Introduction
Wetland
eutrophication has gradually become one of the most important ecological
environmental problems in the world (Tang et
al. 2010a). Phosphorus (P) is one of the main limiting factors of wetland
eutrophication (Qin et al. 2013; Roy
2017; Zhao
et al. 2018) and is a necessary
nutrient element for plant growth. At present, the eutrophication of water
caused by P nutrients is becoming more and more serious, leading to abnormal
ecosystem responses (Tang et al.
2010a; Hou et al. 2018; Zou et al. 2018). Due to the short flow of
tributaries and the long period of water exchange, it is difficult to manage
plateau wetlands (Yu et al. 2010;
Zhou et al.
2016). Wetland plants can effectively purify sewage and root systems are a key
link to material circulation and energy flow between wetland plants and the
surrounding environment (You et al.
2013; Zhang et al. 2018a; Zhang et al.
2018b). Therefore, it is of great significance to study the root exudates of
wetland plants.
Studies have shown that in root exudate, organic acids can affect the
nutrients and energy of microorganisms. Furthermore, they can change the
structure as well as biological activities of root-zone microorganisms; as
such, the role they play is important (Kuang et al. 2003; Zhao et al. 2016). At present, research on organic acids secreted from
root systems under P stress have mostly focused on the physiological and
ecological changes of cash crops (Gardner 1983; Carvalhais
et al. 2011; Khorassani
et al. 2011; Xiao et al. 2014; Li et al. 2016), trees, and their effects on organic acid exudation (Xu
and Ding 2006; Yu et al. 2017). Li et al. (2005) and Wang et al. (2014) studied environments such
as the nutrient in rice roots that influence the exudation of organic acids
under low P stress. Low P stress can accelerate the exudation of more organic
acid from rice root systems. Deng et al.
(2006) studied the influence of P concentration on Pinus seedling
organic acid exudation. They showed that a P deficiency can lead to acid
environment changes in the body. In addition, studies have shown that sweet
potato and beet can promote the release of P in soil by changing root
morphology and organic acid exudation composition under P-deficient
environments (Ma et al. 2017). Chen
(2009) studied the activation effect of P on root exudations in wheat and broad
bean under P stress applied in a pot experiment, showing that the root system
secreted low molecular weight organic acids to improve the biological
efficiency of P. However, there is less research on wetland plant root
exudation in specific organic acid content quality changes under P stress. Part
of the study on wetland plant roots concentrated on the total root exudation
and analysis of the influence of the rhizosphere effect (Tang et al. 2010b). Liu et al. (2009) compared water grass and willow roots exudation to
determine various plant roots exudation ability. Furthermore, they obtained
three kinds of plant root exudation that were mainly composed of organic acids
and aromatic proteins. Huang et al.
(2014) discussed the change of root exudation amount in Pinellia
and Canna with time. Although previous research on plant root
exudation and P stress had certain understanding of its influence on plant
roots, the wetland floating plant root exudation of organic acid changes has
been rarely a research topic, specifically under P stress. This is especially
true for wetland plant root exudation system research in Pistia
stratiotes; therefore, this experiment has very high scientific value and
practical significance.
With the increasing P accumulation in wetlands, the number of floating
plants will increase and become the main source of primary productivity of
wetland ecosystems (Scheffer et al. 2001). P stratiotes, a typical perennial floating
herb, survives easily and has strong sewage purification capacity, which can
effectively improve eutrophicated water bodies (Li et al. 2012; Victor et al.
2016). This research takes Pistia as
experimental material, analyzes the wetland floating plants in different
concentrations of P stress at different times that influences on root exudation
of organic acids, and tries to find the floating plants eutrophication
adaptation mechanisms under P stress to provide basic reference data for root
micromanagement measures for plateau wetlands.
Materials and Methods
Experimental details and
treatments
Water
culture and solution ratio: In this
experiment, Hoagland nutrient solution was used in the Southwest Forestry
University laboratory at the beginning of March 2018 to cultivate the seedlings
at 2030°C. P stress was carried out at the beginning of April, then, measured
for 7, 14, 21 and 28 days. The concrete operation was as follows: 80 plastic
buckets were taken as hydroponic equipment and the Hoagland nutrient solution
was added. Approximately 160 healthy seedlings with the same age were selected
and two seedlings were cultured in each plastic barrel. In the plastic barrel
of hydroponically grown plants, a 300 mm rubber disc microporous aerator was
installed to prevent the plant root system from rotting through the proper
amount of aeration. To avoid the proliferation of algae in the barrel, aluminum
foil was used for shading (Horchani et al.
2008). The culture medium was changed once in a week. After one month of
culture, plants with good growth and similar plant height were selected and
transplanted into the experimental device. KH2PO4 was
used as the P source and the P concentration gradient was 0, 0.2, 1, 5, 10 and
20 mg/L. There were 72 plastic boxes (4 groups of experiments, 3 replicates in
each group) and 2 plants were cultured in each box. Experimental design was
that completely randomized design (CRD).
