Opportunities from opportunists: native Australian legumes Cullen cinereum and C. graveolens: traits of interest for low-rainfall agriculture

This thesis found the highly opportunistic biology of Australian native legumes Cullen cinereum and C. graveolens can provide a novel, ephemeral pasture system for low rainfall agriculture in the eastern wheatbelt of Western Australia (WA). These species occur as ephemeral herbaceous legumes, responding to sporadic summer rainfall from a soil seed-bank, across the arid interior of Australia. Field experiments conducted in the eastern wheatbelt of WA, found C. cinereum and C. graveolens to be completely dormant until summer, even when sown in autumn. Following thunderstorms, they grow rapidly, which can offset the degradation of the dry crop and pasture residues that typically feed livestock over the dry Mediterranean-type summers. Simulation with the bio-economic model MIDAS found growing ephemeral Cullen species between winter-spring grown crops or pastures on 10% of the model farm area increased total profit by 10%, increased stocking rates by 8% and decreased supplementary feeding by >50%. This was due to the replacement of degraded dry feeds by high-quality legume forage, at little cost.

Eco-physiological studies of Cullen species recruits in later summers following rainfall found they were able to achieve rapid C4-like photosynthetic rates, despite being C3 plants. This meant that they have a necessarily high stomatal conductance, which results in greater water use but also greater transpirational cooling, which was associated with a more stable photochemistry in hot conditions. It is expected that these levels of water expenditure are sustained by subsoil water use.

Subsoil salinity is considered the most widespread subsoil chemical constraint in Australian agricultural soils and is common in the soil type of the field site. A pot experiment which manipulated the field subsoil by leaching and then adding back NaCl, found that C. cinereum was the most negatively affected of the species tested including Medicago sativa, Triticum aestivum and T. turgidum. However, there were no definitive signs of specific ion toxicity in the shoots as all, except T. turgidum, excluded Na+ to very low concentrations. It is likely that osmotic stress was the main effect of the saline subsoil, although other ion imbalances are possible.

When the saline subsoil was placed below the field topsoil in deep pots, it generally had no impact on plant growth, physiology or mineral nutrition. Withholding water from half of the pots did reduce growth and physiology (gas exchange and leaf water potentials) and altered mineral nutrition, irrespective of the subsoil. Impact on roots varied with reductions in root dry mass and root length density in the subsoil layers for C. cinereum and M. sativa when well watered, but diminished when water was withheld. In this experiment, C. cinereum was able to use the saline subsoil water to the same extent as M. sativa, which suggests that salt tolerance as typically assessed under uniform salinity is poorly correlated to subsoil salinity tolerance. Triticum aestivum failed to reach the bottom of the pots of most topsoil over subsoil treatments and had a very stratified water use from the saline subsoil. However, this had no negative impact on parameters measured and some observations suggested that the saline subsoil may have delayed dehydration in T. aestivum. This suggests Cullen species are sustaining their rapid summer growth in the field from subsoil moisture, despite subsoil salinity.