared range (wavelengths greater than 700 nm). These photons lack the necessary energy for the photosynthetic process, making them ineffective for carbon fixation. It is only light in the 400-700 nm range, known as photosynthetically active radiation (PAR), and leaves utilize it for the process of carbon fixation.
• Light reflection: Not all solar light absorbed by the leaves is used. Within the PAR range, some portion, around 5%, is reflected or transmitted. For example, green wavelengths are reflected, which is why leaves look green. Blue and red wavelengths are beneficial for photosynthesis.
• Non-green pigments: Some portions of the light (1.8%) are absorbed by pigments such as anthocyanin, but cannot pass the energy for use in photosynthesis. Therefore, only the light (43.2%) absorbed by chlorophyll is used to fix carbon.

Energy conversion into biomass: The energy lost during ɛc processes occurs for the following reasons:

• Photochemical inefficiency: The energy in blue photons is significantly higher, and some of this energy is lost as heat, thereby degrading it to the lower energy levels of red photons. Here, 8% of energy is lost.
• Photosynthesis type: Around 66% of the remaining energy is lost during carbon fixation. Eight mols of photons are required to convert 1 mol of carbon dioxide to carbohydrates; that is, the energy-to-biomass process has an efficiency of 34%. The C4 processes use more ATP than C3 plants to produce carbohydrates and have an efficiency of only 29%.
• Photorespiration and dark respiration: During photorespiration and dark respiration, 3.5 and 3.4% of energy is lost in C3 plants. The C4 plants have no photorespiration and suffer only a 4% loss of energy during dark respiration.

Hence, theoretically, C3 plants convert only 5.1%, and C4 plants 6% of the solar energy that falls on them to biomass.

In the field, the average photosynthesis efficiency is lower, at 2.4% for C3 crops and 3.4% for C4 crops.

As mentioned earlier, yield has increased in the past decades by optimizing the harvest index (η) and vegetative traits (ɛi). The harvest index (η) for grains is 0.6, and grains form 60% of the above-ground biomass. Increments in the harvest index will be less as some biomass is needed to maintain vegetative parts and conduct the carbon to the grains. Research to date has optimized ɛi to nearly 0.9 during the growing season, and further improvement is limited.

Therefore, further significant yield potential can only come from improving photosynthesis efficiency.

Recent Developments

However, the ability to increase photosynthesis efficiency in the field has been limited until recently, except in Arabidopsis and tobacco.

Recent scattered reports of increased photosynthesis efficiency associated with higher yields since the turn of the millennium suggest that the process can be improved.

• Elevated carbon dioxide (CO2) levels have increased leaf photosynthesis by 22.6% to give a boost of 18.8% in εc and a 15% improvement in soybean yields.
• In 2020, a 7-28% increase in rice yields was reported due to improvements in photosynthesis efficiency resulting from the overexpression of Rubisco in paddy.

These encouraging developments are driving scientists to focus on various methods to improve photosynthesis efficiency by addressing the reasons for energy loss during primary production and reducing CO2 levels. It is predicted that we will need to improve photosynthesis efficiency by 50% to double global food production to achieve food security.

Infrared Photosynthesis Analyzers

Infrared analyzers have been the standard equipment used in experiments to measure photosynthesis over several decades. CID Bio-Science Inc. offers state-of-the-art infrared technology to measure photosynthesis and chlorophyll fluorescence in the field. CID Bio-Science’s CI-340 Handheld Photosynthesis System is a small, portable device that provides real-time results and features modules to control environmental factors, making it suitable for various types of experiments aimed at improving photosynthesis efficiency.

Find out more about CID Bio-Science Precision tools for your photosynthesis research needs.

Sources

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