Plant Responses as a Result of the Heat Wave

Posted: August 8, 2011

I hope that you and your crops successfully made it through the last heat wave. We managed to keep the crops in our research plots well watered. As it turned out, this wasn't easy because a gasket blew in our sand filter at the beginning of the heat wave. Fortunately, we have colleagues, who are also good friends that allow us to use their sand filters until we got a replacement gasket. This article is about how high temperatures, like those of the heat wave, can affect crop yield and quality.

Most plants have a narrow temperature range for optimal growth and development.  When temperatures get too high (or too low) the result may be as extreme as plant death.  For most vegetables, the optimal temperature range is between 50° and 75°F.  Generally, plants will tolerate higher temperatures and, in fact, adapt to higher temperatures over the growing season.  Heat tolerance varies by cultivar.  Plants also have mechanisms that allow them to acclimate; transpiration can reduce heating by 15-25%.

However, at a certain point, if temperatures get too high, damage can occur.  Sunburn and sunscald are common symptoms when fruit are injured by high temperatures and high solar radiation.  Other potential problems from exposure to high temperatures include bolting in lettuce and cabbage, catfacing in tomato and increased pungency in onion and radish.  Leaves can exhibit spots or scorching that may be mistaken for a disease.

Poor fruit set in tomato, pepper and cucurbits can occur when temperatures get above 90°F.  For example, tomatoes will not set fruit effectively when temperatures reach 94°F.  High temperatures have been shown to decrease the formation of flowers and result in abnormal growth/development of male and female flower tissues.  For example, one study showed that heat stress (above 77°F) hinders pollen and anther development.  As temperatures increase, so does the rate of fruit growth and maturity with the result of smaller fruit that ripen more quickly.  Fruit may also have fewer seed.  For cucurbits, temperatures reaching 90°F promote male flowers and delay the appearance of female flowers which results in poor fruit set.  Once temperatures decrease, flowering and fruit set should resume.