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PHOSPHORUS UPTAKE AND RELEASE IN AN AQUATIC
MACROPHYTE (VALLISNERIA SPIRALIS) IN RELATION

 

T0 ALGAL PRODUCTIVITY

By

Lois G. Wolfson

A THESIS

Submitted to
Michigan State University
in partiaT fu1fi11ment of the requirements

for the degree of
MASTER OF SCIENCE
Department of Botany and Plant Pathoiogy

1978

ABSTRACT

PHOSPHORUS UPTAKE AND RELEASE IN AN AQUATIC
MACROPHYTE (VALLISNERIA SPIRALIS) IN RELATION

TO ALGAL PRODUCTIVITY

 

By

Lois G. Nolfson

Phosphorus uptake, translocation to and release from leaves were

found in the submerged aquatic anqiosperm Vallisneria spiralis L. when
33

 

P04 was injected into the root environment. Rates of translocation
and release were related to the concentration of P04-P in the medium
and to root biomass. Autoradiographs indicated that both acropetal and
basipetal transport occurred in the plant. Epiphytic and planktonic al-

gae rapidly absorbed 33

P04 released, and contained up to 37 percent of
the amount released to the foliage medium.

Net productivity of phosphorus—starved Scenedesmus cultures increased

 

when given phosphorus released from the Vallisneria foliage. It appears

 

that phosphorus can be transported from the sediments through the
plants and released into the water where it can subsequently be utilized

by surrounding algae.

ACKNOWLEDGMENTS

My sincere thanks goes to my major professor, Dr. Peter Murphy. His
constant help, suggestions and encouragement were extremely beneficial
and fully appreciated. I would also like to thank my committee members,
Drs. Robert Wetzel and Clarence McNabb for their critical reading and
reviewing of my thesis and for use of equipment. Dr. Edward Farnworth
and Carolyn Vann also read the thesis and offered constructive comments.
Special thanks go to Dr. Hans Kende for his help and use of equipment
while I was using radioisotopes and to Dr. Gary Mills for introducing me
to some analytical procedures. I would also like to acknowledge and
thank the graduate students with whom I shared a lab for useful conversa—
tions and for helping to make my studies at MSU well worthwhile. Finally,
I wish to express my gratitude to my parents for their unending support

in all my endeavors.

ii

TABLE OF CONTENTS

LIST OF TABLES .................................................. ... iv
LIST OF FIGURES .................................................... v
INTRODUCTION ....................................................... l

Literature Review ................................................ 1
METHODS AND MATERIALS .............................................. 9
RESULTS ............................................................ 17
DISCUSSION ......................................................... 4O

Phosphorus uptake, translocation and release ..................... 4O

Macrophyte—algal interactions .................................... 42
CONCLUSIONS ........................................................ 46
APPENDIX ......................... . .............. . .................. 47

-Nutrient composition of modified Gaudet's medium (Gaudet 1963) ... 47

Nutrient composition of modified Bold‘s Basic Medium
(James 1974) ..................................................... 48

LIST OF REFERENCES ................................................. 49

Hr
I-‘v
r-‘r

LIST OF TABLES

 

 

Page
Table l. Comparison of phosphorus in Vallisneria compartments
at day ll for six concentrations ....................... 26
Table 2. Percent Phosphorus in Vallisneria and the growing
medium at day l2 ....................................... 30
Table 3. Percent Phosphorus at day 7 in different compartments
at three concentrations ................................ 35

‘iV

LIST OF FIGURES

Page

FigUre 1. Experimental apparatus partitioning the Vallisneria
foliage and roots ....................................... l0

Figure 2. Autoradiographs of plants, whole and sectioned, given
33P to the root medium for 30 sec, 2, 5, and lo minutes
showing rapid initial uptake. A photograph is to the
right of each autoradiograph ............................ l8

Figure 3. Autoradiographs of whole plants given 33P to the root
medium for (l-r) 20 min., 40 min., one, five, 10, and
24 hours indicating little difference in foliage
translocation and a significant difference in root
absorption .............................................. 20

Figure 4. Autoradiograph negative of plant given 33P to foliage
. medium for one and 10 hours (l-r) showing significant
difference in foliage absorption and little translocation
to roots ................................................ 22

Figure 5. Average P release to foliage medium from Vallisneria
grown in three substrates. Vertical bar represents i one
standard error of the mean .............................. 23

Figure 6. Regression of labelled P present in Vallisneria leaves
and in the nutrient medium in which it grew ............. 28

Figure 7. Average 33P release from Vallisneria foliage to nutrient
medium with varying P concentrations in the foliage
compartment ............................................. 29

Figure 8. Release of 33P from foliage to surrounding medium with
three concentrations of PO -P in root compartment.
Vertical bar represents i ne standard error of the
mean .................................................... 33

Figure 9. Net productivity of Scenedesmus quadricauda after addition
of foliage medium from P enriched and P starved
Vallisneria. Vertical bar represents i one standard error
ofimean ................................................. 37

 

Figure 10. Net productivity of P starved and P enriched Scenedesmus
grown with constant shaking ............................. 38

V

INTRODUCTION

The importance of phosphorus in the aquatic environment is major

because it is a limiting element and an essential nutrient for

growth. Whereas the water is generally low in available phosphorus,

the sediments usually contain much greater concentrations (Hepher 1966).
Sediments rich in available nutrients may contribute to the productivity
of rooted aquatic plants by ion uptake through roots. Various studies
have demonstrated that the roots of aquatic plants serve a function in
nutrient uptake by absorbing nutrients through the root and root hairs
(Lundegardh 1966) and are not present only as a means of support in the
substratum as has been argued previously (Sutcliffe l962, den Hartog and
Segal l964).

This study investigates the extent to which phosphorus confined in
the sediments can be absorbed by the roots of a submerged aquatic plant,
translocated through the foliage and released into the water. The ef-
fect of phosphorus concentration on translocation and release is consi-
dered. Upon release from the plant, phosphorus movement is traced to
observe the response of algal and epiphytic populations surrounding the
plant. If phosphorus can be recycled from the sediments through plants
to plankton, a phosphorus source otherwise unavailable for metabolism

becomes accessible.

