Root response of different cotton genotypes to local phosphorus supply

Objectives In soil, nutrients are distributed in a heterogeneous or patchy manner. It is of great significance to explore response of roots of different cotton genotypes to the heterogeneity of nutrients, which is of great importance to improve phosphorus (P) utilization efficiency of cotton.

Methods In this soil culture experiment, two kinds of phosphorus supply methods are used: uniform phosphorus supply and local phosphorus supply. The root box is divided into three layers from top to bottom (upper layer 0–20 cm, middle layer 20–40 cm, lower layer 40–60 cm). The amount of phosphorus fertilizer in each layer of the root box is P₂O₅ 100 mg/kg under the uniform phosphorus supply. Phosphorus fertilizer dosage is P₂O₅ 300 mg/kg under local phosphorus supply, all concentrated in the middle layer (20–40 cm), and no phosphorus is applied to the upper and lower layers. Nitrogen and potassium fertilizers are uniformly added to each layer of the root box according to N 150 mg/kg and K₂O 5 mg/kg under both phosphorus supply methods (calculated at 75 kg/hm² of Xinjiang potassium fertilizer), a soil box experiment simulated the localized supplies of P (20–40 cm) is conducted to study root response to local P supply in two cotton genotypes.

Results Local P supply significantly modified root morphological parameters in two cotton genotypes. Total root length, root surface, root volume, specific root length, fine root length of low phosphorus efficiency genotype (XLZ13) and high phosphorus efficiency genotype (XLZ19) in the local P supply treatment were increased by 38.0%, 41.9%, 97.6%, 27.3%, 35.9% and 34.5%, 21.7%, 39.0%, 22.5% and 42.8%, respectively. There were significant differences in root parameters of the two cotton genotypes with the local P supply. The total root length, root surface, root volume, specific root length and fine root length in middle layer of the high phosphorus efficiency genotype (XLZ19) were 1.23, 1.31, 1.73, 1.07 and 1.30-folds of those of the low phosphorus efficiency genotype (XLZ13). The results of the principal component analysis showed that P supply could affect the plasticity of cotton root system, while cotton genotype could modify basic structure of root system. Partial least squares regressive analysis showed that the VIP values of total root length, fine root (0–0.4 mm) length in middle layer (20–40 cm), root surface and root volume were larger than 1, which played important role in aboveground phosphorus uptake. The fine root (0–0.4 mm) length in middle layer (20–40 cm) made greater contribution to P uptake of cotton in soil, i.e, the increase of 10% of fine root (0–0.4 mm) length in middle layer (20–40 cm) could improve 2.33% of the aboveground phosphorus uptake.

Conclusions The high phosphorus efficiency genotype cotton has higher environmental adaptability in the local P supply. For efficient and inefficient genotypes, the local P supply should be adopted to optimize root distribution and growth, and to improve cotton’s ability to obtain heterogeneous phosphorus nutrients so as to exert the maximum biological potential of cotton, improve nutrient utilization, reduce fertilizer dosage, and protect ecological environment.