Objectives To explore the coupling effects of foliar application of different types of microbial inoculants during the fruit expansion period on the physiological quality and rhizosphere microenvironment of ‘Zhongmi-2’ kiwifruit (Actinidia chinensis cv. ‘Zhongmi-2’), and to screen out the optimal microbial inoculant application scheme.

Methods A 3×3 two-factor full factorial experiment was conducted, 10 treatments was composed of a water control (CK) and different inoculant types (A1, Bacillus; A2, Trichoderma; A3, composite inoculant) and dilution ratios (B1, 200-fold; B2, 300-fold; B3, 400-fold). Leaf physiological parameters, fruit quality, and physicochemical indicators of the rhizosphere soil were measured, and a comprehensive evaluation was performed utilizing the membership function method.

Results 1) Bio-fertilizers effectively improved the physicochemical properties of the rhizosphere soil: A1B3 and A2B3 treatments decreased the rhizosphere pH from 6.92 in CK to 6.46 and 6.50, respectively (P<0.05); A3B3 treatment mitigated rhizosphere soil organic matter depletion at fruit maturing stage, compared to CK (P<0.05). (2) Nerther inoculant nor delution fold showed effect on leaf chlorophyll and soluble starch contents in leaves, the dilution fold exhibited an effect on leaf soluble protein (P=0.001), with the peak value in the A3B3 treatment reaching 21.48 mg/g, which was significantly higher than that of CK (18.28 mg/g); inoculant type, dilution fold, and their inter action showed effect on leaf soluble sugar content at 0.05,0.001, and 0.001 significant levels, respectively. A1B1 treatment was recorded the highest soluble sugar content in leaves (41.15 mg/g), showing low export of photo-assimilates, whereas A2B3 and A3B3 were recorded soluble sugar content 26.39 mg/g and 28.16 mg/g,showing high outward allocation of photoassimilates. 3) All the treatments did not change the standard short-cylindrical appearance of the fruit (fruit shape index: 0.99–1.04). The dilution fold demonstrated a distinct effect on single fruit weight, with the average single fruit weight under 400-fold dilution (B3) treatments was higher than that of B1 treatments. Notably, the single fruit weight of A2B3 achieved 105.55 g, which was significantly higher than that of the A1B1 treatment (91.40 g, P<0.05). 4) The combination of inoculant type and dilution ratio significantly improved internal fruit quality. The A3B2 treatment notably enhanced the fresh-eating flavor, achieving a soluble sugar content of 8.28% (a 68.29% increase over CK, P<0.05), a sugar-acid ratio of 6.94, and an ascorbic acid content of 133.94 mg/100 g, thereby ranking first in the comprehensive membership evaluation (0.838). In contrast, the 400-fold composite inoculant (A3B3) increased the accumulation of storage reserves, with its dry matter (24.78%) and total starch (31.37 mg/g) significantly higher than those of CK (19.65% and 24.38 mg/g, respectively, P<0.05).

Conclusion The dilution folds of microbial inoculants for foliar application intensively affect the regulation effect on the aboveground physiological quality and the underground rhizosphere microecology of kiwifruit. 200-fold dilution induces leaf carbon retention and inhibits single fruit development; 300-fold dilution is optimal for promoting the accumulation and conversion of soluble sugars and vitamin C; 400-fold dilution facilitates the maintenance of leaf nitrogen metabolism homeostasis, source-to-sink translocation, starch and dry matter accumulation, and rhizosphere soil carbon pool conservation. In agricultural practice, the 300-fold dilution of the composite microbial inoculant (A3B2) is preferred for the production of high-quality fresh-eating fruit, the 400-fold dilution of the composite microbial inoculant (A3B3) is recommended for ecological cultivation prioritizing orchard soil carbon conservation and the enhancement of long-term fruit storability, and the 400-fold dilution of the Trichoderma inoculant (A2B3) is recommended for improving the appearance specifications of commercial fruit.