Detecting moisture and heat stress are important steps to any effective irrigation management system. In the process of scouting your crops, you or the person doing it needs to understand some of the basics that exist with field moisture stresses and to be sure not to confuse them with heat stress.
Plant Stress in General:
Aside from, or in conjunction with physical injury, plant stresses which can lead to yield and/or quality loss happen as a result of either Biotic or Abiotic processes. Examples of Biotic effectors include insects and diseases. Abiotic effectors relate to chemical activity within the plant, including water. The presence of a biotic stressor on the plant results in the plant using its abiotic processes such as lignin production and metabolism to battle the ‘invader’. When the plant is predisposed to stress from abiotic barriers due to drought, chill, flooding, etc. it cannot respond to biotic stress as well. This is true across similar stressors within each causal group as well. For example, research has shown that a corn plant with anthracnose stalk rot is less likely to suffer economic yield loss if the presence of European Corn Borer is well below economic threshold, and its presence when corn borer is near threshold will accentuate the damage to the plant and increase yield loss even more. The same holds true with abiotic stressors. For example, when a plant takes up significant water during an active vegetative stage, and a drought period kicks in where the soil is approaching the wilting point, the stomata in the leaves will close as a protective mechanism, slowing metabolism. If extended for long, salts can accumulate in the plant and not be respired properly; resulting in salt injury and necrosis. Therefore, the interrelationship between water availability in the soil and plant metabolism is very important when focused on maximizing yield, quality, and plant health.
Heat stress is not the same as drought stress. It can be a serious condition in some areas of North America and on certain crops at certain times in their growth cycle. Like chilling stress, it involves the rapid slowing or even shutting down of some metabolic processes within the plant. Let’s examine some of the primary facts and factors relating to heat stress, according to The Prudent Garden (Garden Heat Stress, July 2016):
Sometimes we think that watering is the best response to Heat Stress…it is not always the answer! Many times you will see heat shock and stress when the soil is actually at acceptable moisture levels. Overwatering during Heat Stress can make the problem worse, especially if you’re not monitoring soil moisture and avoiding waterlogging the roots! Fungal diseases thrive in such conditions. The plant is shutting down (abiotic) due to heat and if you over water, you also introduce an opportunity for (biotic) pests and influencers to reduce yield potential even further. Watering should be done to only cool the plant and the environment around it when the soil is in adequate moisture status.
One of the more common diseases in tomato, pepper, squash, and others is blossom end rot. In summers when we have rapid heat accumulation plus inconsistent watering, this fungus will thrive. Technically caused by a calcium deficiency, the deficiency is actually caused when the soil goes from too dry to wet and back again in conjunction with heat stress. Adding calcium isn’t the answer, consistent soil moisture is.
Flower set and pollination are also affected by heat stress. If heat stress occurs in the bud stage, the plant may not even set flowers until cooler conditions prevail. If it occurs after the flowers are out and before pollination, they may just drop their flowers to protect themselves.
Drought stress can come on insidiously in crops. The plant responds to this abiotic stressor in different ways at different stages. Symptoms of drought stress are usually related to stomata response, whereby the respiratory organ of the leaf closes to retain water. Corn leaves curling onto themselves is a good example. Yield has probably already been lost once a plant is showing drought stress for several days on end. If the crop is near pollination, the yield and quality loss is even greater. In fruits, at fruit set, yield quality and tonnage can be extremely impacted.
The best way to prevent these stressors, whether biotic, or abiotic, is to provide consistent moisture at an optimum level throughout the season according to the plants needs at that point in time. Technology in wireless soil moisture monitoring, when combined with reliable data to estimate evaporation rate, soil texture effects, and crop needs; holds great promise for farmers who are scheduling irrigation to maximize yield.
Apple tree exhibiting moisture stress. In this instance, it is a young transplant that has not been properly set into an appropriate planting medium and the roots are unable to take up water at the appropriate rate.
Example of a Pepper plant after leaf drop from Heat Stress, putting on new growth after temperature normalizes
Source: Garden Heat Stress
Resources and Suggested readings/presentations:
LinkedIn Presentation: Plant Response to Stress, W. McLaughlin
LinkedIn Presentation – Plant Stress Responses, Anand Reghuvaran