The composition of the Hoagland nutrient solution was: K2SO4
0.75Χ10-3 mol/L, MgSO4 0.75Χ10-3 mol/L, KCL
1Χ10-3 mol/L, Ca(NO3)2 2.0Χ10-3 mol/L,
H3BO3 1Χ10-5 mol/L, CuSO4 1Χ10-7
mol/L, MnSO4 1Χ10-6 mol/L, ZnSO4 1Χ10-6
mol/L, (NH4)6Mo7O24 5Χ10-6 mol/L,
Fe-EDTA 1Χ10-4 mol/L. This was the mother liquor of the nutrient
solution, diluted four times, and used as the solution of the hydroponic culture
(Chen 2009).
Collection and isolation of root exudates: The entire root system of the
plant cultured under P stress was washed with deionized water for 7, 14, 21 and
28 days respectively, and then collected with root exudates (Collection liquid ratio: H3BO3 5 μmol/L, CaCl2 600 μmol/L,
KCl 100 μmol/L, MgCl2
200 μmol/L, pH 5.6). After three repeated
washings, the entire root system was covered with a black plastic bag and
transferred to a beaker containing 50 mL of root exudates. The root exudates
were collected under natural light for 4 h (9:0013:00), transplanted to a solution containing 1 L of 0.5
mmol/L CaCl2 for 4 h (13:0017:00). Then the extraction solution was
obtained by a CH2Cl2 extraction and root lotion, three
times (40 mL/times) (Tian et al. 2003; Zhang et al. 2007). Finally, a 200 mL extraction solution was
extracted at 38°C, dried with anhydrous Na2SO4, and concentrated to
dry reserve by rotating evaporation with a vacuum rotary evaporator (Wei et
al. 2016).
Determination of root exudates: The liquid was extracted with a syringe after a full
shake of the CH2Cl2, which was added to the rotating
evaporation bottle operated by 1.2 knots for 0.5 mL, through a 0.45 μm needle filtration membrane. At the same time, the
membrane was filtered by 0.45 μm needle and then
put into a small brown bottle for GC-MS analysis. The organic acids in the root
exudates were determined by GC-MS (Agilent 7890B). The chromatographic
conditions (Yu et al. 2013; Liu et al. 2017) were as follows: The capillary column was an HP-5 ms column (30 m Χ 250 μm Χ
0.25 μm); the injection port temperature was 260°C, the carrier gas was He (purity is not less than
99.99%), the flow rate 1 mL/min, the injection 1 μL,
the flow valve was opened after 1 min, the column temperature was programmed, the starting temperature was 50°C, and the flow rate was 2 min, 20°C per min, programmed to 150°C, @ 5°C per min, programmed to 220°C, then, 6°C per min, programmed to 250°C, for 15 min.
The mass spectrometer conditions (Liu et al. 2017) were: The electron bombardment source (Ei), ionization energy was 70 eV, ion source temperature
was 200°C, interface temperature 280°C, quadrupole temperature 150°C, solution delay time 3.75 min, scanning mode (SCAN), the
scanning range M/Z 33453, and the tuning file was standard tuning.
Statistical
analysis
The identification
of organic acids in the root exudates was done by artificial analysis of total
ion flow map and checked with the standard map of the NIST08 mass spectrometry
database; the determination of the root exudates was carried out by computer
search. The relative content of substances was calculated according to the peak
area (%) of the components that was detected in the chromatogram. In this
study, Excel WPS2016 and SPSS21 software were used for data processing and
statistical analysis. The Kernelized Stein Discrepancy (KSD) test was used for
multiple comparisons. The significant level α was 0.05, highly significant
level α was 0.01, and the scanning map was drawn by Origin 8.5.
Results
GC-MS scanning
pattern of root exudates from Pistia
In this study, only
the scanning atlas in four different periods when the P concentration was 1
mg/L is listed. The scanning results showed that the scanning spectra were
different in different periods; the number of characteristic peaks and the area
of high and low peaks were not consistent. Each characteristic peak represented
a compound, so it can be seen from the spectra that root exudation under P
stress is different in different time periods from Pistia
(Fig. 1).
Differences in organic acids in root exudates under
different P treatments and stress time
Changes in organic acid exudation under different P
treatments during the same stress period: Table 1 shows that, except for the difference of the
relative content of phthalic acid in the different concentrations on the 7th
day, there were significant differences (p<0.05) in the amount of organic
acid secreted in each period of time. When the P stress time was 7 days, the
exudation of sulfurous acid (SA) and phenyldicarboxylic
acid (PCA) increased first and then, decreased with the increasing P
concentration. The minimum exudation of SA was 1.06% when the concentration was
0 mg/L. When the maximum relative content was 1.39% in 5 mg/L, the content of
sulfite in 5 mg/L was significantly (P<0.05) higher than that in 0 and 0.2
mg/L and the relative content of sulfite in 20 mg/L was significantly
(P<0.05) lower than that in 5 mg/L. There was no significant (P>0.05)
difference in the relative content between other phosphorous sulphite concentrations. The excretion of PCA began to
decrease when concentration was 10 mg/L, and the minimum exudation was 0.28% at
0 mg/L concentration. The relative content of carbonated stone and BA increased
with the increasing P concentration and the relative content of carbonic acid
in 20 mg/L and 10 mg/L was significantly (P<0.05) higher than that of the
other concentrations.
