Thidiazuron (TDZ) is the main defoliant used in production to promote leaf abscission for machine-picked cotton. Under low temperatures, the defoliation rate of cotton treated with TDZ decreases and the time of defoliation is delayed, but there is little information about this mechanism. In this study, RNA-seq and physiological analysis are performed to reveal the transcriptome profiling and change in endogenous phytohormones upon TDZ treatment in abscission zones (AZs) under different temperatures (daily mean temperatures: 25 °C and 15 °C). Genes differentially expressed in AZs between TDZ treatment and control under different temperatures were subjected to gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses to compare the enriched GO terms and KEGG pathways between the two temperature conditions. The results show that, compared with the corresponding control group, TDZ induces many differentially expressed genes (DEGs) in AZs, and the results of the GO and KEGG analyses show that the plant hormone signaling transduction pathway is significantly regulated by TDZ. However, under low temperature, TDZ induced less DEGs, and the enriched GO terms and KEGG pathways were different with those under normal temperature condition. Many genes in the plant hormone signal transduction pathway could not be induced by TDZ under low temperature conditions. In particular, the upregulated ethylene-signaling genes and downregulated auxin-signaling genes in AZs treated with TDZ were significantly affected by low temperatures. Furthermore, the expression of ethylene and auxin synthesis genes and their content in AZs treated with TDZ were also regulated by low temperature conditions. The upregulated cell wall hydrolase genes induced by TDZ were inhibited by low temperatures. However, the inhibition of low temperature on genes in AZs treated with TDZ was relieved with the extension of the treatment time. Together, these results indicate that the responses of ethylene and auxin synthesis and the signaling pathway to TDZ are inhibited by low temperatures, which could not induce the expression of cell wall hydrolase genes, and then inhibit the separation of AZ cells and the abscission of cotton leaves. This result provides new insights into the mechanism of defoliation induced by TDZ under low temperature conditions.

Straw returning is an important method of improving soil fertility and reducing environmental pollution. Controlled-release nitrogen fertilizer (CRN) is regarded as an effective way to reduce nitrogen (N) loss and increase N-use efficiency and crop yield. In order to determine the combined effects of straw management (straw removal and straw returning) and N-fertilization strategy (CK (no N), urea, CRN, and a mixture of urea and CRN (UC)) on lint yield, N utilization, and soil properties at harvest of field-seeded cotton, field experiments were conducted from 2018 to 2019. The results demonstrated that the lint yield was the highest with a combination of straw returning and UC, increasing by 4.2–46.9% over other combinations. Straw returning combined with UC facilitated biomass-accumulation and N-uptake from squaring to the boll-opening growth stage, contributing to higher N agronomic-use efficiency and apparent recovery-use efficiency. Moreover, regardless of the straw management, CRN or UC treatment increased the soil microbial N content and sucrase activity at harvest compared to urea or CK treatment. In summary, straw returning combined with UC was beneficial to the lint yield, N utilization, and soil N availability, which might be an optimizing strategy for field-seeded cotton.