Even when everything is right—the optimum seedbed, nutrients and crop protection—yields can still fall short of expectations if the weather doesn’t cooperate. Abiotic stress, including temperature extremes, drought and excess soil moisture as well as salts, toxins and even excess light, can all impact yields. The most common and greatest stress is water-related, says Tom Sinclair, adjunct professor in North Carolina State University’s department of crop science.
“When we talk about abiotic stress in the Midwest and West, 80 to 90 percent of the time we’re talking about water and how the plant uses it,” says Sinclair. “In soybeans we can have severe nitrogen deficiency due to drought before we even know the plant is getting dry. We don’t see stress until the plant wilts, but by then we’ve lost at least half the nitrogen fixation capability. And a lot of other things have started to shut down as well, including photosynthesis and leaf growth.”
Water—or the lack of it—is only one of many stresses crops deal with each year. High temperatures, especially at night, can steal up to a bushel of corn a day from potential yields, points out Fred Below, professor of crop physiology at the University of Illinois. While 86 degrees is the optimum daytime temperature for corn and 98 degrees or above causes heat stress during the day, 73 degrees is the tipping point for nighttime temperatures.
“The plant has no way to cool itself at night with the stomates closed,” says Below. “Without transpiration, it is like putting the plant in an oven.”
During the day, the opening of the stomates cools the plant. In the case of drought, the stomates close to conserve moisture. That, too, can be a problem, Below says. “I believe plants over-respond to stress,” he says. “When it is dry, you want the stomates to close, but not too soon or for too long.”
Plant stress response timing can be a problem, says Sorina Popescu, assistant professor at Cornell University’s Boyce Thompson Institute for Plant Research (BTIPR).
“Protein receptors in cell membranes are sentinels that continuously monitor the environment; when they perceive a stress signal, they become active and recruit other proteins in an enzymatic cascade that transmits the stress signal all the way to the cell nucleus,” says Popescu. “In the nucleus, where gene transcription takes place, the stress signals change the gene expression to prepare the plant for a response. This begins to affect the plant. This signaling pathway is triggered seconds after the receptors perceived the stress signal; however, the changes in gene expression can last for days or weeks and considerably strengthen the plant’s ability to withstand stress.”
In fact, the gene expression can last the rest of the season, says Sharon Clay, an agronomist at South Dakota State University. Clay’s research team has found that stress can permanently down-regulate the genes responsible for immune response insect damage response and phosphorous uptake, as well as for photosynthesis.
“The genetic response takes place before any obvious signs of stress are seen,” says Clay. “The crop doesn’t have to be severely stressed before the down regulation occurs, and it can occur at early as well as later stages.”
Under optimum conditions, the down regulation may not noticeably impact yield. However, should the crop be stressed later in the season, yield loss could occur. An affected plant may be less able to handle a stress-induced buildup of reactive oxygen species (ROS). These chemically reactive molecules containing oxygen include peroxides and oxygen ions that are constantly being produced in a healthy plant as part of oxygen metabolism. Normally, plant enzymes break them down to be used in plant growth. In plant tissue faced with an invading stress, ROS may initially act as a plant protectant in ways such as forming hydrogen peroxide that strengthens the plant cell wall. However, too much ROS can damage surrounding cells or lead to plant death.
“The commonality to all abiotic stress is that it leads to increased accumulation of ROS,” says Popescu. “Depending on the stress, the receptors and the signaling pathways, ROS can accumulate in different places, in the chloroplast of the cell, outside the cell in the apoplast or even in the mitochondria or cytoplast.”
Differences in how much and where ROS locates, as well as how it affects gene expression, creates a specific response, such as the leaf rolling associated with drought stress. Such visible signs are often not seen until late in the stress response, perhaps long after the short- or even long-term damage to yield has been done.