Brassinosteroids Regulate Antioxidant System and Protect Chloroplast Ultrastructure of Autotoxicity-Stressed Cucumber (Cucumis sativus L.) Seedlings

Ping Yang,Jie Li,Muhammad Azher Nawaz,Fuxin Li,Lisha Bai

doi:10.3390/agronomy9050265

Ping Yang, Jie Li + Show 3 more

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https://doi.org/10.3390/agronomy9050265

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Abstract

Autotoxicity is a common problem being faced in protected vegetable cultivation system. Phytoremediation of plant autotoxicity is an emerging concept to minimize deterioration of soil environment and reduction of yield and quality of vegetable crops. Brassinosteroids (BRs) have been reported as a potential phytohormone to assist phytoremediation. However, the effects of BRs-induced autotoxicity stress on plant growth, photosynthesis and antioxidant defense system are poorly understood. Hence, we focused on the changes in physiological characteristics and ultrastructure of cucumber leaves in response to the application of 24-epibrassinolide (EBR) under autotoxicity stress conditions. The results showed that leaf area, plant height, fresh weight and dry weight of cucumber were obviously decreased under autotoxicity stress conditions. EBR application obviously improved the phenotypic characteristics of cucumber seedlings. Chlorophyll content, net photosynthetic rate, stomatal conductance and transpiration rate of cucumber leaves were markedly reduced under autotoxicity stress conditions. Application of EBR improved the photosynthetic pigments (chlorophyll a by 15.80%, chlorophyll b by 18.70% and total chlorophyll content by 17.30%), net photosynthetic rate by 36.40% and stomatal opening of leaves under autotoxicity stress conditions. EBR application also maintained the integrity of chloroplast and thylakoid structures under autotoxicity stress conditions. The activity of catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX) and antioxidative compounds ascorbate (AsA) and reduced glutathione (GSH) contents were markedly decreased, however, these were obviously increased after EBR application under autotoxicity stress. EBR application also increased the soluble sugar and protein, and proline concentration by 59.70%, 7.22% and 36.58%, respectively in the leaves of cucumber, decreased malondialdehyde by 24.13% and reactive oxygen species contents (H2O2 by 35.17%, O2− by 12.01% and •OH by 16.59%), and reduced the relative permeability of the cell membrane by 14.31%. These findings suggest that EBR application enhanced the photosynthetic capacity of leaves, maintained the integrity of chloroplast and thylakoid structures, and effectively alleviated the damage of membrane caused by lipid peroxidation and root damage under autotoxicity stress conditions. The growth inhibition effect of autotoxicity stress on cucumber was reduced by EBR application.

Highlights

The rapid developments in protected cultivation and intensive cropping pattern have led to an increased index of multiple cropping [1,2]
Autotoxicity is a common problem being faced in protected vegetable cultivation system because after harvesting the roots remains in the soil [7]
The findings reported in this study may provide a theoretical basis for BRs as a potential phytohormone that can be utilized to alleviate autotoxicity stress of cucumber

Summary

Introduction

The rapid developments in protected cultivation and intensive cropping pattern have led to an increased index of multiple cropping [1,2]. The continuous cropping results in the deterioration of soil environment, serious diseases and pests attack on vegetables, and reduction of yield and quality of vegetable crops [3,4]. If the same type of vegetable is cultivated continuously, it affects the species and quantity of soil microbes. The root system of the vegetable secrete the same substances (roots exudates) for a long time that may affect microbial diversity and microbial population and increase the incidence of soil-borne diseases [6]. Autotoxicity is a common problem being faced in protected vegetable cultivation system because after harvesting the roots remains in the soil [7]. A range of secondary metabolites in the residual root of vegetables, such as cinnamic acid (CA), flavones and terpenoids are the potential autotoxins [8,9]

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