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Article

Sex-Specific Physiological Responses of Populus cathayana to Uranium Stress

by
Hongxia Xia
1,
Xinyan Cheng
1,
Liuliu Zheng
1,
Hui Ren
1,
Wanting Li
1,
Yanbao Lei
2,
Anđelka Plenković-Moraj
3 and
Ke Chen
1,*
1
School of Life Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China
2
Biodiversity Conservation Key Laboratory of Sichuan Province&China-Croatia “Belt and Road” Joint Laboratory on Biodiversity and Ecosystem Services, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China
3
Department of Biology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, 10000 Zagreb, Croatia
*
Author to whom correspondence should be addressed.
Forests 2022, 13(7), 1123; https://doi.org/10.3390/f13071123
Submission received: 15 June 2022 / Revised: 14 July 2022 / Accepted: 15 July 2022 / Published: 17 July 2022
(This article belongs to the Special Issue Adaptation of Trees to Abiotic Stress Induced by Environmental Change)
Browse Figures
Versions Notes

Abstract

:
Attention is increasingly being paid to the contamination of soil by the radioactive element uranium (U). Phytoremediation of contaminated soil by economically advantageous and environment-protective plants shows considerable potential for addressing this problem. Populus cathayana is a species with high heavy-metal tolerance, economic value, and notable potential for phytoremediation. Plant-sex-related differences can lead to differences in vegetative growth and tolerance to various stressors. As such, in this study, we designed a pot experiment to analyze the responses of male and female trees of P. cathayana to 50 mg kg−1 U stress in contaminated soil for 3 months. We studied the U uptake and distribution, photosynthesis, chlorophyll fluorescence, active oxygen species, and antioxidant enzymes of P. cathayana. The results showed that the photosynthetic activity and chlorophyll fluorescence of male and female trees were similar, and U stress mainly affected the nonstomatal factors and photosystem II during photosynthesis. Regarding the physiological and biochemical processes, male and female trees showed different defense strategies: male trees had higher peroxidase (POD), H2O2, and soluble sugars, but lower malondialdehyde (MDA), superoxide dismutase (SOD), and soluble proteins. Under U stress, the active oxygen produced by male trees could be cleared by antioxidant enzymes, preventing damage to the cell membrane. Male trees accumulated a higher U concentration in their roots than female trees, whereas the transportation of U from roots to leaves in male trees was lower than that in female trees. Therefore, our results suggested that male trees have a higher tolerance capacity and greater ability to remediate U-polluted soil than female trees. Future phytoremediation studies should consider the differences between plant sexes in the tolerance to U-contaminated land.