Litenatune Reuiuu
In early investigations Pond (l9OS) found that if the roots of

submerged aquatic plants were prevented from entering the substratum,
l

2
optimal growth was not achieved. Plants rooted in sand grew better
than plants in tap water but not nearly as well as plants rooted in a
soil with a higher organic content. Correlations between the distribu-
tion of certain aquatic macrophytes and chemical properties of the sub-
stratum indicated that plants largely depend on nutrient uptake from
the soil via the roots (Misra l938).

Denny (1972)planted floating-leaved and submerged-leaved plants
in artificial ponds where the sediment.conditions ranged from nutri-
ent-rich to nutrient-poor. The overlying water was similar in all ponds.
Results showed that all plants grew faster on mud than sand, with dif-
ferential species response to substrate type. Potamogeton thunbergii
had a growth rate four times greater in mud than sand. Signs of nutri-
ent deficiency were observed when the nutrient concentration available
to the roots was suboptimal even though the foliage portion of the
plant was in a mesotrophic water environment. Denny concluded that
nutrients could be absorbed from either the substrate or water and that
sometimes both could be sources of nutrient uptake.

Reviewing studies concerning the role of roots and leaves in nut-
rient uptake, Sculthorpe (l967) noted that as long as oxygen was avail-
able to root tips, roots were capable of absorbing and accumulating
ions. Bristow and Nhitcombe (l97l) used radioactive tracers in nutri-
ent uptake studies to determine the relative importance of root and fo-
liage absorption. Nith species of Myriophyllum, they examined the
plant's ability to translocate radioactive phosphorus (32F) from the
roots to the growing regions of the stem. Normally, roots absorb phos-
phate from the soil which then enters the xylem tissue to be rapidly

transported to other plant parts. A two-compartment chamber was

3

constructed that partitioned root environment from shoot environment, and
32F was injected into the lower root environment. After a ten-day growth
period, the specific activity of 32F in the shoots was determined for
three species. Upward translocation of phosphate from the roots to the
growing portions of the stem occurred in Myriophyllum brasiliense, M,
spicatum and Elodea densa. With E, ggggg, the phosphate concentration in
the foliage environment had no effect on the Upward translocation. When
32F was added to the foliage environment, downward translocation to the
roots was minimal. Also axillary shoots absorbed a very low amount of
the total phosohate present in the medium. Bristow (l975) noted that
when nutrients are made available to both roots and shoots, root uptake
exceeds foliage uptake in most cases. The availability of nutrients to
either compartment had no immediate influence on the uptake ability of

the other. In all experiments with Elodea and the Myriophyllum species,

 

labelled phosphorus was not recycled from the substrate into the water by
the plant unless plant decay occurred.

It has been demonstrated that submergent plants play a passive role
in releasing nutrients upon death and decomposition at which time nutri-
ent availability to surrounding algae increases. Recently, however, it
has been sugested that during times of active growth, rooted submerged
plants can serve as nutrient pumps. Uptake of trace metals has been
found to occur in aquatic macrophytes by Mayes and McIntosh (1977). Dif-
ferent levels of the metals cadmium and lead in water and soil resulted
in significantly different amounts in the plants. Also, the authors sug-
gested that release of these metals from the stem and leaves occurs, im-

plying a role of macrophytes in trace metal cycling.

4

Studies of nutrient absorption in vascular plants have demonstra-

ted that phosphorus is absorbed by the roots, translocated to the foli-
age and released into the water. McRoy and Barsdate (l970) set up ap-

32

paratus similar to Bristow §t_al, (1971) to observe P movement

through Zostera marina (eelgrass). I__§jtg_experiments in natural eel-
grass habitats were also performed. Phosphate absorbed through roots
was translocated through the eelgrass and often accumulated in areas of
active growth and 6611 division. In experiments in light where phosphate
absorption was greatest, 33 percent of the phosphate absorbed was even-
tually released from the eelgrass beds into the open water. McRoy gt_
g1, (l972) further evaluated the role of phosphorus transfer in a lagoon
ecosystem dominated by eelgrass. Reactive phosphorus translocation rate
through eelgrass was estimated to be 62.4 mg P/mz-day. Of the reactive
phosphorus excreted, three metric tons P/day was exported from the la-
goon into the sea. This study gave further support to the importance of
littoral vegetation in phosphorus cycling in lakes.

32 59 45

Using P, Fe, and Ca to trace the movement of ions in Myrio-

phyllum exalbescens Fernald, DeMarte and Hartman (l974) constructed a

 

two-chambered apparatus. Elodea canadensis Michx. was placed in a tray
that contained the foliage portion of Myriophyllum, and radioactive mat-
erial was put into a beaker which contained the Myriophyllum roots.
Under different conditions of light and substrate, experiments were car-
ried out to determine if translocation and excretion of ions occurred.
With autoradiography they observed that §19g§g_absorbed labelled ions
released from the leaves of Myriophyllum. When a rooted plant was in-
jured due to broken offshoots, the amount of 32F released into the water

increased significantly.

5

Spartina alterniflora Loisel was studied by Reimold (1972) to de-

 

termine pathways of phosphorus in a salt marsh ecosystem. When tidal
inundations occurred, labelled phosphorus previously injected into the _
sediments was released from Spartina leaves into the water approximately
10 to 15 days after its introduction into sediments. Maximum uptake of
phosphorus was greatest when productivity was at a maximum. Twilley
§t_gl, (1977) observed absorption, translocation and secretion of label-

led phosphorus in Nuphar luteum (L) Sibthorpe and Smith. Secretion by

 

submersed leaves was found only in the summer. Of the phosphorus trans-
ported from roots and rhizomes, 86 percent was released into the water.
Studies concerning the cycling of phosphorus have been widespread,
reflecting its acknowledged status as a major limiting factor in the
QVOWth Of 51939, The concentration of inorganic or total phosphorus
in water gives little indication of the amount available overall. The
amount of phOSDhorus in the sediment in comparison With the phosphorus
in the water may differ by several orders of magnitude (Wetzel 1975).
High phosphorus concentrations of 7000 ug/g in the sediment Were sev-
eral orders of magnitude greater on a volume basis than the phos-
phorus in the overlying water in some oligotrophic lakes (Williams and
Mayer 1972).
Based on the previously cited studies it is concluded that rooted
aquatic plants take up phosphorus and release this nutrient to
the surrounding water. Few studies, however, have dealt with the effect
of phosphorus on surrounding organisms following release from rooted
plants. McRoy and Goering (1974) showed that labelled nitrogen was

15

transferred from Zostera to an algal epiphyte with the N content being

greater in the epiphyte than in any part of the plant. McRoy cites a

6

I4 32

study where it was found that C and P could also be transferred
from Zostera to epiphytes. Wetzel (1969 a,b) monitored dissolved or-

ganic compound excretion in the macrophyte Najas flexilis (Willd.)

