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High daytime temperatures can reduce photosynthesis, resulting in lower cotton yield potential. High nighttime temperatures can increase plant respiration, consuming energy that could otherwise be used to increase cotton yield potential. Sustained periods of above average day or night temperatures can explain lower than expected cotton yields.
There are several factors involved in the ability of a cotton crop to regulate temperature. Air temperature is important, as well as sunlight, soil moisture, relative humidity, and air movement. Optimal plant tissue temperature for cotton growth and photosynthesis is between 74 and 90˚F. Cotton can lower tissue temperature by opening leaf stomates to allow water to evaporate. For example, cotton plants with adequate soil moisture can be 10 degrees cooler than the air temperature.1
Figure 1. Developing cotton squares within the canopy.
The ability of a cotton crop to regulate temperature can be reduced with the following conditions:
High yield potential cotton that is heat-stressed can produce yields 20 to 30% below expectations. Research implicates higher than average day and/or night temperatures. High temperatures can decrease photosynthesis and increase respiration, leading to reduced seed production, reduced lint development, and unexpectedly lower yields. While cotton typically maintains canopy temperatures lower than air temperatures, high humidity, coupled with high air temperature, can lead to canopy temperatures above the optimum.
Scientists at the University of Arkansas exposed cotton plants to normal (89.6˚F daytime temperatures and 75.2˚F nighttime temperatures) and normal daytime temperatures with increased (86.0˚F nighttime temperatures) for seven days following pinhead square stage (approximately 4 weeks after planting).2 Although the cotton was not taken to harvest, the studies showed significant effects on crop productivity brought on by the higher nighttime temperatures. Plants grown with higher nighttime temperatures had 23% and 36% lower photosynthesis rates compared to the control, one and seven days after treatment initiation, respectively. Respiration in the heat-stressed plants increased 54% compared to the control on the first night. Seven days into the trial, heat-stressed plants had a 68% higher respiration rate compared to the control.2 Further analysis of the heat-stressed plants showed reduced carbohydrate content in the flower buds and a decrease of almost 50% in the number of flower buds per plant, thus reducing the plant's ability to reach optimum yield potential.
Optimal performance occurs at canopy temperatures below 82.4˚F. Heat stress on cotton, resulting from canopy temperatures above 82.4˚F, can significantly reduce overall lint yield, delay crop maturity, and reduce lint quality. Heat-stressed cotton flowers produce little or no pollen which can lead to young bolls (3-5 day old) to abort. Young squares may also be damaged, leading to low fruit retention. These damaged squares typically do not abort, but can develop into smaller flowers that do not fully open, produce sterile anthers, and have what appears to be an elongated stigma resulting from the filaments supporting the anthers failing to elongate properly.3
If available, apply irrigation frequently and lightly. Avoid cultivation as this can damage roots and increase square and boll shed.
Planting varieties that are described as more heat tolerant or earlier maturing varieties that may pollinate before high temperatures occur may help reduce potential heat stress. However, predicting high temperature extremes prior to planting is nearly impossible. Agronomists generally recommend planting varieties with high yield potential and good insect and disease resistance within the recommended planting window established for local growing conditions.
Sources: 1 Hake, K. and Silvertooth, J. 1990. High temperature effects on cotton. National Cotton Council. Physiology Today. Vol. 1, No 10. 2 Loka, D.A., and Oosterhuis, D.M., 2016. Increased night temperatures during cotton's early reproductive stage affect leaf physiology and flower bud carbohydrate content decreasing flower bud retention. Journal of Agronomy and Crop Science. Volume 202, Issue 6. ISSN 0931-2250. 3 Brown, P. 2008. Cotton heat stress. The University of Arizona. AZ1448. https://extension.arizona.edu/. Snider, J.L., Oosterhuis, D.M., Skulman, B.W., and Kawakami, E.M. 2009. Heat stress-induced limitations to reproductive success in Gossypium hirsutum. Physiol Plant. 137(2): 125-38. Web sources verified 06/04/18. 180604114002