The efficacy of chemical topping in field-grown cotton is mediated by drip irrigation amount in irrigated agricultural area

Abstract Background Cotton production in China is challenged by high labor input including manual topping (MT). Recently, to replace MT in the Xinjiang cotton region of China, mepiquat chloride (MC) was applied once more than the traditional multiple-application; this was designated as chemical topping (CT), but it is unclear whether the amount of irrigation needs to be adjusted to accommodate CT. Results The main plots were assigned to three drip irrigation amounts [300 (I1), 480 (I2), and 660 (I3) mm], and the subplots were assigned to the CT treatments [450 (MC1), 750 (MC2), and 1 050 (MC3) mL·hm−2 25% MC] with MT as a control that was performed after early bloom. The optimum drip irrigation amount for CT was explored based on leaf photosynthesis, chlorophyll fluorescence, biomass accumulation, and yield. There were significant influences of drip irrigation, topping treatments and their interaction on chlorophyll fluorescence characteristics, gas exchange parameters and biomass accumulation characteristics as well as yield. The combination of I2 and MC2 (I2MC2) performed best. Compared with I2MT, the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and photochemical quenching coefficient (qP) of I2MC2 significantly increased by 4.0%∼7.2%, 6.8%∼17.1%, 5.2%∼17.6%, and 4.8%∼9.6%, respectively, from the peak flowering to boll opening stages. Moreover, I2MC2 showed fast reproductive organ biomass accumulation and the highest seed cotton yield; the latter was 6.6%∼12.8% higher than that of I2MT. Further analysis revealed that a 25% MC emulsion in water (MCEW) application resulted in yield improvement by increasing Pn, φPSII, and qP to promote biomass accumulation and transport to reproductive organs. Conclusion The results showed that the 480 mm drip irrigation combined with 750 mL·hm−2 MC increased the rate of dry matter accumulation in reproductive organs by increasing Pn, φPSII, and qP to improve photosynthetic performance, thus achieving higher yield.