Mepiquat chloride application does not favor leaf photosynthesis and carbohydrate metabolism as well as lint yield in late-planted cotton at high plant density

Late planting of short-season cotton under high plant density and single fertilization after rapeseed harvest is a promising alternative to traditional relay intercropping of full-season cotton in the Yangtze River Valley of China. However, it is not clear if mepiquat chloride (MC) application is still necessary in the new system. We hypothesized that MC application might reduce leaf photosynthesis and economic yield and thus not be necessary in the new system. To test the hypothesis, short-season cotton was sown in mid-May at 10 plants m(-2) and with 225 kg N ha(-1) applied at early flowering, and MC was applied three times at five main-stem leaf intervals initiated from the 6th leaf stage at five dosages (MCO, MC30, MC60, MC90, and MC120 represent 0,30, 60,90, and 120 g ha(-1), respectively) in a 2-year experiment The photosynthesis, chlorophyll contents, carbohydrate contents, carbohydrate metabolic enzyme dynamics, and cotton yield under different MC treatments were determined. Photosynthesis was reduced by 1-28% and lint yield decreased by 6-29% as MC dosage increased. However, chlorophyll a, b, a + b, sucrose, glucose, fructose, and starch contents in the 4th leaf from the top were increased by 3-56%, 2-45%, 3-51%, 4-48%, 3-48%, 2-63%, and 2-43%, respectively. The activities of enzymes involved in carbohydrate metabolism decreased by 2-25% for sucrose phosphate synthase, but increased by 2-40%, 2-30%, and 3-52% for sucrose synthase, soluble acid invertase, and neutral invertase, respectively. These findings suggest that increased accumulation of sucrose, hexose, and starch in the leaf with MC application diminished photosynthesis and thus could explain the yield loss. Therefore, MC application is not necessary for late-planted cotton.