Equal potassium-nitrogen ratio regulated the nitrogen metabolism and yield of high-density late-planted cotton (Gossypium hirsutum L.) in Yangtze River valley of China

Potassium (K) plays a pivotal role in physiological and biochemical processes and is indispensable for efficient production of cotton. The optimal K fertilization ratio relative to reduced nitrogen (N) rate for efficient N metabolism and yield formation in late planted high-density cotton remain to be understood. In the field, three K ratios relative to N [K-1 (K2O: N = 0.8: 1), K-2 (K2O: N = 1: 1), and K-3 (K2O: N = 1.2: 1)] were applied in a randomized complete block design, in 2016 and 2017. Results revealed that increasing the K fertilization ratio exerted a significant effect on leaf area, fresh and dry weight of functional leaf, chlorophyll content, and N metabolism, as well as yield and yield indices in both cropping years. Higher lint yield was obtained with the application of K-2 and K-3 rather than K-1. Similarly, K-2 and K-3 treatments evidently improved the activity of N metabolizing enzymes such as NR (nitrate reductase), NiR (nitrite reductase), GS (glutamine synthetase), GOGAT (glutamate synthase), GPT (glutamic-pyruvic transaminase), and GOT (glutamic oxaloacetic transaminase). Moreover, leaf nitrogen, nitrate-N, free amino acids, and soluble protein contents were efficiently balanced in K-2 and K-3 treatments over K-1. The findings of this study suggest that the application of K ratio must be equivalent to N to achieve an acceptable yield and profitable return in late-planted high-density cotton.