 

Rosth. and Schmidt. noting probable utilization of these compounds by
epiphytic populations. Freshwater algal communities are often limited
in their growth by phosphorus availability (Schindler 1977). Studies of
the mechanisms of phOSphorus transport that result in phosphorus avail-
ability to plankton are clearly warranted.

The forms of phosphorus that can be utilized by algal species vary
as does the amount. Some algal Species possess the ability to take up
phosphorus in forms initially different from inorganic orthophosphate as
a result of their enzymatic activity (Kuhl 1974). Chlorella can deacy-
late phospholipids on its surface and then absorb the phosphorus re-
leased (O'Kelley 1973). The macrophytic alga, Chara, can utilize gly-
cerophosphate to obtain phosphate. Some marine algae are capable of
using glucose-6-phosphate by splitting the compound and using the phos-
phate (Kuenzler 1965). Kuenzler and Perras (1965) found phosphatases
bound to the cell surface of some marine algae that enabled the algae to
obtain phosphorus from phosphorylated compounds.

Nutrient requirements for algae differ (F099 1973, Pubs and Canelli
1970), and any phosphorus released from submersed rooted plants can sub-
sequently affect algal growth providing that the phosphorus is in a form
that can be assimilated. Bacteria and algae have been found to take up
significant amounts of phosphorus from solution. Rigler (1956) found
that rapid turnover of phosphate in a small lake may be due to bacteria
that are competing effectively with algae. Paerl and Lean (1976) used

autoradiography combined with tracer kinetics to demonstrate that

7
initial uptake of phosphorus was by algae and bacteria.

Jordan and Bender (1973) found that the interaction of other nut-
rients with phosphorus could increase the phytoplankton response to
phosphorus assimilation. Growth was enhanced when EDTA, a chelating
agent, and nitrogen were added to cultures containing phosphorus. Half
of the algal species studied were stimulated by the treatments, but
others showed little difference in productivity. These results reflect-
ed those of Lean and Nalewajko (1976). Differences in growth were noted
in axenic cultures of Navicula, Chlorella, Anabaena and Scenedesmus.

Both Chlorella and Scenedesmus could assimilate phosphorus rapidly, how-

 

ever neither culture increased significantly in biomass. In Anabaena,
20 percent of the phosphate present was absorbed before an increase in
growth was noted. No direct correlation between phosphorus uptake and
growth occurred. In Scenedesmus, a lag phase of six to eight days be-
tween phosphorus uptake and cell volume increase was observed. Fogg
(1973) noted that if growth is measured after addition of phosphorus, a
significant increase in population may take several days during which
time changes in the chemistry and flora of the sample may occur.

Faust and Correll (1976) compared the utilization of orthophos-
phate between bacteria and algae by differential filtering of cells.
Depending on the season of the year, assimilation of phosphorus varied
between the two organisms. Bacteria absorbed more phosphorus per cell
than did algae. When concentrations of orthophosphate in water were re-
latively low, phosphorus uptake by bacterial cells_was not significantly
greater than phosphorus uptake by algae, which indicated that phosphorus
uptake per unit biomass and phosphorus concentration in the water were
independent of each other. In this study no competitive advantage was

noted in uptake by bacteria over algae at low phosphorus levels, which

8
suggested that any release of phosphorus from a particular source
could have a nearly equal chance of being taken up by algae or bacteria

in phosphorus limiting waters.

METHODS AND MATERIALS

The submerged rooted aquatic angiosperm Vallisneria spiralis L.
was used for all uptake experiments. This species was chosen since it
has a substantial root system, is small enough for laboratory experi-
ments and is readily available in pet fish stores. A closely related
species, V, americana Michx. occurs as a nuisance plant in many Mich-
igan lakes and streams. The vascular system of Vallisneria is not well-
developed. A central lacuna which results from degeneration of xylem
conducting elements is present, as are conducting elements of the
phloem (Bristow 1975). The stele of the root is surrounded by a well-
developed endodermis. In terrestrial plants the endodermis may regulate
lateral movement of ions and water in the roots (Sculthorpe 1967). Al-
though a reduced vascular system is not directly related to root uptake,
the conspicuous endodermis indicates the importance of roots in ion tran-
sport.

A two-compartment experimental chamber similar to that described by
Bristow and Whitcombe (1971) was used in studies of phosphorus movement.
The intact plant Vallisneria was partitioned into root and foliage com-
ponents so that phosphorus could be introduced into either compartment
(Figure l). The foliage was contained in a 500 ml graduated cylinder.

A 25 m1 Erlenmeyer flask painted black to exclude light fit into the
graduated cylinder and held the roots. By placing the plant through a
hole in a rubber stopper and sealing the area around the hole with sili-
con grease seal and with Parafilm, the roots and root environment was as-

sumed to be sealed from the foliage environment. Two side holes were
9

10

Figure 1. Experimental apparatus partitioning the Vallisneria foliage

and roots

 

 

 

 

 

 

 

 

 

 

 

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Figure l,

12
blown into the flask and covered with septum seals to allow for injec—
tion of phosphorus. A glass tube was placed in another hole in the stop-
per to relieve pressure. As an added measure, 0.1 percent of the inert
blue dye, Evan's blue was placed into the rooting medium. This dye is
not toxic to the plant and is not taken up by plant roots (H. Kende,
pers. comm.); any blue color in the foliage medium would have indicated
leakage. The presence of the blue dye was determined bv placing aliquot
samples of the medium in a spectrophotometer at 620 nm.

