Differences in the Growth and Physiological Response of Eight

Aquatic Botany 139 (2017) 25–31

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Aquatic Botany

jou rnal homepage: www.elsevier.com/locate/aquabot

Differences in the growth and physiological response of eight

Myriophyllum species to carbon dioxide depletion

a a,b,∗

Emin Dülger , Andreas Hussner

Stress Physiology and Photosynthesis of Plants, Heinrich-Heine-University, Düsseldorf, Germany

Institute of Botany, Heinrich-Heine-University, Düsseldorf, Germany

a r a

t i b s

c t

l e i n f o r a c t

Article history: The growth and photosynthesis of submerged aquatic plants is often limited by the CO2 availability in

−

Received 27 June 2016

their habitats, but about 50% of all submerged plants are able to use HCO3 as an additional carbon

Received in revised form 13 February 2017 −

source. This ability to use HCO3 provides a competitive advantage over non-CO2-users under CO2 limit-

Accepted 16 February 2017

ing conditions. Here, we studied the growth and physiological response of eight Myriophyllum species (M.

Available online 21 February 2017

spicatum, M. triphyllum, M. heterophyllum, M. papillosum, M. variifolium, M. tetrandrum, M. tuberculatum, M.

− −

verticillatum) to CO2 and HCO3 use conditions. Physiologically, plants acclimated to HCO3 use showed

Keywords: −

higher net photosynthetic rates under both CO2 and HCO3 use conditions than plants grown under high

CO2 and bicarbonate −

Photosynthesis CO2 conditions. Furthermore, we found signiﬁcant differences in the HCO3 use capacity between the

Growth Myriophyllum species. The long-term exposure to high CO2 conditions during growth caused an accumu-

Mass fraction lation of starch within the leaves, while the chlorophyll content decreased. Moreover, plants allocated

Starch more biomass into roots and reduced the leaf biomass under CO2 enrichment. The growth rates illustrate

−

Pigments that M. spicatum is the most efﬁcient HCO3 user out of the tested Myriophyllum species, followed by M.

triphyllum and M. heterophyllum. The other ﬁve studied Myriophyllum species showed only a minor or no

− −

HCO3 use capacity. We conclude, that the HCO3 use capacity varies greatly even within a single genus,

−

and that the HCO3 use capacity, among others, is an important trait of strong competitive submerged

plants.

1. Introduction induced CO2 limitation in dense macrophyte stands (Santamaria,

2002). Aquatic plants have evolved various adaptations to cope

The growth of submerged aquatic plants is determined by var- with CO2 depletion. Morphologically, their thin and ﬁne dissected

ious abiotic and biotic parameters, including temperature, light, leaves and the low chlorophyll content per surface area increase

nutrients and carbon availability (Sand-Jensen, 1989). Unlike in ter- the diffusional uptake of CO2, and some submerged plants are able

restrial habitats, the dissolved inorganic carbon (DIC) availability of to take up carbon via their root system (Winkel and Borum, 2009).

freshwater ecosystems varies over a wide range between and even Physiologically, about 50% of all submerged plants have carbon con-

within water bodies (Maberly et al., 2015), and the CO2 availability centration mechanisms, like the single cell C4 or the crassulaceen

itself is inﬂuenced by the water pH, which determines the portion of acid metabolism (CAM) (Bowes, 2011). Long-term exposure to CO2

− 2−

CO2, HCO3 and CO3 within the DIC pool (Pedersen et al., 2013). enrichment revealed, beside the effect of increased growth rates,

Even though inland waters are often supersaturated with CO2 increased leaf dry matter content (LDMC, Hussner and Jahns, 2015),

in comparison with water in equilibrium with the atmosphere decreased speciﬁc leaf area (Madsen et al., 1996) and increased

(Raymond et al., 2013), the CO2 availability of freshwater habi- allocation of biomass to roots (Madsen et al., 1996; Hussner et al.,

tats often gets limited for submerged aquatic plants due to the low 2015, 2016). Moreover submerged plants acclimated to CO2 deple-

CO2 diffusion rate in water and the high uptake of CO2 by primary tion had higher chlorophyll content (Hussner and Jahns, 2015) and

−

producers, which increases the water pH and can lead to self- increased afﬁnities to both CO2 and HCO3 , but strong differences

−

in the HCO3 use capacity between species occur (Maberly and

Madsen, 1998, 2002; Maberly et al., 2015; Hussner et al., 2016).

