Planting cotton after harvest of winter wheat may be an alternative to relay intercropping that reduces labor and material inputs under a double cropping system of wheat-cotton in China. However, it is unclear whether lint yield with single dose application of reduced N at early flowering will increase N use efficiency without lint yield reduction in such a system. In a two-year field study, cotton was directly sown after wheat harvest on 20 May and fertilized one time at early flowering with 0 (N0), 90 (N90), 180 (N180), 270 (N270), or 360 (N360) kg N ha(-1). Cotton yield, biomass accumulation and partitioning, N uptake and use efficiency, critical N dilution curve, and physiological parameters related to N metabolism were determined. In the two experimental years, N180 did not differ from N270 and N360 in seed cotton yield but was 55.4% higher than N0 and 11.5% higher than N90. N180 was considerably higher than N0 and N90 in biomass but was comparable with N270 and N360 after peak blooming. By contrast, the harvest index in N180 was comparable with or slightly lower than that in N0 and N90 but was considerably higher than that in N270 and N360. Compared with lower N rates, the increase in biomass in N180 was attributed to the increased duration or rate of biomass accumulation in the rapid accumulation period of biomass. Compared with higher N rates, the increase in harvest index in N180 was largely due to increased partitioning of assimilates to fruiting sites. Total N content in source, flow, and sink organs increased with the increase in N fertilizer rate. Critical N dilution curves showed that the N nutrition index of sink organs (NNIso) increased with the increase in N fertilizer rate. The NNIso in N180 was closer to 1 than the index in the other N rate treatments and therefore should be the optimal N rate in this system. The N use efficiency (NUE) in N180 was similar to or slightly lower than that in N90 but was considerably higher than that in N270 and N360. Nitrate-N and soluble protein contents in petioles or leaves in N180 were comparable with those in N270 and N360 but were much higher than those in N0 and N90, which was consistent with the seed cotton yields under the different N rates. Overall, N fertilization rate can effectively regulate N metabolism, NUE, biomass accumulation and distribution, and seed cotton yield and yield components. Thus, to optimize yield, NUE, and NNIso, 180 kg N ha(-1) applied at early flowering is recommended compared with other N rates in wheat-cotton double cropping systems.

Potassium (K) fertilizer plays a crucial role in the formation of the biological and economic yield of cotton (Gossypium hirsutum L.). Here we investigated the effects of the amount of K on biomass accumulation and cotton fiber quality with lowered N amounts (210 kg ha−1) under late sowing, high density and fertilization once at 2 weeks after squaring. A 2-year field experiment was performed with three K fertilizer amounts (168 kg ha−1 (K1), 210 kg ha−1 (K2), and 252 kg ha−1 (K3)) using a randomized complete block design in 2016 and 2017. The results showed correspondingly, K3 accumulated cotton plant biomass of 7913.0 kg ha−1, next to K2 (7384.9 kg ha−1) but followed by K1 (6985.1 kg ha−1) averaged across two growing seasons. Higher K amounts (K2, K3) increased biomass primarily due to a higher accumulation rate (32.68%–74.02% higher than K1) during the fast accumulation period (FAP). Cotton fiber length, micronaire, and fiber strength in K2 were as well as K3 and significantly better than K1. These results suggest that K fertilizer of 210 kg ha−1 should be optimal to obtain a promising benefit both in cotton biomass and fiber quality and profit for the new cotton planting model in the Yangtze River Valley, China and similar climate regions.

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.

Potassium (K) plays a pivotal role in physiological and biochemical processes and is indispensable for efficient production of cotton. The optimal K fertilization ratio relative to reduced nitrogen (N) rate for efficient N metabolism and yield formation in late planted high-density cotton remain to be understood. In the field, three K ratios relative to N [K-1 (K2O: N = 0.8: 1), K-2 (K2O: N = 1: 1), and K-3 (K2O: N = 1.2: 1)] were applied in a randomized complete block design, in 2016 and 2017. Results revealed that increasing the K fertilization ratio exerted a significant effect on leaf area, fresh and dry weight of functional leaf, chlorophyll content, and N metabolism, as well as yield and yield indices in both cropping years. Higher lint yield was obtained with the application of K-2 and K-3 rather than K-1. Similarly, K-2 and K-3 treatments evidently improved the activity of N metabolizing enzymes such as NR (nitrate reductase), NiR (nitrite reductase), GS (glutamine synthetase), GOGAT (glutamate synthase), GPT (glutamic-pyruvic transaminase), and GOT (glutamic oxaloacetic transaminase). Moreover, leaf nitrogen, nitrate-N, free amino acids, and soluble protein contents were efficiently balanced in K-2 and K-3 treatments over K-1. The findings of this study suggest that the application of K ratio must be equivalent to N to achieve an acceptable yield and profitable return in late-planted high-density cotton.