Light was provided for 16 hours each day by four Gro-lite fluor-
escent bulbs and four lOO-watt incandescent bulbs with an average light
intensity at the water surface of 5380 lux. Temperature was kept at
25°C i 2 degrees.

For all experiments involving uptake and release of phosphorus,

1/10 strength modified Gaudet's medium (Gaudet 1963) in distilled water
was used to meet daily requirements of the plants. However, no supple-
mental addition of vitamins that may have been essential for growth was
given to the plants. Unlabelled phosphorus as KZHPO4 was added to the
root or shoot compartment at various concentrations: 0, 0.001, 0.01,
0.1, 1.0, or 10 mg/l. Two to five replications per concentration were
used. The pH normally ranged from 6.8 at the beginning of an experiment
to 8.5 at the end. Occasionally a pH of 9 was reached when experiments
extended long intervals. In one experiment 0.01M ACES buffer (HZNCOCH -

2
NHZCHZCHZSO3, pKa at 20°C, 6.9) was added to the nutrient medium. All
glassware was acid washed in sulfuric acid and dichromate, washed with
phosphate-free Liquinox (Alconox, Inc.), rinsed three times in single
distilled water and twice in double distilled water.

Initial experiments were conducted to demonstrate that Vallisneria

 

13

could release phosphorus to the surrounding medium. After the plants were

incubated for three days in the nutrient medium, 10 uCi of 33

33 33

P04 (here-

after referred to as P) in one ml as KZH P04], was injected into the
root environment. Three rooting environments were initially used: li-
quid nutrient medium only, autoclaved silica sand and nutrient medium and
autoclaved Lake Lansing sediment and nutrient medium with four replica-

33P were injec-

tions for each substrate type. One mg/l P04-P and 10 uCi
ted into each substrate. Subsequent experiments used liquid nutrient
medium. Release of labelled phosphorus into the foliage environment was
monitored over time by radioassaying aliquot samples by liquid scintilla-
tion. Where the growth medium was changed, counts from the previous sam-
ple were included. Upon termination of the experiment, plants were washed
twice in distilled water, sectioned into foliage and root, dried for 48
hours at 80°C and weighed. The remaining nutrient medium was filtered us-
ing a Buchner filter and flask with Whatman paper #1 to measure the amount

of 33

P in the algae that had colonized in the medium and placed directly
into scintillation vials for counting.
Foliage and root samples were homogenized, subsampled and digested

in 0.5 ml 10N H SO and 0.2 ml 30% H202, and counted in a liquid scintil-

2 4

lation spectrometer (Packard Tri-Carb Model). Scintillation fluid cock-
tail consisted of ToluenezTriton-X 100 (2:1 v/v) with 0.55% 2,5-diphenyl-
oxazole and 0.01% 1,4-bis-(2-(5-phenyloxyazolyl))-benzene (Allamong and
Abrahamson 1973) as the fluors. Counting efficiency for 33P was 79% to

89%, and corrections for background and decay were made. Quenching

l
33904 (Emax - 0.248 MeV, half-life 25.2 days, specific activity 50mCi/mm)

14
did not affect readings of the samples. ~Phosphorus uptake and re-
lease was quantified by monitoring activity in foliage medium, in root
and leaves of plants, in rooting medium, in epiphytes that had been
scraped from the glass and plants and filtered algae surrounding the

plant, and by calculating the initial specific activity of 33

P injec-
ted into the medium. Plant growth.was not measured.

Total phosphorus in the plant and in the medium was determined by
using modified Fiske-Subbarow reagent (Fiske-Subbarow 1925). After the
digestion procedure, ammonium molybdate was added followed by the rea-
gent made of 0.25 grams 1-amino-2 napthol-l sulfonic acid with 15% sodium
bisulfite and 0.5 grams sodium sulfite. Samples were read at 620 nm in a
Beckman DB Spectrophotometer.

Autoradiography was performed using Kodak X-ray film. Labelled
phosphorus was injected into the rooting nutrient medium, and plants
were removed after 30 seconds, 2, 5, and 10 minutes to observe transloca-

33P. Plants were washed twice in double distilled water, sect-

tion of
ioned, numbered and dried in a plant press to avoid diffusional translo-
cation. Plants were placed on top of the film and kept in a folder press
in the dark for seven days. Film was developed according to Kodak speci-
fications. Whole plants were also autoradiographed from 20 minutes to

24 hours following injection of phosphorus into either the rooting or
foliage medium to observe acropetal or basipetal translocation.

To determine the influence of released phosphorus on algal popula-

tions, aliquot samples were taken from two samples of Vallisneria foli-

 

age medium in which the roots of one plant had been given a nutrient med-
ium with KZHPO4 while the other had received a nutrient medium without

phosphorus in the root compartment. When phosphorus release was assumed

15
to be maximal, the aliquot samples were injected into separate cul-

tures of Scenedesmus quadricauda (Turp.) Breb. Scenedesmus was used be-

 

 

cause it has a high tolerance to phosphorus (Fogg 1973) and can be easily

cultured. Scenedesmus was found to occur normally in the macrophyte

 

medium. Epiphytes present included Oscillatoria sp., other filamentous
forms and diatoms. These algae may have been difficult to culture and

remove due to their adhesion to surfaces.