Furthermore, even within a single species, the growth conditions

∗ −

Corresponding author at: Universitätsstr. 1, Geb. 26.13.O2.R32, D-40225 Düs-

affect the HCO3 afﬁnity of plants (Madsen et al., 1996; Hussner

seldorf, Germany. Tel.: +49 211 8114290.

and Jahns, 2015). However, submerged macrophytes with the abil-

E-mail address: [email protected] (A. Hussner).

http://dx.doi.org/10.1016/j.aquabot.2017.02.008

26 E. Dülger, A. Hussner / Aquatic Botany 139 (2017) 25–31

cates the important role of the CO2 availability on submerged plant

performance and the competitive strength of submerged plants.

Within the genus Myriophyllum, recent studies revealed strong

−

differences in the HCO3 capacities of M. spicatum, M. aquaticum

and M. heterophyllum and M. verticillatum (Maberly and Madsen,

2002; Eusebio Malheiro et al., 2013; Hussner and Jahns, 2015). In

this study, we compare the growth and physiological acclimation

of eight Myriophyllum species to ambient air CO2 and elevated CO2

conditions, including species known as strong competitors (M. spi-

catum and M. heterophyllum) and six less competitive species (M.

papillosum, M. tetrandum, M. triphyllum, M. tuberculatum, M. vari-

ifolium and M. verticillatum). Besides the general effects on growth

rate and physiology, we focused on various plant traits (dry mat-

ter content and mass fractions) which have rarely been considered

in aquatic plant research to date, but are widely accepted in ter-

restrial plant research as indicators for plant growth responses to

environmental changes (Poorter et al., 2012). Based on recent ﬁnd-

−

ings, we hypothesize that (i) the capacity for HCO3 utilization

varied strongly between the Myriophyllum spp., (ii) the acclimation

−

to CO2 or HCO3 use causes growth and physiological responses in

submerged Myriophyllum species and (iii) that plant traits provide

valuable information on the submerged aquatic plant response to

changes in the environmental conditions.

Fig. 1. The relative growth rate (RGR) of eight Myriophyllum spp. grown under LC

2. Materials and methods

(low CO2) and HC (high CO2) conditions for 35 days. Mean ± SE of four replicates

are shown. Asterisks indicate signiﬁcant differences between the LC and HC treat-

2.1. Plant material

ment of each species (Tukey’s multiple comparisons of means). The F-values and

level of signiﬁcances for differences between the different carbon treatments (C),

between the species (S) and their interaction (S C) are given from two-way ANOVA. The plant material used was taken either from laboratory cul-

(Signiﬁcance levels: *p-value < 0.05, **p < 0.01, ***p-value < 0.001, ****p < 0.0001.)

tures at the Heinrich-Heine-University of Düsseldorf (M. spicatum

and M. heterophyllum) or was provided from laboratory cultures

− at the Plant Protection Service in Wageningen, The Netherlands

ity to use HCO3 have a major competitive advantage over species

(M. triphyllum, M. tetrandum, M. papillosum, M. tuberculatum and

restricted to CO2 use under CO2-limiting conditions, and show

M. variifolium, provided by Dr. J. van Valkenburg).

higher growth rates when CO2 limitation occurs, e.g. during the day

− For precultivation, plants were grown under standard condi-

in dense macrophyte stands. Consequently, the ability for HCO3

tions in 30 l aquaria, ﬁlled with a standard medium for general

use is considered as one of the major determinants of the distri-

aquatic plant purposes (Smart and Barko, 1985) and a water pH of

bution of submerged aquatic plants in their habitats (Sand-Jensen,

7.05 ± 0.08, rooted in a nutrient rich sediment. In each aquarium,

1989; Maberly and Madsen, 1998, 2002).

