Module
How Plants Live in Poor Soil
From the Expert
Hans Lambers says ...
“Soils in south-western Australia are among the must nutrient impoverished in the world, severely lacking phosphorus, the essential nutrient for all life on earth. Over time, plant species have evolved adaptations to allow them to acquire enough phosphorus.
However, these symbiotic fungi are not as effective in our very poor soils, as is another adaptation. If we are able to dig up a banksia seedling or small plant (on a firebreak), we will see this adaptation when we gently scrape away the top layer of soil. A mat of cluster roots will be revealed. Many of these will be dead; only the white ones are active. These white cluster roots are produced in winter, when the soil is moist, and more nutrients are available.
The cluster roots release organic acids that mobilise the tiny amounts of phosphorus and micronutrients contained in the soil. They, effectively, mine the soil for these nutrients. In sedges, we find carrot-shaped roots and kangaroo paws make sand-binding roots that function like the banksia’s cluster roots.
It is no wonder that we have far more of all these nutrient-mining plant species than anywhere else in the world, where soils are generally richer and symbiotic fungi more able to help the plants.”
From: Hans.lambers@uwa.edu.au
Proteoid Roots – Cluster Roots
Background
Plants belonging to the Family Proteaceae (banksias, grevilleas, and hakeas) have greatly increased the surface area of their roots by growing at the terminal ends of the rootlets an extensive system of finger like projections, called cluster roots. They are only present in the first few weeks of a seedling’s life; the increased surface area allows the seedling to absorb more of the low-level nutrients available in the soil [12-15].
How they work
These roots release massive amounts of organic acids (carboxylates) into the soil, which then mobilise the phosphorus that is bound up in other chemicals and not readily available for the plant. It is a costly strategy, but, on very old and severely P-impoverished soils, a carboxylate-releasing and P-mobilising strategy is the most effective at acquiring growth limiting resources.
Watch YouTube clip with Hans on Cluster roots
Fungi
Fungi are microscopic plant-like cells that can be single celled (eg. yeast) or grow in long threadlike structures or hyphae that make a mass, called mycelium.
• On somewhat richer soils, where there is more phosphorus available, plants rely on a symbiotic relationship with mycorrhizal fungi, an effective and less costly strategy than cluster roots [16].
• Their hyphae explore the soil and secrete digestive enzymes onto their food source, such as dead organisms. Breaking down this organic matter provides nutrients for plant roots to absorb.
Arbuscular mycorrhiza are the most common colonisers, especially in agricultural plant associations, such as wheat and barley, and arid zone plants such as Acacia, Allocasuarina and Jacksonia species.
• These fungi have arbuscles, or growths that form inside the plant root, and have many small projections into root cells in addition to their hyphae outside the root.
• Such a growth pattern increases the plant’s contact with the soil, improving access to water and nutrients.
• The plant makes organic molecules by photosynthesis and supplies them to the fungus in the form of sugars or lipids.
• The fungus, in turn, supplies the plant with water and mineral nutrients, such as phosphorus, mobilised from the soil.
• About twenty percent of the photosynthetic products made by the plant host are consumed by the fungi, and exchanged with equal amounts of phosphate from the fungi to the plant host.
A Story of a Rare Orchid
The Wheatbelt is home to an extremely rare soil organism, an orchid, Rhizanthella gardneri. Discovered in 1928 by a farmer ploughing mallee country to plant wheat, this orchid has its life cycle entirely underground.
From the Expert
Kingsley Dixon calls this orchid ‘the greatest miracle of plant evolution’.
…“What we understand as a plant that derives its food energy from the Sun via its leaves, and its nutrients taken in by the roots, does not apply here. Rhizanthella has no leaves and no roots. Instead, it has an enlarged stem, or tuber, that has developed a special relationship with a soil fungus. Fungi are microscopic plant-like cells that can grow in long threadlike structures or micro-hairs, called hyphae. Many plants have their root structures invaded
by helpful hyphae (or mycorrhizae). The orchid enslaves the fungus, stealing all the nutrients the orchid needs to grow and flower.
