An international team of plant scientists has identified the sensors that
plants use to navigate between drying out and starving in dry conditions.
Led by Rainer Hedrich, a biophysicist from Julius-Maximilians-Universität
(JMU) Würzburg in Bavaria, Germany, the research team published the results
of their research in Nature Plants.

The stomata of plants are comprised of pores and guard cells. The guard
cells must be able to measure photosynthesis and water supply to respond
appropriately to changing environmental conditions. They use a receptor to
measure the CO2 concentration inside the leaf. Sharp increases in CO2 levels
signify that photosynthesis is not running ideally. The pores close to
prevent unnecessary evaporation and reopen once the CO2 concentration has
fallen. Water supply, on the other hand, is measured through the hormone
abscisic acid (ABA). This hormone is produced when water is scarce, and
plants set their CO2 control cycle to water-saving mode. This process is
possible through guard cells fitted with ABA receptors. When the hormone
concentration in the leaf increases, the pores close.

"We conclude from the findings that the guard cells offset the current
photosynthetic carbon fixation performance with the status of the water
balance using ABA as the currency," Hedrich explains. They found that when
water supply is good, ABA receptors evaluate the basic hormonal balance as
quasi 'stress-free' and keep the stomata open for CO2 supply. When water is
scarce, drought stress receptors recognize the elevated ABA level and make
the guard cells close the stomata to prevent the plant from drying out.

[www.uni-wuerzburg.de]-
measure-their-co2-uptake/