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j.flora.2018.08.008

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Flora 248 (2018) 1–9
Contents lists available at ScienceDirect
Flora
journal homepage: www.elsevier.com/locate/flora
Drought stress effects on gas exchange and water relations of the invasive
weed Chromolaena odorata
T
⁎
G. Naidooa, , K.K. Naidoob
a
b
University of KwaZulu-Natal, School of Life Sciences, Private Bag X54001, Westville, 4000, South Africa
Mangosuthu University of Technology, Department of Nature Conservation, P.O. Box 12363, Jacobs, 4026, South Africa
A R T I C LE I N FO
A B S T R A C T
Edited by Hermann Heilmeier
In this study, the effects of drought stress on gas exchange and water relations of the invasive weed Chromolaena
odorata were investigated. Drought stress was induced in potted plants by withholding water. The responses of
drought stressed plants (DS) were then compared to well-watered (WW) controls. Measurements of diurnal gas
exchange and chlorophyll fluorescence parameters were taken initially and every two days after water was
withheld to day 10. Maximum CO2 uptake in WW plants ranged from 10.5 to 12.8 μmol m−2 s−1 while in the DS
treatment values decreased from 9.3 μmol m−2 s−1 on day 0 to 3.95 μmol m−2 s−1 on day 10. Trends in diurnal
leaf conductance and transpiration in DS and WW plants were similar to those for CO2 uptake. In WW plants,
maximum quantum yield (Fv/Fm) of Photosystem II (PS II) was high, while in the DS treatment, it decreased
significantly from day 2 to day 10. Leaf water potential (Ψ) was high in WW plants and significantly lower in DS
plants. Water use efficiency (WUE) and proline concentrations were low in WW plants and increased significantly with drought stress. The photosynthetic rates in C. odorata suggest a high capacity for light utilization
and water absorption and transport to leaves. Early stomatal closure and significant reductions in stomatal
conductance suggest that C. odorata may be considered as a drought-avoider species which is sensitive to water
deficit and responds by rapid wilting and leaf abscission. Our study provides empirical evidence for links between plant functional traits and the ecological success of an important invasive species.
Keywords:
Photosynthesis
Chlorophyll fluorescence
Water potential
Proline
Water use efficiency
Siam weed
1. Introduction
Biological invasions are increasing globally and causing significant
impacts on ecosystem functioning (Chen et al., 2016; Liu et al., 2017).
Single invasive species can alter ecosystem processes and adversely
affect the environment (Ehrenfeld, 2010). Chromolaena odorata (L.) R.
M. King and H. Robinson (Asteraceae) is a worldwide invasive C3
perennial shrub that is also known as Triffid or Siam weed (te Beest
et al., 2009). The species is native to Mexico, the West Indies and tropical South America and was introduced into South Africa from the
northern Caribbean (Goodall and Erasmus, 1996; Von Senger et al.,
2002). The species has a high growth rate, enormous production of
small, wind dispersed seeds and plasticity in biomass allocation (te
Beest et al., 2009, 2013). These traits contribute to its rapid expansion
in southern and eastern Africa (Kriticos et al., 2005; Raimundo et al.,
2007; te Beest et al., 2013). The weed reduces biodiversity and disrupts
the structure and functioning of ecosystems (Foxcroft et al., 2010). Due
to its high competitiveness and ability to establish rapidly, this species
has invaded large areas in Africa (te Beest et al., 2015) and southern
⁎
China and caused serious damage to pastures, crops and plantations
(Liu et al., 2017).
Climatic niche models suggest that drought and cold stress are the
main predictors of C. odorata distribution (Kriticos et al., 2005;
Raimundo et al., 2007). Chromolaena thrives in well-drained soils
(Mandal and Joshi, 2014) and under high light conditions (Zhang and
Wen, 2009). Despite increasing interest in the spread of this species, its
responses to drought stress are poorly understood. Plant responses to
drought have been associated with morphological and physiological
traits that aid survival (Vilagrosa et al., 2014; Chirino et al., 2017).
Monitoring changes in photosynthesis, chlorophyll fluorescence and
water relations parameters under drought stress may provide clues to
the distribution and spread of C. odorata (Vilagrosa et al., 2014; Liu
et al., 2017). Species with less tolerance to drought conditions show
major reductions in stomatal conductance to reduce water loss
(Vilagrosa et al., 2010; McDowell, 2011). Drought-avoider species,
which are very sensitive to water deficit, respond by dropping their
leaves during stress (Esperón-Rodríguez and Barradas, 2015). Species
that present low sensitivity to soil drought conditions with delayed
Corresponding author.
