Impacts of varying day and night environmental conditions on cotton flowering, yield, and fiber quality

Introduction: Increases in the frequency of higher-than-optimum air temperatures can substantially reduce cotton production. Little is known about the influence of different combinations of day/nighttime temperature on cotton flowering and boll maturation under ambient and elevated CO₂ conditions. Methods: This study examined the impacts of air temperature variations on the morphology of cotton flowers and seed yield under air CO₂ concentrations at 425 ppm (ambient, aCO₂) and elevated at 725 ppm (eCO₂) in controlled Soil-Plant Atmospheric Research (SPAR) chambers. The four temperature conditions were: optimum (OT; 33/21°C, day/night), high temperature (HT; 36/24°C, day/night), high nighttime (OT+HNT; 33/24 °C, day/night), and high day/nighttime (HT+HNT; 36/28 °C, day/night). Results: Various reproductive and seed yield traits, as well as the phenology of the plants, differed significantly (p < 0.001) under the treatments. The boll maturation period significantly decreased in plants grown under HT+HNT, with only 39 days under aCO₂ and 38 days under eCO₂ compared to 47 days at OT. In the HT and OT+HNT conditions, the duration was 42 days at aCO₂ and 46 days at eCO₂, as opposed to 41 and 44 days, respectively, under aCO₂. Furthermore, there was a significant reduction in the number of pollen grains per anther, 13% for OT+HNT, 24% for HT, and 39% for HT+HNT, relative to OT treatments. The seed cotton weight also showed a drastic decline, decreasing from 105 g plant^-1 under OT to 90 g under OT+HNT, 47 g under HT, and 12 g plant^-1 under HT+HNT conditions. In the HT+HNT environment, lint percentage and seed weight per plant were reduced by 26% and 86%, respectively, when compared to OT. The eCO₂ did not alleviate the reductions in cotton yield caused by higher air temperature exposure. Discussion: This study highlights that high air temperature induces flower abscission and anther indehiscence, while diverting biomass allocation towards vegetative organs. The resulting source-sink imbalances between vegetative and reproductive structures resulted in significant reductions in seed and lint yield and growth patterns across CO₂ and temperature environments. These findings provide insights into cotton management strategies under future environmental scenarios.