Wilting Point In Plants: Temporary And Permanent Limits

Wilting point is one of the critical indicators of vegetation health. If the wilting point of crops is low or crosses the bare minimum, plants are permanently damaged and will die. So, it is important to prevent them from reaching the permanent wilting point. It is feasible with ongoing soil moisture and crop state monitoring, and satellite imagery assists in the remote control.

What Is The Wilting Point?

It is the soil moisture that crops can absorb; otherwise, they will wither. Wilting point is reached when available moisture drops lower than a plant can get. The deficiency may cause either temporary or permanent withering.

Wilting point is a ratio of moisture content to dry earth weight or volume. Its percent expression (of the dry weight) is the wilting coefficient.

Wilting point of soil is not a unique value. It correlates with multiple factors:

• soil type (texture, porosity, compaction, structure);
• air and soil temperature;
• osmotic stress due to soil salinity;
• spreading of plant roots;
• specifics of transpiration;
• moisture throughout the soil profile.

Does Wilting Point Vary For Different Plant Species?

Yes, it depends on the crop type but is typically about -1.5 MPa (-15 atmospheres/bars), which is common for sunflower, wheat, or corn.

Does The Wilting Point Depend On Soil Texture?

It does. Soil moisture volumes at the soil wilting point are approximately 15-20% in clay soils and just 5-10% in sandy ones and can be measured with soil moisture sensors. Finer soils have better water-retaining capacity. However, wilting point depends on the soil texture and its porosity that predetermines the ease of roots’ growth and their capability to uptake soil moisture. This is why hairy roots in silty sand absorb water better and survive twice longer^{Garg, A., Bordoloi, S., Ganesan, S.P. et al. A relook into plant wilting: observational evidence based on unsaturated soil–plant-photosynthesis interaction. Sci Rep 10, 22064 (2020). https://doi.org/10.1038/s41598-020-78893-z} than coarse root systems in clay loam, even despite moisture present in the soil matrix.

What Happens Once A Plant Reaches A Temporary Wilting Point?

As the term suggests, crops droop at this level but temporarily. It is typical on hot days under a lack of soil moisture. At the temporary wilting point, crops recover once the air temperature falls or after rewetting.

What Is The Permanent Wilting Point?

It is the lowest amount of moisture plants require not to wither irrevocably. Unlike in the state of temporary withering, crops can’t return to their normal activity even after soil moisture is replenished. Drooping usually happens because the transportation of water from the roots decreases. When it occurs, plants limit photosynthesis and transpiration due to lessened stomata turgor pressure^{Competency Area 2: Soil hydrology AEM. Northeast Region Certified Crop Adviser (NRCCA) Study Resources} (stomata are small holes on the leaf surface). If water availability drops below the critical level, it eventually means yield loss.

Permanent Wilting Point Signs

Withering crops reveal drooping leaves that later fade and dry out. Vegetation foliage doesn’t droop simultaneously. Withering starts from basal leaves and then shows in apical ones. These two stages are known as the first and ultimate permanent wilting points correspondingly.

Why Can’t Plants Absorb Water At The Permanent Wilting Point?

When plants wither, it doesn’t mean that there’s no water in the ground at all. There is, yet it is not available to crop roots because soil particles hold moisture too strong and its amount is generally small.

Wilting Point Among Other Causes Of Crop Withering

Lacking moisture in the root zone is the most common cause of withering, and vegetation survival depends on the root system, soil type, and the temporary or permanent limits for the individual crop. If temperatures drop overnight, plants have a chance to recover. Yet, combined with other unfavorable circumstances like dry winds or long droughts, yield losses will be even higher. Also, crops are especially sensitive to water stress at certain stages of plant growth. For example, soybeans are the most vulnerable to water stress during the period of flowering to pod filling^{Chris Hay, Jeppe Kjaersgaard, Todd Trooien. Chapter 49: Soybean Irrigation. extension.sdstate.edu}.

However, foliage drooping does not always mean the crop has approached the soil wilting point. For example, not only water deficiency but overwatering may lead to withering because waterlogged roots are deprived of oxygen and subject to crop diseases like rots. So, it is critical to monitor fields and save plants by managing permanent wilting point signs early or avoiding the problem with properly scheduled irrigation events.

EOSDA Crop Monitoring For Wilting Point Control

The best way to prevent plants from withering is through the ongoing measuring of this parameter considering the crop type and field specifics. However, satellite imagery can be also helpful in wilting plants monitoring, and EOSDA Crop Monitoring provides a lot of useful analytics for farmers. The most valuable features in this regard are:

• monitoring various vegetation indices to identify withering;
• tracking NDMI values to detect water content drops in plants;
• analyzing root zone and surface soil moisture to adjust irrigation to avoid water deficiency in plants.

By comparing vegetation indices and NDMI values, it is possible to understand their correlation. If there is any, the field should be inspected for factors that may cause withering. With no causes detected, it is necessary to measure the wilting point and take proper actions if needed.

EOSDA Crop Monitoring To Meet Your Agribusiness Needs

EOSDA Crop Monitoring is an effective management tool for small and large-scale farming. The EOSDA satellite-based platform can also be beneficial for insurance companies, agricultural equipment producers, chemicals and seed suppliers, and other business entities. Integration of satellite analytics into all existing systems will not only help you get more data faster as such but optimize your facilities’ use. Furthermore, the obtained insights will allow well-grounded business decision-making and understanding of relevant customer needs.

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