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Assessing the impact of high soil boron and salinity on the root growth of potential primer-plant species

Stephen Davies1, James Nuttall2, Roger D. Armstrong2 and Mark Peoples1

1 CSIRO Plant Industry, Black Mountain Laboratories, GPO Box 1600, Canberra, ACT 2601.
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Department of Primary Industries, Private Bag 260, Horsham, Vic 3401.
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Primer-plants are those considered capable of modifying potentially hostile soil conditions for the benefit of following crops. However a primary consideration when selecting a potential primer crop will be its ability to tolerate soil chemical constraints such as boron (B) toxicity and salinity. In one experiment, the impact of increasing levels of B and salinity on root dry weight (DW) was assessed for nine species, where the highest B and salinity treatments were equivalent to levels encountered at depth on alkaline soils in south-eastern Australia. There was a 30-60% decline in root DW at the highest level of soil B (48mg extractable B/kg) across the species, except in sulla in which there was no decline in root DW. The impact of salinity (ECe = 21 dS/m) on root DW was even greater with up to an 90% decline in root DW for some species. There was little difference between species at the highest salinity level (21dS/m), but significant differences (P <0.05) at the moderate salinity level (11.6 dS/m) with wheat and the perennial grasses, tall fescue and phalaris being more tolerant than the sensitive dicots, chickpea and lucerne. Tall fescue and tall wheatgrass were the only species whose root dry weight increased in response to salinity. In a second experiment, the impact of salinity and B toxicity on root development in lucerne and chicory was examined. Both lucerne and chicory compensated for the reductions in rooting depth by increasing lateral root growth in the benign soil layers. Overall, total lateral root length increased by 20% in lucerne in response to salinity but declined in chicory by 35% due to a decline in lateral root number.

Media summary

The root growth of potential primer-plants capable of modifying (priming) the soil for subsequent crops vary in their tolerance to soil salinity and boron toxicity, constraints commonly occurring in the subsoils of south-eastern Australia.

Key Words

primer-plants, boron toxicity, salinity, root development, perennial.


Root growth, and thus grain yields are significantly restricted in large areas of south-eastern Australia by a range of chemical and physical constraints in the subsoil (Rengasamy 2002; Nuttall et al. 2003). When a plant species is grown in rotation with a crop with the specific aim of improving the growth of the crop by modifying the conditions in such soils, the species can be referred to as a primer-plant (Yunusa and Newton 2003). While usage of the term primer-plants is relatively recent the concept of using plants to improve soil conditions is not new, with previous studies often using the term ‘biological ploughing’ or ‘biological drilling’ when referring to the use of plant roots to ‘break-up’ or generate root channels in and through hard soils (Henderson 1989; Cresswell and Kirkegaard 1995). Examples of species capable of ‘biological drilling’ to generate some benefit to following crops include lucerne (Rasse and Smucker 1998; McCallum et al. 2004), lupin (Henderson 1989), stylosanthes (Lesturgez et al. 2004) and bahia grass (Elkins et al. 1977). However, it can be difficult to distinguish between benefits due to physical amelioration and other break-crop effects (Cresswell and Kirkegaard 1995). In alkaline soils of south-eastern Australia physical and chemical constraints tend to co-exist, where subsoil salinity, sodicity and toxic concentrations of boron (B) are common (Nuttall et al. 2003). Suitability of primer-plants to alkaline soils will therefore require tolerance to a range of physicochemical constraints. With this in mind a number of potential primer-plant species were chosen and screened for their relative tolerance to salinity and high B. The impact of these constraints on seedling root development was also assessed on a subset of species.


Screening primer-plant seedlings for tolerance to toxic concentrations of boron or salt

Soil was collected from the 10-20cm layer of an alkaline soil near Birchip in the Mallee region of western Victoria. The soil had low levels of salinity (ECe =1.8dS/m) and extractable B (1.5mg/kg). Two bioassays were conducted, one each for B and salinity. Experiment 1: Soil treatments were established by spiking 1kg batches of air-dry soil (<5 mm) with either boric acid or salt solution (Table 1). After drying the soil was placed in pots lined with a plastic bag to prevent leaching of salts and watered to 90% of field capacity by weight. The range of primer-plant species tested were selected on the basis that they could potentially be adopted into farming systems in a similar way to lucerne. Data for the annual crops chickpea (Cicer arietinum) and wheat (Triticum aestivum), the perennial dicots chicory (Cichorium intybus), lucerne (Medicago sativa) and sulla (Hedysarum coronarium) and the perennial monocots tall fescue (Festuca arundinaceae), phalaris (Phalaris aquatica) and tall wheatgrass (Thinopyron ponticum) are reported here. Due to large species differences in seedling growth rate the species were harvested when root systems in the control (untreated) soil had fully explored the volume of the pot. At harvest shoots were removed and root dry weight (DW) measured after drying at 70C.

