Photosynthetic characteristics of boll subtending leaves are substantially influenced by applied K to N ratio under the new planting model for cotton in the Yangtze River Valley

Abstract A new cotton planting model has been developed for the Yangtze River Valley which involves late sowing, high plant density and less applied N compared with the traditional transplanted cotton production. However, it is unknown if the K application rate can be reduced to achieve a 1:1 or lower applied K 2 O: N ratio. Therefore, we conducted field experiments to investigate the optimal K 2 O: N ratio for cotton under the new planting model. The study involved the evaluation of the photosynthetic attributes of the boll subtending leaves in relation to crop yield. A two-year (2016-17) field experiment was conducted with three K to N ratios [K08 (0.8:1), K10 (1.0:1) and K12 (1.2:1)] with 210 kg N ha −1 applied to all treatments. Our results indicated that the increased K application improved the photosynthetic attributes of the boll subtending leaves, leading to increased crop yield. Compared with K08, K10 and K12, respectively, enhanced the photosynthetic rate by 7–32% and 36–59%, chlorophyll fluorescence by 4–13% and 8–35%, chlorophyll content by 5–61% and 20–75%, total Rubisco activity by 7–23% and 30–55%, leaf area by 21–72% and 32–87%, leaf biomass by 12–36% and 31–51%, and relative water content of the boll subtending leaves by 2–8% and 13–16%. Also, compared with K08, leaf ATP content and SLW (specific leaf weight) were down-regulated in K10 and K12 by 7–23% and 1–26%, respectively. Moreover, statistically similar contribution to yield was found between K10 and K12, suggesting that under the late sowing, high density and low applied N model, a 1:1 K 2 O: N application rate N (K10) is the optimal choice for maximum financial return without reducing cotton yield.