Plants are more sensitive to changes in temperature than humans. Not only are they stuck where they are planted, temperature plays a critical role in plant processes.
Photosynthesis, the process by which green plants capture the energy of the sun and convert it to sugar energy (plant food) using carbon dioxide, nutrients from the soil and water, increases with heat.
Respiration is the process by which plants burn those sugars to yield energy for growth, reproduction and other life processes. It also rises with increasing temperatures. If sugar production cannot meet respiration demand, plants stop growing and producing flowers, leaves or fruit. For tomatoes, growth peaks at 96 degrees. In addition, increased respiration rates reduce sugar content of produce. Fruits and vegetables grown in heat are less sweet (Colorado State University Extension, or CSUE).
The loss of water vapor through leaves by transpiration helps cool a plant, just as an evaporative cooler does a house. When temperatures climb, a plant needs more water to cool itself and to maintain cell water pressure throughout the plant.
“Water maintains the pressure in cells much like air inflates a balloon allowing a plant to remain stiff and upright. This pressure, turgidity, helps leaf cells regulate water loss and carbon dioxide uptake. It also is the force that pushes roots through the soil,” (CSUE). Plants that lose turgidity wilt.
When it is extremely hot, a plant may not be able to pull the water up out of the soil and through its vascular tissues quickly enough to meet transpirational demand.
When we set our irrigation timers in April or May and then leave them at that amount all summer, we deprive plants of sufficient water for plant health. Leaf scorch with marginal burning is one sign of a plant either not being watered sufficiently, or not being able to suck water up out of the soil quickly enough because of the heat.
Additional symptoms of water stress besides decreased growth, reduced vigor or leaf scorch include small, off-colored leaves and decline from the top of the plant down. Water-stressed plants also suffer from nutrient deficiencies because there isn’t enough water to move nutrients in from the soil and through the plant. Root growth is reduced, root health declines and so does overall plant health. Flower colors fade and flowers have a shorter life.
To help your plants through the heat spell, increase the amount of water you apply, water deeply and mulch to keep soil cool and soil evaporation to a minimum.
JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at skellyj@unce.unr.edu.