Soil available phosphorus (AP) deficiency and shortage of phosphate rocks limit cotton production in China. Therefore, pool-culture experiments were conducted in 2019 and 2020 using two cotton cultivars (CCRI-79, low-P tolerant; SCRC-28, low-P sensitive) under three soil AP levels (P0: 3 ± 0.5, P1: 6 ± 0.5, and P2 (control): 15 ± 0.5 mg kg−1) to ascertain the effect of soil AP on boll biomass and lint yield. P0 and P1 decreased the P concentration and net photosynthetic rate (Pn) of subtending leaves, thus, reducing boll biomass and lint yield. Additionally, soil AP deficiency decreased boll wall:boll, lint:boll, and lint:seed, and increased seed:boll ratio. Upper fruiting branch positions (FB9–12) had higher lint:seed ratio and proportion of the total lint yield under low soil AP. Moreover, soil AP deficiency also reduced the sucrose transformation rate (Tr) and activities of sucrose-metabolizing enzymes, such as ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), sucrose phosphate synthase (SPS), and sucrose synthase (SuSy), while increased carbohydrate levels (soluble sugar, sucrose, and starch) and the activity of cytosolic fructose-1,6-bisphosphatase (cy-FBPase) in the subtending leaves. The sucrose and starch contents, cy-FBPase, and SPS activities of SCRC-28 were more sensitive to low soil AP than CCRI-79. Higher Tr and activities of initial Rubisco and SuSy in the subtending leaves improved boll biomass and lint yield.

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Background Potassium (K) is an essential nutrient for plant growth and development. However, plant fertilization ignoring the soil K level is very likely to cause excessive fertilizer use, and further arouse a series of side effects. This study investigated the response of cotton growth to different soil K levels and the uptake of major nutrients, aiming to evaluate the appropriate K supply level for cotton growth. Using a random block design with 6 soil K levels, we conducted 18 micro-zones field experiments over two continuous years. The soil available K concentration of each treatment was K1 (99.77–100.90 mg·kg−1), K2 (110.90–111.26 mg·kg−1), K3 (123.48–128.88 mg·kg−1), K4 (140.13–145.10 mg·kg−1), K5 (154.43–155.38 mg·kg−1), and K6 (165.77–168.75 mg·kg−1). Cotton nutrient contents, soil nutrient contents, accumulation and distribution of dry matter in cotton were determined, and the relationships between K content in soil and plants and dry matter accumulation were analyzed. Results The soil K content had a significantly positive relationship with dry matter and K accumulation in cotton plants. There were significant differences in dry matter accumulation, single-plant seed cotton yield, mineral nutrient uptake and the proportion of K accumulation in reproductive organs among different soil K levels. The results showed that there was significant difference between K4 and lower K level treatments (K1 and K2), but no significant difference between K4 and higher K level treatments (K5 and K6) in dry matter, single-plant seed cotton yield, or accumulation, distribution and seed cotton production efficiency of N, P and K. Conclusion The soil K level of K4 was able to provide sufficient K for cotton growth in our experiment. Therefore, when the soil K level reached 140.13 mg·kg−1, further increasing the soil K concentration no longer had a significant positive effect on cotton growth.

Estimating the precise nutritional status of crop nitrogen (N) after flowering period is not only important to predict deficiency but the excess that could be revised by fertilization in future crops. Critical N dilution curves describing the critical N concentration ([N]c) in plant tissues during crop growth have been used to estimate the N status of whole plants in cotton. Little is known, however, about the critical N dilution curve for specific plant organs such as cotton fruits. The objective of this study was to verify the feasibility of fruits-based critical N dilution curve as a useful diagnostic tool for diagnosing the N status of cotton crops. A 3-year field experiment was conducted with seven N application rates (0–360 kg N ha–1) using the high-yielding cultivars Jimian 228 and Lumian 28, which differ in maturity. The relationship between fruits dry mass (DM) and N concentration ([N]) was analyzed, and a model of [N]c for cotton fruits was constructed and validated. The results showed that fruits [N]c decreased with increasing fruits DM. The critical N dilution curve based on cotton fruits was described by the equation [N]c = 2.49 × DM–0.12 (R2 = 0.649, P < 0.0001) across cultivar-years. The N nutrition index (NNI) of the fruits (NNIf) with the N dilution curve was significantly related to the NNI of shoot DM, relative yield (RY), and boll density at most sampling dates. For an NNIf of approximately 1, the RY was nearly 95%, while it decreased with a decreasing NNIf below 1. The petiole nitrate-N (NO3-N) concentration was also linearly related to the NNIf, suggesting that the NO3-N concentration in the petiole was a good predictor of the NNIf. Therefore, fruits-based critical N dilution curve and the derived NNIf values will serve as a useful diagnostic tool for diagnosing N status in cotton crops.