However, that’s not the end of the story! The fungus has a special relationship with a nearby plant, a Melaleuca species, M. uncinata, which also has a mycorrhizal partnership with the orchid’s fungus. So here we have a 3-way relationship. But again, that’s not the end of the story!
Plants need to reproduce. In May, Rhizanthella flowers are developed underground, and therefore rely on pollination by termites and a tiny gnat. Unlike all other orchids, Rhizanthella develops its seeds inside an underground fleshy fruit (or berry) that has a sweet smell. How to disperse the seeds? They have obviously been designed to be eaten. Two species of ground-foraging marsupials are contenders, the quenda and the woylie, the former declared Rare, Near-Threatened and the latter as Critically Endangered.
The dependency of this tiny orchid on its ecosystem remaining undisturbed has made it critically endangered and close to extinction with only a handful of plants remaining due to land-clearing. It is listed today in the Federal Government’s EPBC Act as Critically Endangered.”
This is a complex symbiotic relationship between an orchid, a plant and a fungus. (See Yr3/4 Ecosystem for more on this subject). Furthermore, as an example of a plant growing and reproducing in the absence of sunlight, it is a worthy subject for student exploration.
Rhizanthella gardneri, Fred Hort
Activity
Locking for Mycorrhiza:
MATERIALS
• Petri dishes
• Forceps
• Glass or plastic test-tubes with caps
• Potassium hydroxide (KOH)
• Vinegar
• Black ink
• Dissecting microscope (20 x –40 x mag)
• A crop plant, or wattle seedling in the schoolground
METHOD
Collect the sample by gently removing a crop plant (wheat, barley, canola), or a wattle seedling, from the soil and gently shake to remove excess soil. Clean the roots in tap water and cut into small pieces (1-2 cm).
Prepare the staining solutions
• 10% KOH (10g KOH in 100ml dH2 O)
• 5% vinegar (5ml vinegar + 95ml dH2 O)
• 5% black ink (5ml black ink + 95ml of 5% vinegar)
Stain the roots
1. Place each root sample in the lidded test-tube and cover with 10% KOH.
2. Keep the roots for 5-7 days at room temperature, or boil for 10-30 mins.
3. Gently rinse the root several times with tap water.
4. Gently rinse the root with 5% vinegar, once.
5. Add 5% black ink and leave overnight, or boil for 1 – 5 mins.
6. Gently rinse with tap water once.
7. Place the root onto a flat dish (Petri) and cover with water.
8. View the root under a dissecting microscope. Mycorrhizal hyphae if present, are easily identified at this magnification. Using a compound microscope, greater detail of hyphae and the plant root can be seen.
9. Draw and label mycorrhiza in association with the plant’s roots.
Arbuscular mycorrhiza invading plant root. Photo B. Mickan
Effects of colonisation of plant by arbuscular mycorrhiza) From: https://turf.umn.edu/news/arbuscularmycorrhizal-fungi-tiny friends-big-impact
Glossary
Absorption – ability of a substance to hold water between its particles. If the substance is permeable (ie. has pores), water can enter the particle.
Acidity – concentration of hydrogen ions (H+)
Adsorption – molecules are held loosely on the surface of a particle, and not easily removed.
Adhesion – the process of adsorption.
Anaemic – low levels of haemoglobin in blood
Bioturbation – disturbance of top layers of the soil ecosystem
Calcareous – describing soils containing calcium
Carboxylates – organic compounds such as citrate and malate (oxidised versions of citric and malic acids), that take the place of phosphorus tightly bound to soil particles, pushing it into solution. This effectively mobilises the phosphorus, making it available for the plant’s roots to take in.
Cobalt – atomic number of 27, present in Earth’s crust in combination with other elements
Cohere – stick together
Enzyme – a protein that stimulates the rate of a chemical reaction
Reference
1. Elias, M., C. Chartier, H. Prévot, H. Garay, and C. Vignaud, 2006, The colour of ochres explained by their composition. Materials Science and Engineering B. 127: p. 70-80.
2. Wyrwoll, K.-H., B.L. Turner, and P. Findlater, 2014, On the origins, geomorphology and soils of the sandplains of south-western Australia, in Plant Life on the Sandplains in Southwest Australia, H. Lambers, Editor., UWA Publishing: Crawley, Western Australia. p. 3-22.
