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. 2023 Mar 31;18(3):e0283787.
doi: 10.1371/journal.pone.0283787. eCollection 2023.

Changes in growth, physiology, and photosynthetic capacity of spinach (Spinacia oleracea L.) under different nitrate levels

Kangning Han  1 , Jing Zhang  1 , Cheng Wang  1 , Yan Yang  1 , Youlin Chang  1 , Yanqiang Gao  1 , Yang Liu  1 , Jianming Xie  1
Affiliations

Changes in growth, physiology, and photosynthetic capacity of spinach (Spinacia oleracea L.) under different nitrate levels

Kangning Han et al. PLoS One. .

Abstract

Nitrate content is an essential indicator of the quality of vegetables but can cause stress at high levels. This study aimed to elucidate the regulatory mechanisms of nitrate stress tolerance in spinach (Spinacia oleracea L.). We studied the effects of exogenous application of 15 (control), 50, 100, 150, 200, and 250 mM NO3- on spinach growth, physiology, and photosynthesis. The results showed that all the nitrate treatments inhibited the growth of the aerial parts of spinach compared to the control. In contrast, low nitrate levels (50 and 100 mM) promoted spinach root formation, but this effect was inhibited at high levels (150, 200, and 250 mM). Treatment with 150 mM NO3- significantly decreased the root growth vigor. Low nitrate levels increased the chlorophyll content in spinach leaves, whereas high levels had the opposite effect. High nitrate levels also weakened the net photosynthetic rate (Pn), the actual photochemical efficiency of PSII Y(II), and increased non-photochemical quenching (NPQ), reducing photosynthetic performance. Nitrate stress increased the activity of nitrate reductase (NR) and promoted the accumulation of nitrate in spinach leaves, exceeding the health-tolerance limit for nitrate in vegetables, highlighting the necessity of mitigating nitrate stress to ensure food safety. Starting with the 150 mM NO3- treatment, the proline and malondialdehyde content in spinach leaves and roots increased significantly as the nitrate levels increased. Treatment with 150 mM NO3- significantly increased soluble protein and flavonoid contents, while the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) were significantly reduced in leaves. However, spinach could resist nitrate stress by regulating the synthesis of osmoregulatory substances such as proline, thus showing some nitrate tolerance. These results provide insights into the physiological regulatory mechanisms of nitrate stress tolerance and its mitigation in spinach, an essential vegetable crop.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Effect of different nitrate levels on spinach.
Fig 2
Fig 2. Effect of different nitrate levels on the morphology of spinach roots.
Fig 3
Fig 3. Effect of different nitrate levels on the root activity of spinach.
Mean values with different alphabets differ significantly at (p < 0.05) by Duncan’s test.
Fig 4
Fig 4. Effect of different nitrate levels on chlorophyll a, chlorophyll b, and total chlorophyll contents of spinach.
Mean values with different alphabets differ significantly at (p < 0.05) by Duncan’s test.
Fig 5
Fig 5. Effect of different nitrate levels on photosynthetic gas exchange parameters of spinach.
Mean values with different alphabets differ significantly at (p < 0.05) by Duncan’s test. (A) Net photosynthetic rate, Pn. (B) Stomatal conductance, Gs. (C) Intercellular CO2 concentration, Ci. (D) Transpiration rate, Tr.
Fig 6
Fig 6. Effect of different nitrate levels on chlorophyll fluorescence parameters of spinach.
Mean values with different alphabets differ significantly at (p < 0.05) by Duncan’s test. The coloured bar at the top of the leaf images (A) represents the chlorophyll fluorescence parameters range black (0) to purple (1.0) and how they mapped to the colour palette. (A) Images of Fv/Fm, Y(II), qP and NPQ, respectively. False colors are used to represent values of the parameter ranging from 0 (black) to 1.00 (purple). (B) The electron transport rates, ETR. (C) The maximum photochemical efficiency of PSΠ, Fv/Fm; PSII actual photochemical efficiency, Y(II); Photochemical quenching, qP; Non-photochemical quenching, NPQ.
Fig 7
Fig 7. Effect of different nitrate levels on the accumulation of nitrate and NR activity of spinach leaves.
Mean values with different alphabets differ significantly at (p < 0.05) by Duncan’s test.
Fig 8
Fig 8. Effect of different nitrate levels on SOD, POD, CAT, and APX activities of spinach leaves and roots.
Mean values with different alphabets differ significantly at (p < 0.05) by Duncan’s test. (A) superoxide dismutase, SOD. (B) peroxidase, POD. (C) catalase, CAT. (D) ascorbate peroxidase (APX).
See this image and copyright information in PMC

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