By David L. Bush, Ph.D.
First, let me explain the title of this essay. Chilling injury is cellular damage to meristematic plant root tissues that often occurs when soil temperatures fall below 50 degrees Fahrenheit during the imbibition phase of seed germination. The Artful Dodger part of the title is a literary analogy. The Artful Dodger, a character in the Charles Dickens’ novel Oliver Twist, was a skilled pickpocket who could lift a gentleman’s wallet or cut a lady’s purse without being detected. And that is exactly what chilling injury does: it robs a farmer of yield, maturity and fiber quality without being detected.
Warm-season crops with centers of origin in the tropics or sub-tropics are often chilling sensitive, with cotton classified as very sensitive. When cotton seeds are subjected to soil temperatures below 12 Celsius (54 F) during the imbibition phase of seed germination, permanent damage to the roots can occur. This damage includes death of the developing roots (radicles), which results in nub-roots, sloughing of the root cortex and loss of tropism.
Loss of tropism as a chilling injury symptom was demonstrated to me in 1979 while working as a cotton breeder in West Texas. A customer called to complain that the seed we sold him was “coming up upside-down.” I was sent to investigate, and sure enough, about one-third of the emerging seedlings had the roots sticking up in the air and the cotyledons trying to go down unto the soil. A group of meristematic cells in the radicle senses gravity and signals the developing roots to go down and the developing main stem (shoot) to go up. Chilling temperatures can scramble these signals. The customer assured me that “soil temperatures were fine” when he planted. “Of course,” he went on to say, “a cold front dropped temperatures into the 40s that night.” Elementary, my dear Watson.
In research trials, cotton plants from seed exposed to chilling temperatures showed reduced plant height, delayed fruiting, reduced fiber quality, reduced fresh and dry weight, reduced root development, reduced hypocotyl elongation, reduced leaf size and stunting. A problem with chilling injury is that it seldom kills the plant; it just debilitates it. The cotton farmer may have noticed that his crop was a little slow to emerge and the stand was a bit ragged, but the crop grew out of the slow start. What he did not realize was that the roots were damaged, preventing optimal crop performance. Like high blood pressure, there are no obvious symptoms until it kills you from a stroke or heart attack.
Much of the research on the site of chilling injury has focused on cellular membranes. Plant cells are bounded by the plasma membrane. Organelles – the nucleus, mitochondria and chloroplasts – are also membrane bound and can be damaged by chilling temperatures. Membranes are complex organelles containing protein binding sites, ion pumps and cellular communications portals. Any damage to these structures impairs function and reduces metabolic efficiency. When a cottonseed is in the resting stage, its water content is less than 20 percent. At this moisture level, the plasma and organelle membranes are folded into a hexagonal crystal configuration. During the water imbibition phase of germination, the molecular structure of the membrane must transition from the hexagonal crystal structure to a phospholipid bilayer. This is the point where imbibitional chilling injury can occur. At chilling temperatures (60 F at 6 inches or >68 F at 2 inches; 2) a five-day forecast of >25 DD60s; and 3) forecast low air temperatures are >50 F for five days following planting.
I think chilling injury is more widespread and does more damage than farmers realize. Farmers know their farms and can predict how each one should perform each year. Sometimes they are disappointed, but can’t say exactly what reduced crop performance. Usually, they just blame it on the variety — an attitude we cotton breeders do so enjoy — but chilling injury may have been what robbed them of yield, maturity and quality. Just like the ole Artful Dodger.
Contact Dr. Bush at firstname.lastname@example.org.