What is plant water stress?
Plant water stress, often times caused by drought, can have major impacts on plant growth and development. When it comes to crops, plant water stress can be the cause of lower yields and possible crop failure. The effects of plant water stress vary between plant species. Early recognition of water stress symptoms can be critical to maintaining the growth of a crop. The most common symptom of plant water stress is wilt. As the plant undergoes water stress, the water pressure inside the leaves decreases and the plant wilts. Drying to a condition of wilt will reduce growth on nearly any plant.

From an irrigator's perspective, managing water to minimize stress means knowing plant water availability, recognizing symptoms of water stress, and planning ahead. This article outlines how water stress impacts plant growth and development and how to anticipate plant water stress to minimize negative consequences.

How does water stress impact plant growth and development?
Plants take in water through their roots. The amount of force needed for a plant to remove water from the soil is known as the matric potential. When soil moisture is low, plants have to use more energy to remove water from the soil, thus the matric potential is greater. When the soil is dry and the matric potential is strong, plants show symptoms of stress. This is known as the matric effect.

Plants can also have difficulties extracting water from the soil if salts are present in the root zone. Generally, when the soil solution is more saline than the plant, more energy is needed to uptake water than if the soil solution is not saline. Plant water stress caused by saline conditions is known as the osmotic effect. Stress from the osmotic effect will cause the same symptoms as stress from the matric effect.

Salinity as a stress factor
In places where both salinity and drought are present, plants must overcome matric and osmotic forces to uptake water. If the osmotic effect is strong (high salinity), then the matric force (how dry the soil is) needs to be minimized to reduce the combined effect of matric and osmotic forces. Simply stated, more water needs to be added to the soil to minimize the effects of plant water stress in saline soils.

Most water for irrigated crops is extracted from shallow soil depths where the majority of the roots are. In the first few days following irrigation, 40% or more of plant water comes from the top twelve inches of soil. Plants use roots to uptake water from the soil, so adequate root density, distribution and conditions conducive to root growth are important in aiding plant utilization of water.

Why does water stress impact plant growth and development?
The main consequence of moisture stress is decreased growth and development caused by reduced photosynthesis. Photosynthesis is the process in which plants combine water, carbon dioxide and light to make carbohydrates for energy. Chemical limitations due to reductions in critical photosynthetic components such as water can negatively impact plant growth.

Low water availability can also cause physical limitations in plants. Stomates are plant cells that control movement of water, carbon dioxide, and oxygen into and out of the plant. During moisture stress, stomates close to conserve water. This also closes the pathway for the exchange of water, carbon dioxide, and oxygen resulting in decreases in photosynthesis. Leaf growth will be affected by moisture stress more than root growth because roots are more able to compensate for moisture stress.

How can plant water stress be managed?
Crop selection can be a key component when dealing with or anticipating moisture stress. Generalizations about plant groups and how they behave under moisture stress can be used to guide decisions about crop selection for drought and saline conditions.

• Determinate crops: Resistant to moisture stress during vegetative stages, determinate crops are grown for harvest of mature seed and include small grains, cereal crops, peas, beans, and oil seed crops. Determinate crops show a linear relationship between water stress and seed production. These crops are most sensitive to stress during seed formation including heading, flowering, and pollination. Each has a minimum threshold growth and water requirement for seed production. This process can be interrupted by stress and generally can't be recovered with removal of the stress.
• Indeterminate crops: Indeterminate crops include tubers and root crops such as potatoes, carrots, and sugar beets. These crops are relatively insensitive to moisture stress in short intervals (4-5 days) throughout the growing season and have no specific critical periods. If and indeterminate crop is subject to moisture stress, quality will be affected rather than yield. Harvestable yield increases as water use increases. Indeterminate crops are more directly related to climatic demand and cumulative water use during the season than to stress during any particular growth stage.
• Forages: Forage crops are grown for hay, pasture, and biomass production. In comparison to determinate and indeterminate crops, perennial forages are impacted least by moisture stress. Perennials usually have deep well established roots systems. Forage yields are typically in response to climatic conditions. Forages that have undergone moisture stress will have lower yields than those that have not. Annual forages are an effective way to take advantage of early season moisture and cooler temperatures. In general, as water stress is increased, forage nutritional value is increased, yet overall yield and harvestable protein is decreased.

How can the consequences of plant water stress be minimized?
Producers can minimize the consequences of plant water stress by learning how to manage it ahead of time. Water stress timing and crop selection choices can be very important details. If one had to prioritize to deal with moisture stress, the list might look a lot like this:

Water stress timing

• 1st choice - Early in the crop growth cycle
• 2nd choice - Close to harvest

• 1st choice - Perennial forages
• 2nd choice - Annual forages
• 3rd choice - Short season indeterminate crops
• 4th choice - Short season determinate crops
• 5th choice - Long season determinate crops

A great source of readily available information can be found in online journals:

• Soil Science Society of America Journal
• Agronomy Journal
• Crop Science
• Journal of Environmental Quality

Other articles that may answer further questions about plant water stress and crop selection include:

• Forage Species in Montana
• Irrigating With Limited Water Supplies
• Crop Water Use and Growth Stages

¹This presentation was given at the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America 2003 Annual Meetings held in Denver, Colorado.

Home Page