There was no significant (P>0.05) difference in the exudation at 0, 1
and 0.2 mg/L concentration, However, when the concentration was 20 mg/L, BA
exudation increased significantly (p<0.05). When the P concentration was 0.2
mg/L, 1 mg/L, 5 mg/L and 10 mg/L, there was no significant (P>0.05)
difference in the exudation of BA, CA, while OA tended to decrease with the increasing
concentration. At most, when the concentration of CA was 10 mg/L, the amount of
CA secreted by 0.2 mg/L was significantly (P<0.05) different from that the P
concentrations of 1, 5 and 10 mg/L. There was no significant (P>0.05)
difference in CA exudation between 0, 0.2 and 20 mg/L, and between 1, 5 and 10
mg/L. OA exudation was mainly in the range of 120 mg/L P concentration, and
when the concentration of OA was 1 mg/L, the OA exudation decreased
significantly (p<0.05). When the total organic acid content of root exudates
was the highest at 1 mg/L under P stress, the minimum exudation was 0.2 mg/L
concentration. There was no significant (P>0.05) difference between 0, 5, 10
and 20 mg/L concentrations in organic acid exudation (Fig. 2).
When the time of P stress was 14 days, except for the minimum exudation
of SA that was 0.49% in 0.2 mg/L, the exudation of other organic acids and
total organic acids were least in the condition of no P stress. The contents of
carbonate, CA, and PA appeared at the same time. The minimum exudation of OA,
PCA, BA, and total organic acid was as follows: 1.24, 0.44, 5.84, 0.18, 0.41,
0.24, and 9.36%. When the highest concentration of phosphorous acid was 1 mg/L,
the exudation was significantly (P<0.05) higher than that at 0, 0.2, 5 and
20 mg/L (Fig. 3). The exudation of carbonates and CAs increased significantly
(P<0.05) at the concentration of 01 mg/L (p<0.05), the exudation of 15
mg/L decreased significantly, and the exudation increased in the range of 520
mg/L. When the concentration was 20 mg/L, the content of CA was 11.53 and 4.71%
at 1 mg/L (Table 1). The exudation of both PA and PCA increased 01 mg/L. The
15 mg/L exudation decreased significantly (p<0.05), and the 510 mg/L
exudation increased significantly (p<0.05). 1020 mg/L decreased
significantly (p<0.05) and the maximum exudation occurred at the time of the
10 mg/L P concentration. OA increased gradually with an increase in P
concentration; the relative content of 20 mg/L concentration was 1.055.
Furthermore, it was significantly (p<0.05) higher than that of other
concentrations. There was no significant (P>0.05) difference in the
exudation of BA between 0 and 2 mg/L, but there was a significant (p<0.05)
difference between other P
concentrations. There was no significant
(P>0.05) difference in the total organic acid exudation except for the 0.2
and 20 mg/L concentrations. The relative contents of total OAs in the other P
treatments were significantly (p<0.05) different, and the total OA
excretions most frequently occurred at a 10 mg/L concentration; the exudation
was as high as 52.62% (Table 1).
Table 1: Comparison of relative content of organic acids in root exudates under
different phosphorus concentration and time treatment (%)
|
Organic acid |
Concentration (mg/L) |
Duress time (day) |
|||
|
7 |
14 |
21 |
28 |
||
|
Sulfurous acid |
0 |
1.06±0.07 b-A |
1.01±0.04 b-A |
0.51±0.03 b-C |
0.89±0.09 d-B |
|
0.2 |
1.19±0.16 b-A |
0.49±0.04 c-BC |
0.36±0.06 c-C |
0.56±0.09 d-B |
|
|
1 |
1.32±0.08 ab-A |
1.19±0.04 a-B |
0.42±0.05 bc-D |
0.99±0.05 c-C |
|
|
5 |
1.39±0.15 a-B |
1.01±0.12 b-C |
0.55±0.08 ab-D |
2.00±0.10 a-A |
|
|
10 |
1.35±0.04 ab-B |
1.11±0.11 a-C |
0.60±0.06 ab-D |
1.51±0.09 b-A |
|
|
20 |
1.11±0.11 b-A |
