By Kirsten Slemint, Danielle Jones and Brittaniee-Jane Lawler
Australia’s Salinity Solution
The Oldman Saltbush (Atriplex nummularia) thrives in extremely saline soils with highly evolved biochemical mechanisms and by utilising underground saline water with a root system several metres below the surface (Ben Salem et al., 2010). It is distributed throughout the semi-arid and arid zones of southern and central Australia where it has evolved despite grazing by macropods such as kangaroos (Ben Salem et al., 2010).
A study conducted by Deescheemaeker et al., (2014) reviews the potential biomass production and regrowth rates of the Oldman Saltbush in varying soil types and landscape positions; while Ben Salem et al., (2010) examines its use as sheep and goat feed showing major limitations in its digestibility.
Hussin et al., (2012) attempts to uncover the interconnected relationships between the underlying mechanisms the plant utilises to adapt to these extreme environments. In particular, the adaptation of salt accumulation in the bladder hairs and the independent evolution of the C4 pathway of the Oldman Saltbush results in a low transpiration rate and high osmotic potential.
Biological and Physiological Advancements of the Oldman Saltbush
At present, salinity affects over 7.5 million km² of Australia. Modelling suggests that the area of saline-affected land will double to 15% in Australia by 2050 (Reece et al., 2014). This highlights the importance of halophyte species such as the Oldman Saltbush in their adaptability to these edapho-climatic conditions (eg. high soil salinity, rainfall below 300mm, high temperature, etc) (Ben Salem et al., 2010).
The Oldman Saltbush has many alternative metabolic mechanisms such as the C4 pathway in the Calvin Cycle (Reece et al., 2014) and the ability to regulate its ion concentration by osmosis and bladder hairs located on the leaf epidermis (Hussin et al., 2012). The C4 pathway is an evolved adaption of increased glucose production in which CO₂ is pumped from the mesophyll cells into the bundle sheath cells where there is a lower concentration of O2, therefore, minimising photorespiration where ATP is consumed with no output of glucose (Reece et al.,2014).
As saline areas are high in osmotically active ions (Na+, K+ and Cl-) this hypertonic environment can result in a plasmolysed plant cell (Lluka, 2016). However, the Oldman saltbush is observed to remain turgid under these conditions as it is able to hold onto water by sending these ions from the roots via the xylem system into the leaves. A buildup of these ions can result in ion toxicity.
In response, the Oldman Saltbush will excrete excess ions via bladder hairs on the leaf epidermis to maintain the internal salt concentration (Hussin et al., 2012). According to Hussin et al. (2012), there is not much known about the mechanisms of these bladder hairs however they are pivotal to the success of the Oldman Saltbush in these conditions.
A deep fibrous root system in the Oldman Saltbush reaches saline water tables while a lower average stomatal density allows for a substantial reduction in transpiration rate and loss of water (Descheemaker et al., 2014). Research shows that long-term average leaf biomass production is shown to decrease significantly when rooting depths are restricted to less than 1m (Descheemaker et al., 2014) which highlights the importance of these adaptations.
Hussin et al., (2012) aimed to examine which metabolic and physiological features have allowed the Oldman Saltbush to be so well adapted to arid and saline environments, with particular emphasis on CO₂ exchange and salt accumulation in bladder hairs. This was achieved by planting seeds of the Oldman Saltbush into a greenhouse with controlled conditions exposed to varying degrees of seawater salinity (sws) and observed. The osmotic potential, CO₂ gas exchange and ion concentrations in the roots, leaves and bladder hairs were measured throughout the duration of the experiment and total fresh weight was determined upon conclusion at 11 weeks (Hussin et al., 2012).
Results from the study revealed that the Oldman Saltbush achieved a greater biomass in all saline concentrations than compared with the control group. In particular, it showed optimal growth occurring at 50% sws (Hussin et al., 2012). It was also evident that increasing saline levels increased NaCl concentrations. Most of the NaCl ions were evident in the adult leaves and bladder hairs with the least concentration found in the roots (Hussin et al., 2012).
This highlights the importance of the plants’ ability to regulate osmotic potential by excreting excess ions via the bladder hairs and increased glucose production through its C4 pathway mechanism. Hussin et al.,(2012) has also compared studies conducted on other halophyte species and found similar responses.
These results conclude that hyperosmotic conditions are not a limiting factor for the growth of the Oldman Saltbush as it is able to maintain sufficient water influx in high saline conditions (Hussin et al., 2012).
Research conducted by Descheemaker et al., (2014) aimed to capture the effect of soil variability on growth and recovery rates of the Oldman Saltbush. Field research was conducted by planting the Oldman Saltbush in alley systems in three different landscape positions (dune, mid and swale) with different soil types from loamy sands to heavier clay. Over time, the growth rates were measured and following exposure to grazing events the recovery rates were also measured.
