Plant Stress & Proline

In his 1995 State of the Union speech, President Bill Clinton highlighted a USDA program addressing plant stress as an example of wasteful “pet project” government spending. We knew then, and know even more now, that plant stress is a very significant yield-and-quality-robbing factor in agricultural crops to which little attention has been paid.

Test tubes in a row of plant proline samples

Samples ready for instrument analysis. Even without precision and absolute instrument analysis, comparative differences in proline (plant stress) levels can be clearly seen.

We try to optimize fertility, irrigation, weed and pest management practices to achieve the best production under the constraints of environment and economics. However, it has become clear that plant stress comes from everything we try to control; it is additive and it can be cumulative — resulting in loss of yield and quality potential.

For the grower, visual detection of plant stress often comes too late to do anything more than damage control by preventing further loss of yields and quality for the season. One visually obvious “too late” example is dropped squares and bolls in cotton. Another is shed flowers and pods or a predominance of two and three-bean pods in soybeans if stress is present early on, or empty pods if stress occurs later.

In plant health, as in human health, there are signs, although they may not be obvious, that stress is present. The trick is in detecting and interpreting those signs – ideally, before they can be seen. This is evidenced by many of us having annual physical check-ups and blood (in the case of plants, sap) tests to detect “hidden” problems. Certain biological signals accumulate in the plant during periods of stress. They are produced in response to environmental stresses such as water, light, temperature and salinity. Their appearance signals that something is hindering normal plant growth and development with consequent loss of yields and quality.

One of the common stress molecules in plants is proline. Proline is an amino acid normally produced during protein production. In times of stress, plants over-produce this molecule which can be measured with whole leaf analysis. Elevated proline indicates stress responses have been initiated by the plant. The excess proline stimulates metabolic changes in the plant to cope with the stress.

However, proline is not only a signal but also directly helps maintain internal cell turgor, preventing electrolyte losses by supporting good osmotic balance. Proline also reduces the undesirable reactive oxygen species molecules (ROS) which, in times of stress, can overcome the natural ability of plants to dispose of these harmful molecules that are normally produced during metabolism.

Good crop nutrition is fundamental to warding off harmful effects of environmental stresses. Therefore, proline is both a plant stress response and a signal. Increased proline production allows a plant to marshal its physiology to accommodate the stress. But generally, the internal changes the plant makes to manage stress reduce its productivity in favor of survival advantage.

In short, high proline values are simply symptoms of larger problems in the plant. The primary use of the proline data is to formulate a plan to ameliorate or manage the stresses causing the proline response. With high proline, the plant is telling us that something is wrong.

A field full sugar beet leaves wilted due to stress

A dramatic field example of extreme plant stress resulting in unusually high proline levels, with the leaves on these sugar beets having collapsed. While this is a remarkable situation, the point of proline testing is to prevent unplanned crop performance-robbing stresses long before they happen.

closeup of healthy sugar beet leaves

For comparison, healthy sugar beet leaves.

We don’t want to eliminate stress, but control it so that the plant can continue to grow and produce well during the season. Elimination of stress is not a goal because the plant uses certain stresses as timing functions for several important metabolic processes.

While the overall concern is in reducing chronic or unanticipated stresses, there are certain conditions during crop development under which induced and carefully controlled plant stress can be of great financial benefit to the grower: Scheduled and managed stress can result in higher nutrient content of the crop (premium quality), greater yields, or both. This is demonstrated by field results.

Accordingly, the goal is not to minimize stress across the board throughout the season, but to carefully time the inducement, control and manage it.

Regardless, carefully managing overall stress responses during growth promotes better total plant performance. Your lab’s in-season recommendations will include steps to minimize destructive unplanned stresses and manage desired ones detected by increased proline production. As an example, for a low-moisture stress, the lab may recommend increasing potassium fertility to increase internal water use efficiency. Additionally, based on other plant tests, other nutrients may be recommended to maintain canopy structure and function and to reduce or eliminate disease and insect pressures.

The plant lab uses the proline test to monitor total stress in crop plants. If stress levels are high, the results, together with sap and tissue nutrient test results, guide the lab in prescribing changes to your management that will control those stresses. Remember, elevated proline levels indicate stresses that are most often caused by multiple factors. Your lab will consider those factors to formulate a complete remediation plan that addresses all stresses.

Proline may have other effects on plants as well. Collection of proline data has been ongoing for some time. Differences have been seen between reproductive tissues from stressed plants and those of low-stress plants. Therefore, if you hold seeds, tubers or bulbs over to plant yourself or sell them for planting, controlling stresses may influence future crops as well. These effects continue to be evaluated.

By Larry Zibilske, Ph.D. This article appeared in the December 2018 issue of Acres U.S.A. magazine.

Larry Zibilske, Ph.D., is vice president of research at TPS Lab in Edinburg, Texas. Zibilske holds degrees in microbiology and soil science from Texas A&M and a Ph.D. in soil microbiology from the University of Missouri-Columbia.


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