Table 1. Rates of application of boron and salt solution applied to soil and measured levels of extractable boron and salinity.

Boron applied (mg/kg)

Extractable boron

Salt applied

Electrical conductivity (dS/m)

















Impact of toxic concentrations of boron and salt on seedling root growth

Experiment 2: Pots (160 mm diam. 450 mm height), split-vertically, were used to avoid roots from touching the casing. Soil treatments were the same as those used in the above bioassays except that only the two highest levels of soil B or applied salt were used in addition to the control soil. The pots were packed with 9kg of the treated soil with an additional 1kg of benign (control) soil on top. This allowed the seeds to germinate and root growth to commence in relatively benign soil before reaching the hostile soil layer. At harvest, one half of the split pot was removed and the roots carefully washed and excavated from the soil. Rooting depth was measured and the number of lateral roots counted, the roots were then stained and scanned (Fig. 3) with total length of the first-order lateral roots determined using WINRHIZO. Only results for chicory and lucerne are reported here.

Results and Discussion

Screening primer-plant seedlings for tolerance to toxic concentrations of boron and salt

Boron had a significant impact on root DW with the highest level of soil B causing a 30-60% decline in most species compared with the control, apart from sulla in which there was no decline in root DW (Figure 1a).

Figure 1. Change in relative root dry weight as a percentage of the control in a range of species subject to toxic levels of boron (B) and salt.

Figure 2. Scanned images of lucerne (a-c) and chicory (d-f) roots and the impact of toxic levels of boron (b,e; 48mg B/kg) or salt (c,f; ECe = 21dS/m) in comparison with roots in benign soil (a,d)

Salinity had a greater impact on root DW than B with the highest level of salinity reducing root DW up to 90% (Figure 1b). At the highest level of soil salinity there was little difference between the species, except that the relative root DW of canola was higher than the most sensitive species. At the lower level of salinity (6.9dS/m) the root DW of tall fescue and tall wheat grass increased while at the moderate level of salinity (11.6dS/m), wheat, phalaris and tall fescue were more tolerant than the more sensitive dicots (Figure 1b). Of the dicot species, canola was the only species able maintain root DW to the same extent as the monocots with the root DW of all the other dicot species declining more rapidly with increasing salinity (Figure 1b). Chickpea maintained root DW from the moderate to highest level of salinity, however this was not true of shoot DW, which declined markedly over this salinity range resulting in a large increase in root to shoot ratio from 0.6 to 2.6 (data not shown). This suggests that root growth was maintained in chickpea, probably using seed reserves, despite the large decline in shoot growth.

Impact of toxic concentrations of boron and salt on seedling root growth

The effect of toxic levels of B and salinity on root architecture revealed a similar response to that found for root DW with salinity having a greater impact on root growth than B toxicity in both lucerne and chicory (Figure 2). High B caused a significant decline in rooting depth (by 9% in lucerne, Figure 2a&b, and 20% in chicory, Figure 2d&e). Similarly, B also reduced lateral root number by 3% in lucerne and 26% in chicory. However, both lucerne and chicory were able to compensate for the decline in rooting depth by increasing the length of the lateral roots, particularly in the benign soil layer. The average length of the lateral roots increased from 2.7 to 3.6cm for lucerne and from 4.8 to 5.5cm for chicory (data not shown). Similar compensation occurred in the plants subject to salinity with the lateral roots in the upper layers growing longer to compensate for the 44% reduction in rooting depth that occurred for each species. However, while average lateral root length increased (4.1cm in lucerne and 6.0cm in chicory) the total number of lateral roots was reduced by 20% for lucerne and 45% for chicory (data not shown). Overall the total lateral root length increased by 20% in lucerne subject to salinity, but declined in chicory by 35% (data not shown).


Primer-plants have been proposed as a biological means of creating porosity in soils that have naturally high bulk density at depth. Along with high bulk density, many alkaline soils of south-eastern Australia also have high levels of B, salinity and sodicity. We found that considerable variation existed in root growth of a range of primer-plants to high levels of soil B and salinity. Dicots appear to respond to high salinity by using an ‘avoidance’ mechanism, where lateral roots concentrate in the topsoil, and so have limited capacity to colonise saline subsoils. If primer-plants are used for physically ameliorating alkaline subsoils, then tolerance to a range of physicochemical constraints needs to exist. Although lucerne is commonly used in farming systems as a break crop, its effectiveness at creating biopores in many alkaline soils may be limited given its poor capacity to tolerate salinity. The data imply that species such as tall fescue and phalaris may be better adapted to alkaline soils where multiple physicochemical constraints exist.


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