Heat and drought push plants to their limits

Prolonged heat and water shortages can kill plants by preventing them from cooling themselves, ORF Tirol reported, citing Gilbert Neuner of the University of Innsbruck’s Institute of Botany. Plants exposed to temperatures of 40 degrees Celsius or more for several hours can suffer severe damage, he said.

When water is scarce, plants can no longer cool themselves through transpiration. Heat and drought then disrupt their natural temperature regulation, eventually causing proteins in their leaves to denature, similar to egg white hardening when cooked.

The protective wax layer on leaves, known as the cuticle, also begins to melt. The hotter it becomes, the more porous the layer gets, sharply increasing water loss at a time when plants are already under drought stress.

Short heat exposure is more tolerable

In laboratory experiments, Neuner found that almost all plants can withstand an ambient temperature of 44 to 55 degrees Celsius for about half an hour. This applied both to cold-adapted Alpine flowers and heat-loving tropical plants. Water plants were somewhat more sensitive, while some desert plants survived temperatures of up to 66 degrees.

The longer the heat lasts, the lower the temperature needed to cause damage. Leaves generally tolerated 45 degrees Celsius when the exposure lasted only half an hour, but damage appeared at 40 degrees when the heat continued for four hours. Temperatures of 50 degrees caused effects within minutes.

Neuner said the relationship between temperature and exposure time was similar for plants in temperate Austria and the hot tropics. A photograph of the Alpine plant bearberry shows leaves flowering on a north-facing slope while plants on a south-facing slope have heat-related damage.

Limited safety margin

Heatwaves are becoming longer as a result of climate change, increasing the harmful heat doses received by plants, Neuner said. The safety margin for vegetation is no longer particularly large because plants appear to be reaching a shared physiological upper temperature limit worldwide.