E-mail address: naidoogn@ukzn.ac.za (G. Naidoo).
https://doi.org/10.1016/j.flora.2018.08.008
Received 27 August 2017; Received in revised form 6 August 2018; Accepted 11 August 2018
Available online 15 August 2018
0367-2530/ © 2018 Elsevier GmbH. All rights reserved.
Flora 248 (2018) 1–9
G. Naidoo, K.K. Naidoo
Table 1
Durban climate from 2000 to 2015 (www.durban.climatetemps.com).
Climate variable
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Mean
Average max
temperature (oC)
Average temperature (oC)
Average min
temperature (oC)
Average precipitation (mm)
Number of wet days
Percentage of sunny (cloudy) daylight hours
22
22
23
23
25
26
27
28
27
26
24
22
25
17
11
17
12
19
15
20
17
22
18
23
19
24
20
25
21
24
20
22
18
19
14
17
11
20
16
39
3
67
(33)
70
62
4
63
(37)
70
73
6
49
(51)
70
9
10
41
(59)
75
10
11
42
(58)
80
102
12
43
(57)
80
134
11
48
(52)
80
113
9
51
(49)
80
126
9
51
(49)
80
73
7
60
(40)
80
59
4
70
(30)
80
28
3
68
(32)
75
828
89
53
(47)
76.7
Relative humidity (%)
2. Materials and methods
2.1. Plant material and experimental design
The study was undertaken in the University of KwaZulu-Natal
Conservancy (29° 49′ S, 30° 56′ E), Durban, South Africa. Seedlings of C.
odorata growing in a grassland were selected for experiments. The climate is subtropical with hot, humid summers and warm dry winters
(Table 1). The mean temperature is 25 °C in summer and 17 °C in
winter. The mean annual temperature is 20 °C, relative humidity is 77%
and the mean annual rainfall 828 mm. The dry season (winter) is from
June to August with June being the driest month.
Thirty uniform, young seedlings of C. odorata were selected and
transplanted in 10 L plastic pots filled with a loamy soil, consisting of
equal quantities of sand, silt and clay. Plants were collected intact with
soil after a rain when the soil was wet. Rooting depth of plants was less
than 30 cm and all roots were recovered intact. All plants were vegetative, unbranched and healthy. Plants were collected in early March
2015 and acclimated for three months till the end of May. The pots
were maintained in the open under natural conditions (Table 1). Plants
were watered daily until water drained from the pots (field capacity).
At the end of three months (June) plants were assigned randomly into
two groups, well-watered (WW) and drought-stressed (DS). The mean
height of plants at the commencement of treatments was 45 cm. In the
WW treatment, plants were watered daily after sunset until water
drained freely from the pots. The DS plants were not watered for the
duration of the study. The surface of the soil in each pot was covered
with plastic film and aluminium foil to reduce evaporation, to minimize
temperature fluctuations and to prevent rain from entering the pots.
Gas exchange measurements were taken initially and every two days
after water was withheld to day 10 when further measurements were
not possible due to severe wilting. During this drought period of 10
days, average day and night temperatures were 19 °C and 11 °C respectively, average relative humidity 66%, mean maximum photosynthetic photon flux density (PPFD) 1538 μmol m−2 s−1 and there was
no rain. At the end of the 10-day drought stress period, plants were rewatered daily for two weeks and monitored for re-sprouting and stem
regeneration.
Fig. 1. Changes in gravimetric soil moisture in pots of well-watered (WW) and
drought stressed (DS) Chromolaena odorata plants over 10 days. Vertical lines
represent standard error (n = 5). Bars with different letters are significantly
different at p ≤ 0.05 using Tukey-Kramer multiple comparisons test.
Fig. 2. Changes in midday minimum leaf water potential in well-watered (WW)
and drought stressed (DS) Chromolaena odorata over 10 days (n = 2). Other
details as for Fig. 1.
stomatal closure under low soil water conditions can be considered
drought-tolerant species (Levitt, 1980). These species have low gas
exchange rates, which allow water to be maintained longer in the soil
thereby prolonging water availability. The role of photosynthesis-related traits in invasiveness of introduced plant species is still not well
elucidated (te Beest et al., 2015; Liu et al., 2017). The objective of this
work was to determine changes in the physiological traits of C. odorata
in response to depleting soil moisture.
