Carbon source and sink size affect seminal root system architecture in maize

Establishment of a seminal root system in maize is a crucial step in supporting the growth of nascent seedlings, until the adventitious root system is established. Nonetheless, little is known about this process in maize due to the hidden nature of roots and the difficulty to precisely assess the complicated architecture of a tangled root system in soils. The dynamics of root growth and of the concomitant carbon (C) allocation during this initial step can affect seedlings’ survival chances, growth, and yield at later stages over the life course of maize plants. Assimilated C is allocated to different competing sinks in the plant, such as the shoot and different roots, as a function of C availability and the size of the different sinks (the larger the sink size, the more C is allocated to it). Changes in roots’ C supply or sink size may affect the competition for C supply between the different sinks, and may re-shape C allocation patterns and thus root architecture. Such changes can happen due to decreased photosynthesis or stomatal closure (or both), thus affecting C supply, or due to the potential effect of heterogenous soil, that can lead to some roots outperforming others, and thus affecting the different roots’ sink size within a root system. Architectural differences in the seminal root system in maize, which consists of one primary and a few seminal roots, can result from changes in the density along the main root’s axis, diameter, length, growth rate, and the diversity of the lateral roots of the primary and seminal roots. We used aeroponics and high temporal and optical resolution monitoring to test the effect of changes in C source and sink sizes on maize seminal root architecture, and more specifically, on lateral root density, dimensions, growth, and diversity. Shading and excision of roots were used to manipulate C source and sink size, respectively. Root system images were taken every 2.5 hours, and root growth was assessed by repeatedly measuring dimensions of individual roots over a 2-3 days period using ImageJ software, combined with the SmartRoot plug-in. Under high-light conditions, the growth of the primary root’s laterals was more vigorous compared to the seminal roots’ laterals. Shading led to decreases in lateral root growth, density, and diversity, compared to well-lit seedlings. Root excision lead to changes in root architecture, probably due to changes in C allocation patterns, with increased growth of the laterals in the remaining roots (either the primary or the seminal roots). Selection for maize genotypes that are able to maintain seminal root system growth under limited water and nutrient availability conditions by maintaining (a) C allocation to the root system even when its sink size decreases, and (b) plasticity of C allocation patterns within the root system in such a way that the proportion of water and nutrient gain to C investment in constructing and maintaining roots is maximized, will contribute to increased seedling survival and crop yield under unpredicted and unfavorable conditions, typically experienced in low-input agriculture.