Morphophysiological and biochemical responses of cotton (Gossypium barbadense L.) to nano zinc (ZnO-NPs) and Azospirillum sp. under water deficit stress conditions

Water deficit stress (WDS) is one of the most significant abiotic limiting factors in cultivated crops, including cotton (Gossypium barbadense L.). With global climate change and the destruction of ecological balance, the frequency and severity of drought events are increasing in many regions around the world. Therefore, the aim of the present work was to investigate the potential of using nano zinc particles (ZnO-NPs), plant growth-promoting rhizobacteria (Azospirillum sp.), and their combination to mitigate the negative impacts of WDS on cotton var. Giza 96. Extended irrigation intervals of 30 and 45 days led to considerable decreases in plant height, chlorophyll content, relative water content (RWC), yield, and cotton fiber quality compared to the optimal irrigation interval (every 15 days). When applied individually or in combination, ZnO-NPs and Azospirillum sp. can ameliorate the negative effects of WDS on cotton growth and productivity. Overall, the use of Azospirillum sp. and ZnO-NPs, either individually or in combination, has demonstrated their potential to enhance cotton growth and yield parameters (plant height, dry weight, leaf area, chlorophyll pigment, seed index, seed yield, and lint%) under prolonged irrigation intervals during 2021 and 2022 seasons. Antioxidant enzymes activity comprising catalase (CAT), peroxidase (POD), and polyphenol oxidase (PPO) and proline content were increased under drought stress. Cotton fiber quality parameters including length, strength, and micronaire were insignificantly affected under low irrigation rate. The combination of ZnO-NPs and Azospirillum sp. provided the most effective mitigation of WDS during both experimental seasons, outperforming the effects of individual applications.