Effect Of Sowing Date And Planting Density On Nitrogen Metabolism Of Cotton Under Summer Direct Sowing

Based on the objective of “Minimizing cotton input without yield reduction” for sustainable development, a field trial was conducted on the experimental farm of Huazhong Agricultural University in 2014-2015, to understand the effect of sowing date and planting density on cotton(Gossypium hirsutum L. cv Huamian 3109) yield and nitrogen metabolism under late and direct seeding after winter crops(canola/whea t) harvested and once fertilization at first flower stage. A split-plot design was employed with sowing date(m-d)(S1, 5-30; S2, 6-14) as the main plot, and planting density(plants/m2)(D1, 7.5; D2, 9.0; D3, 10.5) as the subplot. Cotton lint yield, nitrogen metabolism in main leaf with different positions and in fruiting leaf, and photosynthet ic rate were checked. Results indicated:1. The characteristics of nitrogen metabolism in cotton main leaf was varied from leaf positions, sowing date and planting density. From the top to the bottom, nitrate content in petiole decreased during squaring, but an increase and decrease trend happened during first bloom and peak bloom; NR activity decreased continuo us l y all the way through different periods; contents of free amino acid and soluble protein dropped to the bottom or reached to the peak in the 4t h leaf from the top, respectively, after first bloom.2. The characteristics of nitrogen metabolism in cotton main leaf was varied from sowing date and planting density. When sowing date was delayed, average of petioles nitrate contents with different positions rose significantly during first bloom and peak bloom, but average of NR activities, free amino contents or soluble protein contents with different leaf positions showed a gradually decreasing tendency; In terms of increasing planting density, average of petiole nitrate contents with different positions assumed a gradually declining trend during first bloom, which was reversed to tendency of NR activities with different positions; on the contrary, average of petiole nitrate contents and NR activities with different positons showed the same tendency that was increasing firstly then decreasing during peak bloom. However, average of free amino acid contents and soluble protein contents decreased permanently during first bloom and peak bloom.3. Nitrogen metabolism in main stem leaf and fruiting leaf was varied from cotton growth periods. From squaring to peak bloom, nitrate content firstly increased then decreased with the highest value during first bloom, with main stem leaf ≤ fruiting leaf; NR activity decreased continuously, with main stem leaf > fruiting leaf; nitrite content increased gradually in main stem leaf, whereas decreased in fruiting leaf, with main leaf < fruiting leaf; Ni R activity increased firstly then decreased in main leaf, but increased permanently in fruiting leaf, with main leaf = fruiting leaf during squaring, main leaf > fruiting leaf during first bloom, and main leaf < fruiting leaf during peak bloom; soluble protein content increased steadily, with main leaf > fruiting leaf during squaring and first bloom, but main leaf < fruiting leaf during peak bloom. In addition, soluble sugar content firstly increased then decreased with the highest value during first bloom, with main leaf > fruiting leaf all the way.4. Nitrogen metabolism in main stem leaf and fruiting leaf was different fromsowing date and planting density. It was easy to observe that delaying sowing date could not only rapidly elevate nitrate content in main leaf and fruiting leaf, but also decreased NR activity and soluble protein content in main leaf and fruiting leaf without influence on nitrite content in main leaf and fruiting leaf during first bloom and peak bloom, compared to Ni R activity that was decreased significantly in main leaf during first bloom or in fruiting leaf during peak bloom respectively. However, increasing density decreased nitrate content, NR activity, Ni R activity and soluble protein content in main leaf and fruiting leaf simultaneously during first bloom and peak bloom, regardless of nitrite content in main leaf and fruiting leaf.5. Difference existed in nitrogen metabolism with variation in sowing date and planting density. NR activity(inhibition) was verified as the limiting factor among nitrogen metabolism when sowing date was delayed. However, when planting density was increased, reduction in nitrate content of petiole might be concluded as the dominant factor, which induced the decline of NR activity.