Potassium relative ratio to nitrogen considerably favors carbon metabolism in late-planted cotton at high planting density

A new planting model characterized with late sowing, high planting density, low N and single fertilizer application is much competitive to combat the present high cost cotton production. However, what is the optimal relative ratio of K to N under this model? as we understand it certainly plays a critical role in cotton production through a series of physiological processes. A two year (2016-2017) field experiment was conducted in a randomized complete block design with three K ratios relative to N [0.8 (K-1), 1.0 (K-2), and 1.2 (K-3)] with four replications. Results showed that relative increase in K ratio had significant effect on functional leaf biomass, specific leaf weight, carbon metabolism as well as cotton yield in both growing seasons. K-3 and K-2 produced the similar seed cotton yield but higher than K1 in both years. Similarly, K-3 and K-2 prominently up-regulated the activity of ribulose 1, 5-bisphosphate carboxylase oxygenase (Rubisco), sucrose phosphate synthase (SPS) and sucrose synthase (SS) but down-regulated the activity of soluble acid invertase (SAI), glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH). Moreover, the balance of carbohydrates (sucrose, starch, glucose and fructose) and ATP was more efficient under higher K ratio (K-3 and K-2) as compared with K-1. These findings might suggest that K rate should be equal to N as for promising yield and profitable return are concerned under newly proposed cotton planting model in the Yangtze River Valley of China.