Effect Of Potassium Ratio To Nitrogen On Cotton Carbon,Nitrogen,and Antioxidant Metabolism Under Late-Planted,High-Density,and Low-Nitrogen Conditions

Cotton is an important fiber producing crop worldwide.China is the main producer and consumer globally with the highest cotton production for the last three decades.The Yangtze River Valley of China is considered one of the major cotton production regions.Double cropping systems such as cotton-wheat and cotton-rapeseed,have been widely adopted for conventional production.The high input in cotton production,especially the cost of fertilizer,is a big challenge.The extensive use of fertilizer not only reduces final economic profits on cotton production but also causes environmental issues.A new planting model characterized with late sowing,high planting density,low N(nitrogen),and one time fertilizer application is more competitive to replace the present high cost cotton production.However,what is the optimal relative ratio of K(potassium)to N under this model.We hypothesized that cotton yield could be maintained by applying equal or higher K ratio relative to N(KRN)under this newly proposed cotton planting model.A two-year(2016-2017)field experiment was conducted at Huazhong Agricultural University,Wuhan,China using randomized block design with all the fertilizers together with three KRN [K1 {K2O: N = 0.8: 1(168: 210 kg ha-1)},K2 {K2O: N = 1: 1(210: 210 kg ha-1)},and K3 {K2O: N = 1.2: 1(252: 210 kg ha-1)}] applied at first flower appearance stage.The results showed that the average seed cotton yield was enhanced by 15% and 16% in K2 and K3 over K1.Cotton C and N metabolism,C-N balance,antioxidant activity were summarized below:Higher KRN(K2 and K3)application improved the leaf K content over K1 at all crop stages and during both crop seasons.Higher KRN(K3 and K2)up-regulated the activities of ribulose 1,5-bisphosphate carboxylase oxygenase(Rubisco),sucrose phosphate synthase(SPS),and sucrose synthase(SS)but down-regulated the activities 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)was more efficient under higher KRN(K3 and K2)as compared with K1.Similarly,K2 and K3 treatments significantly improved the activities of N metabolizing enzymes,such as NR(nitrate reductase),Ni R(nitrite reductase),GS(glutamine synthetase),GOGAT(glutamate synthase),GPT(glutamic-pyruvic transaminase),and GOT(glutamic oxaloacetic transaminase).Furthermore,the contents of leaf N,nitrate,free amino acids,and soluble proteins were more efficiently balanced in K2 and K3 treatments over K1.Carbon-nitrogen metabolizing enzymes activities and assimilation compounds were more effectively balanced in leaves of K2 and K3 treated plants over K1 plants,leading to the appropriate C/N ratio.Furthermore,leaf K content,C-N compounds balance,and low C/N ratio were observed with K2 and K3 applications compared with K1.Increased KRN(K2 and K3)proved as a regulating factor to keep the antioxidant enzymes(SOD,CAT,and POD)activities at an optimal level and to balance the production of H2O2.Also,the increase in MDA,glutathione,and proline contents suggested that the K1 application is un-favored for the cotton plants.Overall,higher KRN improved cotton yield by balancing C-N metabolism,increasing leaf K content mitigating the oxidative effects.These findings suggest that equal K to N in the proposed new planting model could be a good way to ensure the improvement of cotton profit at the Yangtze River Valley of China.