Developing crops for mild drought tolerance

19159 Whilst we continue to have no rain in southern England (I’m sure this won’t last too long…), drought tolerance must be on the mind of many farmers. World reports on climate change and food security have identified water scarcity as a critical factor for agriculture this century. Breeding for drought tolerance is a major research aim and there have been some successes using both conventional breeding and genetic engineering. Over the last 12 months for instance, new drought-tolerant maize varieties produced by conventional breeding have been released which have shown increases in seed yields of between 5 and 15% under water-stressed field conditions.

Drought tolerance is a complex area. Crops which can withstand severe drought may not perform well under conditions of mild or no drought; the response to drought is intricately woven with other conditions such as soil nutrients, soil quality, temperature and other environmental stresses; physiological responses of plants to increase their water use efficiency depend on the development stage of the crop; and thanks to these complex interactions several hundred genes have been identified which control or modify drought response.

Most research has focused on developing crops for severe drought stress, but new studies published in Nature Biotechnology and Plant Cell focus on the physiological responses of plants to mild drought stress. The team from the University of Gent has identified a mechanism that can be used to develop crop varieties which will produce high yields under mild drought stress, more commonly found in temperate climates. In trials with the test plant Arabidopsis, ethylene inhibited leaf growth at the first sign of limited water availability, and growth resumed after water became available. Overcoming this reaction could lead to crops which keep on growing during mild and temporary spells of drought that occur in the field, avoiding unnecessary yield losses and higher crop productivity.

As an indication of the amount of research, the number of records on “drought resistance” in CAB Direct has increased steadily every year – from one paper (in 1924!) to almost 1000 papers published in 2010. And just to prove the value of older research, the one paper from 1924 is surprisingly topical:
Clark, J. A. Segregation and correlated inheritance in crosses between Kota and Hard Federation wheats for rust and drought resistance. Journal of Agricultural Research, 1924, 29:1, pp 1-45.

Environmental impacts of Bt crops – on target or non-target?

Genetically modified crops containing a toxin gene from the bacterium Bacillus thuringiensis have been used by farmers for 11 years now. These Bt crops were designed to give the plants resistance to important pests. But might they also be harming non-target invertebrates?  A study by Steven Naranjo of the US Department of Agriculture’s Agricultural Research Service looks at the evidence and compares it with the impacts of the pesticides that would otherwise have been used.

Bt maize and cotton have been commercially produced on about 42 million hectares in 20 countries. Their potential non-target effects have been considered in over 360 published research papers. Naranjo, in his paper in CAB Reviews, looks across around 200 of these studies to draw conclusions.

 

Investigations found that the abundance of all non-target invertebrates was slightly lower for Bt crops than in non-Bt crops, but much higher in Bt crops than in non-Bt crops treated with insecticides. Using meta-analysis, a way of doing a meaningful comparison across different studies, Naranjo found that laboratory studies indicated negative effects of Bt on some non-target invertebrates, though these depended on how the trials were done and which invertebrates were being looked at. However, few harmful effects of Bt crops were shown in field studies. One factor may be that exposure to the Bt toxin is higher in the laboratory experiments than in the field. It was also clear that nontarget effects for insecticides are much greater than for Bt crops.

 

While Bt crops mean that some specialist parasitoids that would otherwise attack pests of maize have less to feed on, the overall levels of predation on pests have not been shown to drop. Naranjo believes Bt crops could enhance the role of biological control in integrated pest management.

 

Naranjo's paper emphasises that a key comparison to make is what would have happened without Bt crops. Bt maize and Bt cotton are believed to have led to a 136.6 million kg reduction in insecticide active ingredient, and rootworm-resistance crops will reduce the levels of insecticide present in the soil.

 

The paper, "Impacts of Bt crops on non-target invertebrates and insecticide use patterns" by Steven E. Naranjo appears in CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, 2009, 4, No. 011, 23 pp.

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Why Can’t GM and Organic Just Get Along?

Growing of organic and genetically modified crops on neighbouring farms continues to be contentious, especially in Europe, but the issue of coexistence of same-species crops for different markets is not limited to GM. In a paper entitled “Can GM and organic agriculture coexist?”, Eberhard Weber points to the need for oilseed rape for cooking and industrial oil to be segregated, and similarly for crops for human food and animal feed for both maize and barley. Writing in CAB Reviews, he says that such situations work on the principle that some contamination will happen, but that “threshold values above zero for adventitious presence must be defined for coexistence rules.”

