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A new approach to improving yield of white clover - role of roots

S.J. Blaikie and W.K. Mason

Department of Agriculture, Research Institute, Kyabram VIC 3620 NSW Agriculture, Tamworth NSW 2340

Summary. A novel approach to identifying ways of improving the yield of white clover is by studying the performance of the roots, their relationship with shoot production and the way they respond to changes in soil conditions. These experiments examined the effects of changing soil physical conditions with depth in the root zone. Although roots grew preferentially in the most favourable soil, a strong correlation was maintained between shoot growth and the sum of root growth from both soil layers. The presence of an unfavourable layer inhibited root growth in the other layer, with the result that shoot growth was reduced.

Introduction

White clover, Trifolium repens, is an important component of irrigated pastures in northern Victoria. Past research with white clover pasture has focused on measuring responses in shoot growth to new or modified techniques of management and has led to an improvement in annual yield from about 15 t DM/ha to 20 t DM/ha. However, given the ready availability of irrigation water, much higher yields of pasture should be possible (1). The soils of the area are duplex, with a loam topsoil (0.05-0.20 m) and dense (bulk density 1.6 t/m3) clay subsoil. Conditions in the soil are generally sub-optimal for roots with the result that their growth is restricted. Work with other species (e.g., 2) has shown that an approach to improving shoot yields lies in developing an understanding of the factors which control root growth and the relationships between root and shoot production.

Previous work with white clover (3) has demonstrated that a strong correlation exists between the size of the root system and the size of the shoot system it supports. This highly coordinated growth of white clover roots and shoots is consistent with the idea (e.g., 4) that a line of communication exists between the two systems by which the roots send signals to the shoots that influence their growth by reflecting the capacity of the soil environment to support root growth and function.

In the field, the root system grows in two soil layers of contrasting physical conditions (the loam topsoil and the clay subsoil). This paper reports the results of two experiments that investigated the degree to which such a change in soil physical conditions with depth affects the relationship between root and shoot growth and whether extra root growth in favourable layers can compensate for the lack of growth in unfavourable soil layers.

Methods

The experiments were conducted in a glasshouse at the Kyabram Research Institute. Single, vegetative cuttings of white clover cv. Haifa (2 to 3 leaves) were planted in soil cores containing one of a range of treatments. The plants were regularly watered and fertilised and the cores were drained on sand columns to avoid the effects of any water, nutrient or oxygen deficiencies on plant growth. Destructive harvests were taken at intervals (at 19 and 32 days Expt 1; at 22, 33 and 41 days Expt 2) to monitor root and shoot development.

Soils were collected from field sites on the Institute. All field soils were Lemnos loam, a red-brown earth, which typically has 0.15 m of loam over clay subsoil. Cores (0.11 m inside diameter; 0.075 m deep) contained either topsoil (top), subsoil (sub) or modified soil (mod). The modified soil was from a highly productive site which had been created for an earlier experiment. Typical annual yields of pasture (DM) from these sites are: top 20 t/ha; sub 10 ti ha; mod 25-30 t/ha. In addition to the field soils, some cores contained a sand-based potting mix (sand) which allowed comparison with earlier work.

Treatments were created by stacking two cores one on top of the other and binding them firmly together to form a layered core of overall depth 0.15 m. Lifting the cores from the sand columns each day halted the growth of roots that had protruded through the base of the cores. Thus, root growth was confined within the cores. At each destructive harvest, root growth in each layer could be determined by slicing the cores into their original components and washing the roots from each separately. Roots were stored in formalin and their number was determined by sub-sampling and counting branch points prior to drying and weighing. Throughout this paper treatments will be named according to the format: soil in top core/soil in bottom core. The treatments for experiment 1 were: sand/sand, sand/sub, mod/sub, top/sub, sub/sub, sub/sand and for experiment 2 were: sand/sand, sand/nil, sand/mod, sand/top, sand/sub. There were four replicates in each experiment and in both cases the data were analysed using analysis of variance with l.s.d. calculated at P=0.05.

