Minirhizotron imaging reveals that nodulation of field-grown soybean is enhanced by free-air CO2 enrichment only when combined with drought stress

Abstract

The rate of N2 fixation by a leguminous plant is a product of the activity of individual nodules and the number of nodules. Initiation of new nodules and N2 fixation per nodule are highly sensitive to environmental conditions. However, the effects of global environmental change on nodulation in the field are largely unknown. It is also unclear whether legumes regulate nodulation in response to environment solely by varying root production or also by varying nodule density per unit of root length. This study utilised minirhizotron imaging as a novel in situ method for assessing the number, size and distribution of nodules in field-grown soybean (Glycine max (L.) Merr.) exposed to elevated atmospheric CO2 ([CO2]) and reduced precipitation. We found that nodule numbers were 134–229% greater in soybeans grown at elevated [CO2] in combination with reduced precipitation, and this response was driven by greater nodule density per unit of root length. The benefits of additional nodules were probably offset by an unfavourable distribution of nodules in shallow, dry soil in reduced precipitation treatment under elevated [CO2] but not ambient [CO2]. In fact, significant decreases in seed and leaf nitrogen concentration also occurred only in elevated [CO2] with reduced precipitation. This study demonstrates the potential of minirhizotron imaging to reveal previously uncharacterised changes in nodule production and distribution in response to global environmental change.

Elevated Carbon Dioxide and Soil Moisture on Early Growth Response of Soybean

“…The result showed that, plants under elevated [103] found that nodule numbers were 134% -229% greater in soybeans grown at elevated [CO 2 ] in combination with reduced precipitation, and this response was driven by greater nodule density per unit of root length. Soil moisture also plays vital role in development of nodules since N 2 -fixation is sensitive to soil moisture than the carbon assimilation but the magnitude of the reduction in N 2 fixation depends on the severity of the drought stress as well as the timing of the drought stress relative to plant growth and development [104].…”

Crops for a future climate

“…The papers by Gray et al (2013) and Vandergeer et al (2013) in this volume emphasise the below ground processes and acquisition side of the equation, which are still considerably less explored than above ground plant functions. Plants appear to be able to capitalise on higher atmospheric CO 2 so long as they have access to adequate N, underlining the importance of nitrogen use efficiency in crops (Reich and Hobbie 2012).…”

“…Legumes such as soybean are putatively better placed to take advantage of increasing [CO 2 ], because of their capability to fix N 2 . The paper by Gray et al (2013) reports direct observations of root nodules in a FACE setup using rhizotrons. Interestingly, elevated [CO 2 ] stimulated nodulation only under drought conditions.…”

“…However, as this work relies on a glasshouse study, field evaluation is still pending. Root traits are more difficult to measure nondestructively, but the rhizotron system described by Gray et al (2013) for soybeans could be adapted for use with other crops. As we progress our understanding of what plant traits determine responses to climate changes and elevated [CO 2 ], and how they interact, measurements could be focused to such traits and better tested in field settings.…”

“…Hence, CO 2 exposure needs more involved and expensive exposure technology, with glasshouse and growth chamber options (e.g. such as those employed in Bourgault et al (2013) and Thilakarathne et al (2013)), field chamber or 'tunnel' technology (e. g. in Dias de Oliveira et al (2013)), to FACE (Free Air CO 2 Enrichment) systems (e. g. Hasegawa et al (2013); Gray et al (2013); Oehme et al (2013); all in this volume). Whilst these technologies are well established for scientific purposes with limited plot size, scaling up to screen many genotypes in large replicated experiments (as required by traditional breeding approaches) is not yet achievable or affordable.…”