Regional

The Effect of Neodymium (Nd3+) on Some Physiological Activities in Oilseed Rape during Calcium (Ca2+) Starvation

Posted on: February 10, 2025

Abstract

It has been reported that Rare Earth Elements (REE) can promote plant growth as well as other physiological activities. Since the ion radius of Nd3+ is very close to that of Ca2+, the interaction of Neodymium (Nd) and Calcium might be one of the important mechanisms to understand such phenomena. Treated with 3×10-6 mol·L-1 Nd(NO3)3 in Ca2+-deficient solution, the effects of Nd on membrane damage in oilseed rape (Brassica napus) were studied. It showed that the symptom of Ca-starvation was relieved, and the peroxidation process in rape was deferred. It indicated that added Nd could lower the root relative permeability and MDA content in leaves, increase CAT, POD, and SOD activities in rape. Likewise, the added Nd to Hoagland solution showed similar results. The interpretation is offered that the effects are a consequence of the Nd function in substituting some roles of Calcium through interacting with the cellular membrane. REE compound fertilizer is extensively applied in China; better understanding of the physiological mechanism of REE could be used to direct agricultural operations.

Key words: Rare Earth, Neodymium, Calcium, Oilseed Rape, Peroxidation

Introduction

As early as 1879, it was reported that there were trace Rare Earth Elements (REE) in barley, tomato, and grape. China has very rich resources of Rare Earth. Since 1972, synthetically studies of interdisciplinary fields have been conducted on agricultural applications of REE in China. Rare Earth compound fertilizers were extensively applied there and increased crop productivity significantly. However, the mechanism of REE’s biological effects has not been very clear yet. Compared to La or Ce, there have been few reports on Nd, while the ion radius of which is most similar to Ca2+, so it is necessary to do further study to clarify the relationship of Nd3+ and Ca2+. It was found that free Nd3+ at low concentrations can specifically bind to the high-affinity Ca2+ binding sites on the Ca-ATPase. In this work, we investigated the effect of Nd on some physiological activities in oilseed rape, especially the influence on the cell membrane.

Materials and Methods

Plant Material

After sterilization and water treatment, seeds of oilseed rape (Brassica napus L.) CV Zheyou You 2 were set on filter paper moistened with deionized water and germinated in the dark at 25°C for 2 days. Then, seedlings were moved into and cultured on silica in a growth chamber for 7 days. Five uniform seedlings were placed in a polyethylene foam float, provided with Hoagland’s solution for 2 weeks for different treatments. There were four treatments in this experiment: control (Hoagland, CK), control added in Nd(NO3)3 (CK + Nd), Ca-deficient solution (-Ca), and Ca-deficient solution added in Nd(NO3)3 (-Ca + Nd). The concentration of Nd(NO3)3 used in this experiment is 3×10-6 mol·L-1. Twenty seedlings were cultured for each treatment. Samples were taken at the 10th day for the following experiments.

Relative Permeability of Root Membrane

The root of rape was excised and rinsed thoroughly with deionized water. After incubation in a 9 cm culture dish with 30 ml deionized water at 30°C for 15 min, the root was washed several times to remove the inorganic ion outside of the membrane. Then, the samples were transferred into tubes with 10 ml deionized water to measure the conducting value (E0); conducting value (E1) was measured after incubating the tubes at 30°C for 30 min; at last, tubes were placed in boiling water for 10 min to make electrolyte in tissues completely leak out, and the conducting value (E2) of the solution was measured after the tubes had gotten cold. The relative permeability of the root membrane was given according to the following formula:

Cell permeability (%) = (E1 - E0) / (E2 - E0) × 100

SOD Activity

Washed 2 g leaves were ground with 0.05 mol·L-1 phosphate buffer at pH 7.8 in an ice bath and centrifuged at 13000g for 20 min. The supernatant was collected to measure enzyme activity using the NBT reduction method.

MDA Content

The procedures of enzyme extraction were the same as above. MDA content was measured by the TBA method: 3 ml extract and 5 ml 0.5% TBA were incubated for 10-15 min in a boiling-water bath, then centrifuged at 1800g for 10 min after it had cooled down. The supernatant was collected to measure the absorbance at 532 and 600 nm, respectively.