Results and discussion

In experiment 1 there was a wide range in shoot and root dry weights between treatments at both harvests although only the final harvest data are presented here (Table 1). Plants in the most productive treatment, sand/sand, grew about three times as much DM as those in the least productive treatments, top/sub, sub/sub and sub/sand.

Table 1. Shoot and root dry weights (g) at the end of Experiment 1.

Although there were large effects of treatments on plant growth, the combined data from all treatments and both harvests demonstrated that the sizes of roots and shoots were correlated (Fig. 1). The good fit of the straight line that was fitted to the allometric (log-log) plot, indicates that plants in all treatments used the same strategy in terms of allocating DM between roots and shoots. Thus, the ratio of the relative growth rates of shoots and roots (i.e., the slope constant) remained constant throughout and was not affected by either plant size or soil treatment.

Figure 1. The relationship between shoot and root dry weight in experiment 1. Open circles - harvest 1; Closed circles - harvest 2. The linear regression is of the form ln(shoot DM)=1.52+1.05*1n(root DM), n=45, R2=0.97, rsd=0.17.

The distribution of roots varied widely between soil treatments (Fig. 2) and in absolute terms, the growth of roots in the different soils (other than sand) reflected the yield of pasture grown in these soils in the field. When both layers were the same (i.e., sand/sand or sub/sub), the roots were distributed approximately evenly between the two. In all other treatments, the roots grew preferentially in the more favourable soil layer.

Figure 2. Root number per plant in each layer of treatments in experiment 1. The top section of each bar represents the growth of roots in the upper soil layer of each treatment. The I.s.d. (P=0.05) are 0.83 upper; 0.87 lower. A-sand/sand, B-sand/sub, C-mod/ sub, D-top/sub, E-sub/sub, F-sub/sand.

It is interesting to note, however, that the plants did not compensate by increasing the number of roots in the favourable soil. This is best illustrated by comparing the growth of plants in sand/ sand and in sand/sub. Although the root growth in sand/sub was restricted in the sub layer, the root growth in the sand layer was no greater than in the upper layer of sand/sand. The question then arises as to why the growth of roots in the sand layer of sand/sub was not greater, to ensure that the sum of root growth in both layers was equivalent to the sum of root growth in sand/sand. This would, in turn, have ensured that the shoot yields of plants in each treatment were similar.

It is unlikely that any water, nutrient or oxygen stresses were restricting the growth of roots in the sand layer of sand/sub because the experiment was managed to avoid these problems. The plants were not 'pot-bound' since earlier experiments had demonstrated that it is possible to grow four to five times as many roots in the same volume of sand without retarding growth. This leaves the possibility that there was an effect from the roots growing in the subsoil that was restricting the overall growth of the rest of the plant.

The second experiment provided further evidence for such a mechanism by demonstrating that when plants were grown in a core of sand alone (sand/nil) root growth was much greater than in the upper layer of sand/sand and, despite the fact that the potential rooting volume was halved, shoot and root DM in the sand/nil treatment were not significantly different from the sand/sand treatment (Table 2). However, when grown in sand/any of the other soil treatments, shoot and root growth were reduced compared with the plants in sand/sand.

Table 2. Shoot and root DM and root numbers in experiment 2.

Thus, it can be concluded that by removing the soil layer from below the sand we removed a limitation to root growth in the sand layer. It follows that the restrictions to root growth caused by an unfavourable layer of soil in the root zone cannot be compensated for by improving the conditions in the rest of the root zone.

References

1. Cooper, J.P. 1970. Herb. Abstr. 40, 1-15.

2. Richards, D. and Rowe, R.N. 1977. Ann. Bot. 41, 729-740.

3. Blaikie, S.J. and Mason, W.K. 1990. Aust. J. Agric. Res. 41, 891-900.

4. Passioura, J.B. 1988. Aust. J. Plant Physiol. 15, 687-693.

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