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Some plants like it hot. Cotton with superior heat tolerance can be a profitable crop for warmer climates, so Agricultural Research Service (ARS) scientists are identifying tolerance-specific genetic selection tools to assist breeding efforts.

Unfortunately, it's nearly impossible to differentiate between heat tolerance and heat avoidance simply by examining the quantity and quality of final crop yields. Heat avoidance refers to characteristics that enable a plant to withstand the heat with similar, but less reliable, results?for example, by shifting the bulk of metabolic activity to cooler, evening periods.

At the U.S. Arid-Land Agricultural Research Center in Maricopa, Ariz., ARS scientists are investigating the process known as «dark respiration.» This research could make it easier to differentiate between heat-tolerant and heat-avoidant plants.

Dark respiration is a continuous process in which mitochondria within a plant's cells oxidize carbohydrates to create energy. Cotton plants make more starch during the day than they require for growth. The excess starch is stored in plant cells' chloroplasts, where photosynthesis occurs. At night, that starch is broken down via respiration and other metabolic processes and used to support new growth, such as cotton bolls.

To determine the relationship between efficient nocturnal carbon use and heat tolerance, plant physiologist Steven Crafts-Brandner and plant geneticist Richard Percy?now with the ARS Southern Plains Agricultural Research Center in College Station, Texas?selected three upland and three pima cotton cultivars, choosing a mix of heat-tolerant and heat-susceptible plants. They have been monitoring the cultivars' rates of dark respiration and photosynthesis throughout the day.

Percy and Crafts-Brandner have already made some significant observations. For example, the cultivars with the greatest heat tolerance generally have lower rates of dark respiration and more efficient use of carbohydrates. If ongoing studies support these observations, the scientists may be able to use these traits to improve the cotton breeding program.