0.52±0.04 c-B |
0.65±0.07 a-B |
1.06±0.11 c-A |
|
|
Phenol |
0 |
0.21±0.02 d-C |
1.24±0.11 f-B |
2.05±0.06 c-A |
0.08±0.01 e-D |
|
0.2 |
0.30±0.04 cd-C |
1.58±0.08 e-B |
10.26±0.06 ab-A |
0.21±0.04 d-C |
|
|
1 |
0.34±0.04 cd-C |
5.57±0.07 c-B |
11.21±0.74 a-A |
0.24±0.04 d-C |
|
|
5 |
0.43±0.12 c-B |
4.00±0.23 d-A |
10.99±0.46 ab-A |
0.35±0.06 c-B |
|
|
10 |
0.61±0.06 b-B |
10.20±0.18 b-A |
10.09±1.13 b-A |
0.48±0.04 b-B |
|
|
20 |
0.84±0.13 a-C |
11.53±0.15 a-A |
10.19±0.15 ab-B |
0.58±0.06 a-D |
|
|
Carboxylic Acid |
0 |
1.12±0.11 ab-A |
0.44±0.04 d-C |
0.32±0.03 cd-D |
0.68±0.04 d-B |
|
0.2 |
1.03±0.07 b-B |
0.58±0.05 d-C |
0.30±0.03 d-D |
1.61±0.14 b-A |
|
|
1 |
1.16±0.03 a-C |
4.71±0.03 a-A |
0.40±0.04 c-D |
1.49±0.11 b-B |
|
|
5 |
1.20±0.05 a-A |
0.91±0.14 c-B |
0.69±0.08 b-C |
1.18±0.09 c-A |
|
|
10 |
1.21±0.03 a-B |
1.01±0.13 c-C |
1.26±0.04 a-B |
2.16±0.07 a-A |
|
|
20 |
1.14±0.13 ab-B |
1.20±0.11 b-B |
1.34±0.08 a-B |
2.20±0.11 a-A |
|
|
Phthalic acid |
0 |
3.22±0.04 a-C |
5.84±0.11 e-B |
6.11±0.09 c-A |
0.43±0.08 d-D |
|
0.2 |
3.21±0.22 a-B |
27.60±1.38 b-A |
28.74±1.02 a-A |
0.49±0.03 d-C |
|
|
1 |
3.37±0.18 a-C |
28.14±1.05 b-B |
29.68±0.93 a-A |
0.88±0.09 c-D |
|
|
5 |
3.43±0.06 a-C |
9.81±0.17 d-B |
28.25±1.04 b-A |
2.12±0.11 b-D |
|
|
10 |
3.41±0.11 a-C |
32.10±0.51 a-A |
28.11±0.80 b-B |
3.66±0.22 a-C |
|
|
20 |
3.25±0.14 a-C |
23.01±0.86 c-B |
27.58±0.46 b-A |
2.06±0.04 b-D |
|
|
Oxalic acid |
0 |
0.69±0.08 a-A |
0.18±0.03 c-B |
0.10±0.02 d-B |
0.21±0.09 d-B |
|
0.2 |
0.45±0.04 c-B |
0.21±0.03 c-C |
0.15±0.04 d-C |
1.04±0.08 a-A |
|
|
1 |
0.76±0.07 a-A |
0.25±0.03 c-C |
0.27±0.05 c-B |
0.78±0.12 b-A |
|
|
5 |
0.56±0.06 b-B |
0.32±0.04 bc-C |
0.42±0.04 b-C |
0.79±0.09 b-A |
|
|
10 |
0.42±0.06 c-B |
0.40±0.11 b-B |
0.43±0.04 b-B |
0.61±0.07 c-A |
|
|
20 |
0.39±0.05 c-B |
1.05±0.06 a-A |
0.62±0.05 a-B |
0.94±0.08 ab-A |
|
|
Benzene dicarboxylic acid |
0 |
0.28±0.08 c-B |
0.41±0.04 e-A |
0.32±0.03 d-AB |
0.11±0.02 c-C |
|
0.2 |
0.31±0.03 c-D |
2.59±0.08 b-A |
2.39±0.06 c-B |
0.57±0.07 b-C |
|
|
1 |
0.53±0.05 b-C |
2.51±0.10 b-B |
3.09±0.10 a-A |
0.64±0.08 ab-C |
|
|
5 |
0.58±0.03 b-C |
1.96±0.04 c-B |
3.02±0.15 a-A |
0.70±0.05 a-C |
|
|
10 |
0.74±0.03 a-B |
3.21±0.11 a-A |
3.07±0.16 a-A |
0.62±0.11 ab-B |
|
|
20 |
0.61±0.03 b-B |
0.57±0.09 d-B |
2.65±0.12 b-A |
0.55±0.07 b-B |
|
|
Benzoic Acid |
0 |
0.15±0.02 d-B |
0.24±0.08 e-A |
0.15±0.05 d-B |
0.13±0.02 d-B |
|
0.2 |
0.33±0.03 c-D |
5.55±0.10 a-A |
0.92±0.04 bc-B |
0.52±0.05 c-C |
|
|
1 |
0.38±0.03 bc-C |
1.04±0.07 c-A |
0.81±0.08 c-B |
1.09±0.02 a-A |
|
|
5 |
0.40±0.03 b-D |
2.28±0.07 b-A |
1.34±0.10 a-B |
1.01±0.21 a-C |
|
|
10 |
0.32±0.03 c-D |
0.57±0.04 d-A |
0.94±0.06 b-B |
0.75±0.13 b-C |
|
|
20 |
0.60±0.04 a-B |
0.49±0.10 d-B |
1.34±0.07 a-A |
0.53±0.08 c-B |
|
|
Total organic acid |
0 |
8.41±0.29 b-B |
9.36±0.19 e-A |
9.57±0.17 c-A |
2.53±0.20 f-C |
|
0.2 |
7.08±0.48 c-C |
43.10±1.48 c-B |
54.71±1.09 a-A |
6.15±0.35 d-C |
|
|
1 |
9.36±0.24 a-C |
45.68±1.14 b-B |
49.78±1.36 b-A |
8.69±0.32 c-C |
|
|
5 |
8.20±0.31 b-D |
25.02±0.71 d-B |
48.91±1.34 b-A |
9.82±0.37 b-C |
|
|
10 |
8.71±0.16 b-D |
52.62±0.71 a-A |
49.40±1.29 b-B |
11.37±0.42 a-C |
|
|
20 |
8.95±0.10 ab-D |
41.78±0.95 c-B |
48.97±0.45 b-A |
10.20±0.23 b-C |
|
Note: The figures
in the table were all repeated mean ±standard errors in three groups. The
lowercase letters after the numbers in the table indicate the differences
(longitudinally) in exudations between different concentrations of phosphorus
in the same organic acids at the same time of stress, the uppercase letters
represent the differences (horizontal) between different stress time treatments
