Mar/Apr – 1994
Story Title: Replicating Nature
Author: Roger Fox
By recreating nature in a laboratoy aspects of plant growth can be studied in minute detail. At the University of Sydney, research into soil fungi and mangrove ecologies is revealing much of interest.
Research into plant behavior, when carried out in a laboratory, demands the careful imitation of outdoor growing conditions. Without this, the results and patterns that emerge can’t be reliably applied to normal field conditions. These days, modern technology has put the replication of nature within reach, so that at research centres such as the University of Sydney’s School of Biological Sciences, very specific aspects of plant growth can be studied in detail.
At the heart of the plant research work being carried out by the botany section of the School, are several sophisticated modular growth chambers. Measuring only a few meters square, these units offer the researcher a high level of control over light, humidity and to a lesser extent, temperature. The chambers incorporate both metal arc and incandescent bulbs, resulting in the broad light spectrum that a plant would receive out in the field. This is not the ‘supplementary lighting’ often referred to in the glasshouse industry, which seeks to coax a bit of extra growth out of plants – it is, rattler, total daylight replacement. In charge of overseeing a number of these projects is Mark Curran, Professional Officer with the School of Science. According to Mark, the broad range of plant ‘subjects’ which may be grown in the set-ups means they cannot afford to be plant specific, in the way that commercial operations are.
“We have to provide a more general purpose environment, which is going to be suitable for a wide range of plants,” Mark emphasizes.
But even allowing for a general approach, plants do differ widely in their light needs and no two will respond in the same way. Mark points to the example of beans, which were a ‘disaster’ in this environment of intense lighting, turning purple and ceasing growth. The plants were being overloaded energy wise and could not cope, whereas other plants under exactly the same conditions, will thrive.
“When you’re dealing with really bright lights,” Mark observes , “you have to think about what the plant actually sees in the real world, and it doesn’t see something which turns on in the morning and turns off at night. It sees something that comes up in intensity, peaks and then drops off thing again. And there are some plants that can’t cope with a major change in that.”
Metal arc lighting is used in the growth chambers, because the multi element composition of the light gives a very broad spectrum. A Dichroic filter panel, which uses a metallic mirror to reflect infra red light but allows visible light to pass freely, is used to prevent excess heat from entering the working area of the cabinet. The filter is so efficient that incandescent lights have to be used to provide the ‘far red ‘light needed by the plants. Excess heat from the lamps is extracted by a recirculating cooling water system, but the Dichroic panels halve the amount of refrigeration that would otherwise be needed for temperature control.
“What you’re looking at is a reasonable air circulation so that your temperature doesn’t vary a lot and you have an even light intensity,” Mark says.
The growth chambers are similar to a cold room in structure, though there the similarity ends – their sophisticated control equipment gives them a value of around $100,000 each. The lights are on a four tier system which turns the incandescent lights on, and then brings on the metal arcs in three stages. In some of the units, humidity can be controlled also, for experiments where this factor is the variable input. Humidity can be set anywhere between 60% and 95%, and where necessary, a chemical dryer Will take it down to 25%. The units also provide the scope for C02 and UV research. Through having a number of different systems set up, a variety of plants can be trialled under a variety of conditions.
In one of the current growth chamber experiments, the role of mycorrhizal fungi in plant growth is being put ‘under the microscope”. These fungi live in the root system of plants, and while they have long been known to be beneficial, comparatively little is known about how they work. Conducting this project is Peter McGee, a lecturer with the School of Biological Sciences, who is using the cabinets to produce uniform material under controlled conditions. For Peter, the plants are simply the source of the organic fungus he is growing underneath.
“The fungus is an integral part of the working of the plant,” Peter says. “It’s not separate.”
“The reason why we’re interested in mycorrhizal fungi is because out there, they function as an extension of the root system, so they’re increasing the uptake of mineral nutrients, particularly those nutrients that don’t move through the soil rapidly.”
It is this ability to increase the uptake of less mobile nutrients, such as phosphorus, copper and zinc, which makes mycorrhizal fungi of great potential benefit to the agricultural and horticultural industries. Peter is currently working with cot ton and clover crops and, since the fungi is not carried in the seed, it is added separately at the time of planting and the results observed and recorded.
For the purposes of Peter’s experiment, the growth chambers represent a way of replicating a set of growing conditions that he can’t get by using a glasshouse. He needs light intensities of 600-800 microeinsteins to meet the saturation levels of the plants he is working with, most of which grow in western areas of New South Wales. In natural field situations, these plants can experience light intensifies as high as 2000-2500 microeinsteins in summer. The wavelength of the light is important in these experiments, particularly the balance of infrared to the total amount of light, and the units also offer control over temperature – from O°C to 40°C (32°-104°F).
The plants are grown in a medium of 90% river sand and 10% high nutrient clay. The day is added because it has a very high cation exchange capacity and is a good buffer. The plant containers have no drainage, but are watered to a set weight, representing around 70% of the field capacity of the soil.