− water was ﬁltered by small aquarium ﬁlters (type 2213, Eheim,

Spencer and Bowes (1990) were the ﬁrst to link HCO3 use and

Deizizau, Germany). Plants grew in a 16/8 h light/dark cycle at room

potential plant invasiveness. In a review of alien aquatic plants ◦

temperature of 23 C, and a photon ﬂux density of about 60 ␮mol

in Europe, it was noted that almost all successful and fast grow- −2 −1

photons m s . For the experiments, plants were taken out of

ing alien submerged aquatic plant species in Europe have a carbon

− these precultures and the apical tips of the plants were used for the

concentrating mechanism (CCM) and are thus able to use HCO3

studies, except for M. verticillatum where freshly sampled turions

(Hussner, 2012), which acts as an competitive advantage over

(sampled from the Heider Bergsee near Cologne) were used for the

non CO users under CO limitation. These include species with

2 2 experiments.

known CCMs, such as single cell C4 Hydrilla verticillata and Ege-

ria densa, as well as species with less well described CCMs, like

Elodea canadensis, Elodea nuttallii, Lagarosiphon major, Vallisneria 2.2. Growth conditions

spiralis or Myriophyllum heterophyllum (Bowes, 2011; Hussner et al.,

2016). A number of these submerged aquatic plant species with a For the experiments, unrooted shoot apices of 10 cm in length

type of CCM belong to the Hydrocharitaceae family, which might were taken from the precultures, with initial dry masses of

be a reason for the large number of fast growing species and strong 0.027 ± 0.002 g (M. variifolium) to 0.052 ± 0.004 g (M. heterophyl-

competitors within this family. lum). The freshly collected turions of M. verticillatum had an initial

However, the success of a given species depends on both species dry mass of 0.173 ± 0.016 g. The initial dry masses of the plant

−

traits and habitat characteristics, and the advantage of HCO3 use in parts were calculated based on the dry mass to fresh mass ratio

◦

submerged plants is only present under CO2 limitation. Conversely, of subsamples of the species, determined after drying at 105 C to

species restricted to CO2 use generally have a higher afﬁnity to CO2 constant mass.

−

than species with both CO2 and HCO3 use, and thus a competitive For the experiments, plants were potted into small 200 ml

advantage under high CO2 conditions (Maberly and Madsen, 1998). glasses, ﬁlled with a nutrient-rich mixture of nutrient-rich natural

−

In aquatic plants with a high capacity to use HCO3 , the effect of sediment and compost (provided by the Düsseldorf Botanic Gar-

light and temperature on growth are stronger than the effect of CO2 den, containing 103.9 ± 1.7 mg NO3–N per kg soil and 31.1 ± 0.5 mg

depletion (Eller et al., 2015; Hussner et al., 2015), but conversely, P2O5–P per 100 g soil) and a 0.5 cm layer of washed sand on top,

the negative effects of CO2 depletion overwhelm the positive effects to reduce the nutrient loss into the water column. Four individual

−

of increasing temperature and light in poor or no HCO3 users containers with one plant per treatment were subsequently trans-

(Eusebio Malheiro et al., 2013; Hussner and Jahns, 2015). This indi- ferred into 5 L plastic containers. The containers were either ﬁlled

E. Dülger, A. Hussner / Aquatic Botany 139 (2017) 25–31 27

Fig. 2. The dry matter contents of roots (RDMC), stems (SDMC) and leaves (LDMC) and the mass fractions of roots (RMF), stems (SMF) and leaves (LMF) of eight Myriophyllum

spp. grown under LC and HC conditions for 35 days. Mean ± SE of four replicates are shown. Probabilities and F values as in Fig. 1.

with the medium according to Smart and Barko (1985) with dis- where Wi is the initial DM and Wf the ﬁnal DM after T = 32 days

solved inorganic carbon content (DIC) of 0.85 mM or with a medium Dry matter content of leaves (LDMC), stems (SDMC) and roots

with reduced DIC of 0.4 mM. For the low CO2 treatments (LC), plants (LDMC):

were grown in the medium with 0.85 mM DIC and water was con-

DMC gdrymass/gfreshmass

tinuously mixed with air, resulting in a pH of 8.3 ± 0.2. For the high

CO2 treatments (HC), the medium with 0.4 mM DIC was contin-

Mass fraction of roots (RMF), stems (SMF) and leaves (LMF):

