The creeping trends of a changing climate
This article concludes a 4-part series focusing on agriculture, climate, and change. The first three articles in the series, “Is Kansas agriculture likely to be affected by a changing climate?”, “A brief on global research on drought monitoring”, and “Water, temperature, and crop productivity research” can be found on the eUpdate website in Issues 710, 712, and 713, respectively.
Like the proverbial frog in the heating pot of water, we may not notice the creeping trends of a changing climate. Farmers in the semi-arid Central High Plains encounter dramatic year-to-year fluctuations in weather patterns, which tend to overwhelm the long-term trends. We are just learning how to recognize and interpret long-term climate signals, such as the El Nino-Southern Oscillation (fluctuations in the surface temperatures of the equatorial Pacific Ocean). What has to be done to ensure global food security in the face of these changes?
Semi-arid cropping systems include 19.5 million acres of winter wheat production in regions such as the western U.S., Argentina, western and central Asia (Figure 1); here the evaporative demand for water can exceed annual precipitation by three to five times.
Is there opportunity to increase crop productivity, given limited and untimely water supply? Two approaches include: improving crop water productivity and increasing stress tolerance of critical processes such as ovule fertilization by pollen.
Figure 1. Mega-environments (ME) identified by the International Maize and Wheat Improvement Center (CIMMYT). The water-limited ME12 includes 19.5 million acres of winter wheat production, sustaining over 140 million persons. Sonder, Kai, 2016
The fundamentals of carbon-water exchange, which underlie crop water productivity, are ‘managed’ by leaf stomata—openings that allow carbon dioxide (CO2) entry to leaf biochemistry as well as the exit route for water vapor diffusing into the atmosphere. This linked diffusion of CO2 and water vapor supports the theory that the carbon-water exchange rate is closely regulated, affected by biochemistry and atmospheric humidity.
Greenhouse and field studies (Xin et al., 2009; Narayanan et al., 2013) indicate that sorghum cultivars do differ in carbon-water exchange rates (Figure 2) with parallel differences in radiation use efficiency (Figure 3). Current results from Eastern Colorado indicate corn requires approximately 2,500 gallons of water per bushel. Improving crop water productivity may reduce this water requirement (Mowitz, 2012). This evidence supports accelerated investigations into the mechanisms driving differences in plant carbon-water exchange rates as well as development of high-throughput screening tools to identify desirable germplasm.
Figure 2. Evidence that sorghum cultivars can differ in the carbon-water exchange rate. Here, instantaneous transpiration efficiency (nTE), normalized by vapor pressure deficit (VPD)—a measure of atmospheric aridity, is shown in relation to the ratio of leaf internal CO2 concentration to air (Ci/Ca). This scaling relationship taken from Xin et al., 2009.
Figure 3. Field evidence that sorghum cultivars differ in biomass productivity in relation to use of water and radiation. Relationships are shown between water use efficiency (WUE) and radiation use efficiency (RUE) among sorghum genotypes; WUE was derived as the slope of the regression of aboveground biomass on cumulative water use, while RUE was derived as the slope of the regression of aboveground biomass on cumulative intercepted photosynthetically active radiation (IPAR). From Narayanan et al., 2013.
Our grain farmers need effective conversion of biomass—improved by WUE and RUE—into grain. Heat stress and water deficits impair pollen development and fertilization of ovules, resulting in the ‘seed-set’ required for grain production, according to studies conducted by researchers at the Feed the Future Innovation Lab for Collaborative Research on Sustainable Intensification (Dr. Vara Prasad, Director). Species of Aegilops, a relative of wheat, provide sources of genetic traits conveying heat and drought tolerance to pollen development and ovule fertilization (Pradhan et al, 2012). Scientists in the Wheat Genetics Resource Center are systematically integrating these traits with elite breeding lines, developing wheat varieties with increased stress tolerance.
Sustaining the increased crop productivity to provide global food security will require continued innovation, collaboration, and vision.
Xin, Z. R. Aiken and J. Burke. 2009. Genetic diversity of transpiration efficiency in sorghum. Field Crops Research 111:74-80. doi:10.1016/j.fcr.2008.10.010
Narayanan, S., R.M. Aiken, P.V.V. Prasad, Z. Xin and J. Yu. 2013. Water and radiation use efficiencies in sorghum. Agron. J. 105:649-656. doi:10.2134/agronj2012.0377
Pradhan, G. P., P. V. V. Prasad, A. K. Fritz, M. B. Kirkham, and B. S. Gill. 2012. High temperature tolerance in Aegilops species and its potential transfer to Wheat. Crop Science 52:292-304. doi:10.2135/cropsci2011.04.0186
Mowitz, D. 2012. 2,500 gallons per bushel. Successful Farming. www.agriculture.com/machinery/irrigation-equipment/drip-irrigation/2500-gallons-per-bushel_272-ar22678
Rob Aiken, Crops Research Scientist, Northwest Research-Extension Center
Xiaomao Lin, State Climatologist