1. Introduction

Uranium is an important energy material that has been widely used in various fields. The U content in the Earth’s crust is very low [1]; however, owing to extraction, mining, smelting, and reprocessing processes, U has been leaked, causing serious soil pollution [2,3]. The most popular method of remediating U contamination is through planting on the contaminated soil, which is both economical and environmentally friendly [4]. Plants uptake the U in the soil through their roots. Poplar is an economic tree with rapid growth, large biomass, long root systems, and tolerance to heavy metal pollution [5,6,7].
U is generally absorbed by plant roots; only a small amount is transferred to other parts. U absorption capacity of plants is related to soil properties, such as pH and organic matter. Low pH promotes the absorption of U [8]. Additionally, U ions can affect the physiological and biochemical reactions of plants, such as by inhibiting photosynthesis and stimulating active oxygen production and lipid membrane oxidation. Under U stress, the detoxification mechanisms used by plants include the immobilization of U through cell walls and storage of U in a vacuole medium. The antioxidant enzyme system is an important detoxification mechanism for plants. Excess amounts of reactive oxygen species (ROS) can damage cell membranes and proteins, leading to lipid peroxidation [9,10]. Plant leaves are adapted to different environmental stresses in many ways, and their structural characteristics can reflect the influence of environmental factors on them [11]. Previous studies indicated that high concentration of heavy metals could induce palisade and sponge tissues thickness in leaf of Syringa microphylla. Owing to thickness of leaves and increase of chlorophyll contents (Chl) in S. microphylla induced by heavy metals, better light quality and higher light quantum were captured by photosynthetic organ, and the photosynthetic rate (Pn) and gas exchange parameter (Gs) of leaves increased subsequently [12,13]. U affects plant photosynthesis to different degrees through different mechanisms. U affects plant leaves; by affecting stomatal conductance, U slows photosynthesis by affecting the water transport in the roots and the water supply in the leaves. The nonstomatal functions of leaves include a series of reactions, the most important of which is the conversion of light energy into chemical energy through photosystem I (PSI) and photosystem II (PSII) in the light system and the subsequent Calvin cycle. However, little attention has been paid to the effects of U on stomatal and nonstomatal factors.
Dioecious plants play an important role in maintaining the structural stability and functions of the terrestrial ecosystem [14,15]; approximately 6% of angiosperm species are dioecious [16]. When females and males of a species are substantially different, this is termed dimorphism, which includes differences in sexual features and other vegetative traits [17]. The females of a plant species generally exert a higher reproductive effort than the male because the female plants produce not only flowers but also fruits and seeds. If resources are limited, reproduction directly competes with the other two processes (vegetative growth and stress resistance) [17,18,19]. Male trees experience non-biological stresses, such as ultraviolet radiation, temperature, humidity, and heavy metals, and are more resistant to these stressors than females [20,21]. With limited resources, female trees often invest more energy in reproduction, whereas male trees devote more resources to vegetative growth and stress resistance. In addition, dioecious plants often exhibit spatial sex segregation, with female plants occupying fertile habitats and male plants growing in more hostile environments. Therefore, considerable differences exist in the absorption, migration, distribution, and detoxification mechanisms of heavy metals between male and female plants. In particular, female and male poplar trees exhibit different physiological and biochemical responses to environmental pressure [22]. However, the photosynthetic and physiological effects of contaminated soil on female and male poplars remain poorly understood, especially contamination by U.
In summary, poplar is a tree species that shows potential use for U-contaminated soil remediation; however, little is known about the physiological and biochemical differences between the female and male trees under U treatment. As such, in this study, we hypothesized that U would reduce photosynthesis and antioxidant enzyme activity in P. cathayana. With limited resources, owing to different reproductive costs, female and male trees would have different tolerances to U. We conducted pot experiments with female and male trees of P. cathayana to explore changes in photosynthetic capacity, chlorophyll fluorescence, and antioxidant enzymes in soil with a U concentration of 50 mg kg−1.

2. Methods

2.1. Planting of Populus cathayana and Experimental Treatments

Cuttings of P. cathayana were from Maoxian Ecological Station, Chengdu Institute of Biology, Chinese Academy of Sciences. Uranyl nitrate [UO2(NO3)2·6H2O, 238U] (Mianyang City Letter Jie Trade Co. Ltd., Mianyang, China) was dissolved and added into the soil at the concentration of 50 mg kg−1 (pH 5.6, EC 0.9 mS cm−1). The soil contaminated with U was balanced for 2 months before being used for treatments in this study. After sprouting and growing for about a month, healthy cuttings of P. cathayana with approximately equal height were selected and replanted into 5 L plastic pots filled with 3 kg homogenized soil for treatments. Therefore, this study was designed including four treatments: female P. cathayana without U treatment (CK-F), female P. cathayana with U treatment (U-F), male P. cathayana without U treatment (CK-M), and male P. cathayana with U treatment (U-M). There were three replicate pots for each treatment. The growth environment of P. cathayana was in the greenhouse (temperature 25 ± 5 °C, and watered in the morning and evening every day). After treatment for three months, plants were measured for U enrichment, biomass, photosynthetic capacity, chlorophyll fluorescence, and antioxidant enzymes.

2.2. Measurement of Gas Exchange Parameters

The leaves of poplars from top to bottom sixth were measured for gas exchange parameters with GFS-3000 (Walz, Germany) from 10 AM to 12 AM on a sunny morning. The environment temperature was 28 °C and CO2 concentration was 400 ppm. Parameters measured included net photosynthetic rate (A), intercellular CO2 concentration (Ci), water use efficiency (WUE), and stomatal conductance (Gs). Photosynthetic light response curves were obtained, varying light intensity from 0 to 2000 μmol photons m−2 s−1.

