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Within-plant phenotypic plasticity of root system architecture of cereal crops under vertical soil factor gradients - An integrative approach to study the effects of soil compaction

José Correa

Research question/general hypothesis
We will test the hypothesis that genotypes exhibiting a high within-plant phenotypic plasticity of root system architecture (RSA-WPV) have an advantage over those with low RSA-WPV with respect to nutrient acquisition, water uptake, plant biomass, and, ultimately, yield; under conditions of vertical soil factor gradients.

ABSTRACT
Rootsystem architecture (RSA) describes the spatial arrangement of root components and have a crucial role in determining the ability of a plant to better explore the soil for water and nutrients. However, soils not often supply the most favorable conditions for crop growth and they impose physical, chemical and biological stresses on plant roots restricting plant growth and development especially in degraded soils. Furthermore, soils are heterogeneous: their physicochemical and biological properties vary in both time and space. Despite that, plants are able to adjust and compensate to such constrains through their phenotypic plasticity. However, it is not a straightforward task to determine if a particular phenotypic change is an active response focused to maximize the plant fitness or only a simple passive response with no relevant consequences on plant performance. Based on that, the aim of this thesis is to study the cereal RSA into a changing soil environment. We will focus on space gradients of soil compaction to evaluate their effects on the within-plant phenotypic plasticity of RSA.As a first step, three preliminary experiments using one genotype of sorghum (Sorghum bicolor L. Moench) were done. We estimated that a minimum of 12 replicates and 2 plants per pots are needed to detect a significant effect of soil compaction on plant phenotype at a root penetration resistance ca. 1.2 MPa. In addition, a possible decoupling response between shoot and root growth to soil compaction is proposed for this genotype in particular since no effect on shoot traits were found whereas fine roots showed a significant plasticity (Figure 1, 2 and 3). As immediate steps (in progress), we proposed to identify those sorghum genotypes with contrasting plasticity to soil compaction and to elucidate the actual effect of plant size on this response. To define a plastic response we will focus on those phenotypic changes associated with the following RSA traits: number and length of lateral roots (lateral root density), total length of root system (total sum of root length), root dry mass, specific root length [ratio of root length to root dry mass (m/g)], root axis diameters, lateral root angles and root system tortuosity (ratio of root length to the shortest distance between two arbitrary points in the root such root origin and tip). To test whether within-plant plasticity of RSA is an active response to tolerate vertical gradients of root penetration resistance gradients in sorghum, experimental and simulation studies will be carried out using those genotypes, growing in a system of vertical gradients of soil bulk density. Finally, we will propose an ideotype for soil compaction conditions. The analysis of within-plant phenotypic plasticity of root and shoot (within the same plant) will be focused on an integrative approach in order to interpret it in a biological sense as a whole.

Keywords
Phenotypic variation/variability; soil bulk density; adaptive plasticity; genotype×environment interaction

 Figure 1. Distribution (A) and accumulated (B) distribution of root length according the root diameter classes under loose and compacted soil conditions for pot tray experiment. The root length was recorded in 25 ranges of root diameter between 0 and 2.5 mm.

 Figure 2. Specific root length per plant under loose and compacted soil conditions across experiments. The significant result is according to two-sample t-test for each experiment and highlighted by red lines connecting loose and compacted condition and the following code (P value): ***0–0.001; **0.001–0.01; *0.01–0.05.

 Figure 2. Specific root length per plant under loose and compacted soil conditions across experiments. The significant result is according to two-sample t-test for each experiment and highlighted by red lines connecting loose and compacted condition and the following code (P value): ***0–0.001; **0.001–0.01; *0.01–0.05.


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