uously added from a commercial container. Due to the addition

= /

of CO2, the DIC in the water increased to 0.85-0.95 mM at a pH MF gdrymassofplantfraction gtotalplantdrymass

of 6.8 ± 0.1. The pH was controlled on a daily basis and the CO2

addition was regulated with needle valves (one needle valve per

2.4. Net photosynthesis measurements

container), and due to the ongoing mixing, the diurnal variation in

pH within the containers was low (<0.2). Due to the different pH,

Net photosynthesis was determined using apical shoots with

the LC and HC treatments differed in the availability of both CO2 and

−

7.5–10 cm in length. The shoots were incubated for 45 min at room

HCO3 (Hussner and Jahns, 2015). The gas exchange measurements

◦ −2 −1

temperature (23 C) and 60 ␮mol photons m s in 100 ml stop-

were carried out 34 days after the initiation of the experiments, and

per bottles, placed on a shaking plate. The stopper bottles were

one day prior to the plant harvest.

ﬁlled with Smart and Barko (1985) media with varying DIC lev-

els of 0.1, 0.5, 1, 2 mM. The pH was adjusted to pH 6 (by adding

2.3. Plant harvest and determination of growth parameters −

0.1 mM HCl) and pH 9 (0.1 mM NaOH) to induce CO2 or HCO3 use,

and two bottles without plants were used as controls. The oxygen

After 35 days, plants were harvested in the morning, 2–4 h after

measurements were carried out using an oxygen electrode (type

the light was switched on, and separated into stems, roots and

OXI-340, WTW, Weilheim, Germany). The net photosynthetic rate

leaves and then gently blotted dry on soft paper tissue. The fresh

was calculated on a dm basis.

masses of the plant parts were determined, subsequently shock

frozen in liquid nitrogen and lyophilized thereafter. After lyophili-

2.5. Chlorophyll and anthocyanin analysis

sation, the dry masses were determined and the leaf material was

ground to a ﬁne powder in a bead mill (Retsch MM 301, Haan,

Chlorophyll content was analyzed using the freeze–dried leaf

Germany) for pigments and starch analysis.

material following the method of Arnon (1949). Chlorophyll was

The following growth parameters were calculated based on the ◦

extracted in 80% acetone for 12 h in the dark at 4 C from 1 to

determined fresh mass and dry mass of roots, stems and leaves:

2 mg leaf material, subsequently centrifuged at 13,000 rpm and the

−1

= W − W T

Relativegrowthrate(RGR) (ln f ln i) chlorophyll content of the supernatant was measured spectropho-

tometrically at 645 and 663 nm.

28 E. Dülger, A. Hussner / Aquatic Botany 139 (2017) 25–31

◦

Anthocyanins were extracted with −20 C methanol–HCl (0.1%

◦

HCl), held at 20 C for 24 h and quantiﬁed photometrically accord-

ing to Murray and Hackett (1991).

2.6. Leaf starch analysis

The starch analysis was carried out following a protocol of

Smith and Zeeman (2006). After the removal of free glucose, the

starch within 2 mg of the lyophilized leaf material was solubi-

lized and subsequently degraded over night by the addition of

2.5 U amyloglucosidase and 3.5 U ␣-amylase. Thereafter, glucose

was phosphorylated by adenosine triphosphate (ATP) in the reac-

tion catalyzed by hexokinase. Following this, the base absorbance

at 340 nm was measured and the glucose-6-phosphate was oxi-

dized in the presence of nicotinamide adenine dinucleotide (NAD)

to 6-phosphogluconate. During this oxidation, NAD was reduced

to NADH in an equimolar amount. After 30 min, the absorbance at

340 nm was determined again and the increase in the absorbance

was directly proportional to the glucose concentration.

2.7. Statistical analyses

Overall differences between species, between CO2 treatments

and interactive effects were tested with a two-way ANOVA

(alpha = 0.05) using R statistics (version 3.2.3; R Core Team 2015,

http://www.R-project.org). A three-way ANOVA was carried out to

test for the effects of DIC level and pH during the measurements and Fig. 3. The starch content of eight Myriophyllum spp. grown under LC and HC condi-

tions for 35 days. Mean ± SE of four replicates are shown. Probabilities and F values

growth conditions on net photosynthetic rate. Differences between

as in Fig. 1.

the LC and HC treatments of each species were tested using Tukey’s

multiple comparisons of means.

