Cottonseed oil and protein content as well as germination traits are major indicators of seed quality. However, the responses of these indicators to plant density and mepiquat chloride (MC) are still uncertain. To investigate plant density and MC effects on cottonseed yield and main quality parameters, we conducted a two-year field experiment including four plant densities (1.35, 2.55, 3.75 and 4.95 plants·m− 2) and two doses of MC (0 and 135 g·hm− 2) in Dafeng, Jiangsu Province, in 2013 and 2014. The application of MC reduced plant height, fruit branch length and fruiting branch number under different plant densities, resulting in a lower and more compact plant canopy. Cottonseed yield showed a nonlinear increase as plant density increasing and achieved the highest value at 3.75 plants·m− 2, regardless of MC application. No significant interactions were found between plant density and MC for cottonseed yield and quality parameters. The 100-seed weight, cottonseed oil content and vigor index significantly decreased as plant density increased, while these parameters significantly increased with MC applying under different plant densities. Seed vigor index was positively correlated with 100-seed weight and seed oil content across different plant densities and MC treatments. Thus, application of MC could realize a win-win situation between cottonseed yield and main quality parameters under various densities; and plant density of 3.75 plants·m− 2 combined with 135 g·hm− 2 of MC applying is optimal for high cottonseed yield and quality in this cotton production area.

Cotton in double cropping systems (rapeseed-cotton or wheat-cotton) is often late-planted and experiences a much longer period of cool temperature at the late growth stage, which impairs seed quality. A two-year field experiment with two planting dates (normal and late planting date) was conducted to investigate the effects of late planting on cottonseed vigor and its relationship with seed biomass accumulation and kernel antioxidant metabolism during seed development. With the delay of planting date and the increasing height of fruiting branches, the mean daily temperature during boll period decreased from 25.9 degrees C to 20.7 degrees C. Meanwhile, the seed, kernel and vigor index decreased by 2.6-12.1%, 4.7-15.0% and 2.5-16.0% respectively. The direct path coefficient of growing degree days (GDD) and the indirect path coefficients via GDD for seed, kernel and vigor index were positive and high. Seed vigor was more sensitive to the change of kernel index than seed index and the cool temperature limited the accumulation of kernel biomass mainly via reducing the maximum velocity of kernel biomass accumulation (Vmax), which led to a significant decline in seed vigor especially in the late planting date. Under cool temperature caused by late planting, the time of the maximum activity for each H2O2 scavenging enzyme was postponed with their maximum activities being decreased, which caused slow reduction in the content of malondialdehyde and H2O2 content during the late stage of seed development. In addition, peroxidase (POD) had the closest correlation with cottonseed vigor compared with catalase, ascorbate peroxidase and glutathione reductase. Altogether, low seed vigor occurred in late planting could be attributed to the decreased kernel biomass and prolonged duration of oxidative damage on membranes caused by cool temperature. The GDD of boll period is valuable for estimating cottonseed harvesting time especially under late planting conditions and seeds with high POD activity are more tolerant to cool temperature.