Wirrapunga was open to the public on the 8th and 9th September 2012 from 10am to 4:30pm as part of the Australian Open Garden Scheme. I spoke at 12noon and 3pm each day. The text of my talk follows:
Plants and Partnerships
It is not difficult to find examples of partnerships between plants and other forms of life. For example, consider the relationship between nitrogen-fixing bacteria and legumes. In this case the plant gives the bacteria a home within nodules on the roots. It also provides the bacteria with other needs. In return the bacteria give the plant nitrogen in a form it can use.
However the evolution of plants, in itself, gives an example of one of the most amazing partnerships of all. When the earth formed some billions of years ago there was no free oxygen in the atmosphere. However there was much carbon dioxide. We learn in school that oxygen came about by plants photosynthesising carbon dioxide to give oxygen. It is therefore reasonable to assume that the percentage of carbon dioxide in the atmosphere several billion years ago was at least as much as that of oxygen today – about 20%.
Given the amount of carbon dioxide together with abundant water and sunshine, organisms evolved that could use the energy of the sun to convert carbon dioxide and water to food – sugar – by a process we call photosynthesis. We know when these organisms evolved because the oceans then had dissolved ferrous oxide in them. Ferrous oxide is partially oxidised iron and is soluble in water. Fully oxidised iron is insoluble and this settled to the bottom of the oceans in the warm shallow seas where these bacteria lived. This is the iron ore being mined in Australia now that saved us from the Global Financial Crisis. They can date the beginning and ending of this iron ore formation. It took 500 million years for all the ferrous oxide in the oceans to be oxidised. Clever little bacteria weren’t they?
Given there was sugar and now oxygen available other organisms evolved that could use this oxygen to convert sugar to energy and other useful compounds. This process we call respiration. These organisms then formed a truly remarkable partnership to become the early plants.
These original partners still exist in plants today. The photosynthesising bacteria became chloroplasts. The sugar converters became the mitochondria in plants today. These cell organs became the engine rooms of the plant as we know it today.
Originally these plants just floated around in the water, absorbing the other nutrients they needed. However, when they moved to land and grew roots things became more complicated. Now they had to suck up nutrients from the soil. This process is called transpiration.
So the plant development was complete. We have the power plants producing all the energy the plant needs – photosynthesis. We have the factories producing the thousands upon thousands of compounds the plant needs– called respiration and we had the transport system – called transpiration.
In theory there is no problem with this but in practice there was a basic problem. These nutrients can only be sucked up, dissolved in water, if they are in soluble form. In nature they normally occur in an insoluble organic form.
This is normally referred to as the “nutrient cycle”. The nutrients are taken up by the plant in soluble inorganic form, used by the plant and then returned to the soil as litter or via animals in insoluble organic form. Once in the soil the organic nutrients are converted to soluble inorganic nutrients by soil organisms. The nutrients then become available again to the plants.
The amount of nutrients available is limited. There is fierce competition for these nutrients. As expected some wonderful partnerships have evolved. One partnership is very well known to all the gardeners here today. It is that partnership which has evolved between plants and we gardeners. We plant our plants. We supply them with all their nutrient needs - fertilizer. We destroy their competitors – weeds. It is one of the most successful partnerships that have ever evolved.
Just hop in your car and head north. Those beautiful yellow paddocks of canola. Canola is a member of the brassica family. Cabbages are about the best example, I can find, of a plant that is nearly totally dependent on gardeners for its survival. In the wild it survives in a few special places. Wikipedia says about wild cabbage, “Its tolerance of salt and lime and its intolerance of competition from other plants typically restrict its natural occurrence to limestone sea cliffs, like the chalk cliffs on both sides of the English Channel “
A few facts that might interest you:
About 7,000 species of plants are cultivated as crops, including about 2,000 for food. Thirty species provide 90% of our food while just four – wheat, rice, maize and potatoes provide 60% of the world’s food.
Another 28,000 species are cultivated in gardens.
In total about 35,000 species are cultivated in gardens or on farms.
This is about 14% of the world’s 250,000 species.
This accounts for how 14% of the world plant species solved their nutrient problem. What about the other 86%?
The most successful organism that has evolved that converts organic nutrient to inorganic nutrient is fungi. There are over a million different species.
However today I want to talk about their partnerships with plants.
Basically there are two important families of fungi which have formed a beneficial relationship with plants. Firstly there are the large fungi – the ones we can see. We notice them when they send fruiting bodies above the soil – mushrooms.
Large fungi tend to be associated with large plants – trees. The fungal filaments grow between the external root cells forming a characteristic highly branched network, called the Hartig net, which eventually surround many of the external root cells. The Hartig net functions as the interface between the fungus and the plant. In addition to the Hartig net, the fungal filaments cover the root surface. Strands extend from the mantle into the surrounding soil. These fungi are called ectomycorrhizal (outside, small, root) fungi. At least 5000 species of fungi are involved in ectomycorrhizal associations, often with a high degree of specificity.
So! If you walk in a eucalypt forest you will see the beautiful purple, green and cinnamon fungi of the Cortinarius family associated with eucalypt trees. If you walk in a pine forest you will see the red with white spotted Amanita and the yellow sponge gilled boletus. If you walk in an oak forest you may find truffles.