2.2. Gas exchange
Gas exchange measurements were taken at hourly intervals on the
abaxial surface of the most recently expanded mature leaves, immediately below the shoot tip. Measurements commenced at 08h00 and
terminated at 16h00. Leaves of five separate plants per treatment were
monitored on each day and measurements taken with a portable photosynthesis system (Li-6400, Li-Cor, Nebraska, USA). Measurements
were taken on the same five plants throughout the experiment.
Individual leaves were supported and held in position in the assimilation chamber with the aid of an extendable, ultrafluid action tripod
(Maxtec – model 1493). Leaf temperature and PPFD were measured
2
Flora 248 (2018) 1–9
G. Naidoo, K.K. Naidoo
Table 2
Effect of duration of drought stress on ecophysiological parameters in Chromolaena odorata. Maximum values per day ± standard error are given, except for
water potential where minimum values are given. Values with different letters are significantly different at p ≤ 0.05 using Tukey-Kramer multiple comparisons test
after repeated measures ANOVA, n = 5 for all parameters except Ψ where n = 2. Abbreviations: A = CO2 uptake, g = leaf conductance, E = transpiration,
WUE = water use efficiency, ΦPSII = quantum yield of PSII, ETR = electron transport rate, Fv/Fm = ratio of variable to maximal fluorescence (maximum quantum
yield of PSII), Ψ = leaf water potential.
Parameter
Duration of stress (days)
−2 −1
A (μmol m s )
g (mol m−2s−1)
E (mmol m−2s−1)
WUE (μmol CO2/mmol−1 H2O)
ΦPSII
ETR (μmol m−2s−1)
Fv/Fm
Ψ (MPa)
Soil moisture (%)
0
2
4
6
8
10
9.30 ± 0.25a
0.12 ± 0.00a
2.88 ± 0.12a
3.23 ± 0.32d
0.65 ± 0.00a
110.27 ± 4.01a
0.83 ± 0.00a
−0.26 ± 0.02a
43 ± 0.57a
9.11 ± 0.28b
0.11 ± 0.00a
2.52 ± 0.07b
3. 62 ± 0.25d
0.62 ± 0.00a
88.20 ± 3.28b
0.81 ± 0.00a
−0.68 ± 0.07b
33.67 ± 0.88b
7.81 ± 0.23c
0.05 ± 0.00b
0.91 ± 0.05c
8. 58 ± 0.79a
0.57 ± 0.01b
80.76 ± 2.01b
0.77 ± 0.00b
−0.88 ± 0.07c
27.33 ± 0.33c
6.46 ± 0.27d
0.03 ± 0.00c
0.75 ± 0.06d
8.61 ± 0.87a
0.47 ± 0.00c
67.08 ± 2.24c
0.61 ± 0.00c
−1.28 ± 0.07d
19 ± 0.57d
5.12 ± 0.13e
0.03 ± 0.00c
0.68 ± 0.03e
7.53 ± 0.82b
0.36 ± 0.00d
57.50 ± 2.11d
0.44 ± 0.00d
−1.31 ± 0.01e
13 ± 0.57e
3.95 ± 0.12f
0.02 ± 0.00c
0.66 ± 0.04e
5.98 ± 1.32c
0.31 ± 0.01e
36.44 ± 1.40e
0.30 ± 0.00e
−1.86 ± 0.03f
7.66 ± 0.33f
2.5. Leaf water potential
with a thermocouple and a silicon photodiode respectively, both housed
within the 0.25 L cuvette of the Li-6400. Gas exchange measurements
were taken under ambient conditions of air temperature (mean 18 °C),
incident PPFD (mean maximum 1538 μmol m−2 s−1), CO2 concentration (400 μl l−1) and vapour pressure deficit (VPD, 0.85 kPa). Air
temperature and VPD values within the cuvette were maintained close
to those of the ambient air. The leaf chamber was held at right angles to
incident radiation to prevent shading inside the cuvette.