3. Willis, K.J., E.S. Jeffers, and C. Tovar What makes a terrestrial ecosystem resilient? 2018, Sciencemag. org. 359, 988-989.
4. Bardgett, R.D. and W.H. van der Putten, 2014, Belowground biodiversity and ecosystem functioning. Nature. 515: p. 505-511.
5. Halsmith, R., 2020, Oral McGuire: Cultivating Connections. Landscape Architecture Australia. 167: p. 54-55.
6. Wagner, D. and E.F. Nicklen, 2010, Ant nest location, soil nutrients and nutrient uptake by ant-associated plants; does extrafloral nectar attract ant nests and thereby enhance plant nutrition? Journal of Ecology. 98: p. 614-624.
7. Martin, G., 2003, The role of small ground foraging mammals in topsoil health and biodiversity:Implications to management and restoration. Ecology Management Restoration. 4: p. 114-119.
8. Valentine, L.E., K.X. Ruthrof, R. Fisher, G.E.S.J. Hardy, R.J. Hobbs, and P.A. Fleming, 2018, Bioturbation by bandicoots facilitates seedling growth by altering soil properties. Functional Ecology. 2018: p. 1-11.
9. Valentine, L.E., M. Bretz, K.X. Ruthrof, R. Fisher, G.E.S. Hardy, and P.A. Fleming, 2017, Scratching beneath the surface: bandicoot bioturbation contributes to ecosystem processes. Austral Ecology. 42: p. 265-276.
Ions – are electrically charged atoms or molecules. They are produced by the addition, or removal of electrons from a stable atom or molecule. For example, sodium (Na) can become a positively charged ion (Na+) when it loses an electron from the outer shell. Chlorine (Cl) becomes a negatively charged atom (Cl-) when it has an extra electron.
Lethargic – lacking energy
Organic – describes material whose origin is living matter, such as plant or animal.
Oxidised – compounded with oxygen
Pigment – a substance (mostly insoluble), that gives colour to other materials, by absorption of differential wavelengths in visible light.
Rumen – first chamber in the alimentary tract of a grazing animal
Symbiosis – describes the interaction of 2 organisms that live together and provide mutual benefit one to the other.
Vitamin – a contraction of ‘vital’ and ‘mineral’
10. Clemente, C.J., C.E. Cooper, P.C. Withers, C. Freakley, S. Singh, and P. Terrill, 2016, The private life of echidnas: using accelerometry and GPS to examine field biomechanics and assess the ecological impact of a widespread, semi-fossorial monotreme. Journal of Experimental Biology. 219: p. 3271-3283.
11. Eldridge, D.J. and A. Mensinga, 2007, Foraging pits of the short-beaked echidna (Tachyglossus aculeatus) as small-scale patches in a semi-arid Australian box woodland. Soil. Biol. Biochem. 39: p. 1055-1065.
12. Mucina, L., E. Laliberté, K. Thiele, J.R. Dodson, and J. Harvey, 2014,Biogeography of kwongan: origins, diversity, endemism and vegetation patterns, in Plant Life on the Sandplains in South-western Australia, H. Lambers, Editor., UWA Publishing: Crawley, Western Australia. p. 35-79.
13. Byrne, M., et al., 2014,A diverse flora - species and genetic relationships, in Plant Life on the Sandplains in South-western Australia, H. Lambers, Editor., UWA Publishing: Crawley, Western Australia. p. 81-99.
14. Lambers, H., M.W. Shane, E. Laliberté, N. Swarts, F. Teste, and G. Zemunik, 2014,Plant mineral nutrition, in Plant Life on the Sandplains in South-western Australia, H. Lambers, Editor., UWA Publishing: Crawley, Western Australia. p. 101-127.
15. Lambers, H., et al., 2014, Carbon and water relations, in Plant Life on the Sandplains in South-western Australia, H. Lambers, Editor., UWA Publishing: Crawley, Western Australia. p. 129-146.
16. Raven, J.A., H. Lambers, S.E. Smith, and M. Westoby, 2018, Costs of acquiring phosphorus by vascular land plants: patterns and implications for plant coexistence. New Phytologist. 217: p. 1420-1427.