Higher Yields, Higher Densities Leave Crops Parched

The warming of our planet has myriad effects on climate, including changes to precipitation patterns, extreme weather, extreme temperatures (both hot and cold), and relative humidity of the air. The relative humidity is particularly important for water loss in plants. Relative humidity represents the amount water in the air as a percentage of the total amount of water that the air can hold. For example, if 100 litres of air can hold 10 litres of water, and there are 5 litres of water in the air, then the relative humidity of the air is 50%. The capacity of air to hold water is not constant however, it changes with temperature. Warmer air can hold more water. This means that if you took air at 20°C with 50% relative humidity and warmed it to 30°C, the relative humidity would be much less than 50%.

We all know what happens when we don’t water our plants: they wilt and in many cases perish. For the most part, plants get their water from the soil, and lose most of it to the air. The less water in the air, the faster water loss occurs. Plants do not respond to the relative humidity of the air – instead, they respond to the lack of water in the air called vapour pressure deficit. Vapour pressure deficit is the difference between the amount of water in a volume of air and the amount of water that the air can hold. A high vapour pressure deficit causes plants to lose more water, and in severe cases, shutdown photosynthesis to conserve water – especially when there is little water in the soil.

So why are all these concepts important? One word: agriculture. Lobell et al. (Lobell et al., 2014, Science 344:516-519) set out to determine how sensitivity to drought has changed in soybean and maize (corn) crops in the United States over the past 17 years or so. The past 50 years have seen dramatic increases in crop yields and planting densities. Planting density in particular may create issues with drought stress: i.e. more plants in a given area mean more water loss from the soil through the plants, exacerbating drought stress. Given that drought is expected to increase in the United States with climate change, knowing the drought sensitivity of staple crops is important for global food supplies. Using data from maize and soybean fields, Lobell et al. found that vapour pressure deficit had the greatest impact on yields, with a higher deficit (drier conditions) causing reduced yields. Further, they found that drought sensitivity increased over their study period, meaning that less severe droughts caused greater yield losses by the end of their study than in the beginning.

So why are these crops becoming more sensitive to drought? One idea proposed by Lobell et al. for maize is that planting densities are increasing – with more plants in the soil, more water is needed to support the crops. A moderate drought in a field with low crop density becomes a severe drought in a densely planted field. Since precipitation patterns are changing and the US is getting drier as well, this means that crops are especially sensitive to drought in the future. More than ever, there is a need for crops that are very resistant to drought, that use very little water, and have high yields. Solutions to this very eminent problem include breeding more drought resistant crops from available varieties, and genetic engineering of hardier crops (although this is generally frowned upon). And who will develop these solutions? Plant biologists and farmers – they are the people who can keep the world fed.

Stay safe and stay informed,
Joe

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