Leaf water potential (Ψ) was measured at midday with a Scholander
pressure chamber (Soil Moisture Equipment Corp., Santa Barbara,
USA). Measurements were made on plants adjacent to those used for
gas exchange. Selected twigs from two different plants were fitted
snugly into the rubber stopper. Water loss after excision was kept to a
minimum by covering the healthy twigs with aluminium foil. In addition, water loss in the pressure chamber itself was reduced by lining the
chamber with moistened filter paper. Each measurement represented
the average of two separate plants. In the results, mean midday
minimum Ψ is presented.
2.3. Chlorophyll fluorescence
Diurnal changes in chlorophyll fluorescence parameters were made
on the same leaves that were used for gas exchange measurements
(n = 5). Chlorophyll fluorescence measurements were taken with a
portable pulse amplitude modulation fluorometer (PAM-2000, Walz,
Effeltrich, Germany). The procedure was similar to that described in
Rossa et al. (1998). Measuring and saturating light pulses were introduced through fibre optics that were aligned at an angle of 60° to the
leaf. Fluorescence measurements were taken simultaneously with those
of gas exchange. Leaves were dark adapted for 15 min with a dark leaf
clip (Walz, Effeltrich, Germany) before measuring the maximum
quantum yield of PSII (Fv/Fm). The ratio of variable to maximal fluorescence (Fv/Fm) estimates the efficiency by which the excitation energy
is captured by open PSII reaction centres. This ratio represents the
fraction of PPFD that is used in photochemistry. Values of Fv/Fm below
0.8 suggest stress-dependent photo-inhibition (Adams and DemmigAdams, 2004). Dark adaptation results in relaxation of all fast components of non-photochemical quenching, qn (Keiller et al., 1994).
Quantum yield of PSII (ΦPSII) was calculated as [(F′m–Ft)/F′m] (Genty
et al., 1989). In this equation, Ft is the light adapted fluorescence during
the prevailing irradiance of 1000 μmol m−2 s−1 immediately before a
saturating light pulse of 7500 μmol m−2 s−1 PPFD is superimposed for
700 ms, and F′m the maximum light adapted fluorescence at saturating
light (Schreiber and Bilger, 1993). The equations of Krause and Winter
(1996) were used to calculate electron transport rates through PSII.
Fluorescence parameters were corrected for temperature dependence,
as recommended by Herppich et al. (1994).
2.6. Proline
Proline was determined following the procedure of Bates et al.
(1973). Ten leaves from five different, randomly selected plants, adjacent to those used for gas exchange and chlorophyll fluorescence
measurements, were removed, frozen in liquid nitrogen immediately
after collection and ground with a pestle and mortar. The homogenate
was mixed with 1 mL aqueous sulfosalicylic acid (3% w/v) and filtered
through Whatman #1 filter paper. To this solution, an equal volume of
glacial acetic acid and ninhydrin reagent was added. The ninhydrin
reagent comprised 1.25 mg ninhydrin in 30 mL of glacial acetic acid
and 20 mL 6 M H3PO4. The mixture was incubated for 1 h at 95 °C, and
then placed in an ice bath to terminate the reaction. The contents were
mixed vigorously with 2 mL toluene, warmed to 25 °C and the absorbance of the chromophore measured with a Beckman DU-600 spectrophotometer at 520 nm using L-proline as a standard.
2.7. Data analyses
Data were first screened for normality and uniformity using the
Kolmogorov-Smirnov test and then subjected to repeated measures
ANOVA (SPSS 11.0, SPSS Inc., Chicago) and Tukey-Kramer multiple
comparisons test (P ≤ 0.05). Mean ± standard error (SE) are presented.
3. Results
2.4. Water use efficiency and soil moisture content
3.1. Soil moisture content and water potential
Water use efficiency was calculated as the ratio of CO2 uptake (A)
and transpiration (E), (WUE = A/E) at midday. Soil moisture was determined by the gravimetric method. Soil samples were removed from
each pot and the mass of water contained relative to the mass of dry soil
was determined after drying at 110 °C for 72 h. Results were expressed
as % soil water by weight.
Soil moisture content at the commencement of treatments was 42%.
With progress of the drought stress experiment, soil moisture decreased
significantly from 42% on day 0 to 8% on day 10 (Fig. 1). In WW plants,
midday minimum Ψ ranged from −0.25 to −0.3 MPa, while in the DS
treatment, Ψ decreased significantly from −0.26 MPa on day 2 to
3
Flora 248 (2018) 1–9
G. Naidoo, K.K. Naidoo
Fig. 3. Diurnal course of CO2 uptake (A, D), leaf conductance (B, E), and transpiration (C, F) in well-watered (left) and drought stressed (right) Chromolaena odorata.
