Reallocation to lateral and early-emerging axial roots allows maize (Zea mays L.) with reduced nodal root number to more efficiently forage for nitrate

Abstract

Short title:Reduced nodal rooting leads to more foraging roots Summary:Reallocating root biomass from nodal roots to lateral and early-emerging axial roots allows grasses to capture more nitrogen under limiting conditions, including by increasing foraging at depth. ABSTRACTPrevious simulations indicated reduced nodal root number (NRN) was promising for maize (Zea mays L.) breeding, and were partially confirmed using variation in NRN among inbreds. However, the exact mechanism was unknown, therefore manipulative experiments were conducted in hydroponics and tall solid-media mesocosms with treatments involving no nodal root excision (0% NRE) or excising either 33% or 67% of the nodal roots (NR) as they emerged under high or low levels of nitrogen (N). Reduced NRN was hypothesized to increase elongation of all remaining root classes, increase N acquisition under low N, and increase shoot mass. In both experiments, plants with 67% NRE had 12% and 19% less root fraction of total biomass, 61% and 91% greater lateral-to-axial root length ratio regardless of N levels; and 61% and 182% greater biomass of embryonic roots under low N, compared to 0% NRE for hydroponics and mesocosms studies, respectively. In hydroponics, regardless of NRE level, specific root 2 respiration under high N was 2.6 times of low N, and was greatest at depth. Under low N in mesocosms, plants with 67% NRE had 52% greater shoot biomass, 450% greater root length at depth, and 232% greater deep-injected 15 N content in the shoot relative to 0% NRE, however biomass in hydroponics did not differ based on NRE. These results reveal the mechanism by which plants with fewer nodal roots increase N capture and shoot mass by reallocation of biomass to lateral, embryonic, and first whorl nodal roots that increases foraging efficiency in solid media. Lynch JP (2013) Steep, cheap and deep: an ideotype to optimize water and N acquisition by maize root systems. Ann Bot 112: 347-357 Lynch JP (2018) Rightsizing root phenotypes for drought resistance. J Exp Bot 69: 3279-3292 Lynch JP, Brown KM (2012) New roots for agriculture: exploiting the root phenome. Philosophical Transactions of the Royal Society of London B: Biological Sciences 367: 1598-1604 McCarthy MC, Enquist BJ (2007) Consistency between an allometric approach and optimal partitioning theory in global patterns of plant biomass allocation. Functional Ecology 21: 713-720 Meister R, Rajani MS, Ruzicka D, Schachtman DP (2014) Challenges of modifying root traits in crops for agriculture. Trends Plant Sci 19: 779-788 Mi G, Chen F, Yuan L, Zhang F (2016) Ideotype root system architecture for maize to achieve high yield and resource use efficiency in intensive cropping systems. In Advances in Agronomy, Vol 139. Elsevier, pp 73-97 Poorter H, Niklas KJ, Reich PB, Oleksyn J, Poot P, Mommer L (2012) Biomass allocation to leaves, stems and roots: meta-analyses of interspecific variation and environmental control. New Phytol 193: 30-50 R Core Team (2018) R: A Language and Environment for Statistical Computing, R Foundat...

Barley shoot biomass responds strongly to N:P stoichiometry and intraspecific competition, whereas roots only alter their foraging

“…In contrast, N (as nitrate) is relatively mobile in the soil compared to P and moves down the soil strata with irrigation and precipitation events. Fewer crown roots in maize, for example, can potentially improve N acquisition by exploring deep soil strata, a key root system response (Saengwilai et al 2014;Guo and York 2019). Therefore, the coordinated uptake and utilization of both N and P are essential in relation to optimal plant growth.…”

Barley shoot biomass responds strongly to N:P stoichiometry and intraspecific competition, whereas roots only alter their foraging

“…Given that nutrient foraging by roots is an active process (Zhang et al 2019), it is very likely that plant biomass allocation and root system responses will be driven by the nutrient which is limiting plant growth the most (Poorter et al 2012). It has previously been shown for many crops how eco-physiological (Gastal and Lemaire, 2002), morphological (Fransen et al 1998), architectural (Williamson et al 2001; Lopez-Bucio et al 2003; Postma and Lynch, 2012; Lynch, 2013; Guo and York, 2019), and anatomical (Wahl et al 2001; Postma and Lynch, 2011) root traits respond to nitrogen (N) and phosphorus (P) availability in soil. For instance, Wang et al (2015) showed contrasting root morphological and physiological trait responses of canola, barley, and potato in relation to low P availability.…”