Title: The effect of subambient to elevated atmospheric CO2 concentration on vascular function in Helianthus annuus: Implications for plant response to climate change Authors
Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 16, 2013
Publication Date: August 1, 2013
Citation: Rico, C., Pittermann, J., Polley, H.W., Aspinwall, M., Fay, P.A. 2013. The effect of subambient to elevated atmospheric CO2 concentration on vascular function in Helianthus annuus: Implications for plant response to climate change. New Phytologist. 199:956-965. Interpretive Summary: Plants grow by capturing carbon dioxide (CO2) that diffuses from air into leaves through small openings called stomata. CO2 uptake is unavoidably accompanied by the loss of water vapor from the hydrated inner portions of leaves through stomata into the atmosphere. Plants must continually balance their need for CO2 with the requirement that they not become dehydrated by excessive water loss. The capacity of plants to remain hydrated yet continue to grow was severely challenged during most of the last 15,000 years when the CO2 level in air was 70% of today’s level. Growth at low CO2 requires that stomata open wider or remain open longer, entailing significantly greater water loss and requiring significantly greater capacity to transport water from soil to leaves. We measured components of the water-transport system of sunflower plants grown at low, current, and high CO2 levels to determine how plants regulate the conflicting demand to both grow and remain hydrated as CO2 levels change. Plant grown at low CO2 increased the number and average size of stomata and capacity to transport water from the soil through stems to leaves. These changes increased growth of well-watered sunflower plants at low CO2 but at the expense of a greatly increased risk of dehydration during drought. Water transport capacity changed little from the current to elevated CO2 concentration. Our results imply that plant growth was much more sensitive to drought periods during the Pre-Industrial period than during the last 40 years or more when atmospheric CO2 concentration was significantly higher.
Technical Abstract: Plant gas-exchange is regulated by stomata, which co-ordinate leaf-level water loss with xylem transport. Stomatal opening responds to internal levels of CO2 in the leaf but changing CO2 can also lead to changes in stomatal density that influence transpiration. Given that stomatal conductance increases under sub-ambient levels of CO2 and conversely, that plants lose less water at elevated levels, can downstream effects of atmospheric CO2 be observed in xylem tissue? We approached this problem by evaluating leaf stomatal density, xylem transport, xylem anatomy and resistance to cavitation in Helianthus annuus plants grown under three CO2 regimes ranging from pre-industrial to elevated levels. Xylem transport, conduit size and stomatal density all increased at 260 ppm relative to ‘ambient’ and elevated CO2 levels. The shoots of the 260-ppm grown plants were most vulnerable to cavitation whereas xylem cavitation resistance did not differ in 370 and 480-ppm grown plants. Our data indicate that even as an indirect driver of water loss, CO2 can affect xylem structure and water transport by coupling stomatal and xylem hydraulic function during plant development. This plastic response has implications for plant water-use under variable levels of CO2, as well as the evolution of efficient xylem transport.