2.3. Measurement of Chlorophyll Fluorescence

Before the measurement, P. cathayana was dark-adapted for 1 h, and then the fast fluorescence curves of the fifth, sixth, and seventh leaves of poplar were measured by dual-pam-100 dual channel chlorophyll fluorometer (Walz, Germany) and chlorophyll fluorescence imaging by MINI-Imaging-PAM (Walz, Germany).
The parameter calculation of PSI is as follows:
Y(I) = (Pm′ − P)/Pm; Y(ND) = P/Pm; Y(NA) = (Pm − Pm′)/Pm.
The parameter calculation of PSII is as follows:
Y(II) = (Fm′/Fs)/Fm′; NPQ = (Fm − Fm′)/Fm′; Y(NO) = Fs/Fm.
The photosynthetic electron transfer rate is calculated as follows:
ETRI = PPFD × Y(I) × 0.84 × 0.5; ETRII = PPFD × Y(II) × 0.84 × 0.5. ETRI and ETRII represent electron transfer rates through PSI and PSII, respectively, ETRI-ETRII represents ring electron transfer rates [23], and PPFD represents photosynthetically active radiation intensity.

2.4. Measurement of Soluble Proteins, Soluble Sugars, and Proline for P. cathayana

The soluble proteins content was determined by Coomassie Brilliant Blue G250 (CBB) staining [24]. The dye reagent was prepared with 0.01% (w/v) CBB. In total, 0.2 g of male and female leaves was milled evenly with liquid nitrogen. The final volume of the extract was 2 mL (potassium phosphate buffer at pH 7.8). Homogenate was centrifuged at 4 °C for 6 min at 12,000 r min−1. The supernatant (1 mL) was mixed with 5 mL of CBB evenly and then the absorbance value at 595 nm was measured after standing for 10 min.
The content of soluble sugar was determined by the phenol-sulfuric acid method [25]. In total, 0.2 g of crushed leaves was heated in a sealed test tube in boiling water for minutes. After cooling, we took 0.1 mL of abstracting liquid and put it into a 10 mL test tube with a stopper, added 0.3 mL of distilled water, 0.1 mL of anthrone ethyl acetate (0.02 g·mL−1) and 1 mL of concentrated sulfuric acid (98%), shook it fully, put it into boiling water immediately, kept it warm for 1 min, then took it out and cooled it naturally, used a spectrophotometer to measure the absorbance value at 630 nm.
The content of proline was determined by the sulfosalicylic acid method [26]. In total, 0.2 g leaves were quickly ground in an ice bath with 3% sulfosalicylic acid solution. The final volume was 2 mL after being transferred into a centrifuge tube. Homogenate was sealed and soaked in boiling water for 10 min. After cooling, the homogenate was centrifuged at 3000 r·min−1 for 10 min. A total of 1 mL of supernatant was separated and mixed with 1 mL glacial acetic acid and 2 mL 2.5% ninhydrin color developing liquid in boiling water for 40 min. Thereafter, 2 mL toluene was added in the mixture solution for absorption and static delamination. Finally, the absorption value was measured at 520 nm using a photometer.

2.5. Measurement of Hydrogen Peroxide (H2O2), Malondialdehyde (MDA), Superoxide Dismutase (SOD), and Plant Peroxidase (POD)

The content of hydrogen peroxide was determined by TCA and KI [27]. In total, 0.1 g leaves were mixed with 1.7 mL 0.1% TCA solution and ground in liquid nitrogen. Homogenate was centrifuged at 4 °C for 12,000 r·min−1 for 15 min. In total, 1 mL of supernatant with 0.2 mL 1 M KI solution and 0.1 M potassium phosphate buffer (pH 7.0) was used to conduct a dark reaction (standing in the dark at 25 °C) for 1 h. Then, the supernatant was used to measure the absorbance value at 390 nm by spectrophotometer.
The content of MDA was determined by the barbituric acid method [28]. In total, 0.2 g leaves were weighed and ground with 10% trichloroacetic acid solution 1.7 mL. The homogenate (2 mL) was centrifuged for 10 min at 3000 r min−1. Then, 1 mL of supernatant was mixed with 2 mL of 0.6% thiocuronic acid solution incubated in a boiling water bath for 20 min. Finally, the supernatant was taken to determine the absorbance value of 532, 600, and 450 nm.
SOD activity was measured in plants by nitrotetrazolium chloride blue (NBT) photoreduction. The activity of POD occurred by guaiacol oxidation. All reagent kits were provided by Nanjing Jiancheng Institute of Biological Engineering.

