This year’s wheat harvest is currently underway across the country after a difficult growing season, with harvest itself being delayed due to intermittent stormy weather. The high levels of rainfall at the start of the growing season meant that less winter wheat could be planted and dry weather in April and May caused difficulties for spring wheat as well. This decline in the wheat growing area has caused many news outlets to proclaim the worst wheat harvest in 40 years and potential bread price rises.
Difficult weather during this year’s growing season. Photo: Joe Oddy
This is also the first wheat harvest in which I have a more personal stake, namely the first field trial of my PhD project; looking at how asparagine levels are controlled in wheat. It seemed like a bad omen that my first field trial should coincide with such a poor year for wheat farming, but it is also an opportunity to look at how environmental stress is likely to influence the nutritional quality of wheat, particularly in relation to asparagine.
The levels of asparagine, a nitrogen-rich amino acid, in wheat grain have become an important quality parameter in recent years because it is the major determinant and precursor of acrylamide, a processing contaminant that forms during certain cooking processes. The carcinogenic risk associated with dietary acrylamide intake has sparked attempts to reduce consumption as much as possible, and reducing asparagine levels in wheat is a promising way of achieving part of this goal.
Asparagus, from which asparagine was first discovered and named.
Previous work on this issue has shown that some types of plant stress, such as sulphur deficiency, disease, and drought, increase asparagine levels in wheat, so managing these stresses with sufficient nutrient supply, disease control, and irrigation can help to prevent unwanted asparagine accumulation. Stress can be difficult to prevent even with such crop management strategies though, especially with environmental variables as uncontrollable as the weather, so it is tempting to speculate that the difficulties experienced this growing season will be reflected in higher asparagine levels; but we will have to wait and see.
On average, 10% of all crop production is lost annually to drought and extreme heat, with the situation getting worse year on year. Heat stress happens over short-time periods, but drought happens over longer timescales and is linked to drier soils. Maize and wheat are especially hard hit, with yields falling by up to 50% if drought hits.
On the High Plains, the largest US wheat-growing region, drought is a possibility every season. ‘Drought stress can be a key concern, especially in dry lands, but even in irrigated areas we can’t expect the same levels of water in future and farmers face restrictions,’ says Chris Souder at Monsanto.
So, this is not simply a developing world problem. Pedram Rowhani, University of Sussex, UK, found cereals in more technically developed agricultural systems of North America, Europe, and Australia suffered most from droughts. Yield losses due to drought were 19.9% in the US compared to almost no effect in Latin America.
Crop breeders in the past paid a great deal of attention to yield, but not enough to resilience to extreme events such as drought, Rowhani says, but this is changing. Growers increasingly want built-in drought resilience and plant scientists are looking for novel solutions. New, unconventional approaches based on novel insights from basic science might be necessary.
Hundreds of genes and proteins are involved in the complex trait of drought resistance. Plants avoid drought stress by shortening their life cycle with accelerated flowering, or cut down water loss by closing leaf pores called stomata. One approach by breeders is to target specific traits by crossing individual plants that perform best under drought conditions.
Stomata are found of the underside of leaves and are used for gas exchange. Image: Pixabay
‘About 97% of plant water loss occurs through the stomata. If you want to regulate the amount of water a plant uses, regulate the stomata,’ says Julie Gray, University of Sheffield, UK. Gray has been genetically tweaking wheat, barley, and rice plants so they have fewer of these pores.
She believes rising CO2 levels in the atmosphere means that they do not suffer from less carbon dioxide from opening their stomata. ‘CO2 levels have gone up 40% over the last 200 years. It’s quite possible they are producing more stomata than they need,’ says Gray.
Power plant in Tihange, Belgium. CO2emissions continue to increase. Image: Hullie@Wikimedia Commons
Gray reports that plants grown at 450ppm CO2 with reduced stomatal density, but increased stomatal size, had larger biomass and increased growth tolerance when water was limited. ‘Plants can operate with perhaps half as many stomata before you see significant effects on photosynthesis, so you can definitely reduce water loss this way,’ says Gray.
Root of the issue
At the other end of the plant plumbing system are roots. Susannah Tringe, Joint Genome Institute, UK, is seeking microbes that can gift stress-tolerance to their plant hosts. ‘The microbes associated with plants are likely to be just as important for plant growth and health as the microbiome of humans,’ says Tringe.
Though a lot of work has focused on finding the ‘magic microbe,’ Tringe believes whole communities will be necessary in real field conditions, whereas a single strain could be out-muscled by competitors.
Regular bouts of drought are leading to famine in developing countries. Video: Food and Agriculture Organization of the United Nations
Sugar and drought
‘Drought is probably the most widespread abiotic stress that limits food production worldwide. There is always need to improve drought tolerance,’ says Matthew Paul, Rothamsted Research Institute, UK.
‘Sucrose is produced in photosynthesis,’ Paul explains. ‘During drought conditions, plants will withhold sucrose from the grain, as a survival mechanism’. This can terminate reproductive structures and abort seed formation, even if drought is short-lived, greatly compromising yield.
A plant scientist studying rice plants. Image: IRRI Photos@Flickr
Rothamsted researchers have looked at modifying plants so sugar keeps flowing. ‘If you can get more sugar going to where you want it […] then this could improve yields and yield resilience,’ enthuses Paul. Field studies show that GM maize improved yields from 31 to 123% under severe drought, when compared with non-transgenic maize plants.