Researchers identify mechanisms behind plant response to warming

Microscopic pores on the surface of leaves called stomata help plants ‘breathe’ by controlling how much water they lose to evaporation. These stomatal pores also enable and control carbon dioxide intake for photosynthesis and growth. With global temperatures, widening stomatal pores are considered a key mechanism that can minimize heat damage to plants. University of California San Diego researchers have constructed a detailed picture of increased stomatal ‘breathing’ and transpiration processes in response to elevated temperatures. Their findings (New Phytologist) identify two paths that plants use to handle rising temperatures.

For decades, scientists struggled to find a clear method to decipher the mechanisms underlying rising temperature-mediated stomatal openings due to the intricate measurement processes required. The difficulty is rooted in the complex mechanics involved in setting air humidity (also known as the vapor pressure difference, or VPD) to constant values while the temperature increases, and the trickiness of picking apart temperature and humidity responses. The researchers developed a novel approach for clamping the VPD of leaves to fixed values under increasing temperatures. They then teased out the genetic mechanisms of a range of stomatal temperature responses, including factors such drought hormones, carbon dioxide sensors and temperature-sensitive proteins.

The researchers found that carbon dioxide sensors are a central player in the stomatal warming-cooling responses, and can detect when leaves undergo rapid warming. This starts an increase in photosynthesis in the warming leaves, which results in a reduction in carbon dioxide. This then initiates the stomatal pores to open, allowing plants to benefit from the increase in carbon dioxide intake.

Interestingly, the study also found a second heat response pathway. Under extreme heat, photosynthesis in plants is stressed and declines and the stomatal heat response was found to bypass the carbon dioxide sensor system and disconnect from normal photosynthesis-driven responses. Instead, the stomata employ a second heat response pathway, not unlike gaining entry through a backdoor to a house, to ‘sweat’ as a cooling mechanism.