** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** EXCESSIVELY HIGH TEMPERATURES DURING FLOWERING AND BOLL DEVELOPMENT ARE IMPORTANT DRIVERS OF YIELD LOSS IN THE SOUTHERN UNITED STATES. MANY COTTON PRODUCTION REGIONS ROUTINELY EXCEED THE OPTIMUM TEMPERATURE RANGE FOR MULTIPLE PHYSIOLOGICAL PROCESSES, AND THE FREQUENCY, DURATION, AND INTENSITY OF EXTREME HEAT WAVE EVENTS IS ONLY EXPECTED TO INCREASE IN THE DECADES TO COME. ADDITIONALLY, LOW GENETIC DIVERSITY AMONG MODERN COTTON CULTIVARS HAS RESULTED IN A SHALLOW GENE POOL FROM WHICH TO SELECT FOR THERMOTOLERANCE, AND TARGETED SELECTION FOR HEAT TOLERANCE HAS BEEN NON-EXISTENT. THE OVERRCHING GOAL OF THE PROPOSED RESEARCH IS TO ACCELERATE THE DEVELOPMENT OF TARGETED HEAT TOLERANCE SELECTION IN UPLAND COTTON. OBJECTIVES AIMED AT MEETING THIS GOAL INCLUDE 1) CHARACTERIZING REPRODUCTIVE TOLERANCE TO A SIMULATED HEAT WAVE EVENT FOR A DIVERSITY PANEL OF UPLAND COTTON THAT ACCOUNTS FOR 97% OF ALL ALLELIC DIVERSITY WITHIN THE SPECIES, 2) IDENTIFYING FUNCTIONAL SOURCE LEAF TRAITS THAT ARE INDICATIVE OF GENETIC VARIATION IN REPRODUCTIVE THERMOTOLERANCE, AND 3) MAPPING FUNCTIONAL TRAITS CONFERRING HEAT TOLERANCE TO SPECIFIC REGIONS OF THE COTTON GENOME TO DEVELOP MOLECULAR MARKERS FOR HEAT TOLERANCE SELECTION. TO DATE, NO SUCH EFFORT HAS BEEN SUCCESSFULLY CONDUCTED FOR UPLAND COTTON, AND ACHIEVING THESE THREE GOALS ARE THE DELIVERABLES OF OUR PROPOSED RESEARCH.