Dynamics of Plant Root Growth under Increased Atmospheric Carbon Dioxide

Abstract

Plant growth is influenced by above- and belowground environmental conditions and increasing atmospheric carbon dioxide (CO2) concentrations enhances growth and yield of most agricultural crops. This review covers current knowledge on the impact of increasing CO2 concentration on root dynamics of plants in terms of growth, root/shoot (R/S) ratio, root biomass, root length, root longevity, root mortality, root distribution, root branching, root quality, and the response of these root parameters to management practices including soil water and nutrients. The effects of CO2 concentration on R/S ratio are contradictory due to complexity in accurate underground biomass estimation under diverse crops and conditions. Roots become more numerous, longer, thicker, and faster growing in crops exposed to high CO2 with increased root length in many plant species. Branching and extension of roots under elevated CO2 may lead to altered root architecture and ability of roots to acquire water and nutrients from the soil profile with exploration of the soil volume. Root turnover is important to the global C budget as well as to nutrient cycling in ecosystems and individual plants. Agricultural management practices have a greater impact on root growth than rising atmospheric CO2 since management practices influence soil physical, chemical, and biological properties of soil, consequently affects root growth dynamics in the belowground. Less understood are the interactive effects of elevated CO2 and management practices including drought on root dynamics, fine-root production, and water-nutrient use efficiency, and the contribution of these processes to plant growth in water and nutrients limited environments.

Global 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; Lotze- Campen 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 stres