Say it with flowers: plants today release less moisture Plants release less water into the atmosphere than they did 150 years ago, according to a Dutch-American team of researchers who found that the density of stomata on plants has dropped approximately by a third over the past century and a half. Based on an analysis of common sp... Plants release less water into the atmosphere than they did 150 years ago, according to a Dutch-American team of researchers who found that the density of stomata on plants has dropped approximately by a third over the past century and a half. Based on an analysis of common species from Florida, the scientists conclude that many plants have scaled back in response to soaring levels of atmospheric carbon dioxide (CO2). Reducing the amount of water vegetation is able to release into the air could have potentially lasting implications for the freshwater cycle and the climate. Most plants breathe through stomata, tiny pores on the underside of their leaves through which they absorb CO2, emitting water vapour as part of the process. The incoming CO2 is transformed into sugars, which the plant uses for energy or integrates into cell walls. The outgoing moisture helps to cool the plant and water the roots. Known as transpiration, these vapour emissions also contribute to the total amount of water in the air and in the soil. Plants open and close stomata to regulate this gas exchange, and longer-lived species further adapt by adjusting the number or size of stomata on new growth. This process is triggered, among other things, by varying levels of CO2. Faced with the ongoing rise in CO2, Florida's flexible flora appears to have been plugging up its pores. Researchers from Utrecht University (Netherlands) and Indiana University (US) compared the density of stomata on a range of living plants with information from herbaria and peat formations. Current specimens were found to be 34 % short compared to those dating back a century and a half. 'The increase in carbon dioxide by about 100 parts per million has had a profound effect on the number of stomata and, to a lesser extent, the size of the stomata,' says David Dilcher of the Department of Biology at Indiana University's Bloomington campus (US). As the maximum capacity for CO2 absorption drops, so does the maximum capacity for vapour emission. 'Our analysis of that structural change shows there's been a huge reduction in the release of water to the atmosphere,' says Dr Dilcher. 'The carbon cycle is important, but so is the water cycle,' he explains. 'If transpiration decreases, there may be more moisture in the ground at first, but if there's less rainfall that may mean there's less moisture in ground eventually. This is part of the hydrogeologic cycle. Land plants are a crucially important part of it.' Dr Dilcher co-authored two papers presenting the team's findings in the Proceedings of the National Academy of Sciences of the USA (PNAS). 'Our first paper shows connection between temperature, transpiration, and stomata density,' he notes. 'The second paper really is about applying what we know to the future.' And indeed, the long-term implications for the environment could be significant. According to the authors of the second paper, 'plant adaptation to rising CO2 is currently altering the hydrological cycle and climate and will continue to do so throughout this century.' The team's projections, which focused on subtropical vegetation in Florida, suggest that the amount of water released into the air through transpiration could be halved if current CO2 levels were to double from the current 390 parts per million (ppm) to 800 ppm.For more information, please visit:Indiana University, Bloomington:http://www.iub.edu Utrecht University:http://www.uu.nl/EN/Pages/default.aspx PNAS:http://www.pnas.org/ Countries Netherlands, United States
