Abstract
Hypoxia induces plant stress, particularly in cucumber plants under hydroponic culture. In plants, calcium is involved in stress signal transmission and growth. The ultimate goal of this study was to shed light on the mechanisms underlying the effects of exogenous calcium on the mitochondrial antioxidant system, the activity of respiratory metabolism enzymes, and ion transport in cucumber (Cucumis sativus L. cv. Jinchun No. 2) roots under hypoxic conditions. Our experiments revealed that exogenous calcium reduces the level of reactive oxygen species (ROS) and increases the activity of antioxidant enzymes in mitochondria under hypoxia. Exogenous calcium also enhances the accumulation of enzymes involved in glycolysis and the tricarboxylic acid (TCA) cycle. We utilized fluorescence and ultrastructural cytochemistry methods to observe that exogenous calcium increases the concentrations of Ca2+ and K+ in root cells by increasing the activity of plasma membrane (PM) H+-ATPase and tonoplast H+-ATPase and H+-PPase. Overall, our results suggest that hypoxic stress has an immediate and substantial effect on roots. Exogenous calcium improves metabolism and ion transport in cucumber roots, thereby increasing hypoxia tolerance in cucumber.
Highlights
Of plant metabolism to be affected by oxygen shortage
Plants receiving exogenous calcium exhibited enhanced aboveground growth compared to hypoxia-treated plants, including less etiolated stems, larger leaf areas and an increased number of leaves
Our previous research revealed that exogenous calcium enhanced the biomass and soluble protein content of cucumber seedlings under
Summary
Of plant metabolism to be affected by oxygen shortage. Hypoxic stress interferes with electron transport chains, and a lack of suitable electron acceptors leads to the saturation of redox chains, accumulation of NAD(P)H and decreased synthesis of ATP8. Intracellular Ca2+ signals are realized by spatially and temporally defined changes in the free Ca2+ concentration in the cytosol[14] Environmental stimuli, such as salt stress[15], hypoxia[16], and chilling[17], can alter the cellular Ca2+ concentration. When oxygen is in short supply, these energy-consuming processes affect cell metabolism and the overall plant nutritional status in response to stress. A previous study showed that both hypoxia and anoxia rapidly depolarize the plasma membrane (PM) of higher plants by approximately 50 mV, presumably by inhibiting electrogenic H+ pumps[22] It remains to be determined whether all ion transporters are affected to the same extent by hypoxia. The spatial profile of ions in root tissues is unknown[23]
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