case study 18.1  Soybean: the unsuspected paludophyte

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R. Lawn

Commercial varieties of soybean (Glycine max L.) are intolerant of drought and transient waterlogging when grown as a summer crop in northern Australia. In this regard, soybean behaves as a typical upland grain legume. It is therefore not surprising that reports in the early 1980s of soybean growing and yielding well on soils with a water table maintained just below the soil surface were met with scepticism. Slowly, however, agronomists accepted that soybean can acclimate to sustained waterlogging of most of the soil profile; the physio-logical basis of this phenomenon is the subject of this case study.

Seedling acclimation

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Figure 1 Diagrammatic representation of soybean growing in a partially waterlogged soil bed ('saturated soil culture'). The water table is maintained 5-15 cm below the soil surface while roots with associated nodules develop in the aerobic surface soil. Water uptake, nutrient acquisition and nitrogen fixation in these plants is adequate to sustain healthy, high-yielding plants. (Courtesy R.J. Lawn)

Soybeans acclimate to waterlogging if the water table is kept at least 3–5 cm below the soil surface. If a surface zone of freely drained, albeit very wet, soil is available, roots and nodules proliferate enough to support a viable plant (Figure 1). In practice, seedlings emerge from a well-drained soil bed before being irrigated to impose a high water table.

Tap roots and associated lateral roots previously established below the new water table die from the effects of flooding (Section 18.1). Shoot growth also exhibits symptoms of flooding, primarily seen as checked leaf expansion rates and sometimes a slight inhibition of photosynthesis. The nitrogen and carbon economy of plants is severely disrupted after flooding. Nitrogen uptake is impaired presumably through death of much of the existing root system and denitrification removing available nitrates from soil. Leaf nitrogen concen-trations fall from 4.5–5.0% to 2.5–3.5%, causing transient yellowing of leaves.

Remarkably, roots in the damp, aerobic surface soil become strong sinks for nitrogen and carbon as fibrous secondary roots develop and rhizobia infect these roots to form nodules. There is also extensive growth of thin-walled aerenchyma tissue from lenticels on the surface of roots and nodules, aiding gaseous exchange (Section 18.1).

Within 10–14 d of saturation, N2 fixation commences and leaves become green again, supporting accelerated photo-synthesis and recovery of shoot growth rates. While shoot biomass and leaf area is reduced by 10–50% in plants two weeks after waterlogging is imposed, subsequent rates of accumu-lation of dry matter and nitrogen exceed those of well-drained plants by 50–100% (Troedson et al. 1989).

Why should partially waterlogged soil improve plant performance?

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Figure 2 Dry weight seed per plant increased very rapidly when soybeans (cv. Fitzroy) were grown with a water table maintained c. 10 cm from the soil surface (W) compared to plants in well-drained soil throughout the growing season (D). Note the relationship of timing of podfill to N2 fixation rates shown in Figure 4 (Troedson et al. 1989; reproduced with permission of Field Crops Research)

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Figure 3 Yield response of nodulating and non-nodulating isolines of soybean (cv. Clark 63) to fertiliser nitrogen applied throughout crop growth. Plants were grown in drained soil (D) or with a water table maintained c. 10 cm from the surface (W). When fertiliser nitrogen was supplied, both isolines yielded more when partially waterlogged than in drained soil. Without fertiliser nitrogen, nodulating isolines produced about 4 t seed ha-1 while non-nodulating soybeans yielded almost no seed. Even when fertiliser nitrogen was supplied, nodulated soybeans yielded more than non-nodulating isolines grown under the same conditions. (Troedson et al. 1983)

 

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Figure 4 Soybeans growing with a water table maintained c. 10 cm from the soil surface (W) fixed N2 at higher rates than plants growin in well-drained soil throughout the growing season (D). Additional N2-fixing activity was especially important about 70 d after sowing when rapid seed-fill provided a major sink for nitrogen. (Troedson et al. 1989; reproduced with permission of Field Crops Research)

Waterlogged soil beds promote more sustained, vigorous shoot growth than is possible in drained beds by doubling root biomass and elevating nodule numbers in surface soil by up to 170%. These efficient root systems thrive above a waterlogged subsoil and provide plants with abundant water and inorganic nutrients during podfill while soybeans in drained soil fill pods more slowly (Figure 2). New nodules form even after flowering, sustaining nitrogen delivery to pods and arresting leaf senescence through mobilisation of nitrogen from older leaves. Evidence that nitrogen is the primary factor in improved yield of soybeans under partial waterlogging is seen when nodulating and non-nodulating soybeans are supplied nitrogen at rates of up to 400 kg nitrogen ha–1 (Figure 3). Both nodulation and added fertiliser nitrogen enhance the benefits of high water tables by overcoming nitrogen limitation.

