Sowing Density: A Neglected Factor Fundamentally Affecting Root Distribution and Biomass Allocation of Field Grown Spring Barley (Hordeum Vulgare L.)

Studies on the function of root traits and the genetic variation in these traits are often conducted under controlled conditions using individual potted plants. Little is known about root growth under field conditions and how root traits are affected by agronomic practices in particular sowing density. We hypothesized that with increasing sowing density, root length density (root length per soil volume, cm cm−3) increases in the topsoil as well as specific root length (root length per root dry weight, cm g−1) due to greater investment in fine roots. Therefore, we studied two spring barley cultivars at ten different sowing densities (24–340 seeds m−2) in 2 consecutive years in a clay loam field in Germany and established sowing density dose-response curves for several root and shoot traits. We took soil cores for measuring roots up to a depth of 60 cm in and between plant rows (inter-row distance 21 cm). Root length density increased with increasing sowing density and was greatest in the plant row in the topsoil (0–10 cm). Greater sowing density increased specific root length partly through greater production of fine roots in the topsoil. Rooting depth (D50) of the major root axes (root diameter class 0.4–1.0 mm) was not affected. Root mass fraction decreased, while stem mass fraction increased with sowing density and over time. Leaf mass fraction was constant over sowing density but greater leaf area was realized through increased specific leaf area. Considering fertilization, we assume that light competition caused plants to grow more shoot mass at the cost of investment into roots, which is partly compensated by increased specific root length and shallow rooting. Increased biomass per area with greater densities suggest that density increases the efficiency of the cropping system, however, declines in harvest index at densities over 230 plants m−2 suggest that this efficiency did not translate into greater yield. We conclude that plant density is a modifier of root architecture and that root traits and their utility in breeding for greater productivity have to be understood in the context of high sowing densities

Simulation of root architecture and nutrient uptake in order to understand plant nutrient foraging strategies on low fertility soils

Increasing effort in root phenotyping and the development of noninvasive techniques for visualizing root system architecture reveals large genotypic and environmental variation in various root traits. The functions and trade-offs of these traits are not well understood. By simulating root system architecture and the nutrient uptake by that architecture, I try to develop theory on how roots forage for nutrients

Complementarity in Root Architecture for Nutrient Uptake in Ancient Maize/Bean and Maize/Bean/Squash Polycultures

During their domestication, maize, bean and squash evolved in polycultures grown by small-scale farmers in the Americas. Polycultures often overyield on low-fertility soils, which are a primary production constraint in low-input agriculture. We hypothesized that root architectural differences among these crops causes niche complementarity and thereby greater nutrient acquisition than corresponding monocultures.A functional-structural plant model, SimRoot, was used to simulate the first 40 d of growth of these crops in monoculture and polyculture and to determine the effects of root competition on nutrient uptake and biomass production of each plant on low-nitrogen, -phosphorus and -potassium soils.Squash, the earliest domesticated crop, was most sensitive to low soil fertility, while bean, the most recently domesticated crop, was least sensitive to low soil fertility. Nitrate uptake and biomass production were up to 7 % greater in the polycultures than in the monocultures, but only when root architecture was taken into account. Enhanced nitrogen capture in polycultures was independent of nitrogen fixation by bean. Root competition had negligible effects on phosphorus or potassium uptake or biomass production.We conclude that spatial niche differentiation caused by differences in root architecture allows polycultures to overyield when plants are competing for mobile soil resources. However, direct competition for immobile resources might be negligible in agricultural systems. Interspecies root spacing may also be too large to allow maize to benefit from root exudates of bean or squash. Above-ground competition for light, however, may have strong feedbacks on root foraging for immobile nutrients, which may increase cereal growth more than it will decrease the growth of the other crops. We note that the order of domestication of crops correlates with increasing nutrient efficiency, rather than production potential

CH02 - Advances in root architectural modeling during the last decade

Root architectural models (RSA) have become important tools in root research and plantphenotyping for studying root traits, processes, and interactions with the environment. Themodels have been used to simulate how various root traits and processes influence waterand nutrient uptake. At a more technical level, they have been used to develop phenotypingtechnology, particularly for testing algorithms for segmenting roots. To compute thesequantitative estimates regarding plant nutrition and root functioning, much developmentoccurred in the last decade increasing the complexity of the models. In this chapter, wedescribe first the application of the models to questions in plant biology, breeding, andagronomy, and second the development of the models. We end with a small outlooksuggesting that models need benchmarking and validation and that new developments arelikely to include better descriptions of root plasticity responses and focus on biologicalinteractions among (soil) organisms, including mycorrhizal fungi