Abstract
Soil compaction of arable land, caused by heavy machinery constitutes a major threat to agricultural soils in industrialized countries. The degradation of soil structure due to compaction leads to decreased (macro-) porosity resulting in increased mechanical impedance, which adversely affects root growth and crop productivity. New crop cultivars, with root systems that are adapted to conditions of increased soil strength, are needed to overcome the limiting effects of soil compaction on plant growth. This study aimed (i) to quantify the genetic diversity of early root system development in wheat and to relate this to shoot development under different soil bulk densities and (ii) to test whether root numbers are suitable traits to assess the genotypic tolerance to soil compaction. Fourteen wheat genotypes were grown for 3 weeks in a growth chamber under low (1.3 g cm-3), moderate (1.45 g cm-3), and high soil bulk density (1.6 g cm-3). Using X-ray computed tomography root system development was quantified in weekly intervals, which was complemented by weekly measurements of plant height. The development of the root system, quantified via the number of axial and lateral roots was strongly correlated (0.78 < r < 0.88, p < 0.01) to the development of plant height. Furthermore, significant effects (p < 0.01) of the genotype on root system development and plant vigor traits were observed. Under moderate soil strength final axial and lateral root numbers were significantly correlated (0.57 < r < 0.84, p < 0.05) to shoot dry weight. Furthermore, broad-sense heritability of axial and lateral root number was higher than 50% and comparable to values calculated for shoot traits. Our results showed that there is genetic diversity in wheat with respect to root system responses to increased soil strength and that root numbers are suitable indicators to explain the responses and the tolerance to such conditions. Since root numbers are heritable and can be assessed at high throughput rates under laboratory and field conditions, root number is considered a promising trait for screening toward compaction tolerant varieties.
Highlights
It is estimated that an area of 68 million hectares of arable land is degraded by soil compaction (Hamza and Anderson, 2005; Batey, 2009), which is caused by the increasing use of heavy agricultural machinery in modern agriculture (Tracy et al, 2011)
Two and 3 weeks after emergence leaf number and plant height under high soil bulk density were around 35% lower compared to the plants grown under low and moderate bulk density (Table 3)
Moderate soil compaction led to a slight reduction in leaf number and plant height of around 7% compared to the low bulk density treatment
Summary
It is estimated that an area of 68 million hectares of arable land is degraded by soil compaction (Hamza and Anderson, 2005; Batey, 2009), which is caused by the increasing use of heavy agricultural machinery in modern agriculture (Tracy et al, 2011). In comparison to undisturbed soils, compacted soils are characterized by lower (macro-) porosity and decreased pore connectivity (Bottinelli et al, 2014; Chen G. et al, 2014; Kuncoro et al, 2014b) This soil structural degradation adversely affects soil physical functions, which limit root growth and decrease agricultural productivity (Barraclough and Weir, 1988; Botta et al, 2010; Arvidsson et al, 2014). Crop growth in compacted soils may be limited due to low levels of plant available water (Lipiec and Hatano, 2003) and decreased fluid transport rates (Kuncoro et al, 2014a; Colombi et al, 2017) Together these adverse changes of soil physical functions lead to decreased physical soil fertility caused by soil compaction (Abbott and Murphy, 2007). The integration of root traits into breeding programs is suggested to increase the tolerance of crops to soil derived abiotic stress and to contribute to crop productivity under limited soil fertility (York et al, 2013)
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