How Leaf Area Index Affects Yield

• Leaf Area Index (LAI) varies with species, cultivars, growing conditions, and crop stages.
• The influence of LAI on yield will depend on the harvested portion of crops.
• An increase in LAI above a specific optimum value for fruits and seed crops will decrease yield.

Leaf Area Index (LAI) is a standard vegetative trait correlated with yield parameters like fruit size, panicle number, kernel weight, etc. Various factors like cultivar and growing conditions will moderate its effects on yield. To better use this trait, knowing how it influences yield is necessary.

Leaf Area Index

Leaf Area Index (or LAI) is half the total leaf area per ground area. It helps understand the foliage’s density, shading, photosynthesis amount, and plant water use.

LAI is one of the main vegetative traits used to determine plant physiology, functions, and their effects on yield. LAI will differ based on species, cultivar, habit, growing conditions, and years.

Since LAI determines light and water use efficiency, it influences biomass accumulation and allocation to reproductive parts, such as seeds and fruits. However, more LAI is not always better.

Light and Water Use Efficiency

Research efforts to improve agricultural yields have focused on increasing canopy and light interception. However, when LAI is high, for example, 5-7, many leaves are shaded. Since lower canopy leaves do not intercept light, they have low photosynthetic rates. So, the extra leaves and LAI don’t help in carbon fixation and biomass accumulation. Yet these leaves respire and use the plants’ carbon resources. The plant also invests biomass and nutrients in growing the leaves. The marginal increases in carbon fixation produce little gains in primary net benefits for plants due to high respiratory costs. For example, in soybeans, an LAI increase of 40% increased yields only by 3%.

Plants with fewer leaves and lesser LAI increase the light use efficiency and photosynthetic rates of lower canopy leaves, save carbon investments in more leaf production, and reduce respiratory costs. Hence, more biomass is left and diverted for fruit and seed production. For this reason, small or large losses of leaf area due to pest damage or weather don’t reduce yield. For example, wild soybeans can lose 50% of leaves without losing seed yield.

A growing crop needs to grow leaves and reach optimum LAI. However, a further rise in LAI does not increase light interception, as shown in Figure 1, and will diminish yields. So modern cultivars reduce leaf area. It is estimated that cultivars selected for optimum LAI could increase crop yield by 8% to produce an additional 6.5 million metric tons of food annually in the USA alone.

Figure 1: “Overall relationship between light interception and LAI for three species, sorghum, maize and rice in South East Queensland,” RSECO.org. (Image credits: P. Inthapan and S. Fukai, Aust J Exp Agric 28: 243-248, 1988)

Water use efficiency

A lower LAI cuts crop water demand. Plants with fewer leaves have less transpiration and need less water to grow. If the LAI is optimum, the reduction in leaves number doesn’t affect biomass accumulation and yield. Higher crop water use efficiency is good news for future agriculture.

Factors Moderating LAI Influence

However, the influence of LAI on yield is not the same for all agricultural and horticultural crops. The contribution of LAI to yield will differ on various factors such as,

• Crop and growth type
• Crop lifecycle phase
• Growing conditions

Understanding how and when LAI affects yield can help select relevant cultivars and develop optimum agricultural practices.

Crop Type

Crop type, plant part harvested, and leaf orientation can moderate LAI influence on yield.

Crop species: The optimum LAI differs for species. For example, apples have an LAI of 1.5 to 5, while mangoes have an average LAI of 2.94. Sorghum requires a much lower LAI than rice for improved yields.

Leaf orientation: Erect leaves have an LAI that is more suitable for increasing yield than horizontal leaves, as they do not shade lower canopies, and the plant has more photosynthesis.

Plant part harvested: The plant part harvested will determine the importance of LAI for yield. High LAI is always optimum for leafy vegetables, where leaves are the commercial products. For vegetables (where the fruit is harvested), fruits, cereals, and pulses, younger plants must grow to achieve an optimum LAI. However, additional leaves and higher LAI will reduce yield and/or quality.

Crop Stage

As a plant grows, the canopy changes, and so will LAI. Studies show LAI at various crop cycle times can be crucial for determining yield. Species and cultivars will show different patterns. For example, in sown paddy, LAI at the early tillering stage was essential and correlated with all yield parameters like panicle, kernel number, and grain weight. For transplanted paddy, LAI at the middle tillering stage was crucial. LAI at early growth stages is essential for paddy to yield well.

Cultivation Practices

Cultivation practices alter growing conditions that control plant growth and health and will change LAI for the same species and cultivars.

The effects of various levels of fertilizers on yield show that after increasing nutrient application beyond the optimum levels, yield plateaus or even decreases. Increasing nutrient application can be targeted to get the optimum LAI, but further fertilizer addition should be avoided. Soil water levels that reduce leaf development or increase LAI will affect yield.

Planting density also matters as the LAI will increase with plant numbers. An excessive increase in LAI due to high-density planting lowers grain-filling in rice.

LAI is used in orchard management to improve fruit yield and quality through canopy management, as light is crucial for fruit development.

Hence, a change in LAI for the same cultivars in the same region due to different cultivation practices will produce different yields.

Climate Change

Carbon dioxide (CO2) levels were expected to increase crop yields. However, the yields have not increased as predicted. In response to higher CO2 levels and more carbon fixation, plants increased leaves, leaf area, and LAI. The over-investment in leaves occurs by diverting fruit and seed development resources and is accompanied by higher respiration costs. The increase in LAI is partly the reason for lower-than-expected yield benefits from climate change. Therefore, using cultivars with less LAI will be necessary to improve agriculture and horticulture in the future.

Figure 2: “Leaf area index of guava (Psidium guajava) by digital plant canopy imager (CID BioScience Inc), Patil et al. 2018. (Image credits: https://doi.org/10.20546/ijcmas.2018.704.059).

Measuring LAI

Research shows that LAI affects all yield components significantly. However, it cannot be used in isolation to predict or monitor yield by scientists and farmers. Several field methods exist for measuring LAI. Scientists prefer canopy analyzers for their precision. Devices like CID Bio-Science’s CI-110 Plant Canopy Imager, shown in Figure 2, which non-destructively records and analyzes LAI in real-time in the field, are a great asset to speed research work to improve food production and security for current and future generations.

Sources

Hashimoto, N., Saito, Y., Yamamoto, S., et al. (2023). Relationship between Leaf Area Index and Yield Components in Farmers’ Paddy Fields. AgriEngineering, 5, 1754-1765. https://doi.org/10.3390/agriengineering5040108

Patil P., Biradar P., Bhagawathi A.U., Hejjegar I.S. (2018). A review on leaf area index of horticulture crops and its importance. International Journal of Current Microbiology and Applied Sciences. 2018; 7(4):505-513. doi.org/10.20546/ijcmas.2018.704.059.

RSECO.org (n.d.). Plants in Action. Retrieved from https://www.rseco.org/content/1221-%E2%80%82leaf-area-index-and-canopy-light-climate.html

Srinivasan, V., Kumar, P., & Long, S. P. (2017). Decreasing, not increasing, leaf area will raise crop yields under global atmospheric change. Global change biology, 23(4), 1626–1635. https://doi.org/10.1111/gcb.13526