2.6. Measurement Growth and Determination of U Enrichment in Leaves and Roots of P. cathayana

Plant height and root length were recorded. The roots and leaves were washed with distilled water and placed in an oven at 105 °C for 30 min, then dried at 80 °C until constant weight was achieved (about 72 h). The dried plant tissue was ground into powder, 0.1 g of sample was accurately weighed and put into the digestion tank, 5 mL nitric acid and 2 mL hydrogen peroxide were added, and it was digested with microwave digester (CEM, Matthews, NC, USA). When the liquid was clear and transparent in the digestion tank, solution was filtered with 0.22 mm before being fixed to volume at 50 mL, and was sent to the Southwest University of Science and Technology Analysis Test Center for ICP-MS (Agilent, Stevens Creek Blvd Santa Clara, CA, USA) for analysis of U content.

2.7. Statistical Analysis

Data were presented as the means ± SD (standard deviation) of independent measurements. Differences among the groups were tested by two-way analysis of variance (ANOVA) followed by Tukey’s multiple-comparison test at a significance level of p < 0.05 using SPSS 18.0. In the figures, the same letter indicates no significant difference (p > 0.05), and different letters indicate significant differences (p < 0.05). The figures were made by Origin 2021 and SigmaPlot 14.0.

3. Results

3.1. Distribution and Content of U for Female and Male of P. cathayana

Table 1 shows the U contents in different parts of P. cathayana in this study. The content in the roots of male trees was 13.06 mg kg−1; this content was three times that of the female tree roots (Table 1). In addition, the U content in the leaves of female trees was 0.24 mg kg−1, but we detected no U in the leaves of male trees (Table 1). The results also showed that the U in P. cathayana was mainly stored in the roots, and only a small amount was transferred to the leaves.

3.2. Effects of U on the Growth of Female and Male Trees of P. cathayana

The roots and stems of both male and female trees were not affected by U-contaminated soil, but the leaf biomass was affected (Figure 1). Under 50 mg kg−1 U stress, the biomass of the female tree leaves considerably decreased. Compared with the CK treatment, the height of female trees under U stress notably decreased, and they were substantially shorter than the male trees. Conversely, the height of male trees under U stress was not affected (Table 2). The changes in the root lengths of female and male trees were not significant.

3.3. Effects of U on Gas Exchange between Male and Female of P. cathayana

The water use efficiency (WUE) of male trees under U stress was highest when the light intensity was less than 500 μmol photons m−2 s−1, followed by that of female trees in the CK group, female trees in the U treatment group, and male trees in the CK group (Figure 2A). The stomatal conductance (Gs) of male trees in the CK group was the highest, followed by that in the CK group. The stomatal conductance of the male trees in the U stress group remarkably decreased, but was almost the same as that of female trees in the CK group, and the stomatal conductance of female trees in the U stress group was lower (Figure 2B). U stress reduced the Gs in P. cathayana. The net photosynthetic rate (A) of male trees in the CK group was higher than that of female trees, whereas that of P. cathayana in the U-stress group was lower. When the light intensity was less than 1000 μmol photons m−2 s−1, the A of female trees was higher than that of male trees, but that of the male trees was higher when the light intensity was more than 1000 μmol photons m−2 s−1 (Figure 2C). U stress increased the intercellular CO2 concentration (Ci) in P. cathayana, which was higher in male trees than in female trees (Figure 2D).

3.4. Effect of U on Chlorophyll Fluorescence for Female and Male of P. cathayana

As shown by the fast light response curves, the PSI photochemical quantum yield, Y(I), decreased as the light intensity increased (0–1800 μmol photons m−2 s−1). That of male trees under U stress rose when the light intensity was 200 μmol photons m−2 s−1, and reduced above 400 μmol photons m−2 s−1 (Figure 3A). The reduction ratio, Y(NA), of the PSI response center P700 was almost the same in male and female trees under U stress and 400 μmol photons m−2 s−1, for which it was much lower than that in the CK group (Figure 3B). Y(ND) is the oxidation ratio of the PSI reaction center; U stress increased Y(ND), with female trees having the highest Y(ND). In the CK group, Y(ND) was higher in female than in male trees (Figure 3C).
U stress increased the photochemical quantum yield Y(II) of PSII in male trees (Figure 3D), and reduced the regulatory heat dissipation rate yield Y(NO) in both female and male trees (Figure 3E), but the Y(NO) of male trees remained higher than that of female trees. The NPQ was highest in female trees and lowest in male trees under U stress (Figure 3F). ETRI and ETRII represent the electronic transmission rate of PSII. The ETRI and ETRII of unstressed male trees were higher than those of male trees under U stress. CEF U stress was lower than that of female trees (Figure 3G–I).
Table 3 and Figure 4 display the chlorophyll fluorescent parameters and images of leaves in CK-F, U-F, CK-M, and U-M treatments. No significant differences in minimal fluorescence yields of PS II (F0), maximum fluorescence yields of PS II (Fm), ptimal/ maximal photochemical efficiency of PS II in the dark (Fv/Fm), and photochemical quenching (qP) were detected between CK-F and CK-M, indicating similar photosynthesis ability in both sexes. In addition, U stress significantly increased Fm, Fv/Fm, and qP in males compared to the control (Table 3), but this trend was only observed in fluorescent images of qP (Figure 4). In contrast, there was no significant difference in the four fluorescent parameters in females between U stress and control condition (Table 3).

