Research crop scientist from the K-State Northwest Research-Extension Center, Professor Robert Aiken explores agriculture and crop research issues in the United States today
As an agricultural scientist, I consider it my duty to anticipate questions and problems which may confront farmers in the future. When I’m successful, designing and conducting effective field studies, we have the information needed to formulate feasible solutions, before problems get out of hand.
In my semi-arid region of the U.S. Central High Plains, our crop systems contend with heat stress, desiccating winds, lack of rainfall, flood-generating rains and unexpected arctic air masses, inducing winter-kill or bringing the season to a chilling conclusion. Adapting to climate change? In a sense, we prepare for climate change by helping farmers adjust to the challenges of the current growing season.
Our growers recognise long-term warming trends and shifts in weather patterns. A recent report (1), prepared by the State Climatologists of Texas, Oklahoma and Kansas, indicates climate change has been written into the historical weather record. Below are three quotes from the report:
“Both temperature and precipitation have increased across the Southern Plains since the beginning of the 20th century. Temperature increases so far have averaged about 1.5ºF (0.8ºC) over the 20th century and precipitation has increased by as much as 5%, albeit with large variations from year-to-year and decade-to-decade. Heavy rainfall events have increased in frequency and magnitude. Historical data for tornadoes and hail are not reliable enough to be used to determine whether a trend is present in these types of severe weather.” (1)
“Variations in drought conditions from year-to-year and decade-to-decade are triggered by changes in sea surface temperature patterns in the Pacific and Atlantic oceans. The Dust Bowl drought is thought to have been exacerbated by poor land use practices, while precipitation may have been enhanced in recent decades by growth in irrigated agriculture and surface water.” (1)
“Temperatures will continue rising over the long-term, as carbon dioxide and other greenhouse gases continue to become more plentiful in the atmosphere. By the middle of the 21st century, typical temperatures in the Southern Plains are likely to be 4ºF to 6ºF (2.2ºC to 3.3ºC) warmer than the 20th century average, making for milder winters (with less snow and freezing rain), longer growing seasons and hotter summers. Rainfall trends are much less certain. Most climate models favour a long-term decrease, but most projected changes are small compared to natural variability. Extreme rainfall is expected to continue to become more intense and frequent.” (1)
I have specific concerns deriving from these warming trends: declining yield potential because of increased night temperatures, diminished photo-protection systems under persistent heat stress, increased risk of reproductive failure with heat stress at critical development stages, increased crop water requirements, degradation of soil with intensive rainfall events and increased potential for large-scale methane emissions unleashed by thawing permafrost (2). These concerns rise to the top of my “watch list” for climate change impacts.
Crop productivity is expected to benefit from historic and on-going annual increases in global CO2 concentrations. Assimilation rates can be maintained with modestly reduced crop water requirements. Cool-season grass crops and broadleaf crops will likely gain photosynthetic efficiencies. However, warming trends can detract from the beneficial effects of elevated CO2 levels.
“When elevated temperatures exceed optimal conditions for assimilation, stress responses can include damage to the light-harvesting complex of leaves, impaired carbon-fixing enzymes, thereby reducing components of yield including seed potential, seed set, grain fill rate and grain fill duration. Field studies conducted under conditions of elevated CO2 indicate that benefits of elevated CO2 are reduced by heat-induced stress responses.”(3)
Warmer temperatures, the most reliable feature of climate change, can extend the growing season, but also impair plant productivity. Persistent heat stress pushes plant metabolism to the edge of toleration. The complexity of plant metabolic processes can be astounding. Many of these processes are temperature-sensitive, with optimum temperatures for photosynthesis ranging from 25 to 30ºC (77 to 86ºF) for winter wheat (4), up to 32ºC (90ºF) for soybean (5) and up to 38ºC (100ºF) for maize (6). Chronic heat stress, with daily temperatures exceeding this range, can accelerate the breakdown of thermo-protective mechanisms and can result in permanent damage to crop canopies.
Hot conditions prior to and during flowering can result in crop failure. Grain production requires effective pollination of ovules for ‘seed set’, followed by development and growth of the kernels, harvested as grain. Excessive temperatures (i.e., daily mean temperatures > 25ºC for grain sorghum (7), wheat (8)) for a few days in the ~15-day period around flowering can decrease yield potential due to impaired pollination and seed-set; complete failure can occur with daily mean temperatures of 35ºC (wheat) or 37ºC (sorghum).
Night temperatures drive the metabolic rates of a plant, with the associated respiratory release of CO29 as well as cell degradation (10). In a sense, plant respiration depletes the supply of carbohydrates available for plant growth and development. As a long-term trend, warmer night temperatures can sap crop productivity.
Chronic high temperatures add to the evaporative demand on crop systems. This increases the water requirement for crop growth. Warmer temperatures can sap yield potential by impairing heat-tolerance protective mechanisms; by reducing the duration of grain-filling; and by increasing the respiratory cost, the water requirement for growth and the risk of reproductive failure of cereal crops. Warmer temperatures carry a complex drum-beat of warnings for crop productivity. Needed research is underway to adapt crop cultural practices to avoid heat stress; and to seek genetic advances for crop cultivars that are capable of tolerating or resisting effects of warming temperatures.
1 “Climate Considerations.” John Nielsen-Gammon, Gary McManus, Xiaomao Lin and David Brown. White paper developed for “Resilient Southern Plains Agriculture and Forestry in a Varying and Changing Climate: Conference Report” July 18-19, 2017; El Reno, OK. http://twri.tamu.edu/el-reno
2 https://nsidc.org/cryosphere/frozenground/methane.html
3 Aiken, R. “Climate change impacts on crop growth in the Central High Plains.” Proceedings of the 21st Annual Central Plains Irrigation Conference. Colby, Kansas, February 24-25, 2009.
4 Yamasaki, T., T. Yamakawa, Y. Yamane, H. Koike, K. Satoh and Katoh. 2002. Temperature acclimation of photosynthesis and related changes in photosystem II electron transport in winter wheat. Plant Physiol. 128:1087-1097.
5 Vu, J.C.V., L.H. Allen, Jr., K.J. Boote and G. Bowes. 1997. Effects of elevated CO2 and temperature on photosynthesis and Rubisco in rice and soybean. Plant, Cell and Environment 20:68-76.
6 Crafts-Brandner, S.J. and M.E. Salvucci. 2002. Sensitivity of photosynthesis in a C4 plant, maize, to heat stress. Plant Physiol. 129:1773-1780.
7 Prasad, P.V.V., M. Djanaguiraman, R. Perumal and I.A. Ciampitti. 2014. Impact of high temperature stress on floret fertility and individual grain weight of grain sorghum: sensitive stages and thresholds for temperature and duration. Front Plant Sci. 6:820.
8 Prasad, P.V.V. and M. Djanaguiraman. 2014. Response of floret fertility and individual grain weight of wheat to high temperature stress: sensitive stages and thresholds for temperature and duration. Functional Plant Biology 41:1261-1269.
9 Tan, K.Y., G.S. Zhou and S.X. Ren. 2013. Response of leaf dark respiration of winter wheat to changes in CO2 concentration and temperature. Chines Science Bulletin 58(15):1795-1800.
10 Narayanan, S., P.V.V. Prasad, A.K. Fritz, D.L. Boyle, B.S. Gill. 2014. Impact of high night-time and high daytime temperature stress in winter wheat. J. Agronomy and Crop Science 201(3):206-218.
Please note: this is a commercial profile
Robert Aiken, Associate Professor
Research Crop Scientist
Northwest Research-Extension Center
Tel: +1 785 462 6281
