Heat bad for wheat Global warming may affect wheat yield in the Gangetic plain adversely, finds T.V. Jayan

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Heat bad for wheat

Global warming may affect wheat yield in the Gangetic plain adversely, finds T.V. Jayan

It is a question that has no clear answer yet. How will the impending climate change affect the Indian food basket? Researchers specialising in agriculture, climate change and mathematical modelling have been trying to piece together this complicated jigsaw puzzle. For, understanding how major food crops will respond to extreme heat is critically important for ensuring the food security of a billion-plus Indians. Some studies have even suggested there could be 10 to 40 per cent loss in overall crop production in India by 2080-2100 because of increase in temperature. How this excess heat would hamper yield in individual crops still not very clear. But now a team of scientists, led by a researcher at Stanford University in the US, show what rising temperatures may do to wheat, the second-largest cereal crop in India. They argue that extreme heat may accelerate ageing in wheat plants, reducing crop yields. For every degree of increase in temperature, the crop yield can come down by 10 per cent, says David Lobell, who led the study which appeared online in the journal Nature Climate Change yesterday. An earlier study in Australia too demonstrated this fact. It showed that the yield was 50 per cent lower in years that were 2°C warmer compared to those that were 2°C cooler than average temperatures. Indian scientists working on climate proofing of important crops agree. "For every one degree increase in mean temperature, yield of rice and wheat goes down by six to 10 per cent," says Bandi Venkateswarlu, director of the Central Research Institute for Dryland Agriculture in Hyderabad. Climate change projections have indicated that global temperatures may increase by as much as 4°C over pre-industrial levels by the 2060s. More important, most crop models currently used to forecast the impact of climate change on food grain production fail to take into account the impact of high temperatures on ageing of the crop. On the other hand, they capture some beneficial effects of heat on wheat. For example, warming will speed up crop development and affect the rate of moving carbohydrates to the grain. However, most models do not include the effects of extremely high temperatures on cell damage in general, and senescence (ageing) in particular, says Lobell. "They are missing a piece of the picture," he told KnowHOW. Venkateswarlu, however, says unlike the models cited in the study, the models Indian scientists are working with take into account the adverse effects of heat on wheat. Venkateswarlu is a key coordinator for the National Initiative for Climate Resilient Agriculture, which intends to develop heat-tolerant crops. Wheat is now grown on more than 220 million hectares, making it the most widely grown crop in the world. Since the crop prefers cool temperatures, it is sown in late autumn or early winter and harvested before summer. For their study, Lobell and his colleagues used data available from the Indo-Gangetic plains (IGP). The scientists say they chose IGP for two reasons: first, India is a major wheat producer and second, it also regularly experiences high temperatures. For instance, in 2010 wheat yields were adversely affected by a sudden rise in temperature just before the harvest season. "So, India was a good place to test how important late-season heat is for wheat yields," says the Stanford scientist. The study says that extreme heat affects wheat growth at many stages of development and through different mechanisms. Grain yields are decided mainly by two things: grain number, which is determined from 30 days before flowering until shortly after flowering; and grain size, which is determined during grain filling. Lobell admits that increase in temperature just before the harvest season is not all bad. If temperatures hover around 20°C, it can increase the rate of grain filling. Above 30°C, warming can slow grain filling, in part because the photosynthetic apparatus on the leaf is damaged, resulting in an acceleration in the process of ageing. "Farmers all over the world use various strategies to deal with heat. Some rely on using as much water as possible to keep the canopy cool. Others rely on varieties that can avoid damage at really high temperatures. And some rely on early maturing," says Lobell. "There's a lot to be done to figure out what combinations of these strategies deal best with heat, but still produce yields that are attractive to farmers," he observes. Venkateswarlu feels Indian scientists may be able to take a few lessons from this study. One of them would be to use satellite data of crop acreage for making projections of climate change impact.