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Carbon dioxide (CO2), the primary greenhouse gas produced by human activities, is causing global warming and climate change, but higher levels of CO2 in the air can also assist plant growth and improve their ability to cope with drought and water stress. Against a backdrop of imminent climate change that will affect crops contributing to world food supply, a team of scientists from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia is focussing their research efforts on producing “climate ready” sugarcane for future generations.
Sugarcane is one of Australia's most important rural industries and export crops are currently worth about AUS$2 billion to the Australian economy. CSIRO systems ecologist and senior research scientist Dr Chris Stokes, who is heading up this research, defines “climate ready” sugarcane as being those varieties developed to anticipate and thrive in future climatic conditions.
“We’re seeing a number of trends occurring in our climate, including increasing levels of CO2, rising temperatures and changing rainfall patterns,” Dr Stokes explains. “Since these are also the main factors that affect agriculture, knowing the conditions under which crops will be grown in 50 years’ time will ensure that we prepare sugarcane varieties that will continue to provide necessary crop volumes in these future environmental conditions.
“There is a seasonal pulse to the planet, but the CO2 trends are definitely exceeding past fluctuations. In the next 20 years, atmospheric CO2 will increase noticeably and other factors such as temperature and rainfall may also alter beyond the bounds of past variations.
“Our research focuses specifically on the CO2 factor. We’re working to understand how CO2 affects plant water use and drought responses, and to identify new varieties of sugarcane that will be the most responsive to CO2 increases in the atmosphere and can therefore be incorporated into breeding programmes of the future.”
CSIRO is a very mission-orientated organisation that focuses on research directions of national and global significance. Sugarcane research falls into an area called the Climate Adaptation Flagship, a research programme initiated to deliver the best available scientific information, expertise and solutions to support Australia's efforts to adapt to climate change in a practical and effective way.
The research project is being funded by the Sugar Research Development Corporation and the CSIRO Climate Adaptation Flagship, and the research team is working closely with plant breeders within their own organisation and in BSES Limited, a quasi-autonomous non-governmental organisation that is the principal provider of research, development and technology transfer to the Australian sugar industry.
Increase in atmospheric CO2
Over the last decade the average annual increase in atmospheric CO2 was 2.07 parts per million (ppm) per year. The average for the prior decade was 1.6 ppm per year and current atmospheric CO2 levels are about 30% higher than they have been for at least 800,000 years prior to the industrial revolution. Since pre-industrial times, CO2 levels have risen from about 280 parts per million (ppm) to close to 400 ppm and this is having a significant effect on plants, particularly on their water stress responses. Exploiting these responses is key to offsetting indirect climatic effects of increasing CO2 and other greenhouse gases, including higher daytime vapour pressure deficits – as temperatures rise, the air becomes ‘drier’, increasing the amount of water required by plants to maintain the same growth.
The CSIRO team is working to breed sugarcane varieties that use water more efficiently in an effort to make it possible to grow more crops for the same amount of irrigation water, or in stress conditions, to stretch the same amount of water further.
Globally, most research funding has concentrated on the so-called C3 crops cultivated in the major temperate food production zones of the world. These include the cereal grains, wheat, rice, barley and oats, as well as peanuts, cotton, sugar beets, tobacco, spinach and soy beans. Dr Stokes says C4 tropical crop species like sugarcane have been less in the limelight and this is part of the novelty of his team’s research activities. His team’s work on the effects of CO2 on tropical pastures is helping to quantify an underappreciated aspect that will need to be understood as part of adapting to climate change in tropical areas. This work is now being incorporated into systems modelling to evaluate climate change impacts and adaptation options.
"Given the inevitability of greenhouse warming, we must learn how to capitalise on any potential benefits, while identifying the risks far enough in advance to offset their effects," says Dr Stokes. “All this underlines the importance of building in a CO2 component to this type of research. Even with the introduction of strong emission reduction measures, it is almost inevitable that CO2 levels will continue to rise for at least several decades, creating a warmer planet and altering rainfall patterns around the world, with major consequences for agriculture.
“Most of our sugarcane varieties are bred from ancestral stock that was originally sourced by breeders about 100 years ago, but since then shifts in climatic conditions demand that we continually adapt the breeding and agronomy of our crops and our cropping practices.
“Future changes in rainfall are the least predictable. Simplistically, as global warming increases, the hydrological cycle is expected to intensify and result in increased and more variable rainfall. Generally the tropics are expected to become wetter, although certain bands of latitude will become drier.
“To prepare for a future in which we cannot know for certain exactly what the conditions will be in any given location, our work is more about making sure there will be a choice of plant varieties to suit these agricultural environments when the need arises. At such a time, growers will have to make decisions at the farm scale and the work we’re doing today will ensure that a viable choice of sugarcane varieties are available when required. Essentially, we’re attempting to anticipate the type of plants that will do better in conditions such as higher water demand (from ‘drier’ air) and periodic water stress. At the same time, other researchers are working on complementary aspects, such as agronomy and business management.
