Dynamics of Plant Root Growth under Increased Atmospheric Carbon Dioxide

Published in Agron. J. 105:657–669 (2013) doi:10.2134/agronj2013.0018 Copyright © 2013 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. G climate change has emerged as an important environmental challenge due to its potential impact on biological systems on Earth. Atmospheric concentrations of CO2 have steadily increased from approximately 315 μmol mol–1 in 1959 to a current atmospheric concentration of approximately 385 μmol mol–1 which converts to an average annual increase rate of nearly 2 μmol mol–1 (Keeling and Whorf, 2005). At this rate of increase, concentrations are projected to reach levels between 500 and 1000 μmol mol–1 by 2100 (IPCC, 2007). Carbon dioxide is not only a major greenhouse gas, but essential to plant growth (Kramer, 1981; Dahlman et al., 1985; Warrick, 1988; Kimball, 2011). The flow of C from photosynthesizing tissues of higher plants, through the roots and into the soil is one of the key processes in terrestrial ecosystems. Increases in atmospheric CO2 concentration will have direct and indirect effects on crop plants and increases in CO2 will generally increase plant productivity and water-use efficiency (Drake and Gonzalez-Meier, 1997; IPCC, 2007). The long-term response to CO2 remains uncertain and will depend on environmental constraints. Yields of most agricultural crops will increase under elevated CO2 with productivity increases in the range 15 to 41% for C3 crops and 5 to 10% for C4 crops (Cure, 1985; Kimball, 1983; IPCC, 2007; LotzeCampen and Schellnhuber, 2009). Analyses of plant responses to elevated atmospheric CO2 have focused largely on aboveground processes; however, understanding the effects on photosynthesis are insufficient to answer questions about overall plant response to a changing atmosphere. A whole-plant perspective is required to understand the critical feedbacks and adjustments occurring within a plant and between plants and soil. An overlooked and under studied aspect of plant response to rising CO2 is on belowground processes. The vital role of roots as an interface between the lithosphere and biosphere is necessary to understand plant response to elevated CO2. Despite the important role roots play, they have been an understudied component of agricultural research since they exist underground. Root health in crop plants will play a major role in providing sustainable highly productive crops with the ability to cope with climate changes; however, the effect of increasing CO2 on root growth and development is poorly understood. Climate change is expected to increase the incidence of extreme weather events, such as drought, heat waves, and heavy precipitation and floods, causing crop production to become more variable (IPCC, 2001, 2007; Hatfield et al., 2011). Under these conditions, many of the environmental factors, for example, water, temperature, light, nutrition, salinity, air pollutants, and competition have significant interactions with CO2 concentration on root responses for numerous species as summarized by Rogers et al. (1994). Understanding of dynamics of crop roots is important from the point of view of management of available resources to increase the productivity of crops and resilience of crops to climate stress. AbstRAct