 

Orthophosphate (P04-P) in the Vallisneria medium was measured using
a Hach kit Stanna-Ver packet. This particular method is not very accur-
ate; however, it was only necessary to know that phosphorus was being
released from the plant that was given phosphate. Cultures of Scenedes-
Egg were grown in modified Bold's Basic Medium (James 1974). Culture
medium was autoclaved before addition of the algae. For experiments,
cultures were phosphorus-starved, and KCl was substituted for NaCl to
satisfy the potassium requirement that previously was met by KZHP04. Two
ml of suspended phosphorus-starved algae were placed into each of 12 Er-
lenmeyer flasks with 100 m1 of nutrient solution. Twenty-five m1 from
the foliage medium of the growing plant was injected into six flasks.
Aliquot samples of phosphorus-starved Vallisneria injected into the other
six flasks were used as controls. Net productivity over time was measur-
ed as increases in organic carbon using a Beckman Total Organic Carbon
Analyzer (Model 915A). Two samples from each culture, one of which was
filtered with a Sum Nucleopore filter to trap algae and let most bacteria
and dissolved compounds pass, were analyzed for total carbon. Prior to
analysis, the samples were acidified with 10N HCl to pH 2 to stop all
reactions and drive off 002 and bubbled with nitrogen for five minutes

immediately before running through the analyzer. By subtracting the

16
filtered from the non-filtered sample, the amount of carbon present in
the algal cells was determined. Standards of 100, 75, 50 and 25 mg/l
were made from a stock solution of anhydrous potassium biphthalate dis-
solved in COZ-free water, which was obtained by boiling distilled water
for 20 minutes. A C02 trap using an ascarite drying tube fitted to a
holed stopper was used to seal the stock solution. A plastic tube was
attached to the stopper for use as a siphon. Sampled cultures were main-
tained at a temperature of 25°C i 2 under fluorescent lights at
4304 lux, and were swirled manually three times per day. Cultures were
also placed on a shaker in a growth chamber to observe possible differ-
ences in growth. The pH of the algal cultures was also monitored over

time as an indicator of growth.

RESULTS

Roots of Vallisneria spirali§_absorbed and translocated phosphorus
rapidly to foliage. This was first observed by autoradiography. Au-
toradiographs showed that within 30 seconds of initial injection, some
labelled phosphorus (33P) could be found in the foliage (Figure 2).
Since leaves were sectioned immediately after removal from the labelled
source translocation to the upper parts of the plant must have occurred
rapidly. A notable difference can be seen in root absorption among the

33P medium for 30 seconds, two, five or 10 min-

four plants placed in the
utes, respectively. Precautions, as previously mentioned, were taken to
prevent and detect leakage. In order to assure that the small amount of
phosphorus found in the foliage medium after a short time was from foli-
age release and not from leakage, the two-minute treated plant was not
replaced in the foliage medium. After injection, a plastic bag was
placed over the plant to prevent dessication. Only roots were submerged
in the nutrient medium. After two minutes, the plant was removed from
the flask, sectioned, dried and mounted on Kodak X-ray film. Labelled
phosphorus was found in the leaves, indicating that translocation had
occurred. Some leakage, however, was detected in the foliage medium of
the 10-minute treated plant which could explain the heavier labelling
seen in the foliage. Little difference in translocation to foliage was

found for the other three treatments.

Whole plants were also autoradiographed over time to observe areas

33

of accumulated P (Figure 3). Higher activity appeared in the midvein

17

18

Figure 2. Autoradiographs of plants, whole and sectioned, given 33P
to the root medium for 30 sec, 2, 5, and 10 minutes show-

ing rapid initial uptake. A photograph is to the right of

each autoradiograph

     

30 SECONDS Two MINUTES
FIVE MINUTES ' 10 MINUTES

 

Figure 2.

20

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21
region of the foliage in all samples. Little difference in foliage

absorption was seen from times ranging from 20 minutes to 24 hours when

33 33

P was injected in the rooting medium. When P was injected into the

foliage compartment a significant difference in phosphorus movement

could be detected relative to plants absorbing phosphorus by roots.

33

Plants exposed to P for 10 hours compared to those exposed for one

hour had significantly higher 33

P in the foliage tissue (Figure 4).
Basipetal movement to the roots was slight after 10 hours.

Release of 33P into the upper compartment from the rooting medium
was monitored over time. The initial uptake experiment was performed

for the sole purpose of testing whether Vallisneria could release absorb-
33

 

ed phosphorus into the water. Figure 5 shows the average P release

for seven plants grown in a nutrient medium. After one day, 1.0 x 10'4

ug P/mg was released. By day five, 1.5 x 10'4 ug P/mg dry weight was re—
leased. The experiment lasted 19 days at which time plants had begun to
show signs of decay. Additional aliquot samples taken at that time were
questionable since phosphorus is more readily released from decomposing
plants (Wetzel 1975).

After six days, the rooting medium was sampled to determine the per-
cent of labelled phosphorus remaining in solution. Based upon three
replications, 86.5% was still in the medium. Aliquot samples that were
taken from the rooting medium on day 20 revealed that 74.7% of 33P
was still present in the nutrient medium. The other 25.3% was assumed
to have been taken up by the roots. Of that amount taken up, 1.7% was
translocated through the plant and released by the foliage into the up-
per compartment medium.

Upon demonstrating that the macrophyte Vallisneria spiralis could

 

SENSE g
E mozmmmuma 25:25 < 92 29263sz $438 5 mozmmmta HE: 2:583 mSo: am 92 ,2
ms: £5 :22 c: 22:. cm QM: mo“. SEE g u: 8 mm zmaw $25.. 59: ...o madame; .m 952“.

 

21
region of the foliage in all samples. Little difference in foliage

absorption was seen from times ranging from 20 minutes to 24 hours when

33 33

P was injected in the rooting medium. When P was injected into the

foliage compartment a significant difference in phosphorus movement

could be detected relative to plants absorbing phosphorus by roots.

33

Plants exposed to P for 10 hours compared to those exposed for one

hour had significantly higher 33

P in the foliage tissue (Figure 4).
Basipetal movement to the roots was slight after 10 hours.

Release of 33P into the upper compartment from the rooting medium
was monitored over time. The initial uptake experiment was performed

for the sole purpose of testing whether Vallisneria could release absorb-
33

 

ed phosphorus into the water. Figure 5 shows the average P release

for seven plants grown in a nutrient medium. After one day, 1.0 x 10"4

ug P/mg was released. By day five, 1.5 x 10‘4 ug P/mg dry weight was re-
leased. The experiment lasted 19 days at which time plants had begun to
show signs of decay. Additional aliquot samples taken at that time were
questionable since phosphorus is more readily released from decomposing
plants (Wetzel 1975).