Similar to the mass fractions, the dry matter content of roots

3. Results (RDMC), stems (SDMC) and leaves (LDMC) were overall signiﬁ-

cantly higher under HC than under LC conditions, but this increase

3.1. Growth in the dry matter content was more pronounced in the leaves than

in stems and roots (Fig. 2A–C). Additionally, the DMCs were differ-

While signiﬁcant differences in growth rates between the ent between the species, and for the LDMC, these differences were

species were found, all species showed signiﬁcantly higher growth signiﬁcantly affected by the carbon treatments (Fig. 2C). The high-

rates under HC than under LC conditions (Fig. 1). The highest est discrepancy between the LDMC under LC vs HC conditions was

growth rates (RGR) under HC were found for M. heterophyllum present in M. heterophyllum, when under HC, the highest, and under

−1 −1

(0.080 ± 0.004 g d DM d ) and M. spicatum, whilst the lowest LC, the lowest LDMC of all studied species occurred. M. tetrandrum

RGR was found for M. verticillatum (Fig. 1). The highest RGR under had the highest SDMC for both plants under HC and LC conditions,

−1

LC conditions was found for M. spicatum (0.049 ± 0.005 g d DM which were about three times higher than for M. papillosum, which

−1

d ), followed by M. triphyllum and M. heterophyllum, indicating had the lowest SDMC under both LC and HC. The RDMC was highest

−

a good to moderate HCO3 use capacity during growth, while M. in M. triphyllum under both LC and HC, the lowest RMDC was found

verticillatum did not grow under LC. for M. papillosum (LC) and M. verticillatum (HC).

Overall, the biomass allocation into roots (RMF) was signiﬁ- The starch content of the leaves was signiﬁcantly higher in

cantly increased under HC conditions (Fig. 2D), while the leaf mass plants grown in HC than in LC treatments (Fig. 3). This increase was

fraction (LMF) signiﬁcantly decreased (Fig. 2F). Contrary, no signif- signiﬁcantly correlated with increasing LDMC (Fig. 4A), indicating

icant effect of carbon was found for the stem mass fraction (SMF) that the starch accumulation is the major driver for the increase

(Fig. 2E). Besides these overall signiﬁcant changes in the mass frac- of LDMC under HC conditions. Under HC, the leaves of M. spica-

tions due to different CO2 availability, signiﬁcant differences in the tum had the highest starch content of all species (19.89 0.90% of

biomass allocation to roots, stems and leaves were found between DM), which was about 4-times higher than that of HC-grown M.

the species (Fig. 2D–F), and for both SMF and RMF, an interaction papillosum, and 8-fold higher than that of LC-grown M. tetrandrum.

between carbon and species was found.

Under HC conditions, M. papillosum had the highest RMF, fol- 3.2. Physiology

lowed by M. spicatum and M. heterophyllum, while M. variifolium

ranked last (Fig. 2D). Under LC conditions, M. spicatum showed the While the LDMC was generally increased under HC conditions,

highest RMF, which was even higher than the RMF of HC plants the chlorophyll content was signiﬁcantly reduced (Fig. 5A). There is

(except for M. spicatum and M. papillosum), and the lowest RMF a signiﬁcant negative correlation between the chlorophyll content

under LC was found for M. variifolium (Fig. 2D). Contrary to the RMF, and the increase in starch (Fig. 4B), indicating a feed-back inhibi-

the highest LMF was found for LC plants with the highest LMF in tion of starch enrichment on the photosynthetic apparatus under

M. verticillatum. The lowest LMF was documented in M. triphyllum CO2 enrichment. Overall, the chlorophyll content of plants grown

under HC conditions (Fig. 2F). The differences in the SMF between under LC conditions was signiﬁcantly higher than for HC plants. The

the LC and HC treatments were less pronounced than for RMF and highest chlorophyll content was found in leaves of M. heterophyl-

LMF. The highest SMF was found in M. tetrandrum (LC) followed by lum grown under LC conditions, which was almost 5 times higher

M. triphyllum (HC), and the lowest in M. verticillatum (LC) (Fig. 2E). than its chlorophyll content under HC (Fig. 5A), but the differences

E. Dülger, A. Hussner / Aquatic Botany 139 (2017) 25–31 29

Fig. 4. Pearson correlation of the starch content of the leaves and the LDMC (A) and the chlorophyll content (B) of leaves of eight Myriophyllum spp. grown under LC and HC

conditions for 35 days. The p value and the Pearson correlation coefﬁcient (r) are given.