Measurements were taken at two day intervals for 10 days. Vertical bars represent standard error (n = 5).
−1.86 MPa on day 10 (Fig. 2, Table 2).
WW treatment may be attributed to daily variations in microclimatic
conditions (Table 1). In DS plants, maximum CO2 uptake decreased
significantly from 9.3 μmol m−2 s−1 on day 0 to 3.95 μmol m−2 s−1 on
day 10 (Fig. 3D, Table 2). However, in contrast to A, maximum leaf
conductance and transpiration (E) per day decreased more strongly
from day 4 onwards (Fig. 3E, F). Due to the more pronounced decrease
in maximum E than in maximum A, WUE at midday in DS plants increased from 3.23 on day 0 to 5.98 μmol CO2 mmol−1 H2O on day 10
(Table 2).
3.2. CO2 uptake
Diurnal courses of CO2 uptake, leaf conductance and transpiration
for the WW and DS plants are shown in Fig. 3. In the WW plants, CO2
exchange increased from dawn to a mid-morning maximum between
10h00 to 11h00 and thereafter decreased gradually for the rest of the
day. Maximum CO2 uptake in the WW plants ranged from 10.5 to
12.8 μmol m−2 s−1 (Fig. 3A). Diurnal trends in leaf conductance and
transpiration in the WW plants were generally similar to those for CO2
uptake on all days. Slight differences in gas exchange parameters in the
4
Flora 248 (2018) 1–9
G. Naidoo, K.K. Naidoo
Fig. 4. Diurnal course of photosynthetic quantum yield ΦPSII (A, D), electron transport rate through PSII (B, E) and the maximum quantum yield of PSII photochemistry in the dark adapted state Fv/Fm (C, F) in well-watered (left) and drought stressed (right) Chromolaena odorata. Other details as for Fig. 3.
3.3. Chlorophyll fluorescence
stressed (Fig. 4C). In the DS treatment, trends in Fv/Fm on day 2 were
similar to those for WW plants. From days 4 to 10 however, diurnal Fv/
Fm values decreased progressively during the course of the day, with no
recovery at dusk (Fig. 4F). Daily maximum values of Fv/Fm in the DS
treatment decreased significantly from days 4 to 10 with increase in the
duration of stress (Table 2).
Trends in quantum yield of PSII (ΦPSII) in WW plants were generally
similar for all days. Quantum yield decreased from a maximal value of
0.7 at dawn, to a minimal value at midday and thereafter recovered to
the dawn values at dusk (Fig. 4A). In the DS treatment, trends in ΦPSII
were highest on Day 0 and decreased progressively with duration of
stress (Fig. 4D, Table 2). On day 10, however, ΦPSII decreased progressively from dawn to dusk with no recovery. In both WW and DS
plants, ETR through PSII (Fig. 4B, E) followed trends similar to those for
CO2 uptake. In the WW plants, trends in maximum quantum yield of PS
II (Fv/Fm) ranged from 0.75 to 0.80 suggesting that plants were not
3.4. Relationship between photosynthetic parameters and soil and plant
water status
Photosynthetic parameters showed different behaviour in response
to decreasing soil moisture. Whereas CO2 uptake (A) and chlorophyll
5
Flora 248 (2018) 1–9
G. Naidoo, K.K. Naidoo
Fig. 5. Relationship between gas exchange and chlorophyll fluorescence parameters and soil moisture content in drought stressed Chromolaena odorata over all days
(n = 5), (● day 2, ▄ day 4, ♦ day 6, ▲ day 8, ■ day 10).
fluorescence parameters (Fv/Fm, ΦPSII) decreased linearly between 45
and 25% soil moisture content, leaf conductance (g) was highly sensitive to decreasing soil moisture, reaching its minimum level already at
35% soil moisture (Fig. 5). Similarly, A, Fv/Fm and ΦPSII decreased
nearly linearly with decreasing leaf water potential over the whole
range from −0.26 to −1.86 MPa. In contrast, g declined very steeply
already between −0.26 and −0.9 MPa (Fig. 6)
effective in reducing soil moisture from 42% on day 1 to 8% on day 10,
while the difference in Ψ between the WW and DS plants was 1.65 MPa.
