Effects Of Potassium-Nitrogen Ratio On Cotton Photosynthesis And Sucrose Metabolism Under Late Sowing And High Density Conditions

Cotton is the most important fiber crop all over the world.China is a leading cotton producer in the world.Hubei was recognized as one of the top cotton-producing provinces of China.Despite of the introduction of mechanization,high yielding commercialized cultivars and input rate per unit area,cotton yield remained stagnant for couple of decades.Thus,the low profit of cotton production caused the planting area reduction.To improve farmers’income from cotton production,a new planting model characterized with the shorter growing season,delayed and direct sowing,dense plant population,and once fertilization with reduced nitrogen without compromising the cotton yield,has been proved to be a valid alternative management strategy.It was hypothesized that higher K（potassium）ratio should be beneficial to plant photosynthesis and sucrose metabolism,resulting in high productivity under this new cotton planting system.A two-year（2016-17）field experiment was conducted in a randomized complete block design along with four replicates on the experimental farm at Huazhong Agricultural University,Wuhan,China（30°37′N latitude,114°21′E longitude,altitude23 m above sea level）with Huamian 3109（Gossypium hirsutumn L.）.There were three K-N ratio[K08{K（K₂O）:N=0.8:1(168:210 kg ha^-1)},K10{K（K₂O）:N=1:1(210:210 kg ha^-1)},and K12{K（K₂O）:N=1.2:1(252:210 kg ha^-1)}].All the fertilizers(in kg ha^-1)of 210 N,relevant K₂O,63 P₂O₅,and 1.5 B were applied at the appearance of the first flower in the field.The results showed that higher K-N ratio（K10,K12）harvested 15–16%more seed cotton and lint yield,over K08,while,K10 and K12remained statistically similar to each other.The other main findings of the study are as follows:1.Leaf growth attributes including leaf area,leaf biomass and relative water contents were markedly enhanced（26–68%,19–35%,10–14%,respectively）by higher K application ratios（K10,K12）.Whereas,the specific leaf weight was decreased substantively（4–18%）.2.Chlorophyll relative content（SPAD value）of the top 4^th main stem leaf increased from 32 DAE（days after emergence）to 82 DAE,then decreased after a platform up to 92DAE with the maximum at PB stage.Similarly,the chlorophyll content of subtending leaf was the highest at PB stage and lowest at BO stage.Chlorophyll fluorescence was also influenced favorably by higher K-N ratio.3.Plant photosynthetic attributes significantly varied from different light intensities,leaf types,leaf positions,growth stages,diurnal time intervals,and leaf based on an individual or the whole plant.Values of photosynthetic rate（Pn）,stomatal conductance（gs）and transpiration rate（Tr）increased from 8:00 to 13:00 then declined swiftly till18:00.They increased from squaring（SQ）to peak bloom（PB）,then decreased to the lowest at boll open stage.The functional leaf had the highest value compared to the rest main stem leaves.The values of the main stem leaf at 6^th node from the top were higher at FB and BO stages,but lower at PB stage than the boll subtending leaf on the 1^st position of the same node.However,the values of all the photosynthetic attributes were K12>K10>K08,except intercellular CO₂（Ci）had the opposite order.4.Total Rubisco activity was favorably up-regulated（11–62%）,while,ATP contents were down-regulated significantly（10–22%）by higher K-N ratio,leading the cotton plants towards better photosynthetic activity.Carbohydrate metabolic enzymes and contents were substantially influenced by higher K-N ratio.Sucrose metabolizing enzymes i.e.,sucrose phosphate synthase（SPS）and sucrose synthase（SS）activity were favorably up-regulated（8–36%and 7–35%,respectively）while,the sucrose degrading enzymes i.e.,soluble acid invertase（SAI）and glucose-6-phosphate dehydrogenase（G6PDH）activity were substantively down-regulated（5–17%and 7–31%,respectively）by K10 and K12.It was suggested that,higher K ratio favored the translocation of photo-assimilates towards sink leading to a higher cotton yield.Therefore,equal K rate to N（K10）should be optimal for obtaining higher cotton yield and economic return in the new cropping model.