Understanding Drought Impact on Crop Yield: Stages, Mechanisms, and Adaptations.

• Drought’s impact on crop yield affects plant stages differently, moderated by genotype and external environmental factors.
• Very early drought can inhibit germination and seedling establishment, reducing stand density and yield.
• Drought during reproductive and grain filling can reduce yield quantity and quality more than during the growth phases.

Drought is the primary environmental stress crop suffers and is the leading threat to global food security. Increasing drought events pressure the food production systems that will have to produce more to meet the demands of a growing population. Drought effects depend significantly on phenology or plant stages. Find out more about the impact of drought on different crop stages and yield.

Drought Impact on Crop Yield

Figure 1. Morphological, physiological, anatomical, and biochemical dynamics of plants affected by water stress. Seleiman, et al 2021. (Image credits: https://doi.org/10.3390/plants10020259)

Around 80-95 percent of a plant is water, which is essential for many plant processes, such as metabolism, growth, development, reproduction, and biomass accumulation.

The various plant molecular, biochemical, anatomical, physiological, and morphological processes and traits where water is essential will be impaired under water deficiency produced by drought. Ultimately, this reduces crop yield and quality. The various changes that can occur are listed in Figure 1.

The duration, frequency, and intensity of drought determine its effects. Plant response depends on the current drought, as well as the legacy effects of previous events and their intensity. Other factors, like soil characteristics, also strongly influence drought-related symptoms.

Drought effects are also specific to plant species and phenology and can change during the crop cycle. The impact of drought on various stages of the crop is explored in the following sections.

Germination

If the crop experiences drought just after sowing, it can inhibit the establishment of a timely and optimum plant density for maximum yield.

Drought severely reduces germination and seedling survival. Water uptake during the germination’s imbition stage is decreased, so the seeds have less energy and enzymes. For example, in alfalfa, germination is reduced. The germinating seeds have lower hypocotyl length, root, and shoot fresh and dry weight.

Growth

Plant growth happens through cell division, enlargement, and differentiation, which are influenced by interactions of genotype, anatomy, physiology, and morphology. Drought harms anatomy, physiology, and morphology.

Anatomical Changes

Cell enlargement is inhibited by interruption of water flow from the xylem to cells, so a reduction in growth is one of the main effects, as plants can’t elongate through cell expansion. Water deficit causes loss of turgor pressure, which is vital for plants. It maintains plant form, structure, cell functions, and physiological processes.

Figure 2: Drought stress causes stunting and less leaf area in rice cv IR64 after twenty days of drought in the growth phase, Farooq et al. 2009. (Image credits: https://hal.science/hal-00886451/document)

Physiological Changes

The main physiological processes affected by a reduction in water uptake are:

• Photosynthesis: The guard cells must be turgid to open stomata, which lets in carbon dioxide for biomass production. Reduction in leaf area also limits photosynthetic sites.
• Nutrient uptake: Nutrients are transported by water within plants, which reduces during drought. During water stress, applications of fertilizers provide no benefit to crops, as they are not absorbed and assimilated.

Morphology

As a result of the anatomical, biochemical, metabolic, and physiological drought effects, changes are apparent externally in crop morphology. The chief drought effects in the vegetative growth phase are plant height reduction, change in leaf number and area, canopy thinning, and the root system:

• Plant height: Inhibition of cell division and enlargement causes a reduction in plant height and less leaf area.
• Leaf area index (LAI): Drought leads to an increase in leaf browning, drooping, scorching, wilting, brittleness, rolling, early and more senescence. There is also less leaf expansion due to loss of turgor, cell expansion, and tillering. As a result, the LAI, the leaf area ratio on one side to the ground area, is reduced, affecting photosynthesis and transpiration rate.
• Damaged canopy: As drought intensity and duration increase in orchard trees, it can also cause twig cracks, bark cracks, branch dieback, canopy thinning, necrosis, stunted growth, and, in extreme cases, plant mortality.
• Root system: Roots are crucial for water absorption, and the plants change root traits in response to drought. The root-to-shoot ratio in many crops increases as plants prioritize growing more roots. Several species reduce lateral root density and root angle and send roots deeper to explore for water. Root hairs get elongated but thin and have higher mortality. Early drought can destroy the whole root system in species like sugarcane.

