When does a water deficit affect cotton yield the most? According to a study conducted by Derrick Oosterhuis, professor of cotton physiology at the University of Arkansas and Dimitra Loka, doctoral graduate student at UA, cotton is sensitive to water-deficit stress at all stages of growth, but particularly during reproductive development, from first square to peak bloom.

Oosterhuis discussed the impact of water stress on cotton at Cotton Incorporated’s Crop Management Seminar, in Tunica, Miss. Here’s a closer look at what happens to cotton when water becomes scarce – both above and below surface.

Planting to emergence. “Water is critical for germination, noted Oosterhuis, “but prior to first square, there’s not much impact from drought stress or water stress on yield.” Oosterhuis noted that pre-irrigation at this stage “reduces the possibility of seedling disease compared to irrigation shortly after planting.”

At this stage, the most critical development of the plant is taking place under the ground, Oosterhuis noted. “Once seeds have germinated, there must be sufficient water for root proliferation. Often times, we don’t pay enough attention to roots. But it’s critical for us to understand what the roots are doing and where the water for the plant is coming from.”

Cotton seedling root systems develop early and rapidly during this stage, growing up to 2 inches per day.

Emergence to first square. Water demand at this stage is low, according to  Oosterhuis, but root development is still important. “The plant is partitioning a tremendous amount of its resources into the root at this time.”

Oosterhuis noted that unless the soil water deficit is extremely severe, irrigation during this stage contributes relatively little to yield.

In fact, studies have shown that a mild water deficit early in the season can stimulate root production, especially encouraging deeper root systems and helping acclimate plants to scarce water conditions, or “priming plants” to increase water use efficiency, Oosterhuis said.

First square to first flower. Plant activity increases both above and below ground. This is a critical stage for cotton because vegetative growth is very rapid and the number of potential fruiting sites for the crop is determined.

“This is when the plant is taking up a tremendous amount of its nutrients, not only potassium and phosphorus, but also nitrogen, up to 40 percent of what it is going to need for the whole season,” Oosterhuis said.

“During this period, the plant is not only developing its leaf area and squares are setting, but below ground, there is tremendous development of the roots going on. If you don’t have anything slowing down the root system, it can reach its maximum depth in about 50 days. “

Severe water stress during this time is especially damaging to the cotton crop in short-season environments.

First flower to peak bloom. At this stage, most of the crop’s activity moves above ground. Water use by cotton increases from 0.2 inch per day to 0.28 inch per day, its maximum. Water-deficit stress at this stage reduces plant growth and fewer fruiting sites are initiated and fewer seeds set in the boll. Severe water-deficit stress can reduce boll number through shedding of young bolls and results in substantial yield loss, according to Oosterhuis.

“Water stress can also affect the size of the surviving bolls,” Oosterhuis said. “You usually don’t get a decrease in fiber quality unless the stress is extremely severe.”

Rooting depth has reached its maximum at this stage, Oosterhuis noted. “You will get some proliferation on lateral roots, but the bulk of the root system is already there, and you don’t get much increase in root activity. As the reproductive boll load begins to develop, it becomes the major sink for resources at the expense of the root system.”

Oosterhuis noted that the most sensitive stage for a developing boll is in the four to six days after the white flower stage. “Any water-deficit stress once flowering starts is very critical for young bolls. They will fall off very easily, from any stress.”

From peak bloom to open bolls. Water-deficit stress starts to decrease and is less critical during this stage than during squaring and early flowering,” Oosterhuis said. Fiber length and micronaire can be affected by stress, especially in young bolls.

“After bolls start opening, you want water-deficit stress to kick in,” Oosterhuis said. “Stress at this time hastens boll opening, makes defoliation easier and reduces regrowth.”

Oosterhuis noted that cotton varieties react differently to water-deficit situations. Research is under way to pinpoint these mechanisms for drought tolerance, “but we really haven’t exploited these very well.”