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As a result, we found that soil compaction reduces the plant size and biomass, but that the degree of plasticity in biomass varies among genotypes. Although some genotypes were tolerant by having low biomass responses, they did have reductions particularly in fine root length. These reductions were not as strong as in the sensitive lines. Additionally, we showed that plant plasticity responses to soil compaction can be explained to a large degree by allometry, that is the sensitive genotypes are relatively large while tolerant ones are smaller. Unfortunately, this apparent plasticity cannot be exploited in breeding, unless high yields can be obtained using less vigorous genotypes at higher planting densities.
Nevertheless, size-independent responses (true plasticity) were observed especially for number of nodal roots and root length of fine roots, but also for biomass allocation patterns.
As we stated in Chapter 1, soil strength and the other soil physico-chemical properties, which interact and are affected by compaction, are rarely uniformly distributed through the soil profile. Consequently, a plastic root system may direct its growth towards those soil patches with lower mechanical impedance to penetration and where resources are more available than in their surroundings. This proliferation of roots into patches with more favorable soil conditions may be advantageous and a way to compensate for lost root length. However, the results of Chapter 4 were assessed in roots were not only grown in homogeneously compacted soils but were also phenotyped without distinguishing local responses at within-root level. Thus, we do not know if the relative tolerance present in small plants is associated with greater exploration by the roots in other areas of the soil that have better conditions for growth. This response can be masked by using homogeneous soil conditions and / or when the phenotype is measured at the plant level (e.g., root length per plant). That is why in Chapter 5, a study focused on the within-root system plasticity was carried out to test whether plants are able to compensate the effect of very compacted layers with a higher root proliferation where the best condition are found. We found that the effect of a very compacted soil layer not only has a large local impact on the portion of the roots that is being affected but also on the plant as a whole. Based on these findings, the global plasticity of the root system to soil compaction may involve both local and long distance response. This may be a strategy of adaptive plasticity to counterbalance the limited function of an impeded portion of a root system. Thus, plants compensate the lower growth in compacted layers by growing more into the looser zones of the soil. This would involve a complex system of sensing and communication between the different components of the root system that should be
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studied in greater depth in future research.
In Chapter 6, a simulation-based research was conducted to study the consequence of RSA plasticity as a response soil compaction on the plant performance.
Based on that, we tested if those phenotypes with a higher RSA plasticity also express a higher nutrient and/or water uptake per unit of root length than those phenotypes with higher RSA plasticity. In Chapter 4 and 5, we showed that tolerant genotypes expressed low to null effect of soil compaction on shoot traits while the length of fine roots was reduced and there was a higher proliferation of roots (e.g. a higher root length) in the looser and more superficial soil layers. Based on the in silico experiments, we proposed that a tolerant genotype must have mechanisms to compensate a shorter root system by increasing the root uptake efficiency as long as the shoot is not severely affected.
Additionally, we discussed that this higher efficiency must be linked to the facultative expression of high-affinity transporters of nutrients and water. To assess whether greater root absorption efficiency would compensate for the reduction in fine root length, further studies are needed.
…
As a general finding, we showed that less-sensitive genotypes or tolerant are in general smaller sized genotypes. Unfortunately, this apparent plasticity may pose challenges in breeding for soil compaction tolerance. Assuming that a small plant yields less than a larger one, growing small plant genotype for soil compaction tolerance may imply to cultivate a higher plant densities (number of plants per square meter) to compensate for this lower yield. However, some forms of plasticity can be disadvantageous at the population level (Weiner, 2004). For example, increasing population density would increase competition between plants, especially if the response in plasticity is associated with greater proliferation in those areas of the soil with better conditions. Therefore, a strategy of taking advantage of the inherent diversity root plasticity would be the cultivation of heterogeneous crop populations (i.e. genetic mixtures or ‘multilines’) with differential pattern and degree of plasticity. In theory, this may allow the complementary exploitation of distinct soil zones reducing the risk of competition, especially in resource-poor soils (Lynch, 2007b).It is important to emphasize that tolerance is an environment-dependent characteristic, and the list of features that makes a plant tolerant to a specific
constraint may be different for different agricultural conditions.
In conclusion, a high RSA plasticity is associated with a greater tolerance to soil compaction in terms of biomass shoots and leaf area. This RSA plasticity is not only expressed as a reduction of fine root length but also as a greater compensation capacity of root growth. However, this greater tolerance is linked to a smaller plant size and must necessarily be supported by a greater nutrients and water uptake. Additionally, we propose that the understanding of the underlying mechanisms behind RSA plasticity provides a theoretical framework for future cropping techniques or breeding programs focused on minimizing yield penalties where the root plasticity is exploited, which might be of great value for breeding an ‘adaptive’ cultivar in specific low-input farming systems.
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