Among the patterns that have emerged from Peter’s research to date, are that mycorrhizal fungi change the hormone balance in plants, notably cytokinins and gibberellins. The fungi also increase the plant’s tolerance of some pathogens, possibly by increasing the host’s resistance response.
Another effect of mycorrhizal fungi is a change in the water relations of plants, partly due to the hormonal changes that occur. This in turn leads to changed patterns in the opening of leaf stomata. While Peter describes the water responses as icomplex’, two practical results are that plants respond more rapidly to the arrival of rain after periods of drought, and also water moves more quickly through the plant.
Mycorrhizal fungi also lead to changes in the morphology of plants, so that a heavily affected root system will be much smaller than an unaffected one.
“The fungus is functioning as an extension of the root system, ” Peter says. “A plant invests so much in aquiring water and nutrients, but with the presence of the fungus there, it doesn’t have to put so much into it.”
As with all areas of research, one of the greatest limitations to its scope is the funding available. Peter’s work on cotton has been funded by the Australian cotton industry, which potentially has m h to gain from a greater knowledge of the role mycorrhizal fungi play. Current cotton reduction methods tend to discourage the fungi in the soil, because of the use of fungicides, high nutrient application rates and periods of fallow. As a result, subsequent cotton crops have few mycorrhizal fungi in the root zone and growth deteriorates. The practice of leaving soil bare between crops (fallow) adds to the problem, because in the absence of plants there is nothing for the fungus to grow on. The aim of the research is to be able to address the situation before it becomes a problem.
“Ultimately what we want,” Peter says, “is a predictive model to say this is how much fungus you’ve got, this is the likely effect on your cotton crop and therefore now’s the time to do something about building up the fungus, before you plant the crop.”
According to Peter, people have in the past tried to grow rnycorrhizal fungi in hydroponics, but the major problem has been contamination with algae. Other research in this area is being conducted by the CSIRO, which is looking into mycorrhizal fungi on eucalyptus for the development of a timber industry in China.
The growth chambers, with their capacity for reproducing natural levels of radiation, are not the only special purpose environments employed in research at the university. Mark Curran is also involved in operating a series of “mangrove tanks’, where the largely unknown field of mangrove ecology can be studied. The fibreglass tanks work in pairs, whereby the water is pumped from one to the other in an ongoing cycle that replicates the natural tidal conditions in which man grove plants grow. Within a 25 hour period, the plants will experience two high and two low tides.
The purpose of the experiment is to examine the ecology of mangrove roots – how and why they work. The plants seem to be able to store enough air in their roots to supply them from one low tide to the next. But there are differences in how successfully mangrove plants grow in the wild and two of the main variables appear to be the length of time they spend under water and the depth of the water.
The plant subjects in Mark’s study are two of the local man grove species from the Sydney region: Avicennia matina, the Grey Mangrove, and Aegiceris comiculatum, the River Man grove. The plants grow in river sand in pots lined with plastic. Tiny holes in the plastic provide very slow drainage into a solid outer container, so that the plants never completely dry Out. The water is salt, at a concentration of about 70% sea water. The plants are fed slow release nutrients via a plastic tube of fertilizer which is pushed into the soil. In the wild, it may be the availability of nutrients which limits the establishment of mangrove plants. The seeds will grow up to the four leaf stage by using their seed reserves, but they then ‘sit’ for several years at this size and do not grow on unless they get a supply of nutrient -N and/or P seem to be the critical ones.
Mangrove plants have very extensive root systems, which can grow up to 30 meters from the base of the tree. What is most remarkable about the plants is their ability to aerate their roots by transferring oxygen internally – there is no free oxygen in the soil, which is permanently waterlogged. The interior of the root system of the grey mangrove is like a series of air filled tubes, connected to the surface by upward growing roots, called pneumatophores. These are a natural adaptation which allow oxygen to enter the gas spaces of the root while the tide is out, but which are waterproof and keep water out when the roots are covered at high tide.
The clearing of mangroves worldwide is having significant ecological effects. In some countries, they play an important role in protecting coasts from storms and their clearing can lead to problems. Mangrove plants thrive in silt and Mark believes that there are greater numbers now in Port jackson (Sydney), than there were 200 years ago when white settlers first arrived.
Mark decided to create the mini mangrove ecosystem, when he could find practically no information on mangrove plants and mangrove ecology. He now uses the set-up as the basis for student experiments, which involve estimating what the oxygen levels in the roots should be and, through progressive experimentation, ascertaining what they actually are. From an educational perspective, this presents a small discreet problem, one where the students can set up the processes required for solving it.
All horticultural research work is important for the industry, but research at universities is rather unique because of its high level of exposure to students in the scientific disciplines. Not only does it explore scientific issues, while demonstrating processes and procedures, but perhaps just as importantly it can help to kindle the flames of inquiry in undergraduates, who may well become the next generation of researchers.