Effects of combined application of phosphorus and zinc on growth and physiological characteristics of apple rootstock M9-T337 seedlings (Malus domestica Borkh.)

Abstract

Background: Balancing nutrient application is crucial for plant growth. However, excessive fertilizer use, especially imbalanced applications of macronutrients such as phosphate (P), can hinder plant uptake of micronutrients. Balanced P and zinc (Zn) are vital for apple yield and quality, and apple trees are highly sensitive to deficiencies in these nutrients. Therefore, this study was conducted in May 2022, employed a sand culture experiment to investigate the effects of varying P and Zn levels on the growth phenotype, photosynthetic capacity, antioxidant enzyme activity, sugar composition, endogenous hormone levels, and nutrient absorption and utilization of M9-T337 seedlings. Three levels of P (low, medium, high) and three levels of Zn (low, medium, high) were combined to create a total of nine distinct treatment.

Results: The results indicate that combined P and Zn fertilization at various levels exerts either synergistic or antagonistic effects on the growth, nutrient absorption, and utilization of M9-T337 seedlings. Compared to low and medium levels of P, a combination of high P (4 mmol·L^-1) and an adequate amount of Zn significantly enhanced plant growth, root system development, and the microstructure of leaves. Notably, seedlings treated with high P and low Zn (HPLZn) reached a height 1.54 times that of the medium P and medium Zn (MPMZn, control). Physiological indicators under HP conditions revealed significant increases in antioxidant enzyme activity, leaf water retention, photosynthetic pigment concentration, osmotic adjustment substances, and the contents of glucose, sucrose, fructose, endogenous hormones, as well as P and Zn accumulation in the leaves, compared to the control. However, an increase in Zn application led to a declining trend in these parameters. Specifically, the HPLZn treatment exhibited substantial increases in Net photosynthetic rate (Pn), Total chlorophyll (Chl a + b), glucose, fructose, sucrose, and Auxin(IAA), with increments of 7.12%, 27.32%, 11.40%, 23.20%, 16.67%, and 55.11%, respectively, compared to the control.

Conclusion: Based on the comprehensive ranking from principal component analysis, the combination of HP ( 4 mmol·L^-1) and LZn (0.5 µmol·L^-1) was found to be the most effective in enhancing the antioxidant capacity, sugar accumulation, osmotic regulation ability, photosynthetic capacity, endogenous hormone levels, as well as P and Zn nutrient absorption and utilization in M9-T337 seedlings.

Keywords: Carbohydrate composition; Combined application of P and Zn; Endogenous hormone; M9-T337 seedlings; P and Zn nutrients; Photosynthesis.

Fig. 1

Effects of combined application of P and Zn on the growth phenotype of M9-T337 seedlings. LP, MP, and HP represent low, medium, and high P, respectively; LZn, MZn, and HZn represent low, medium, and high Zn, respectively. The same below

Fig. 2

Effect of the combined application of P and Zn on the root con-figuration of M9-T337 seedlings

Fig. 3

Effect of the combined application of P and Zn on the relative electrical conductivity and water content in leaves of M9-T337 seedlings. A: electrical conductivity; B: relative water content

Fig. 4

Effects of the combined application of P and Zn on the leaf ana-tomical structure of M9-T337 seedlings

Fig. 5

Effect of combined application of P and Zn on photosynthetic gas exchange parameters in leaves of M9-T337 seedlings. A: Pn, net photosynthetic rate; B: Gs, stomatal conductance; C: Ci, intercellular CO₂ concentration; D: Tr, transpiration

Fig. 6

Effects of combined application of P and Zn on light and pigment content in leaves of M9-T337 seedlings. A: chlorophyll a; B: chlorophyll b; C: chlorophyll a + b

Fig. 7

Effects of combined application of P and Zn on the contents of osmoregulatory substances (A, B) and starch (C) in M9-T337 seedlings. A: soluble carbohydrate; B: soluble protein; C: starch

Fig. 8

Effects of the combined application of P and Zn on sugar accumulation and sucrose-related metabolic enzyme activities in the leaves of M9-T337 seedlings. A: glucose; B: fructose; C: sucrose; D: sucrose phosphate synthase; E: sucrose synthase; F: acid invertase; G: neutral invertase

Fig. 9

Effect of combined application of P and Zn on antioxidant oxidase activity in leaves of M9-T337 seedlings. A: SOD superoxide dismutase; B: POD peroxidase; C: CAT peroxidase

Fig. 10

Effect of combined application of P and Zn on endogenous hormone content in leaves of M9-T337 seedlings. A: ZT zeatin; B:GA₃ gibberellin; C: IAA auxin; D: ABA. abscisic acid