After six days, the rooting medium was sampled to determine the per-
cent of labelled phosphorus remaining in solution. Based upon three
replications, 86.5% was still in the medium. Aliquot samples that were
taken from the rooting medium on day 20 revealed that 74.7% of 33P
was still present in the nutrient medium. The other 25.3% was assumed
to have been taken up by the roots. Of that amount taken up, 1.7% was
translocated through the plant and released by the foliage into the up-
per compartment medium.

Upon demonstrating that the macrophyte Vallisneria spiralis could

 

 

33

Figure 4. Autoradiograph negative of plant given P to foliage medium
for one and 10 hours (l-r) showing significant difference in

foliage absorption and little translocation to roots

P—33 RELEASE (CPM/ML x A00)

23

7m ..

sun I
552 SEDIMENT

5m ..

LIQUID NUTRIENT

15'!

 

 

N-NHTM P -
E-NflIlfllflbmm------E-EEEN

TIME (DAYS)

22
23

FIGURE 5. AVERAGE 53P RELEASE TO FOLIAGE MEDIUM

FROM VALLISNERIA GROWN IN THREE SUBSTRATES. VERTICAL
BAR REPRESENTS t ONE STANDARD ERROR OF THE MEAN

24

take up 33

P by its roots, translocate it through its foliage and even-
tually release it into the upper compartment, a series of experiments
were carried out to determine if sediment type or varying concentrations
of phosphate influenced phosphorus translocation and release. Figure 5
shows results concerning 33P release when the plant was rooted in nutrient
medium, as described previously, autoclaved silica sand and nutrient med-
ium, or Lake Lansing sediment and nutrient medium. The nutrient medium
contained 10 mg/l unlabelled phosphorus as KZHP04. Total phosphorus
content of Lake Lansing sediment was 200 mg/l. After one day, plants
rooted in the sediment showed a marked increase in release of 33P over
those in sand or water. After four days, those plants rooted in the sand
showed little further release, while those in liquid nutrient medium con-

tinued to release 33

P to the upper compartment.

Fluctuations in the daily amount of 33P released were detected, which
was attributed to reabsorption of phosphorus by leaves of plants, and par-
tially by uptake Of epiphytic and planktonic algae that had colonized the
incubatior apparatus. Epiphytes that were scraped from the plant were

found to contain 0.5% of the 33

P in the foliage medium, which was an un-
derestimate since epiphytes could not be completely removed. Planktonic
algae filtered from the medium contained 1.3% of the 33P in the foliage
medium. Quantitative analysis of epiphytic uptake was not made because
the filter paper was placed directly into the scintillation fluid; epi-
phytes were partially eluted while in the fluid. In one sample with a

sediment substrate, a 2 ml sample of algae was assayed for 33P in their
cells. The upper compartment contained the equivalent of 0.08 ug P04-P.

Of the 33P released by the plant, 8.6% was taken up into the 2 ml sample

of algae.

25
An analysis Of variance was performed to compare daily 33P re-
lease from plants growing on different substrates. A significant dif-
ference at the 5% level (p< 0.05) was found among the three substrates.

An LSD test (Snedecor and Cochran 1967) showed that the release of
from the plants rooted in Lake Lansing sediment was significantly greater
at the 1% level (p<0.01) from those rooted in sand and nutrient medium
only. No significant difference between sand and liquid nutrient medium
treatments was found. Using the Lake Lansing sediment for further study
created a problem. A standard phosphorus concentration in the sediment
could not be predicted, adsorption of labelled phosphorus to sand was
likely, and adequate mixing of the isotope could not be attained. Also,
since Lake Lansing sediment is sandy, it seemed likely that increased
phosphorus release was due to the high concentration of phosphorus and
not to the ability of the plant to take up ions more successfully in
that type of substrate. Therefore, liquid nutrient medium was used for
all further experiments.

Two experiments were performed to determine if D04-P concentration
had an effect on 33P uptake and release. In one experiment, concentra-
tion was varied in the compartment with roots, while in the other exper-
iment, foliage compartment concentration was varied. In the latter,
concentrations of 0, 0.001, 0.01, 0.1, 1.0, and 10 mg/l unlabelled P04-P
were used with two replications. An equal amount of labelled phosphorus
(10 uCi/ml) was injected into the rooting environment of each plant.
Table 1 shows the amount of labelled and total phosphorus in the plant
and labelled phosphorus in the foliage medium at the end of the experi-

ment for the five concentrations. High variability within treatments

was observed. Plants having similar weights were used but differences

26

 

m.F Namap m.m omopm o.o_
m.o muoNN 0.0 NNmNF o._
~.o oooam N.F Nomop _.o
o.p ommmm o.o Fpmm po.o
m.o mmmum F.o mmmm Foo.o
o.~ mOFoN o.o ammo_ o
. . _.,. . . A—\mev ucwspemasou goo;
A“: age ms\m may A»: age ms\Equ mmmrpou Ease Asauv op a wu: op ;p_3 mcopm

mm

mmmvpom :P a ~wuoh unoppom er a umueuopmcegp a . szvumz cw mmm umuum covpmgucmucou niece

mm
,.smco?ungucmocousx?m so» PF.Xmu um macmsugmqaou oweummnfipc> cw ngosamoca mo camwuoasoo ..p «Fame

 

27
could have occurred in root biomass, number of leaves or leaf size, all
factors contributing to possible increased variability.
Linear regression analysis indicated that the only significant dif-

33P in the leaves and

ference between treatments was in the amount of
the foliage medium (p<0.01), where a negative correlation of r = -0.58
was observed. There were indications that one of the treatments using
10 mg/l unlabelled P04-P had leaked to the foliage medium. When this
compartment was excluded from analysis, a greater negative correlation
of r = -0.74 was found (Figure 6). No significant correlation was found

33

between P in the medium and total phosphorus in the plant (r = -0.41)

or 33P in the foliage and total phosphorus in the foliage (r = -0.47).
Differences in concentration in the foliage medium did not have a

significant effect on release of 33

P from the foliage through the first
six days of the experiment (Figure 7). Following that time, phosphorus
release was significantly greater (Pf0.01) in the plant having the high-
est concentration (10 mg/l) as compared to the other concentrations (LSD,
p<0.05). These results exclude the one replication where leakage was
noted. Concentrations from 0 mg/l to 1 mg/l showed no significant dif-
fErences at the 5 percent level. The nutrient medium was changed
frequently, and epiphytes did not colonize the containers. Measure-
ments of the amount Of phosphorus released from the plant were accumu-
lated over time to account for changing of the medium.

