Relative Potassium Ratio Balanced the Carbon-Nitrogen Assimilation in Cotton Leaf Under Reducing Nitrogen Application

Potassium (K) plays a pivotal role in carbon (C) and nitrogen (N) balance via several physiological processes. Conventional K application ratio (0.6–0.8) is too low to meet the optimal K requirement for physiological activities as well as yield benefits in cotton. However, in line with low N application, what is the optimal K ratio relative to N, for efficient C-N balance and its relationship with enzymes activities as well as yield, is the objective of this study. A pot experiment was conducted with three K ratio to N (K1 (0.8), K2 (1.0), and K3 (1.2)) using completely randomized design at the experimental site of Huazhong Agricultural University, Wuhan, China. The morphological attributes, C-N enzyme activities, assimilate contents, and yield-related components were analyzed under different K ratios. Results showed that higher K application ratios (K2 and K3) significantly enhanced the yield, leaf morphological attributes (leaf area and leaf weight), and C- and N-metabolizing enzyme activities. Similarly, C and N assimilation compounds were effectively balanced in leaves of plants receiving K2 and K3 over K1 leading to an appropriate C/N ratio. Conclusively, leaf K content, C-N compounds balance, and narrow C/N ratio in higher K application suggest that the K relative ratio should be equal or even higher than N application rate. However, from an economic point of view, results suggest that the K ratio equal to the N is sufficient to get promising cotton yield with late planting under reducing N rate.