All these internal and external changes caused by drought reduce dry matter accumulation and grain yield. Though drought during the vegetative phase can cause substantial yield losses, the effects of drought occurring during the reproductive and grain-filling phases can have more severe consequences. See Table 1 for a comparison of yield reduction due to drought at various crop phases.

Table 1: “Economic yield reduction by drought stress in some representative field crops,” Farooq et al. 2009. (Image credits: https://hal.science/hal-00886451/document)

Reproduction Phase

Drought occurring before, during, and post-anthesis has different effects and is strongly influenced by species.

Drought at preanthesis reduces the time to flower opening.

Drought at anthesis results in barrenness, as it prevents grain development later due to many reasons.

• Many species reduce tillering, spikes, spikelets, flowers, or achenes, reducing yield. For example, sunflowers produce fewer achenes in their flowers, lowering yield.
• Lack of sufficient assimilates supply leads to ear abortion, for example, in pearl millet.
• In cotton, fewer flowers and more boll abortion reduce lint production.

Drought during post-anthesis reduces the time for grain filling. In this phase, water stress effects on yield are significant regardless of drought severity and duration. For example, in wheat, after heading, drought doesn’t affect the kernel filling, but time to maturation is reduced, so dry matter accumulation is lower.

Grain Filling

Starting from anthesis, the developing grain is a sink for assimiliates. In cereals, these are starch synthesized from simple carbohydrates, which involves enzymes. Drought reduces enzyme activity in sucrose and starch synthesis, limiting grain development.

There are exceptions to this rule. In rice, drought during grain filling increases the mobilization of assimilates to grains and speeds up grain filling.

Crop Length Reduction

Drought affects crop phenology by shortening the crop growth cycle. Plants switch from the vegetative stage to the reproductive phase during drought. The reduction in growth period results in significant yield reductions, for example, in wheat and barley.

Genotype, the timing of phenologies, and structural traits produce a variety of drought resistance mechanisms among crops.

Measuring Drought Effects

The information available so far is being applied to develop cultivars with drought tolerance. Measuring morphological traits is more manageable than studying complex biochemical and anatomical traits while phenotyping cultivars. Still, using non-destructive tools for precise estimations in real-time will benefit morphological trait measurements. Standard tools are already available, such as those offered by CID Bio-Science. The firm produces the following precision tools that several scientists have already used:

• CI-202 Portable Laser Leaf Area Meter and CI-203 Handheld Laser Leaf Area Meter to measure leaf area.
• CI-110 Plant Canopy Imager can measure canopy cover and LAI.
• The minirhizotrons CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager can measure length, width, density, root hairs, etc.
• The CI-340 Handheld Photosynthesis System measures the rate of photosynthesis, transpiration, and stomatal conductance.

Technology and science could help solve one of the biggest challenges of our times: drought’s impact on crop yield.

Sources

Farooq, M., Hussain, M., Wahid, A., & Siddique, K. H. M. (2012). Drought stress in plants: an overview. Plant responses to drought stress: From morphological to molecular features, 1-33.

Farooq, M., Wahid, A., Kobayashi, N. S. M. A., Fujita, D. B. S. M. A., & Basra, S. M. (2009). Plant drought stress: effects, mechanisms, and management. Sustainable agriculture, 153-188.

Lipiec, J., Doussan, C., Nosalewicz, A., & Kondracka, K. (2013). Effect of drought and heat stresses on plant growth and yield: a review. International Agrophysics, 27(4).

Seleiman, M. F., Al-Suhaibani, N., Ali, N., Akmal, M., Alotaibi, M., Refay, Y., … & Battaglia, M. L. (2021). Drought stress impacts on plants, and different approaches to alleviate its adverse effects. Plants, 10(2), 259.

Szira, F., Bálint, A. F., Börner, A., & Galiba, G. (2008). Evaluation of drought-related traits and screening methods at different developmental stages in spring barley. Journal of Agronomy and Crop Science, 194(5), 334-342.

Yang, X., Lu, M., Wang, Y., et al. (2021). Response Mechanism of Plants to Drought Stress. Horticulturae 2021, 7, 50. https://doi.org/10.3390/horticulturae7030050