Generally, plants are stressed when the Ψ difference is more than
1.5 MPa (Flexas and Medrano, 2002). The diurnal gas exchange measurements clearly indicated that Chromolaena exhibits high photosynthetic rates at PPFDs up to 1500 μmol m−2 s−1. Other studies also
indicated that Chromolaena is highly effective in intercepting light (te
Beest et al., 2009; Quan et al., 2015). Physiological traits such as efficient light utilization and maintenance of high photosynthetic rates
under adequate soil moisture conditions probably contribute to the
success and spread of this species. Chromolaena has a high relative
growth rate, high specific leaf area and high total biomass (te Beest
et al., 2009), indicating its acquisitive resource use strategy. Several
other exotic invasive species (Rossa et al., 1998; Liu et al., 2017) also
exhibit these traits.
High gas exchange rates under WW conditions suggest that
Chromolaena has a high capacity for water absorption and transport to
leaves. In DS plants, there were significant decreases in leaf conductance and midday leaf Ψ as soil moisture declined. The similar
diurnal courses of CO2 uptake and leaf conductance suggest that decreased photosynthesis in the DS treatment was caused by stomatal
closure. Moreover, the negative linear correlation between photosynthetic parameters (CO2 uptake, Fv/Fm, ΦPSII) and leaf Ψ indicates the
sensitivity of photosynthesis to reduced leaf water content. However,
the hyperbolic correlation between leaf conductance and leaf Ψ shows
that stomata are much more sensitive to initial decreases of soil and leaf
3.5. Proline
In WW plants, proline concentrations ranged from 50 to 55 μmol
g−1 dry mass with little variation from initial values for the duration of
the experiment (Fig. 7). Concentrations of proline in the DS treatment
increased significantly with increase in duration of stress from
52 μmol g−1 on day 2 to > 100 μmol g−1 on day 10 (Fig. 7).
3.6. Re-sprouting after drought stress
At the end of the 10-day drought stress period, plants were re-watered daily for 2 weeks. All the plants that abscised their leaves resprouted from the rootstock. Stem regeneration also occurred after
dieback.
4. Discussion
In this study, withholding water to induce drought stress was
6
Flora 248 (2018) 1–9
G. Naidoo, K.K. Naidoo
Fig. 6. Relationship between gas exchange and chlorophyll fluorescence parameters and leaf water potential in drought stressed Chromolaena odorata over all days (n
= 5), (● day 2, ▄ day 4, ♦ day 6, ▲ day 8, ■ day 10).
2011; Esperón-Rodríguez and Barradas, 2015).
Healthy plants with an efficient photosynthetic apparatus have typical Fv/Fm values of 0.82–0.83 (Adams and Demmig-Adams, 2004). In
WW plants, early morning Fv/Fm values of 0.81–0.83 indicated that the
photosystems were functioning efficiently. The midday depression in
Fv/Fm in WW plants was probably due to photoinhibition of photosynthesis which occurs when light energy absorption exceeds the capacity for light use (Bjorkman and Demmig, 1987; Adams and DemmigAdams, 2004). In the late afternoon, the complete recovery in Fv/Fm to
their dawn values suggested that photoinhibition was reversible. In DS
plants, Fv/Fm values decreased progressively, suggesting a drastic decrease in photochemical efficiency with increase in the duration of
drought stress. Values of Fv/Fm below 0.6 are indicative of severe
drought stress (Vilagrosa et al., 2010). The lack of recovery in Fv/Fm
values in the late afternoons in DS plants suggests damage to PSII rather
than downregulation of photochemistry (Adams and Demmig-Adams,
2004). Photosystem II activity becomes inhibited only when the rate of
damage exceeds the rate of repair (Tikkanen et al., 2013). In grapevines
(Vitis vinifera L.), a reduction in Fv/Fm and ETR coincided with a decrease in leaf water content (Hailemichael et al., 2016) suggesting damage to PSII reaction centres. Similar results were reported for maize
plants (Zea mays) that were gradually allowed to dry down over several
days until leaf Ψ values were between −1.6 and −2.2 MPa (Gleason
et al., 2017).