3.5. Effects of U on Contents of H2O2 and MDA and Activities of Antioxidant Enzymes in Leaves of P. cathayana

U stress increased MDA levels in female trees, but decreased the levels in male trees (Figure 5A). The H2O2 contents of male and female trees were substantially increased by U stress, and that in male trees was much higher than in female trees (Figure 5B). We found no significant difference between the female trees in the CK and U stress groups, and male trees under U stress had higher values than those in CK groups (Figure 5C). However, the SOD and MDA results were the same. The SOD activity of female trees increased under U stress, but that of male trees considerably decreased (Figure 5D).

3.6. Effects of U on Soluble Sugars, Soluble Proteins, and Proline in Leaves of P. cathayana

Under U stress, the soluble sugars content in the leaves of female trees increased, and was lower than that of the non-stressed female trees (Figure 6A). The effect of U on the soluble proteins content of female trees was not obvious, whereas U stress reduced that of male trees (Figure 6B). We observed no significant difference in the effect of U on proline content between male and female trees (Figure 6C).

4. Discussion

4.1. Sex Differences in Growth, U Enrichment, and Distribution

U can inhibit the growth of poplar [29], including plant height and biomass. In addition, the U content differs between male and female trees [30]. The U concentration is higher in the root system than in other parts of the plant [31]; low U concentrations inhibit the growth of female trees [32]. In this study, the female trees under U stress were substantially shorter than male trees; U stress affected the leaf biomass of male trees, showing that different parts of male and female trees are sensitive to U.
In our results, both male and female P. cathayana mainly enriched U in their roots, which is consistent with previous studies that showed that roots were the primary part of plants such as Amaranthus [33], sunflowers [34], and poplar trees [20,35] to accumulate heavy metals, including U. In further comparison with female trees, the male P. cathayana exhibited significantly higher U accumulation ability in roots without U detected in leaves. Similar results regarding the lower root-to-shoot translocation of heavy metals in males than in females of poplar were also found in Liu et al. [20]. It was deduced that the adaptation strategy of the males to U stress was a kind of higher accumulation with lower root-to-shoot translocation in order to reduce U damage to the plants.