The positives in this relationship are twofold. Although a tree can grow without the fungi, it will not grow as well. However, this type of fungi is generally very mobile. It sends up its fruiting body which releases spores into the air that can blow round the world. You have probably encountered planting a tree or shrub that doesn’t do well for a few years and then suddenly it bursts into life – its fungus has found it. Foresters are beginning to understand these fungi well. In fact to plant a Tasmanian Blue Gum forest without inoculating the seed with its favourite fungus would be a dangerous game indeed.
These fungi also protect the roots from pathogens. Recently the slime (or water) mould phytophthora cinnamomi has been in the news. You may notice the number of dead stringy bark trees is more than normal. A healthy stringybark tree with a healthy mycorrhizal fungal relationship would probably be immune to this sort of pathogen attack.
However little fungi (endomycorrhizal) are by far the most common, occurring in about 80% of all vascular plants. There are less than 200 species involved in such relationships worldwide. Thus, endomycorrhizal relationships are not highly specific. The fungal filaments penetrate the external cells of the plant root, where they form highly branched structures.
When I say the filaments penetrate the root cells they don’t actually enter the protoplast (inside the cell). Think of a blown up balloon as the cell. Now thrust your hand into the balloon without breaking it. Then tie off the balloon round your wrist. Your hand is now inside the balloon yet it isn’t. This facilitates the transfer of metabolites and nutrients between the fungus and the plant. The hyphae extend out into the surrounding soil for several centimetres and thus greatly increase the potential for the absorption of water and essential nutrients. In short the plant gives the fungus sugar and the fungus gives the plant nutrients.
Two other kinds of mycorrhizae are those characteristic of the Epacris family (Ericaceae and Epacridaceae) and those associated with the orchids (Orchidaceae). In Ericaceae, the fungal hyphae form an extensive, loosely organised web over the root surface. The fungus releases enzymes into the soil to break down organic materials and make them available to the plant.
In nature, orchid seeds germinate only in the presence of suitable fungi. The fungus, which is internal in this mycorrhizal association, also supplies its host with carbon, at least when the host is a seedling. The fungi in such associations are the big fungi with more than 100 species involved.
These beneficial fungi also provide other benefits. As well as supplying the nutrient needs they also assist with water needs. They protect the plant from pathogens. Etc.
So! What does all this mean for us today? Consider the present debate concerning car manufacture in Australia. One side says that we would be better off without a car industry and all its public financial support. We can then buy cheaper cars from overseas and we would all be better off.
The other side says, “Hang on a minute. We would lose all our manufacturing skills. What would happen if something then went wrong with delivery? We would have no cars.”
Well! Consider the plants using mycorrhizal associations. They have been trading sugar for nutrients over millions of years. If they lose their fungi they are in serious trouble. Many of them cannot survive without their fungal help. Our indigenous plants evolved in very nutrient poor soils so most of our local indigenous plants will not grow in the conventional garden. The concept of “give them the nutrients they need and remove competition” is not good enough if we wish to save our biodiversity. This means that 80% of the world’s plants cannot be grown in conventional gardens. If I am going to create an indigenous landscape then I am going to have to throw out the gardening handbook.
It is probably easier to talk about what mycorrhizal fungi doesn’t like. If you spread inorganic fertilizer you will probably kill most of your mycorrhizal fungi. If you cultivate your soil you will probably kill most of your mycorrhizal fungi. If you remove all your plants and fallow your paddock you will probably kill most of your mycorrhizal fungi. If you have too many fires, particularly hot fires, you will probably kill most of your mycorrhizal fungi. If you constantly remove the litter you will probably kill most of your mycorrhizal fungi. Therefore, if you do any of the above you will probably lose your indigenous landscape.
Once you lose your mycorrhizal fungi, particularly endomycorrhizal fungi it is very difficult to replace it. Whereas the larger fungi grow above-ground sporing bodies allowing their spores to spread widely and find a suitable host, the tiny endomycorrhizal fungi only spread a few centimetres at a time. You can buy mycorrhizal inoculants which generally contains about 4 species. These are the ones tested for general crops. I do not know how they work on our local indigenous plants. It is probably better to not lose it in the first place.
We are losing our indigenous landscapes and therefore we are losing our biodiversity. It is my strong belief that the loss of our biodiversity is the most important problem facing the earth today. The whole point of this open garden is to get you involved in solving the problem. My philosophy is simple:
THINK GLOBAL – ACT LOCAL
I would like to take this opportunity today to ask you to help us save our biodiversity. I am a friend of Scott Creek Conservation Park. I spend a couple of days a month doing what I can. Scott Creek Conservation Park is just down the road a bit. They have 642 species of indigenous plants on their plant list. 184 of these are rare within the Park. It has the highest biodiversity of any Park in South Australia. They desperately need help to save their biodiversity. We Gardeners have to think a bit more about gardening our bush. But! Not on our terms – on the bush’s terms. Think about donating a little of your time. They have a stand here today. Why not make that important decision today. Go and talk to them. See what you can do. If you cannot spare the time then consider a donation to FAME, our sponsored charity. Of course if you wish to develop your own indigenous landscape I will do all I can to help.
Thankyou.