One company growing GM maize in Germany offered to buy maize from neighbouring non-GM farmers on the same conditions they would achieve without a GM neighbour. “The neighbouring farmers agreed, meaning that no coexistence problem arose,” says Weber, from Martin-Luther-University Halle-Wittenberg.

Weber notes that various organic organisations only require that the production process organic farmers use must not involve GMOs, rather than that they must ensure no GMO presence in their products. He quotes the International Federation of Organic Agriculture Movements: “Organic products are not defined as being free of unwanted pollution. Just as organic farmers cannot guarantee zero contamination from pesticides they do not use themselves, there is no way for them to guarantee that organic products will not be polluted by traces of GMOs.”

Weber looks at the various planting arrangements suggested to minimise gene flow to neighbouring fields. “Many experiments show that the GMO content is reduced with increasing distance as it should be, but no zero level can be achieved”. However, the variability of wind direction means no models can be entirely reliable.

As long as the same threshold value is valid for conventional and organic products, the GM farmer should not have to distinguish between neighbours with conventional and organic production, says Weber. ”The best way will always be agreement between farmers. As long as only farmers are involved, this can mostly be achieved.” However, he says that the involvement of other groups who try to influence political decisions on the rules make it difficult to predict the future of coexistence.

Can GM and organic agriculture coexist?”by  Eberhard Weber appears in CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, 2008, 3, No. 072, 8 pp.

How Green are Biofuels?

Biofuels are often touted as a way of efficiently generating power with lower effects on the environment than existing fuel production methods. But how effective are they if you weigh everything up? A paper by Lucas Reijnders in CAB Reviews looks at life-cycle assessments of biofuels and finds that they are much less efficient than solar cells in energy conversion, and some perform worse that conventional fossil fuels in emission of chemicals contributing to acidification and eutrophication. Some even appear set to worsen climate change.

Life-cycle assessments are very complex to do, because it is hard to work out what effects to rule in or out and on what basis to make comparisons. Factors such as what the land was previously used for need to be taken into account. Thus fuel from sugarcane grown on cleared Cerrado savannah has a lower net effect on greenhouse gas emissions than conventional diesel, but if a rainforest is being cleared, the biofuel does worse than diesel. Biofuels look much better if grown on land that isn’t currently absorbing much CO2. Reijnders, from the Institute for Biodiversity and Ecosystem Dynamics at the University of Amsterdam, points out that incentives may be needed to encourage the use of abandoned soils rather than clearing rainforest, as the rainforest clearance gives producers income from the sale of timber.

The input of fuel into producing biofuel needs to be estimated. Reijnders concludes that total fossil-fuel demand is relatively high for ethanol produced from European grain or US maize, and relatively low for palm oil or ethanol from sugarcane. However, palm oil tends to do worse than fossil fuel in greenhouse gas emissions.

“The ‘seed-to-wheel’ emissions of greenhouse gases associated with current transport biofuels are often higher than the corresponding life-cycle emissions of conventional fossil fuels”, says Reijnders. “Palm oil and ethanol from maize and wheat may contribute to energy security, but are counterproductive in limiting climate change,” he writes.

‘Transport biofuels – a life-cycle assessment approach’ by Lucas Reijnders, CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, 2008, 3, No. 071

Can Bt Maize Beat Down Mycotoxins?

Bt maize (which contains a toxin gene from Bacillus thuringiensis) is genetically engineered to limit damage from certain insect pests. Fungal infestation, which leads to mycotoxin contamination, is known to follow pest damage. So can the Bt toxin also help by reducing mycotoxins in maize?

Felicia Wu from the University of Pittsburgh examines the sometimes conflicting evidence in a paper in CAB Reviews. Aflatoxin is the most serious mycotoxin in terms of financial impact, and it appears that levels of this toxin are not consistently reduced in Bt maize in comparison to non-Bt maize, although future Bt maize varieties may have a more positive effect. However, fumonisin, another important mycotoxin, is reduced in almost all studies. Fumonisin is associated with oesophageal cancer and neural tube defects. Reducing fumonisin through Bt could have significant benefits in developing countries, especially where unprocessed maize is a key part of the diet, and so mycotoxins are present at levels which can health problems. It also could help them avoid losses in the export market through rejection of contaminated maize.

The paper, “Bt corn and impact on mycotoxins” appears in CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, 2007, 2, No. 060, 8 pp.