4.2. Sex Differences in Gas Exchange and Chlorophyll Fluorescence

In angiosperms, photosynthesis is limited by both stomatal and nonstomatal factors. Stomata are prerequisites for photosynthesis. Plants use stomata for gas metabolism processes such as carbon assimilation, respiration, and transpiration, which are important channels for external exchange. Therefore, stomatal conductance indirectly determines the photosynthetic rate of plants. Plants maximize carbon gain by adjusting their stomatal conductance. In the CK group, the A and Gs of male trees were higher than those of female trees, and higher than those of plants under U stress, where Gs was lower in male than in female trees. The level of A was related to the light intensity. When the light intensity was less than 1000 μmol photons m−2 s−1, the A of female trees was higher than that of male trees. When the light intensity was above 1000 μmol photons m−2 s−1, the opposite was observed. We speculated that the light saturation point of the male trees was higher. The drop in A may have occurred because U affects photosynthesis through the Calvin cycle [36]. After U stress, photosynthesis was blocked, A decreased, Ci increased with an increase in dark respiration, and WUE decreased [37]. Ci is an important parameter used to judge the influence of stomata and stomatal factors on photosynthesis [38]: if A decreases while Gs and Ci decrease, stomatal factors affect photosynthesis; otherwise, the nonstomatal factors are at play. Our results showed that the A of both male and female trees under U stress was lower than that of the trees in the CK group [14,36]. However, Ci was higher in trees under U stress, which indicated P. cathayana under U stress was not CO2-limited, confirming that the nonstomatal factor is the main factor reducing A [39].
The nonstomatal factors include PSI and PSII in the leaves, which are responsible for the conversion of light energy into chemical energy. During chlorophyll fluorescence, U negatively affects the components of PSI and PSII, including the oxygen-evolving complex, Cyb6f, and PQ. To reduce the formation of ROS and prevent photoinhibition, plants convert the purple-yellow pigment into zeaxanthin through the xanthophyll cycle. The use efficiency of weak light decreases, which leads to a decrease in the assimilation power caused by photoelectron transfer. PSII is more sensitive than PSI in stressful environments [40], so it is more susceptible to heavy metal ions. One of the important ROS targets is the D1 protein of PSII [41]. U stress caused male trees to have the highest Y(NA) under low light intensity (0–400 μmol photons m−2 s−1). Compared with female trees, in male trees, the Y(NA) indicated the receipt of more electrons and that the PSI was more easily decreased. When the light intensity was more than 400 μmol photons m−2 s−1, the ETRI and ETRII of male trees in the U treatment group were the highest, but their CEF was the lowest. The NPQ of PSII was the main photoprotective mechanism of female trees in the U treatment group [42]. Therefore, under U stress, we found that the photoprotection mechanisms of male and female trees are different, and the photosystems of female trees are more vulnerable.

4.3. Effects of U on ROS and Antioxidant Enzymes in Leaves of P. cathayana

ROS of plants was usually induced under U stress or any other abiotic stress such as light, metals, drought, and flood [43,44]. To protect plants from oxidative stress by ROS, some antioxidants, flavonoids, and secondary metabolites will be produced by plants to adapt to abnormal conditions (i.e., stress). Maintaining a balance between ROS and antioxidants is essential for plant adaptation to and survival in stressed environments. Metal ions can cause the accumulation of ROS in plants, and antioxidant enzymes play an important role in their removal and balancing [6]. The U ions in plants affect the integrity of the cell membrane, to some extent, and produce ROS, including 1O2 and H2O2. In response to heavy metal stress, plants use efficient nonenzymatic and enzymatic detoxification mechanisms through clearing the ROS to reduce the negative effects of heavy metal ions in the plant body. The mechanisms of enzymatic detoxification in plants mainly involve SOD, POD, CAT, and APX. Enzyme activity under heavy metal stress is related to the heavy metal concentration, which is either inhibited or activated.
SOD and POD protect cells from damage by ROS through the process of antioxidation under abiotic stress. SOD is the first line of defense, converting superoxide anions into H2O2. In our study, the changes of POD content were insensitive to U stress both in males and females of P. cathayana, but exhibited notable sexual difference, which is in accordance with the previous results for P. deltoides response to cadmium and salinity stress [29]. The MDA and H2O2 contents were usually used to evaluate the damage level of lipid peroxidation under heavy metals stress, including U [45]. Our results indicated that U exposure induced H2O2 production in leaves of both male and female trees, and there was significant difference between the sexes irrespective of U stress. Moreover, MDA increased significantly in female leaves under U stress, indicating that female trees probably suffered more oxidative stress by U.

4.4. Effects of U on Soluble Proteins, Soluble Sugars, and Proline

When plants are under stress, soluble protein content is an important index for evaluating plant growth and development. The production of soluble sugars and soluble proteins enables enhancement of the water-holding capacity of leaf tissue cells, maintains cell osmotic pressure, protects tissue cells from abiotic stress, and improves tolerance of plants [46]. However, increase of soluble sugars and decrease of soluble proteins were only found in females and males, respectively, in the present study. Further, although it was reported that proline plays a role in protecting the cell membrane of plants against U stress [47], no significant difference of proline content was found among the CK-F, U-F, CK-M, and U-M treatments in this study. Therefore, the osmotic adjustment is definitely important for poplars to abiotic stress [45,46], but the sexual difference in osmotic adjustment of poplar to U stress is still not evidently supported.