33

Translocation of P in roots and foliage varied according to the

concentration of unlabelled phosphorus injected into the rooting medium.

Three concentrations, 0, 0.01, and 10 mg/l P04-P were placed into the

various rooting media along with 10 uCi K2H33P04 in one ml. Table 2

f 33

shows the amount 0 P in each compartment at the end of the

33P IN FOLIAGE MEDIUM (CPM)

28

35000 '

30000 L

25000 :

20000

15000»

10000

5000

 

 

I L l.

0 10000 20000

A J L _A_

30000 40000 50000 60000

J __l

33P IN FOLIAGE (cPM/MG DRY wEIGHT)
FIGURE 6. REGRESSION-OF LABEL-LED P.. PRESENT IN VALLISNERIA

 

LEAVES AND IN THE NUTRIENT MEDIUM IN WHICH IT GREw.

P-33 RELEASE (CPM/ML x 350)

B - N H T H m b m m

29

 

 

 

L.
"U-
~1-

..
G-
G.
q)-

.1.
d»

TIME (DAYS)

FIGURE 7. AVERAGE 33P RELEASE FROM VALLISNERIA
FOLIAGE TO NUTRIENT MEDIUM wITH VARYING P

 

CONCENTRATIONS IN THE FOLIAGE COMPARTMENT.

30

:owuagomee mmepm ece mmepm :o mouxgawaw emaegumcz sop» a

mm eo mmop op mze goo. cage mmmp mpepoh
.4.

 

p._ mn.p m.ee m.m m._m .o.o
o.P mN.m m.mm m.m_ e.mp _o.o
o.o N.Pm m.~ N.~ _.N o
:35
aspemz meoom Espemz A.uz xge may pcmsueeasoo poem
woe c m P o F .e
Ppom w emmx e e._ m 2. ammx cw amma poem =_ ammx mmeEoFm poem cu emeee a on

 

 

N~ xee we sawems mcwzogm use ece e_em:mwp_e> cw wagesamozm “cause; .N mpaeh
a.

31

 

«To, x m.N . m

e._m u me.o ~.Nm o.~ miop x m.¢ I d Fo.o
eiop x ~.~ . m

o.__ . m_.o o~.¢~ e.m mio_ x ¢.m . u Po.o
«lop x ¢.P . a

m.m . no.0 o_._ e.om euop x F.m i m o
aneme mme_Pom mmewPom seem maoog seem Augmwmz Age ms\mnv

Eoge emueuopmceg» & emueuopmcesu Ezwemz ewueoopmcegu Amy muoom ece AP\mEV ucmspgeasoo

-maesgaeaw,=p anme

aaa_.aL e_ Emma

A P

aaae_ae a“ amme New ama_PaE =_ a .aeae

e

Seam a“ canoe 2. ca

.Ae.pea8v N apaae

32

33P injected

experiment (12 days) for two concentrations. Most of the
into the rooting medium was translocated after 12 days. The amount of
phosphorus in the root compared with the phosphorus in the foliage dif-
fered significantly. At 0 mg/l PO4-P, 51.7% of the 33P injected into
the rooting medium was found in the foliage, while only 1.8% was found

33

in the roots. At 0.01 mg/l PO4-P, 55.5 and 66.5% P was found in roots

with 5.75 and 1.75% found in the foliage, respectively, which indicated
that total phosphate was limiting.

Overall, little 33

P was released to the foliage medium. On a per-
centage basis, the amount of phosphorus released to the medium from that
translocated through the foliage ranged from 1.1% in the treatment with
zero phosphate added to 37.7% of 33

0.01 mg/T PO

P released from a sample treated with

4-P.

Figure 8 shows the average amount of 33

P released per ml of media
to the foliage medium for the three treatments based on four replica-

33

tions per treatment when P and P04-P were added to root compartments.

In individual samples, reabsorption of phosphorus by the leaves

may have occurred. A significant difference in release was seen at

days 11 and 12 (p<0.05). The amount Of 33P released from the treatment
with 10 mg/l PO4-P was equivalent to 0.05 pg P04-P. Epiphytes and plank-

33P. Table 2 shows that

ton colonized all cylinders and incorporated
31.6% and 11.6% of 33P released to the foliage medium was taken up by

epiphytes for the two treatments that were given a concentration

of 0.01 mg/l P04-P. Nine percent of the 33

P released from the 0 mg/l
treated sample was taken up by epiphytes.
A fairly high percentage of labelled phosphate was unaccounted for

in all cases, ranging from 18 percent to 38 percent, which could have

33P RELEASE (CPM/ML x 300)

33

 

 

 

325 T
*——— - 10 m 1
3m .L g/
E—I- ‘ 0.011119/1 ‘
27: . X‘“ ‘ 0 "‘9“ ‘
‘1'
25D . . I
i :
225 . 1‘!
2m .1 -
I7: . ,
. l .-
.aa .. . ,,EI
qjé==*F——‘
125 u / x’
//
.aa ~- .2 7
. / ' '
7E _ up- ”’/
P‘>~—- ‘
5D . , ./
3:. ..
25 ., .J_. ‘L
B : :~ : : : i i t i— t ,; :
D - N
a «- 1! II I II II F D In - - -

TIME (DAYS)

FIGURE 8. RELEASE OF 33P FROM FOLIAGE TO SURROUNDING
MEDIUM WITH THREE CONCENTRATIONS OF PDQ-P IN ROOT

COMPARTMENT. VERTICAL BAR REPRESENTS i
ERROR OF THE MEAN

ONE STANDARD

34

resulted from absorption of phosphorus by epiphytes on the glass container
and flask. Much of the algae could not be successfully scraped from the