“To ensure future world food security, we simply cannot just carry on doing things the way they’ve been done in the past. Demand for food is increasing around the world, both as a result of rising populations and rising aspirations in countries that are becoming more affluent. The agricultural sector will need to meet this demand within a context of a changing climate and the nature of that food production will need to change. And, with much of the prime land already taken up, if we are going to expand agricultural production, this expansion has to move into more marginal land, which elevates the importance of efficient irrigation and the ability of crops to cope with stress.”
With research also being conducted into producing ethanol from sugarcane, both from the sugar and fibre components of the crop, it’s clear that crops for biofuel cannot be produced at the expense of existing prime agricultural land and may also have to be cultivated in more marginal land. A lot of work is already being undertaken by sugarcane breeders to examine responses to water stress and to attempt to improve water use efficiency. This will have important implications both for drought tolerance and future variable climates. Australia is a very water-scarce continent and a more efficient approach to the use of water, in both irrigated and rain fed locations, is critical.
Although the sugarcane varieties currently under investigation in the CSIRO research project are predominantly Australian in origin, there is substantial collaboration with researchers and breeders in other major sugarcane producing countries, such as Brazil, India, China, the USA and South Africa. Dr Stokes says a great deal of research findings extrapolate elsewhere, in terms of the avenue of research, including other crops.
CO2 and plant function
The main benefit of CO2 on plant function relates to the central growth process, photosynthesis, during which the CO2 combines with water to produce carbohydrates. Many studies have demonstrated that plants exposed to high CO2 levels in the air use water more efficiently than those grown in normal air. Small pores in the leaves called stomata control both the rate at which CO2 enters the plant and the plant's water loss.
“By increasing the level of CO2 in the atmosphere, as we are doing in research experiments at moment, the CO2 gradient into the leaf increases, while the water gradient out of the leaf stays the same,” Dr Stokes says. “In other words, as CO2 outside the leaf increases, the plant is able to take up more CO2, without losing more water and this is the basis for plants being able to use water more efficiently.
“This is the response we’re trying to understand. We want to capture this benefit as optimally as possible and, based on this understanding, to develop a rapid screening system to screen different varieties of sugarcane to determine how responsive they are to CO2.”
All plant breeding undertaken by Dr Stokes’ team is purely focused on the selection of existing sugarcane genetic stock and does not encroach into the arena of genetic manipulation.
The actual experiments take place in tall growth chambers, high enough to accommodate the fully mature sugarcane plants. These chambers allow CO2 levels, temperature and humidity to be regulated, creating an integrated system for measuring patterns of water use and studying plant responses.
With contemporary CO2 levels at about 400 ppm and ballpark projections suggesting that these levels will have risen to around 550 ppm by 2050, the research team is working with even higher levels of CO2 in the experimental growth chambers — as high as 700 ppm. Dr Stokes says this is to obtain a strong response size from atmospheric CO2 levels that could occur towards the end of the century. The larger treatment effects also allow the research team to observe the differences more clearly. However, each incremental step change in CO2 will have diminishing benefit to the plant, so once levels exceed 700 ppm, each additional increase will have less and less effect on plants.
The chambers use a through-flow system, in which air is drawn in on one side and expelled on the other. In this airstream, the team measures the flow rate of air in the chambers. The difference in the CO2 concentration between the augmented concentration and the outgoing air allows the team to calculate the rate of photosynthesis for the whole chamber at any given time.
Large air blowers mix the air to maintain a relatively uniform CO2 levels, temperature and humidity within the chambers. From a risk management perspective, the air circulation also prevents any toxic build-up of CO2 build-up in these enclosed spaces. In addition, CO2 sensors are located around the chambers as part of a building management system. These sensors operate on a continual basis and are geared to automatically ventilate the area and sound an alarm if the CO2 rises above a certain level.
Bulk CO2 supply
When it came to sourcing CO2 for the research project, the researchers’ fundamental requirements were low levels of plant-active impurities (particularly ethylene), cost and security of supply. The team selected bulk CO2 supply from BOC Australia, part of The Linde Group. In the arenas of agriculture and fishing, BOC works with growers, producers, scientific and research organisations across the world to help customers lower costs, raise productivity, comply with increasingly stringent environmental legislation and meet the high quality standards set by supermarkets.
The CO2 is supplied through gas control equipment from BOC’s GASMATIC® range. This range has been designed to deliver a large, uninterrupted supply of CO2 through a selection of stationary and low-pressure on-site vessels. The GASMATIC system requires no electric motor and no refrigeration unit and has no moving parts. This form of supply was selected as the simplest and most cost-effective route for the research project.
It was important to the research project that the CO2 would not introduce ethylene into the growing chamber. The presence of this gaseous plant hormone has the potential to skew experimental outcomes. BOC Australia worked hard to identify a suitable source of CO2 for this application which fulfilled the low ethylene requirement.