Results from this study show that biomass production levelled off rather than declined in the extremely dry and hot summer (Descheemaker et al., 2014). A low transpiration rate also allows the plant to last 2-3 years without rainfall in deep soils such as the dune position or for <1 year in swale position with depth constraints whilst still maintaining biomass production rate (Descheemaker et al., 2014). Thus, the conclusion is reached that the Oldman Saltbush could be a viable feed alternative in hostile environments as this research also allows predictions to be made on the daily biomass accumulation and regrowth potential.
The conclusions within the research of Descheemaker et al., (2014) and Hussin et al., (2012) highlight the suitability of the Oldman Saltbush to arid and high saline environments and allude to the variety of its potential uses.
Through understanding the physiology and adaptive mechanisms of the Oldman Saltbush this research may also provide possible routes to enhance salt resistance in other crops (Hussin et al., 2012) and by lowering the water table it aids survivability of many other plant species (Descheemaker et al., 2014).
What is the value of this research?
It was evident in the results gathered that the bladder hairs aided in accumulating ions to maintain a negative osmotic potential, however, Hussin et.al, (2012) highlights there is very little information available on the functionality of these bladder hairs.
Understanding the mechanism behind this would help further contribute to the current understanding of the Oldman Saltbush biology and potentially assist our research in developing salt resistance mechanisms in other plant and crop species. More than 30% of the world’s farmland has reduced productivity as a result of poor soil conditions due to chemical contamination, mineral deficiencies, acidity, salinity and poor drainage (Reece et al., 2014). These figures clearly demonstrate the importance of our further understanding of the functionality of the bladder hairs.
Similarly, the experiment conducted by Descheemaker et al., (2014) does not extensively model or investigate the growth, development or function of the Oldman Saltbush root systems. Root data collection would be crucial in order to refine the model allowing its results to support the adaptability of Oldman Saltbush to arid regions.
Descheemaker et al., (2014) suggests that the depth of the roots, lateral root extension, root : shoot ratio and the effects of environmental conditions on root development should be further investigated as the root system plays a crucial role in water retention and the maintenance of low transpiration rates.
Furthermore, exclusion of this data implies uncertainty has occurred in the results of the field experiment and, therefore, also in the model produced.
Although Descheemaker et al., (2014) explores the potential use of Oldman Saltbush as an alternative feed source in some depth there is little to no information on its nutritional content or its viability as an alternative for maintaining the live weight of livestock animals.
In comparison, the results shown in the review by Ben Salem et al., (2010) highlight the importance of this information showing both striking positive and striking negative results for its use. It is noted that the main constraint for farmers looking to adopt the Oldman Saltbush as a feed alternative would be its high mineral concentrations and the need for increased amounts of drinking water for animals to excrete the ingested salt (Ben Salem et al., 2010).
The cost of carrying livestock through periods of drought and food scarcity is currently a major limitation to many in the agricultural industry (Ben Salem et al., 2010). The potential use of the Oldman Saltbush as a supplementary feed option may significantly reduce these costs alleviating this pressure from farmers managing livestock production.
Ben Salem et al. (2010) concludes that Oldman Saltbush alley-cropping could greatly reduce soil erosion, restore soil organic matter, boost crop yields, and provide high returns on farmers’ investments where the focus should be placed for future research and development prospects.
Ben Salem, H., Norman, H.C., Nefzaoui, A., Mayberry, D.E., Pearce, K.L., and Revell, D.K. 2010. Potential use of oldman saltbush (Atriplex nummularia Lindl.) in sheep and goat feeding. Small Ruminant Research, 91: 13-28.
Descheemaeker, K., Smith, A.P., Robertson, M.J., Whitbread, A.M., Huth, N.I., Davoren, W., Emms, J., and Llewellyn, R. 2014. Simulation of water-limited growth of the forage shrub saltbush (Atriplex nummularia Lindl.) in a low-rainfall environment of southern Australia. Crop and Pasture Science, 65: 1068-1083.
Hussin, S., Geissler, N., and. Koyro, H.W. 2012. Effect of NaCl salinity on Atriplex nummularia (L.) with special emphasis on carbon and nitrogen metabolism. Acta Physiol Plant 35: 1025–1038
Lluka, L. 2016. Membrane Structure and Function, BIOL1040 Lecture 2.1., The University of Queensland.
Reece, J.B., Meyers, N., Urry, L.A., Cain, M.L., Wasserman, S.A., Minorsky, P.V., Jackson, R.B., and Cooke, B.N. 2014. Campbell Biology, 10th Ed. Pearson Education, Australia.