Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
The plant temperature at which tissue dies is around 115°F. Normally, plant temperature is just above air temperature. However, plant temperature can rise to a critical level under certain conditions.
Plants have 3 major ways in which they dissipate excess heat: 1) long-wave radiation, 2) heat convection into the air and 3) transpiration.
A critical factor is transpiration. If transpiration is interrupted by stomatal closure due to water stress, inadequate water uptake, injury, vascular system plugging or other factors, a major cooling mechanism is lost. Without transpiration, the only way that plants can lose heat is by heat radiation back into the air or wind cooling. Under high temperatures, radiated heat builds up in the atmosphere around leaves, limiting further heat dissipation.
Dry soil conditions start a process that can also lead to excess heating in plants. In dry soils, roots produce Abscisic Acid (ABA). This is transported to leaves and signals to stomate guard cells to close. As stomates close, transpiration is reduced. Without water available for transpiration, plants cannot dissipate much of the heat in their tissues. This will cause internal leaf temperatures to rise.
Vegetables can dissipate a large amount of heat if they are functioning normally. However, in extreme temperatures (high 90s or 100s) there is a large increase the water vapor pressure deficient (dryness of the air). Rapid water loss from the plant in these conditions causes leaf stomates to close, again limiting cooling, and spiking leaf temperatures, potentially to critical levels causing damage or tissue death
Very hot, dry winds are a major factor in heat buildup in plants. This causes rapid water loss because leaves will be losing water more quickly than roots can take up water leading to heat injury. Therefore, heat damage is most prevalent in hot, sunny, windy days from 11 am to 4 pm when transpiration has been reduced. As the plants close stomates to reduce water loss, leaf temperatures will rise even more. In addition, wind can decrease leaf boundary layer resistance to water movement and cause quick dehydration. Wind can also carry large amounts of advected heat.
Photosynthesis rapidly decreases above 94°F so high temperatures will limit yields in many vegetables. While daytime temperatures can cause major heat related problems in plants, high night temperatures have great effects on vegetables, especially fruiting vegetables. The warmer the night temperature, the faster respiration processes. This limits the amount of sugars and other storage products that can go into fruits and developing seeds.
Heat injury in plants includes scalding and scorching of leaves and stems, sunburn on fruits and stems, leaf drop, rapid leaf death, and reduction in growth. Wilting is the major sign of water loss which can lead to heat damage. Plants often will drop leaves or in severe cases will “dry in place” where death is so rapid, abscission layers have not had time to form.
On black plastic mulch, surface temperatures can exceed 150°F. This heat can be radiated and reflected onto vegetables causing tremendous heat loading. This is particularly a problem in young plants that have limited shading of the plastic. This can cause heat lesions just above the plastic. Heat lesions are usually first seen on the south or south-west side of stems.
High heat and associated water uptake issues will cause heat stress problems. As heat stress becomes more severe this series of event occurs in plants:
1. decrease in photosynthesis
2. increased respiration
3. closing down of photosynthesis -closed stomates stops CO2 capture and increases photo-respiration
4. major slow-down in transpiration (cooling process loss and internal temperature increase)
5. cell membrane leakage (signals changes in protein synthesis)
6. continued physical water loss
7. growth inhibition
8. plant starvation through rapid use of food reserves, inefficient food use, and inability to call on reserves when and where needed
9. toxins generated through cell membrane releases and respiration problems
10. membrane integrity loss and protein breakdown
The major method to reduce heat stress is by overhead watering, sprinkling, and misting for improved water supply, reduction of tissue temperature, and lessening of the water vapor pressure deficit. Mulches can also help greatly. You can increase reflection and dissipation of radiative heat using reflective mulches or use low density, organic mulches such as straw to reduce surface radiation and conserve moisture.
In very hot areas of the world, shade cloth is used for partial shading to reduce advected heat and total incoming radiation. We will be demonstrating the use of shade cloth for summer production of day-neutral strawberries.
Much of this information was adapted from an article Heat Stress Syndrome by Kim D. Coder, Professor, Silvics/Ecology, Warnell School of Forest Resources, the University of Georgia