Novel intra-boll yield components and Q-score can further evaluate the effect of phosphorus fertilizer on cotton yield and fiber quality

• Boll density was the most sensitive yield trait to extra P supply. • P fertilizer increased boll weight mainly by increasing seeds boll-1 and fibers seed-1. • P fertilizer increased lint percentage via increasing fiber biomass seed-1. • P fertilizer increased fiber length, uniformity and strength, finally promoting Q-score. • Some yield and quality traits of different cultivars responded differently to extra P. Novel intra-boll yield component traits and fiber quality indices have been used by breeders to select suitable Gossypium hirsutum L. lines, but there is little research on how these traits change during cultivation practices, especially fertilizer application. We hypothesized that these novel traits have obvious responses to phosphorus (P) fertilizer, which would explain the variation in yield and fiber quality under different P conditions. To address this hypothesis, a field test was conducted with a low-P sensitive cotton cultivar (Lu 54) and a low-P tolerant cotton cultivar (Yuzaomian 9110) under different P fertilizer rates (0 as control, 100, and 200 kg P 2 O 5 ha −1 ) from 2016 to 2018. The results showed that P fertilizer application (100, and 200 kg P 2 O 5 ha −1 ) increased the lint yield, and bolls per ha contributed most to the increase in lint yield, followed by boll weight and lint percentage. The increased bolls per ha under P fertilizer application was due to the accelerated process of boll forming and the slowing of shedding process. The increased boll weight under P fertilizer application could be explained by the higher seed biomass per boll and fiber biomass per boll, especially the latter. The higher seed biomass per boll was due to the increased seeds per boll, and higher fiber biomass per boll should be attributed to the increased fibers per seed under P fertilizer application. The positive contribution of lint percentage to lint yield formation under P fertilizer application could be attributed to the increased fiber biomass per seed. P fertilizer application decreased fiber micronaire, indicating that fiber thickness was decreased by extra P fertilizer, which might be associated with the negative effects of increased fibers per seed and fiber density. In addition, P fertility application promoted the elongation of fiber cells and fiber length uniformity, and increased fiber strength. The end result was an increase in the comprehensive fiber quality index (Q-score) after P application. Moreover, compared to Yuzaomian 9110, yield traits including lint yield, bolls per ha, seeds per boll, seed index, and SSA, and quality indices including fiber length, fiber strength, and Q-score were more sensitive to P fertilizer application in Lu 54.