of the same organic acid at the same phosphorus concentration. In the same
column (or row), the same letter means that the difference is not significant
(p>0.05), and the difference between the letters is significant (p<0.05)
When the P stress lasted for 21 days, except for CA and SA, a least
amount of exudates appeared at the P concentration of 0.2 mg/L, lithic acid,
PA, OA, and PCA. Least amount of BA and total organic acid appeared when the
concentration was 0 mg/L and the relative contents were: 2.05, 6.11, 0.10,
0.32, 0.15 and 9.57%. In the concentration range of 01 mg/L, the exudation of
sulfite was significantly (P<0.05) decreased, the amount of 10 mg/L was less
than that of 5 mg/L, and the exudation of 5 mg/L concentration was
significantly (p<0.05) higher than that at 1 mg/L (Fig. 4). The exudation of
CA and PCA increased significantly (p< 0.05) at the concentration of 01
mg/L. The exudation of 5 mg/L was significantly (P<0.05) lower than that of
the concentration of 1 mg/L. The concentration of 10 mg/L was lower than that
of 5 mg/L (p<0.05). The content of 20 mg/L was higher than that of 10 mg/L.
The amount of BCA (10 mg/L) was higher than that of 5 mg/L and the amount of 20
mg/L was lower than that of 10 mg/L. When the maximum exudation amount of the
stony carbonate and PCA was both 1 mg/L concentration, it was 11.21 and 3.09%,
respectively (Table 1).
%201340-1350_files/image002.png)
Fig. 1: Scanning map of root exudates of P
stratiotes at different times of 1 mg/L
phosphorus concentration
%201340-1350_files/image004.png)
Fig. 2: Correlation analysis between phosphorus concentration and organic acid
at 7 days of stress duration
%201340-1350_files/image006.png)
Fig. 3: Correlation analysis between phosphorus concentration and organic acid
at 14 days of stress duration
%201340-1350_files/image008.png)
Fig. 4: Correlation analysis between phosphorus concentration and organic acid
at 21 days of stress duration
OA excretion increased gradually with the increase of the
concentration. In the range concentration of 0.220 mg/L, the exudation of OA
increased significantly (P<0.05) except for 5 mg/L and the maximum exudation
was 0.62. There was no significant (P>0.05) difference in the exudation of
PA between 0.2 and 1 mg/L, the three concentrations of 5, 10 and 20 mg/L and
the highest relative content was 29.68% when the concentration of 1 mg/L was
the highest (Table 1). The amount of CA secreted from 0.220 mg/L increased
gradually with the increase of P concentration, and the maximum exudation of CA
was 1.34 with 20 mg/L. The highest level of BA exudation was 1.34, and the
difference of BA exudation was significant (p<0.05) at the concentrations of
0, 1, 5, and 10 mg/L. The total OA content of 0.2 mg/L was significantly
(P<0.05) higher than that of 0, 5, 10, and 20 mg/L (p<0.05), which was
lower than that of 0.2 mg/L at 54.71% (Table 1).
When the P stress lasted for 28 days, except for the SA exudation of at
least 0.56% that appeared in the concentration in 0.2 mg/L, the minimum content
of other acids was 0.08%; carbonic acid, 0.68%; CA, 0.43%; PA, OA 0.21%; PCA
0.11%; BA 0.13%; and total organic acid 2.53% in the treatment of concentration
in 0 mg/L. At the concentration of 5 mg/L, the exudation of sulfite was the
largest (2%). The exudation of sulfite increased significantly (P<0.05) in
the 0.25 mg/L region, and decreased significantly (p<0.05) in the 520 mg/L
region. With the increase of the P concentration, the exudation of the stone carbonate
increased significantly (p<0.05), which accounted for 0.58% at the
concentration of 20 mg/L (Table 1).