In many species, a mechanism to reduce photoinhibition includes
production of biochemical osmolytes such as proline. In C. odorata,
increase in drought stress was accompanied by significant decreases in
Fig. 7. Changes in proline concentration in well-watered (WW) and drought
stressed (DS) Chromolaena odorata over 10 days (n = 5). Other details as for
Fig. 1.
water status than the photosynthetic apparatus. Further decreases in
soil moisture and leaf water content resulted in severe wilting followed
by abscission. Chromolaena is sensitive to water deficit as indicated by
decreases in CO2 uptake, leaf conductance, effective quantum yield,
ETR, Fv/Fm, and increases in proline concentration. Moreover,
Chromolaena responds to drought stress by dropping its foliage. Species
that adopt this strategy could be at risk of suffering mortality events due
to lack of carbon fixation during prolonged drought periods (McDowell,
7
Flora 248 (2018) 1–9
G. Naidoo, K.K. Naidoo
leaf Ψ and a significant increase in the concentration of proline. Proline
is an osmoprotector that stabilises membranes and maintains the conformation of cytosolic enzymes to survive drought stress (Peeva and
Cornic, 2009). In a study on C. odorata, Zhang and Wen (2009) demonstrated that accumulation of proline ensured plant survival when
relative soil water content was as low as 3.7% under high light conditions. Many studies on other species have shown that accumulation of
proline prevents reactive oxygen species (ROS) formation thereby improving stress tolerance (Liang et al., 2013; Adnan et al., 2016).
Reports in the literature suggest that C. odorata favours a relative
soil water content of 60–70% and that growth is poor above 80%
(Vanderwoude et al., 2005). Others reported that C. odorata performed
better at relative soil water content of 3.7% compared to 90% under
high light (te Beest et al., 2013). Chromolaena odorata possesses a
shallow, fibrous root system that rarely penetrates the soil to depths
greater than 40 cm. However, roots in close proximity to the surface can
easily access light showers which are frequent in the locality of this
study. Thus, root morphology and allocation patterns are important in
the species’ responses to drought stress (see also Chirino et al., 2017).
Most invasive species, like Chromolaena, exhibit profuse vegetative
growth when water is abundant (Goodall and Erasmus, 1996; Kriticos
et al., 2005; Mandal and Joshi, 2014), but during drought stress, stomatal closure results in decreased leaf conductance, photosynthesis and
transpiration. More conservative water use in DS plants due to a very
sensitive response of leaf conductance to decreasing leaf water potential
resulted in higher WUE, which may be a mechanism to increase the
efficiency of resource utilisation. A study by Chen et al. (2016) demonstrated that net photosynthetic rates and WUE in Mikania micrantha, Ageratina adenophora, Chromolaena odorata and Bidens pilosa
were higher than those of native plants thereby contributing to their
successful invasion in China.
In a field study, M’Boob (1991) suggested that the rapid decay of
copious amounts of abscised leaves contributes to soil organic carbon
and rapid re-sprouting in C. odorata. Mandal and Joshi (2014) reported
that elevated N and P levels in soil under the invader Lantana camara
were due to leaf litter and that these nutrients contributed to its invasion success in India. Whether this occurs in C. odorata, however, is not
known and needs to be investigated. In addition, the abscised leaves of
Chromolaena possess large amounts of allelochemicals which inhibit the
germination and growth of indigenous plant competitors (Ambika and
Jayachandra, 1990). In another study, Chromolaena was shown to allocate carbon to stems during drought stress (Zhang and Wen, 2009).
All these traits demonstrate that a successful, fast growing, invasive
species, like C. odorata, creates positive feedback to increase its persistence, survival and spread (Ehrenfeld, 2010; te Beest et al., 2012).
This study demonstrated that Chromolaena is a drought-avoider species
that responds to drought stress by leaf wilting and abscission. Additional studies should monitor recovery of plants after severe drought
stress, including the efficiency of re-sprouting and subsequent biomass
productivity. Furthermore, studies on interspecific competition between C. odorata and indigenous species under the same environmental
conditions are warranted.
which warrants additional research under competitive conditions in
natural plant communities.
Acknowledgements
The financial support of Mangosuthu University of Technology
(grant number NSCI: 01/2003 to K K Naidoo) and National Research
Foundation (grant number 93560 to G Naidoo) are gratefully acknowledged. The University of KwaZulu-Natal provided technical
support.
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8
Flora 248 (2018) 1–9
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