5. Conclusions

Our results suggest that sexual differences in responses to U stress in females and males of P. cathayana are attributed to differential U accumulation, tolerance, and detoxification strategies. Under the stress of 50 mg kg−1 U, male trees removed the ROS through antioxidant enzymes and osmotic adjustment substances, reducing the damage to cell membranes and other areas. At this concentration, the antioxidant enzymes of female trees were insufficient and could not clear the ROS, leading to cell membrane oxidation. The damage caused by U to P. cathayana was mainly caused due to nonstomatal factors in PSII and decreased the biomass of female trees. In contrast, females of P. cathayana showed higher tolerance, with photoperiod protection, antioxidant enzymes, and U enrichment in their tissues. This study provided new insights into sex-specific accumulation and physiological adaptation related to U tolerance in dioecious woody plants. There are future applications for phytoremediation by woody plants.

Author Contributions

Conceptualization, Y.L. and K.C.; methodology, X.C. and W.L.; software, X.C.; validation, Y.L., A.P.-M. and K.C.; formal analysis, H.X., X.C., L.Z. and H.R.; investigation, H.R.; resources, Y.L.; data curation, H.X. and A.P.-M.; writing—original draft preparation, H.X.; writing—review and editing, A.P.-M.; visualization, H.X.; supervision, Y.L. and K.C.; project administration, K.C.; funding acquisition, Y.L. and K.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by by the National Key Research and Development Program of China (2020YFE0203200), Sichuan Science & Technology Bureau Program (2020YFH0201, 2020YFH0001, 2020YFS0025, 2020ZHYZ0002, 2020ZHFP0207, 2020YFH0215, 2021YFH0008, 2021ZHYZ0002, 2021YFN0105), Chengdu Science and Technology Bureau Program (2021-YF05-02340-SN).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data available on request from the corresponding author.