Fig. 5. The chlorophyll content (A), chlorophyll a:b ratio (B) and anthocyanin content (C) of eight Myriophyllum spp. grown under LC and HC conditions for 35 days. Mean ± SE

of four replicates are shown. Probabilities and F values as in Fig. 1.

in the chlorophyll content of HC and LC plants of the other species Overall, under both pH 6 and pH 9, the net photosynthetic rates

were less. Converse to the strong effects on the chlorophyll content, increased signiﬁcantly with increasing dissolved inorganic carbon

no overall signiﬁcant effects of the growth conditions on both Chl availability, but this increase was signiﬁcantly more pronounced at

a:b ratio (Fig. 5B) and anthocyanin content (Fig. 5C) were found. pH 6 (Fig. 6).

The net photosynthesis measurements revealed that for all

species, the growth conditions and the pH and the DIC content

4. Discussion

during the photosynthesis signiﬁcantly affected the photosynthetic

performance (Fig. 6). In general, all studied Myriophyllum spp.

Carbon availability is one of the most important factors deter-

showed signiﬁcantly higher net photosynthetic rates at pH 6 than

mining submerged aquatic plant growth in freshwater ecosystems

at pH 9, and the net photosynthetic rate (at both pH 6 and pH 9)

(Pedersen et al., 2013). As a response to CO2 limitation, at least 50%

was signiﬁcantly higher in LC than in HC plants. These general dif- −

of all submerged aquatic plants developed various HCO3 uptake

ferences in the net photosynthetic rates between LC and HC plants

mechanisms (Maberly and Madsen, 2002; Raven et al., 2014; Yin

are at least partly a result of the higher chlorophyll content of LC −

et al., in press), to use HCO3 as an additional carbon source. The

plants (Fig. 5A). −

capacity of submerged plants to use HCO3 is considered a major

Highest net photosynthetic rate was found for LC grown M. ver-

parameter determining aquatic plant growth, competitive strength

ticillatum (Fig. 6D) and M. tetrandrum at pH 6 (Fig. 6 H), while the

and thus macrophyte community composition (Pedersen et al.,

highest net photosynthetic rate for HC plants at pH 6 was found for

2013; Maberly et al., 2015; Hussner et al., 2015).

M. variifolium (Fig. 6C), followed by M. triphyllum (Fig. 6A). At pH −

The efﬁciency of HCO3 uptake mechanisms in submerged

9, the highest net photosynthetic rate was found for LC grown M.

plants varies largely between families and species, and while the

triphyllum and M. spicatum (Fig. 6G), and the lowest was measured

single cell C4 mechanisms in H. verticillata and E. densa were found

for M. verticillatum (Fig. 6D).

to be highly efﬁcient, in comparison, M. spicatum and M. hetero-

30 E. Dülger, A. Hussner / Aquatic Botany 139 (2017) 25–31

Fig. 6. The net photosynthesis at pH 6 and 9 and DIC concentrations of 0.1, 0.5, 1 and 2 mM of eight Myriophyllum spp. grown under LC and HC conditions for 35 days.