With the increase of P concentration, the exudation of CA, except for
0.2, 1, 10 and 20 mg/L, increased significantly (p< 0.05). The relative
content of CA was 2.2% when the concentration was 20 mg/L. When the P
concentration was 10 mg/L, PA had the highest exudation, accounting for 3.66%.
The exudation of 1020 mg/L decreased significantly and 110 mg/L increased
significantly (p<0.05). The relative content of OA decreased significantly
(p<0.05) from 0.2 and 1 mg/L as well as 5 and 10 mg/L and increased
significantly (p<0.05) from 0 and 0.2 mg/L as well as 10 and 20 mg/L
(p<0.05). The exudation of PCA increased significantly (p<0.05) in the range
of 05 mg/L concentration (p<0.05), while that of the 520 mg/L decreased
gradually. The highest amount was 0.7% under the treatment of the 5 mg/L
concentration. The 01 mg/L range of BA increased significantly (p<0.05).
The content of 120 mg/L BA gradually decreased with the increase of the P
concentration. The relative content of BA secreted at the 1 mg/L concentration
was 1.09%. The exudation of the total organic acids increased significantly
(p<0.05) in the range of 010 mg/L (p<0.05) and the exudation from 1020
mg/L decreased significantly (p<0.05). Under the treatment of concentration
10 mg/L, the total organic acids secreted the most at 11.37% (Table 1).
Changes of organic acid exudation in different time
periods under the same P stress treatments: When the concentration was 0 mg/L, the organic acids
that were secreted the most at 7 days of stress were sulfite (1.06%), CA
(1.12%), and OA (0.69%). The highest exudations of PCA and BA were 0.41% and
0.24%, respectively, at 14 days of stress and the highest exudation of carbonic
acid was 2.05 and 6.11 for PA, and 9.57% for the total organic acids at 21 days
of stress. SA, OA, and CA had the lowest exudations of 0.51, 0.1, and 0.32%,
respectively, at 21 days after stress. The total organic acid exudations of carbonated
acid, PA, BA, and total organic acid were the lowest in the four stress periods
at 28 days and the relative content was 0.08, 0.43, 0.11, 0.13, and 2.53%
respectively. The amount of sulfite secreted on day 14 was significantly
(p<0.05) lower than that of day 7 (p<0.05) and the amount at day 21 was
significantly (p<0.05) lower than that at day 14. The relative contents of
carbonic acid and PA were significantly (p<0.05) higher at 21 days than in 7
days and 28 days. CA exudation decreased significantly (p<0.05) with the
increase of stress duration. OA excretion at 7 days was significantly
(p<0.05) higher than that in the other three periods (p<0.05). The
exudation of PCA on day 28 was significantly (p<0.05) lower than those of
days 14 and 21 and the exudation of BA on day 14 was significantly (p<0.05)
higher than that at the other three periods. The total organic acid excretion
at 7 days was significantly (p<0.05) higher than that at 14 and 21 days, and
the total organic acid content decreased significantly (p<0.05) from 21 to
28 days (Table 1).
When the concentration was 0.2 mg/L, the exudation of carbonated acid,
PA, and total organic acid was the highest at 21 days, the relative contents
were 10.26, 28.74, and 54.71%, respectively, and the smallest one was at 28
days. The exudation values were 0.21, 0.49, and 6.15%, the highest exudation of
PCA and BA was at 14 days and the least at 7 days, and OA exudation was highest
at 28 days and least at 21 days. The relative content of sulfite was a maximum
of 1.19% at 7 days and 0.36% at the minimum at 21 days. The exudation of SA and
CA decreased significantly (p<0.05) at 21 days and 7 days, respectively, and
increased significantly (p<0.05) at 28 days compared with 21 days. The
exudation of PCA and BA increased significantly (p<0.05) from 7 to 14 days
and decreased significantly (p<0.05) at 28 days compared to 21 days
(p<0.05). The carbonate and total organic acids increased significantly
(p<0.05) from 7 days to 21 days and the exudation at 28 days was significantly
higher than that of the 21 day (p<0.05). The exudation of OA decreased
significantly (p<0.05) from 7 days to 21 days, while that of the PA
increased significantly (p<0.05) between 7 days and 21 days (Table 1).
When the concentration was 1 mg/L, the exudation of carbonic acid, PA,
and total organic acids increased significantly (P<0.05) from 7 days to 21
days and the exudation of 28 days was significantly (p<0.05) lower than that
of 21 days (P<0.05). The exudation of all four acids was the highest at 21 days
and the lowest at 28 days. The exudation of BA and CA was significantly
(p<0.05) higher at 14 days than that at 7 days and higher at 28 days higher
than at days 21 and 14 (P<0.05). However, CA exudation was greatest at 14
days and the least at 21 days. The greatest BA exudation was at 28 days and the
least at 7 days. OA decreased significantly (p<0.05) from 7 days to 21 days
(P<0.05) and the exudation was the most at 28 days and the least at 14 days
(Table 1).