Acknowledgments

We thank Yunlai Tang for instruction of chlorophyll fluorometer.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 13 01123 g001 550
Figure 1. Dry biomass of P. cathayana, including roots, stems, and leaves. Values are expressed as means ± SE (n = 3). Different letters represent significant differences between treatments (p < 0.05).
Figure 1. Dry biomass of P. cathayana, including roots, stems, and leaves. Values are expressed as means ± SE (n = 3). Different letters represent significant differences between treatments (p < 0.05).
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Figure 2. Gas exchange parameters of P. cathayana under U stress: water use efficiency (WUE) (A), stomatal conductance (Gs) (B), net photosynthetic rate (A) (C) and intercellular CO2 concentration (Ci) (D). Values are expressed as means ± SE (n = 3). Different letters represent significant differences between treatments (p < 0.05).
Figure 2. Gas exchange parameters of P. cathayana under U stress: water use efficiency (WUE) (A), stomatal conductance (Gs) (B), net photosynthetic rate (A) (C) and intercellular CO2 concentration (Ci) (D). Values are expressed as means ± SE (n = 3). Different letters represent significant differences between treatments (p < 0.05).
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Figure 3. Effect of sexuality on rapid light induction curve of P. cathayana under U stress: effects on Y(I) (A), Y(NA) (B), Y(ND) (C), Y(II) (D), Y(NO) (E), NPQ (F), ETRI (G), ETRII (H) and CEF (I). Values are expressed as means ± SE (n = 3). Different letters represent significant differences between treatments (p < 0.05).
Figure 3. Effect of sexuality on rapid light induction curve of P. cathayana under U stress: effects on Y(I) (A), Y(NA) (B), Y(ND) (C), Y(II) (D), Y(NO) (E), NPQ (F), ETRI (G), ETRII (H) and CEF (I). Values are expressed as means ± SE (n = 3). Different letters represent significant differences between treatments (p < 0.05).
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Figure 4. Chlorophyll fluorescence images of Fo, Fm, and Fv/Fm in a dark-adapted leaf and NPQ, Y(II), and qP at steady-state with actinic illumination of 400 μmol·photons·m−2·s−1 measured at the end of the experiment in leaves of different treatments. The false color code depicted at the bottom of each image ranges from 0.000 (black) to 1.000 (pink).
Figure 4. Chlorophyll fluorescence images of Fo, Fm, and Fv/Fm in a dark-adapted leaf and NPQ, Y(II), and qP at steady-state with actinic illumination of 400 μmol·photons·m−2·s−1 measured at the end of the experiment in leaves of different treatments. The false color code depicted at the bottom of each image ranges from 0.000 (black) to 1.000 (pink).
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Figure 5. Female and male trees of P. cathayana: MDA (A), H2O2 (B), and the activity of antioxidant enzymes POD and SOD (C,D). Values are expressed as means ± SE (n = 3). Different letters represent significant differences between treatments (p < 0.05).
Figure 5. Female and male trees of P. cathayana: MDA (A), H2O2 (B), and the activity of antioxidant enzymes POD and SOD (C,D). Values are expressed as means ± SE (n = 3). Different letters represent significant differences between treatments (p < 0.05).
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Figure 6. U on female and male trees of P. cathayana in leaves: effects on soluble sugars (A), soluble proteins (B), and proline (C). Values are expressed as means ± SE (n = 3). Different letters represent significant differences between treatments (p < 0.05).
Figure 6. U on female and male trees of P. cathayana in leaves: effects on soluble sugars (A), soluble proteins (B), and proline (C). Values are expressed as means ± SE (n = 3). Different letters represent significant differences between treatments (p < 0.05).
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Table
Table 1. Content of U accumulated in root and leaf of P. cathayana.
Table 1. Content of U accumulated in root and leaf of P. cathayana.
Female TreesMale Treesp-Value
RootLeafRootLeaf
U content
(mg kg−1 DW)		4.462 ± 0.747b0.241 ± 0.150c13.058 ± 2.870aN/A<0.001
Note: Values are expressed as means ± SE (n = 3). Different letters represent significant differences between treatments (p < 0.05). N/A represents data not detected.
Table
Table 2. Growth parameters of P. cathayana under U stress.
Table 2. Growth parameters of P. cathayana under U stress.
FactorTreatmentPlant Height (cm)Root Length (cm)
FemaleCK-F71.0 ± 1.88a30.9 ± 4.98a
U-F44.2 ± 4.87b31.8 ± 2.99a
MaleCK-M82.9 ± 3.29a30.4 ± 3.51a
U-M74.6 ± 5.51a37.1 ± 2.57a
Psex <0.0010.519
PU 0.0030.323
Psex × U 0.0560.456
Note: Values are expressed as means ± SE (n = 3). Values followed by the same letter in the same column are not significantly different (p < 0.05). Psex, sex effect; PU, U effect; Psex × U, sex × U interaction effect (ANOVA).
Table
Table 3. U effects on leaf chlorophyll fluorescence parameters of P. cathayana.
Table 3. U effects on leaf chlorophyll fluorescence parameters of P. cathayana.
FactorTreatmentFoFmFv/FmqP
FemaleCK-F0.475 ± 0.021ab1.151 ± 0.057b0.579 ± 0.035ab0.528 ± 0.063b
U-F0.386 ± 0.051b1.130 ± 0.211b0.637 ± 0.027ab0.666 ± 0.069ab
MaleCK-M0.515 ± 0.026a1.223 ± 0.127b0.551 ± 0.058b0.578 ± 0.050b
U-M0.441 ± 0.090a1.449 ± 0.308a0.575 ± 0.124a0.681 ± 0.135a
Psex 0.0220.0470.8180.111
PU 0.2410.1480.0300.004
PSex × U 0.1980.1160.3640.449
Note: Values are expressed as means ± SE (n = 3). Values followed by the same letter in the same column are not significantly different (p < 0.05). Psex, sex effect; PU, U effect; Psex × U, sex × U interaction effect (ANOVA).
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Xia, H.; Cheng, X.; Zheng, L.; Ren, H.; Li, W.; Lei, Y.; Plenković-Moraj, A.; Chen, K. Sex-Specific Physiological Responses of Populus cathayana to Uranium Stress. Forests 2022, 13, 1123. https://doi.org/10.3390/f13071123

AMA Style

Xia H, Cheng X, Zheng L, Ren H, Li W, Lei Y, Plenković-Moraj A, Chen K. Sex-Specific Physiological Responses of Populus cathayana to Uranium Stress. Forests. 2022; 13(7):1123. https://doi.org/10.3390/f13071123

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Xia, Hongxia, Xinyan Cheng, Liuliu Zheng, Hui Ren, Wanting Li, Yanbao Lei, Anđelka Plenković-Moraj, and Ke Chen. 2022. "Sex-Specific Physiological Responses of Populus cathayana to Uranium Stress" Forests 13, no. 7: 1123. https://doi.org/10.3390/f13071123

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