An additional advantage of maintaining constantly high water tables is the reduced chance of water deficits during periods of high evaporative demand, particularly in the tropics and subtropics where evaporation rates are high. Hence, seed yields from plants grown in partially waterlogged seed beds exceed 8 t ha–1, 10–25% above those from plants in well-drained plots. Larger plants and extended periods of N2 fixation underpin this yield advantage (Figure 4).

Genotypic factors

Soybeans, as with other crop species, have greatly contrasting genetic backgrounds. Phenology of grain crops is defined strongly by the genetically determined shift from vegetative to reproductive growth and how well this shift suits crops for a particular environment. In large trials of many soybean varieties, Asian varieties with short vegetative phases entered reproductive growth before full acclimation to waterlogging and suffered yield reductions when water tables were kept high. Soybeans grown in dry season conditions also matured early and suffered yield reductions when roots were waterlogged. A growth duration of more than 80 d is required for plants to overcome the effects of waterlogging and yield better than those in well-drained soils. Long vegetative phases gave plants time to recover but so much foliage developed that they lodged (fell over) during podfill. The best-yielding genotypes had indeterminate developmental patterns (capac-ity to continue producing leaves after flowering has commenced) giving plants time to overcome the setback to leaf development during early waterlogging and maintain substantial leaf areas throughout podfill. In these varieties, the duration of flowering is long enough that negative effects of nitrogen deficiency on podset during early flowering can be offset later.

Further tailoring of genetic makeup to environment comes through selection of strongly nodulating soybean varieties. Weak nodulators yield very poorly in waterlogged soil through nitrogen deficiency (Figure 3) where ‘hypernodulating’ geno-types offer possibilities of substantial yield advantages when water tables are raised.

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Figure 5 Lowland rice paddies in Southeast Asia showing soybean plants growing in saturated soil on the earthen bunds (banks) that separate rice fields. (Photograph courtesy R.J. Lawn)

Inherited characters conferring tolerance of modern soybean to waterlogging are believed to come from its weedy annual progenitor, Glycine soja, surviving through domestication of soybean in rice-based agriculture. In many parts of Eastern Asia, soybean is still grown on raised earthen bunds separating flooded rice paddies (Figure 5), with very wet soil profiles similar to those we report here. It is therefore not surprising that most genotypes of annual soybean and G. soja can acclimate to raised water tables. However, waterlogging tolerance is not found in the wild, perennial Glycine species indigenous to Australia.

The high degree of tolerance to waterlogging that persists in modern soybean is only now being exploited. Once con-sidered uniquely a crop of well-drained soils, soybean is now shown to have a genetic composition appropriate for more extreme conditions imposed by saturated soils. Intercropping soybean and rice is now an agronomic option, broadening the genetic makeup of paddy crops.

References

Troedson, R.J., Lawn, R.J. and Byth, D.E. (1983). ‘Saturated soil culture of soybeans’, in Annual Report, 35–36, CSIRO Division of Tropical Crops and Pastures: Brisbane.

Troedson, R.J., Lawn, R.J., Byth, D.E. and Wilson, G.L. (1989). ‘Response of field-grown soybean to saturated soil culture. I. Patterns of biomass and nitrogen accumulation’, Field Crops Research, 21, 171–187.

Further reading

Garside, A.L., Lawn, R.J. and Byth, D.E. (1992). ‘Irrigation management of soybean (Glycine max (L.) Merrill) in a semi-arid tropical environment. III. Response to saturated soil culture’, Australian Journal of Agricultural Research, 43, 1033–1049.

Hartley, R.A., Lawn, R.J. and Byth, D.E. (1993). ‘Genotypic variation among Glycine spp. for ability to acclimate to saturated soil’, Australian Journal of Agricultural Research, 44, 703–712.

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