−

phyllum showed only a moderate capacity to use HCO3 (Hussner to leaves (despite the increased dry matter content), and the asso-

et al., 2016). Besides these differences between the species, even ciated increased biomass allocation to roots are most likely a plant

−

the HCO3 uptake capacity for a single species largely depends on response to the starch enrichment. For terrestrial plants, a feed-

the growth conditions. Plants acclimated to CO2 depletion usually back inhibition of starch enrichment was found (Delucia et al.,

−

have a higher afﬁnity to HCO3 than plants grown in CO2 rich water 1985) and thus is likely for the submerged species in our study

(Sand-Jensen and Gordon, 1984; Sand-Jensen and Gordon, 1986; as well. The plants may use the roots as a sink for the nonstruc-

Hussner and Jahns, 2015; Hussner et al., 2016), which we conﬁrm tural carbohydrates to minimize these negative effects. However,

here for all tested Myriophyllum spp. such increased root mass fraction was not reported from terres-

Here, we documented strong differences in the growth rates trial plants (Poorter et al., 2012) but for submerged plants under

of species within the genus Myriophyllum due to different CO2 CO2 enrichment (Pagano and Titus, 2007; Eusebio Malheiro et al.,

−

and HCO3 conditions, when species were cultivated under similar 2013; Hussner et al., 2015), and thus must be considered as a gen-

light, temperature and nutrient conditions. The smaller differences eral response in submerged plants. As a result of the increased root

in the RGR ratio of HC and LC plants compared to the ratio of biomass, plants have an increased root anchorage and increased

net photosynthetic rates of HC plants at pH 6 and LC plants at storage during winter, which will boost plant regrowth in spring

pH 9 can at least partly be explained by the allocation of biomass (Schutten et al., 2005; Wersal et al., 2011). Moreover, the enhanced

into non-productive plant organs (roots), and by the fact that for root allocation will inﬂuence the general persistence of submerged

the photosynthetic measurements, only young apical shoots were plants and allow them to withstand unfavourable conditions, like

used, which might show higher photosynthetic rates than older light limitations during algal bloom events, which will result in

leaves. changing macrophyte community composition.

In general, plant growth was stimulated by CO2 enrichment, but High growth rates are an important trait of highly competitive

the reduced chlorophyll content due to starch enrichment gives plants. Fast growth allows the plants to build up high biomass den-

some evidence for a feed-back inhibition of growth and photo- sities and displace slower growing species, but in many cases, the

synthesis in submerged macrophytes, as suggested by previous CO2 availability is a limiting factor for photosynthesis and growth

studies of Sutherland-Guy and Pip (1989) and Hussner et al. (2016). within dense macrophyte stands (Santamaria, 2002).

Such feed-back inhibitions were known from terrestrial plants, Here, within the species of the genus Myriophyllum, M. spicatum

when after an initial strong enhancement of net photosynthesis is reported as a strong competitor (Aiken et al., 1979) and showed

−

due to CO2 enrichment, these effects decrease in the long-term the highest growth rates under HCO3 use conditions in our study.

(Long et al., 2004; Leakey et al., 2009). However, high irradiances M. triphyllum and M. heterophyllum showed almost similar, but

−

and high temperatures enhance the effect of elevated CO2, result- less than M. spicatum, growth rates under HCO3 use conditions.

−

ing in an even stronger reduction of the chlorophyll content in A recent study documented that the HCO3 use capacity of M. tri-

−

the leaves of submerged Myriophyllum species (Eusebio Malheiro phyllum is signiﬁcantly lower than of the excellent HCO3 users H.

et al., 2013; Hussner et al., 2015). The reduced biomass allocation

E. Dülger, A. Hussner / Aquatic Botany 139 (2017) 25–31 31

verticillata, E. densa and L. major, which displaced M. triphyllum in Leakey, A.D.B., Ainsworth, E.A., Bernacchi, C.J., Rogers, C.J., Long, S.P., Ort, D.R.,

2009. Elevated CO2 effects on plant carbon, nitrogen, and water relations: six

New Zealand waters (Hussner et al., 2015).

− important lessons from FACE. J. Exp. Bot. 60, 2859–2876.

However, with decreasing growth rates and HCO3 use capac-

Long, S.P., Ainsworth, E.A., Rogers, A., Ort, D.R., 2004. Rising atmospheric carbon

ities within the genus Myriophyllum, the competitive strength dioxide: plants FACE the future. Ann. Rev. Plant Biol. 55, 591–628.

Maberly, S.C., Madsen, T.V., 1998. Afﬁnity for CO2 in relation to the ability of

of submerged plants in CO2 limiting waters is reduced. Thus −

− freshwater macrophytes to use HCO3 . Funct. Ecol. 12, 99–106.

the potential of low or no HCO3 users (in the recent study M.

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