When the concentration was 5 mg/L, the relative contents of PA,
carbonic acid, and total organic acids increased significantly (P<0.05) from
7 days to 21 days; at least 7 days for PA and total organic acid and 28 days
for PA and carbonic acid. The relative contents of SA and CA decreased significantly
(P<0.05) from 7 days to 21 days. The exudation of both acids was least 28
days compared with that of the 21st day. The maximum exudations of
SA and CA were at 28 and 7 days, respectively. The maximum relative content of
OA was found at 28 days and the least at 14 days; there was no significant
(p>0.05) difference in this value between days 14 and 21. The exudation of
BA was the most at 14 days and the least at 7 days. The exudations of days 14,
21, and 28 days were significantly (P<0.05) higher than that of the P stress
on day 7 (Table 1).
When the concentration was 10 mg/L, the exudations of PA, carbonic acid,
PCA, and total organic acids at 14 days were significantly (P<0.05) higher
than those at day 7. There was no significant (p>0.05) difference between
lithic acid and PCA on the 14th day and 21st day. The
maximum exudations of stone carbonate and PCA were both at 14 days and the
minimum exudations were at 28 days. The maximum exudations of PA and total
organic acids were at 14 days. At 7 days, the relative contents of the two
acids at 21 days were significantly (P<0.05) lower than those at 14 days. On
the 28th day, the amounts of SA, CA, and OA were the highest, while
the lowest exudations of CA and OA appeared at 14 days. The exudation of SA was
the least at 21 days as it decreased significantly (P<0.05) from day 7 to
day 21. The amount of OA secreted at day 28 was significantly (P<0.05)
higher than that on day 21 but there was no significant (P>0.05) difference
in the exudation of OA at days 7, 14, and 21. The exudation of BA increased
significantly from 7 days to 21 days (P<0.05) and was significantly
(P<0.05) lower at 28 days than that on day 21 (Table 1).
At the concentration of 20 mg/L, the highest exudation of organic acids
for PA, PCA, BA, and total organic acids were as follows: For the total organic
acid and PA, this occurred from days 721. The exudations of the two acids at
28 days was significantly (P<0.05) less than at 21 days, the total organic
acid content was the least 7 days and at 28 days for PA. The exudation of PA
and PCA were the least 14 days. There was no significant (P>0.05) difference
between days 7 and 14 and the exudation of 21 days was significantly
(P<0.05) larger than that of days 7 and 14. The maximum exudation of stony
carbonate and OA were both at 14 days. The exudation of both acids on 14 days
was significantly (P<0.05) higher than that at 7 days and the exudation at
21 days was significantly (P<0.05) lower than that at 14 days. The minimum
exudation of stone carbonate was at 28 days; the same for OA at 7 days of
stress. The exudation of SA at 7 days was significantly (P<0.05) higher than
that of the other three periods and there was no significant (P<0.05)
difference between days 14 and 21; the minimum exudation appeared at 14 days.
The amount of CA secreted at 28 days was significantly (P<0.05) higher than
that on days 7, 14, and 21. There was no significant (P>0.05) difference in
the relative content of CA between days 7, 14, and 21; the minimum amount of
exudation occurred at day 7 (Table 1).
Correlation analysis between P concentration and
relative organic acids content of roots
%201340-1350_files/image010.png)
Fig. 5: Correlation analysis between phosphorus concentration and organic acid
at 28 days of stress duration
Through correlation
analysis, a significant (P<0.05) positive correlation was found between the
concentrations of carbonic acid, PCA, and BA in root
exudates measured at 7 days (Fig. 2). The results showed that with the increase
of the P stress concentration (r=0.897, P=0.000; r=0.650, P=0.004; and r=0.773,
P=0.000), the exudation of carbonic acid, PCA, and BA also increased. OA had a
highly significant (P<0.01) negative correlation (r = -0.643) with P stress
concentration and OA decreased significantly (P<0.05) with the increase of concentration.
There was no significant (P>0.05) correlation between the contents of SA,
CA, and total organic acid at 7 days of culture and the concentration of
stress. When the stress duration was 14 days (Fig. 3), only OA and carbonated
acid, secreted from 7 acids, as well as total organic acids were positively
correlated with the stress concentration (r=0.950, P=0.000; r=0.897, P=0.000),
there was no significant (p>0.05) correlation between the concentrations of
phosphorous acid, sulfuric acid, CA, PA, BA, and total organic acid. The
exudation of SA, CA, OA, and BA was positively (P<0.01) correlated with the
stress concentration at 21 days (Fig. 4). There was no significant (p>0.05)
correlation between the content of PA, PCA, carbonate, and total organic acid
and P concentration. When the stress duration was 28 days (Fig. 5), the
exudation of CA, carbonate, PA, and total organic acid was positively
correlated with the stress concentration (r=0.740, P=0.000; r=0.909, P=0.000;
r=0.633, P=0.005; and r=0.642, P=0.004). There was no significant (P>0.05)
correlation between the concentration and SA, OA, PCA, or BA.
