Reducing water use: more crop per drop
February 2010
The same yield is achieved with up to half the amount of water used
70% of the world’s fresh water extracted for human use is used for agriculture (related link 1). Reducing the amount of water that crops use is an important challenge because as pressure on water use rises, so can the price of food. Furthermore, political tensions over access to water and diverting it for dams, canals and irrigation are ongoing concerns between a number of countries (see ‘Global Issues’).
Farming irrigation systems use a lot of water
If only there were a way to make plants use less water for the same yield. Incredibly, it looks as though there is. The system is called partial root drying (PRD), and it exploits the way that plants’ roots signal that water is scarce.
It works like this: when one side of an individual plant’s roots are starved of water, signals are sent to the leaves to reduce growth and close leaf pores called stomata, which reduces water use, and loss, respectively. Although the growth rate of the leaves declines, because the other side of the plant is being watered the plant does not wilt and carries on producing fruit bodies, like grapes, tomatoes and so on. Finally, alternating which side of the plant receives water prevents roots from dying in very dry soil.
The overall result can be that the same (or nearly the same) yield is achieved with up to half the amount of water used.
With limited access to water but a thriving wine export economy, PRD has been pioneered by Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Adelaide, building on science largely conducted by a research team led by Professor Bill Davies at the University of Lancaster. When the Lancaster team first tried the system with apple trees, leaf growth declined but the water balance of the leaves remained buoyant; a clue that the signal to reduce leaf growth and close the stomata was coming from the roots – not the leaves – and this can be exploited using different watering strategies to reduce water use in agriculture
The system has been trialled in a number of environments and on different crops. Australian scientists at CSIRO and the University of Adelaide found that water use could be reduced by 40%. An unpredicted bonus was that the reduced leaf growth allowed more sunlight to strike the grapes, raising the sugar concentration which can make for a higher quality product.
Drip irrigation systems are ideal for PRD
In Turkey, PRD has been tried on cotton, which is a very thirsty crop and uses more water than wheat or rice. In an EU-funded project co-ordinated by Davies’ Lancaster group irrigation was reduced by half. Again a bonus emerged – the PRD cotton could be harvested three weeks earlier, which reduced the chance of autumn rain wrecking the harvest. In the same project, a research team from The Technical University of Lisbon significantly enhanced grape fruit quality with PRD compared to the quality of fruits irrigated conventionally with the same quantity of water. All of these responses increased the resilience of the food production industry against environmental stresses.
However, not all crops seem to benefit. Pepper, aubergine (eggplant) and citrus crops showed greater yield reductions of a fifth or more with no or minimal increases in water use efficiency. Moreover, Danish scientists recorded no gains with potatoes.
But recent work by Dr Ian Dodd at Lancaster has shown how to maximize the hormone signalling between roots and shoots such that PRD can have highly beneficial effects on the efficiency of crop water use. However, enhanced signalling does not always occur and this is particularly the case at lower soil water contents; an observation that shows how to best employ PRD and explains some of the less significant PRD effects reported by some workers.
The system has also shown particularly promising effects on water use efficiency of rice when soil is allowed to wet and dry (the PRD effect) in response to irrigation. These techniques are currently being trialled in China and in the Philippines, with significant yield increased reported for applications of given quantities of water.
Many rice farmers and farmers of other crops in water-scarce regions accept the long-term need to become more water-use efficient and to try to increase food production while using less water – i.e. producing ‘more crop per drop’.
The Lancaster team’s socio-economic impact assessment, undertaken in Portugal, Morocco, Turkey and the UK, reported a rapid return on investment in extra relatively low tech irrigation equipment. In the UK this arose from both savings in labour and from the production of better value crops. In the Mediterranean, this payback was from a greater resilience of the cropping system against water scarcity.
Related links
- BBSRC: Future directions in research relating to food security
- Professor Bill Davies, The Lancaster Environment Centre, Lancaster University
- Food crop production using partial root drying (PRD): the facts and the fiction
- Partial root drying (PRD): A new technique for growing pants that saves water and improves the quality of fruit
- Biomass allocation in tomato (Lycopersicon esculentum) plants grown under partial rootzone drying: enhancement of root growth
