Abstract:

【Objective】Continuous cropping obstacle is a serious problem in the facility production of strawberry (Fragaria × ananassa Duch.). Phenolic acid, a secretion of strawberry roots, is one of the main factors causing continuous cropping obstacle. The aim of this study was to explore the physiological response by which exogenous phenolic acids affected root mitochondrial function in strawberry roots under the effects of phenolic acids accumulated in the rhizosphere of long-term continuous cropping soil. The results were used to discuss the potential responding mechanism of mitochondria in strawberry roots to allelopathy effect mediating by phenolic acid, which would lay the foundation for the further study of continuous cropping obstacle of strawberry.【Method】In this study, strawberries were used as the test material, and syringic acid, phthalic acid and their mixed solution of 27.0 μg·g^-1 were exogenously applied. Scanning Electron Microscope (SEM), Histochemical Staining, Liquid-Phase Oxygen Measurement and other techniques were applied to observe the changes of ROS content in root system, to measure the changes of mitochondrial oxidative phosphorylation level, respiratory pathway and the activities of key enzymes in respiration, to study the effect of autotoxic phenolic acid-induced reactive oxygen species on mitochondrial function in root systems, and to explore the effects of root systems’ mitochondria on physiological functions, such as respiratory complex metabolism and energy metabolism. 【Result】Applying two exogenous phenolic acids or mixed solution treatment could significantly reduce the strawberry root vigor, affect the total root length, total surface area, total root volume and average diameter, and inhibit the occurrence and growth of roots, with the degree of changes: syringic acid treatment (D)>two solution mixing treatment (D+L)>phthalic acid treatment (L)>CK. The activity of H⁺-ATPase decreased significantly in the early stage with the phenolic acid stress time, and the decline amplitude was the largest at the third day. Compared with CK, D, L and D + L were decreased by 38.8%, 28.8% and 33.7%, respectively, which hindered the ion transport capacity of root cells and became stable in the later stage. The activities of K⁺Na⁺-ATPase and Ca²⁺Mg²⁺-ATPase in roots were also decreased, and the changes were similar. Exogenous phenolic acid could reduce SOD, POD, CAT and APX activity, increase the contents of H₂O₂ and $ \mathrm{O}_{2}^{\mathop{}_{.}^{-}}$, and also increase MPT in root mitochondria, and reduce Δψm and cytochrome Cyt c/a content in root mitochondrial inner membrane. Although the root respiratory pathway was still mainly the EMP-TCA pathway, the contribution rate was significantly reduced, while the PPP pathway showed an obvious increasing trend. In the late stage of applying exogenous phenolic acids, with the extension of stress time, all respiratory key enzyme activities were inhibited, which led to the obstruction of respiratory pathways, with the degree of changes: D>D+L>L>CK. 【Conclusion】 Exogenous phenolic acid treatment could reduce root vigor and hinder the operational capacity of ions inside and outside cells. The treatment could also reduce root antioxidant enzyme activity and root mitochondrial function, leading to the reduction of key enzyme activities in every respiratory pathways, and ultimately obstruct every respiratory pathways.

Key words: strawberry, phenolic acid stress, mitochondria, respiration, physiological response