Discussion
During the entire plant
growth process, the root system is an important organ for the communication
between the plant and the soil environment; it mainly relies on the root system
to absorb the nutrients needed for the growth of the plant from the outside
environment. At the same time, roots also secrete a large amount of
organic-root exudates into growth media. Plants respond to environmental stress
by adjusting the types and contents of root exudates. The diversity of root
exudates is the embodiment of adaptation by different types of plants to their
living environment (Song et al. 2017; 2018). The organic
acids in root exudates are one of the main adaptive mechanisms of plant roots
under environmental nutrient stress. Plant roots change the pH value of the
rhizosphere by secreting low molecular weight organic acids. Insoluble P in the
surrounding environment of roots is activated to improve the utilization
efficiency of nutrient components in plants (Qin et al. 2011; Chen et al.
2017). In wetland ecosystems, the water quality eutrophication caused by P is
becoming more and more serious; therefore, the study of the effects of P stress
on plant root exudates to understand the specific changes of organic acids in
root exudates is important. It is important to better understand the adaptation
mechanism of plant roots to nutrient stress. At the same time, the secretory
characteristics of specific plants in a specific environment (wetland,
woodland, grassland, etc.) can provide basic reference materials for the
rhizosphere measurements of environmental pollution control.
In this study,
with the increase of culture time, there were significant differences in the
amount of organic acid exudates in each time period and the relative contents
of organic acids secreted by large root exudates under different P
concentrations were also discernible. When the P concentration was 20 mg/L, the
organic acids content secreted in each time period decreased, as P is the main
nutrient element necessary for plant growth. The P concentration of 20 mg/L for
wetland plants has exceeded the amount needed by plants, which may destroy
plant root tissue and lead to a decline in plant root exudation ability. At 7
and 28 days of culture, most of the acids showed a decrease in exudation, which
may be due to the fact that at 7 days, the plants did not fully adapt to the P
stress and the exudation was not very stable. After approximately one month,
some plants began to senesce, resulting in a decrease in exudation; the
specific reasons for this need to be further verified. PA and carbonic acid are
the main organic acids exuded by root systems at all culture levels. Previous
studies have shown that many non-mycorrhizal plants, such as white lupin and
rape, secrete a large amount of organic acids into the root environment
under P deficiency stress (Tian et al.
2000). However, the main organic acids secreted by different plant roots are
not consistent. Such is the case of Brassica napus,
which secretes a large amount of organic malic acid in the absence of P (Duan 2003). Alfalfa secretes citric acid, malic
acid, and succinic acid under P deficiency stress. Under P stress, oxalic acid,
citric acid, and malic acid were the main organic acids in the root exudates of
Gymnaceae and Zhuge
(Zhao and Wu 2014). The content of organic acid in soybean root
exudates increased under P stress and the relative content of malic acid was
the highest (Zhang et al. 2011). It
can be concluded that terrestrial plants mainly adjust malic acid and citric
acid to adapt to environmental changes under the stress of nutrient elements.
The reason for this is that each plant has its unique characteristics, for
example, the P stratiotes is a typical wetland phytoplankton and the
growth environment of terrestrial plants is different from that of terrestrial
plants. The concentrations of P stress were P deficiency as well as low and
high concentrations of P; the relative content of PA was the highest under
different culture times, followed by carbonation. This indicates that the high
exudation of PA is the adaptation mechanism of phytoplankton in wetlands under
external nutrient stress.
Conclusion
Under P stress for
7 days, the exudation of SA, carbonic acid, BCA, and BA was the least in the
condition of no P stress. Carbonate, PCA, BA increased substantially with the
increase of stress concentration; OA significantly decreased with increasing
stress. At 14, 21 and 28 days, the exudation of carbonate, PA, OA, PCA, BA, and
total organic acid were the lowest under the condition of no P stress. On the
14th day, the exudation of OA and carbonic acid increased significantly with
the increase of stress concentration; at 21 days, the exudation of SA, CA, OA,
and BA increased significantly with the increase of stress concentration. At 28
days, the exudation of CA, carbonate, PA, and total organic acid increased with
the increase of stress concentration. The results showed that the exudation of
organic acids was closely related to the concentration of stress and the time
of treatment. P stress could increase the exudation of organic acids in plant
roots. PA is the main organic acid exuded by root systems at all culture
levels, indicating that the major regulation of PA exudation by large roots
under P stress is an important mechanism of active adaptation to the
environment.
Acknowledgements
National Science Foundation of China (No. 31760149;31860235), Fund to Key
Research and Development Program of Yunnan Provincial (No.2018BB018), Fund
Project to The forestry science and technology innovation platform project of
The State Forestry Administration (2018-LYPT-DW-162; 2018-YN-12).
Author Contributions
DX and ZJC planned the experiments, made the
write up and statistically analyzed the data, ZYY and
DX interpreted the result, statistically analyzed the data and made
illustrations.
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