POD Activity Measurement

1 g leaves were ground with 0.02 mol·L-1 KH2PO4 in an ice bath and centrifuged at 13000g for 15 min. The supernatant was collected to measure the absorbance at 470 nm.

CAT Activity Measurement

1 g leaves were ground with CaCO3 and H2O, transferred to a 100 ml bottle, and added water to the mark. The supernatant was then used to measure the enzyme activity using the Na2S2O3 titration method.

Results

Effect of Nd on Membrane Permeability of Rape Root during Ca-Starvation

Comparing to the CK treatment, the membrane permeability lowered a little for the treatment of CK + Nd. For the -Ca treatment, the permeability rose significantly as much as 120.6%, which was 97.9% of the control for -Ca + Nd treatment (Fig 1). It indicated that Nd could reverse the membrane damage induced by Ca deficiency.

Effect of Nd on SOD Activity and MDA Content of Rape Leaf during Ca-Starvation

Under the condition of Ca deficiency, the SOD activity decreased significantly, declining 23.9% of the control. When 3 μmol·L-1 Nd(NO3)3 was added into Ca-deficient solution, SOD activity rose to 145.0%, while the activity of CK + Nd treatment was 16.3% higher than the control (Fig 2). It indicated that Nd treatment could stimulate SOD activity, which decreased during Ca starvation.

MDA content is used as an important index reflecting cell peroxidation degree. During Ca deficiency, MDA content of rape leaf increased to 6.19×10-9 mol·g-1, 37.1% higher than the control, while MDA content increased 21.0% for -Ca + Nd treatment (Fig 3). It suggested that Nd application could defer the senescence process induced by Ca starvation.

Effect of Nd on POD and CAT Activities of Rape Leaf

For the treatments of -Ca and -Ca + Nd, POD activity of rape leaf is 91.6, 106.0 OD470 min-1·mg-1 protein, increasing 37.9% and 59.7%, respectively (Fig 4). The POD activity increased 31.2% when Nd(NO3)3 was added into the control, while CAT activity increased to 107.5%. During Ca deficiency, CAT activity declined obviously, only 76.0% of the control (Fig 5). When the plant was treated with Nd, the enzyme activity increased 4.7%. It indicated that POD and CAT had different reactions to Ca starvation. However, Nd treatment could facilitate the activities of both enzymes.

Discussion

Senescence is one of the obstacles to high productivity. In undesirable environments and the process of senescence, oxygen free radicals, which damage membrane lipids, are an important factor for plant damage and senescence. There have been a great many reports on the relationship of calcium and senescence. As one of the stabilizing agents for the cell membrane, calcium can prevent membrane damage and leakage, maintain structural stability and integrity of the cell membrane, strengthen the structure and connection of the cell wall. In addition, it can also lower the peroxidation effects of membrane lipids, increase activities of membrane-protecting enzymes, and keep the balance state of ion transport, to relieve the damage of plants by environmental stresses.

The results of our experiments showed that the rape development was impeded during Ca-starvation: the membrane permeability rose; MDA, the product of membrane peroxidation, increased; and enzyme activities (SOD, CAT) decreased obviously. It indicated that the membrane integrity was destroyed, and the ability to clear free radicals of the protecting enzyme system decreased. SOD, POD, and CAT are clear agents for free radicals. They are important components of the protecting system for plants to defer senescence.

When Nd(NO3)3 (3×10-6 mol·L-1) was added to the solution, the physiological indexes of rape increased in varying degrees. The damage induced by Ca-starvation was improved to a certain extent. The membrane permeability lowered, MDA content decreased, and enzyme activities increased. It suggested that Nd3+ had some biological effects similar to Ca2+ and could perform some functions of Ca2+ under given conditions. The results were consistent with previous reports.

Conclusions

  1. Ca2+ plays an important role in stabilizing membrane structure and maintaining the balance of production and clearance of free radicals. Ca-starvation quickens the process of cell senescence.
  2. Nd treatment can relieve Ca-starvation symptoms of rape, increase the activities of the protecting enzyme system, lower the extent of membrane lipid peroxidation, and inhibit the membrane peroxidation process.
  3. With many characteristics similar to Ca2+, Nd3+ might perform its biological functions as a type of “Calcium-like element” in plants. The cell membrane might be the place at which Nd exerts its biological effects.

Acknowledgment

This research was supported by the National Natural Science Foundation of China.

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