Wet cotton pollen can reduce yields WATER IS far more damaging to the pollen of a cotton flower than has been generally known before - so much so that a Texas-based USDA scientist is urging irrigated growers to find a way of applying water to the crop other than by above-canopy sprinklers.
After four years of study, Dr. John J. Burke of Lubbock suggests producers either water at night with their sprinklers or install drop lines and drag socks down to furrow-level on their center pivot applicators so open blooms on cotton stay dry.
He says water from any source that reaches the flower's tiny pollen on the bloom can lead to a lint yield reduction because, with any moisture at all, the pollen swells, doubles in size and bursts, reducing pollination. Scientific literature as far back as the 1920s mentions pollen-linked square shed and lint yield loss due to rainfall events.
Burke and his team moved their laboratory work on the subject of "sprinkler induced flower loss and yield reduction" into the field for the first time in 2000, and the results were similar to those found in a greenhouse. In the lab, a single mist of water from a squirt bottle into an open white cotton bloom effectively eliminated 55 percent of seed set from a single flower.
The team looked at a wide array of both Upland and Extra Long Staple varieties. Only a small percentage of the total number of the blooms on a cotton plant are open at any given time. Flowers aren't open at night.
The field work this past year compared cotton lint yield results where cotton was sprinkler-watered over the top, wetting the plants and flowers, and where the crop was kept dry using drop lines and drop socks for applying water directly into the furrow. Both methods were with a center pivot unit.
The 2000 season field results suggested a 28 percent lint yield increase where water was applied "dry" by furrow applications between 10 a.m. and 4 p.m. The method of application was less important during the 8 am. to 10 a.m. time period.
On the Plains of Texas, according to Dr. James Supak, a cotton authority with the Texas Agricultural Extension Service, College Station, producers normally will irrigate their cotton three or four times over the most critical 30 day flowering and boll setting span. Night watering, of course, isn't all that practical because most growers have to water 24 hours a day to keep up with plant needs.
"We essentially lost the flowers that were open on the day the sprinkler came across the field," says Burke, who is research leader at the USDA Agricultural Research Service Southern Plains Area Cropping Systems Research Laboratory's Plant Stress and Germplasm Devel-opment Unit at Lubbock. This past season he and his associates went into the fields and tagged 4,800 cotton blooms that were due to be open the following day.
They discovered that fully 80 percent of all the blooms open on the day the sprinkler traversed that portion of a field fell off the plant, resulting in a lower harvested cotton yield compared to drag sock type of "dry" irrigation.
The study conclusion thus far, according to Burke: "Cotton pollen is hyper-sensitive to water, and when pollen comes in contact with it, the water will cause the rupture and destroy the pollen grain.
"In the study we were looking at, the pollination of cotton strictly from the standpoint of various methods of irrigation management in cotton production - and the results - were very dramatic."
Burke's advice to producers: "Lower your sprinkler all the way to the to the ground or install drag socks so that you get less water in the plant canopy, or sprinkle at night when the blooms aren't open and subject to pollen damage, generally from sunset to sunrise."
Burke told a reporter he was "quite amazed at the magnitude of the differences. And there are a lot of flowers open at any one time down in the plant canopy."
The scientist noted that a single spray with a mist bottle is enough water to reduce cotton seed set by 55 percent.
A cotton flower, as most growers know, lasts only for a single day. During this period (when the bloom is white color) the pollen sack (anther) breaks open and pollen drifts out, known as dehiscing. The drifting pollen grain sticks to the side of the stigma (the female portion of the flower) and begins to put out a pollen tube. That tube grows in length enough over a period of roughly six hours to fertilize one of perhaps 30 ovules. The process is then repeated in other open flowers the next day.
The entire process is both light and temperature dependent, and at times can take longer. The process is most rapid during the warmest portion of the day, from 10 a.m. until about 4 p.m. A fully developed cotton boll will have as many as 30 seed inside. This signals that 30 pollen tubes have grown down to fertilize the 30 ovules. Cotton lint is a product of the developing seed inside the boll.
The sight of beak-shaped bolls on a plant is a sign that some ovules weren't fertilized. Once fertilized, the bloom takes on a pinkish color and drops off the plant, leaving the tiny boll in view.
"Our studies suggest that a cotton flower takes a real beating from a passing over-the-top sprinkler," Burke says. "While the scene of a sprinkler passing over a cotton crop may be pretty to look at, our experience with these studies suggest that a crop is actually taking some big time punishment."
Burke may be contacted for additional information at the USDA-ARS Southern Plains Area Cropping Systems Research Laboratory, Plant Stress and Germplasm Development Unit, 3810 4th Street, Lubbock, Texas 79415, or by calling 806-749-5560.