33

walls and counted, and consequently, the amount of P in epiphytes was

33P they incorporated. No

an underestimate of the actual amount of
quantitative measurement of phosphorus per cell was made. Table 3 shows
the results of an experiment similar to the previous one but with some
variations. Five replications per concentration were used. Concentra-
tions were 0, 0.01, and 0.1 mg/l P04-P with the addition Of 5 uCi of

K2H33Po4 in one m1. Little 33

P was translocated to the foliage from the
roots for all concentrations. For this experiment, chewing gum rather
than a grease seal was used as a sealant (Twilley gt_al, 1977) and.

packed extremely tight into the holes of the stopper. This may have
caused some damage to the delicate plant leaves preventing optimal
translocation. Also a buffer, ACES (Norman Good, pers. comm.) was added
to the nutrient medium. It has been noted in algae that pH may change

the rate of P04 absorption in several ways (Wetzel 1975) which may hold
true for submersed macrophytes as well. Plants began to senesce before
epiphytes colonized the container. A similar trend, however, was seen for

f 33

average replicates O P in the foliage for three concentrations as in

the previous experiment. As the concentration of unlabelled P04-P was

33P

increased from 0 to 0.1 mg/l in the root compartment, the amount of
translocated to the foliage decreased. When unlabelled P04-P was not
added, translocation of 33P was greatest. Differences between treat-
ments, however, were not significant (p>0.10). In contrast, similar

trends in 33P root absorption were not found. Most of the 33P should
have been found in the roots for the treatment given 0.1 mg/l PO4—P with

a percentage significantly higher than 46.4, however, this was not

35

 

mo.owo~.o mm.owmm.o N.enp.- m.Nme P.o
No.owop.o a.~nw.a m.NAN.oa m.ma_.mm _o.o
.o.oneo.o m.mn_.N m.o_ae.em m.mnee o

AP\mEV acmEpgeasou goo;

seeeaz e=_eaz ace op eaeea amm _8a m

ama__aL a? amme ama__ae =_ amme muaam e? amme Beam a? amme ;e_z =a_eaceeneeae a-eoa

 

mcopuegacmucoo muggy an macmsugeanu pcmgm$m_e cw m xee am ngocamosm ecmogma .m mpneh

36

33

Observed. Less than 19% of the P injected into the root compartment

was taken up by the roots. The remaining 81% was still in the root med-
ium. Although a similar trend did not emerge for all treatments, there

was some evidence that concentration variation in the root compartment

33

had an effect on P translocation and release.

33P on surrounding

To better understand the effect of released
epiphytic and planktonic algae, cultures of the planktonic green alga,

Scenedesmus ggadricauda were grown in separate flasks. The results of

 

added aliquot samples Of Vallisneria medium to the net primary produc-

 

tivity of the algae is presented in figure 9. The Upper curve repre-

sents those algae given foliage media from Vallisneria with P04-P added

 

to the root compartment. The lower curve represents cultures given

 

foliage media from Vallisneria without P04-P added to the root compart-
ment. After six days, a significant difference was observed (p<0.01) be-
tween the algal cultures. While total organic carbon as a measure of net
productivity had increased to 29.5 mg/l total carbon in the phosphorus-
enriched one, net productivity in phosphorus-starved cultures fell to

9.5 mg/l carbon. The decrease in carbon was likely due to cell lysis of
the algae. In individual samples, five of the six phosphorus-starved
cultures crashed by day six. Only one of the phosphorus-enrichéd cul-

tures showed a decrease in total organic carbon at this time.

Single cultures, one.phosphate—enriched and one phosphate-starved.
were grown on a shaker in a growth chamber to observe if constant shak-
ing of the culture altered the results. It was indicated (Figure 10)
that constant shaking had no significant effect on cultures as compared
to intermittant shaking.

Supplemental experiments were done using pH as an indicator of

TOTAL ORGANIC CARBON (MG/L)

37

 

 

 

 

3" T —::—— ,e
25 I
/
an . ......
.1.
I: I
ID .. J
5 1
a : t : : t t— I
a - N n I u u r

TIME (DAYS)

FIGURE 9. NET PRODUCTIVITY OF SCENEDESMUS QUADRICAUDA
AFTER ADDITION OF FOLIAGE MEDIUM FROM P ENRICHED AND P
STARVED VALLISNERIA. C3-—-- ENRICHED;(>--- STARVED).
VERTICAL BAR REPRESENTS t ONE STANDARD ERROR OF MEAN

 

 

TOTAL ORGANIC CARBON (MG/L)

38

 

 

25 ..
'2
an .
55 db
Ia ‘
E . ,
a t *t : t t :
a - N n T u m

TIME (DAYS)

FIGURE 10. NET PRODUCTIVITY OF P STARVED AND P ENRICHED
SCENEDESMUS GROWN WITH CONSTANT SHAKING. (Cl- ENRICHED;

 

x - STARVED)

39
increased net productivity. No differences in pH were noted between
the two cultures With and Without phosphate. The pH increased from

7.5 to 11 before algae became stressed in both cultures.

DISCUSSION

PhOAphonuA uptake, tnane£ocation and.aeflea32
The amount of labelled phosphorus accumulated in roots or foliage

of Vallisneria, and the amount released by the foliage were dependent

 

upon the concentration of P04-P supplied to root or foliage compartments.

 

Only when Vallisneria was supplied with a high concentration of P04-P did
33P release to the water increase significantly. Experiments of Barko
and Smart (in press) have suggested that nutrients that are available
from the sediment in surplus of the plant's requirement should be re-
leased in significant concentrations from rooted aquatic plants. The a-
mount of total phosphorus present in the foliage for individual plants
differed greatly, which may explain why P04-P concentration did not show
significant differences in 33P release relative to the concentration
over tenfold increases in some cases. It was assumed, in the analysis
of data, that the ratio between 33P and unlabelled phosphorus remained
constant as phosphorus was absorbed, translocated or released from the
plant (Whittaker 1961).

Similar amounts of 33

P release into the foliage medium resulted
whether PO4-P was supplied to the root or shoot medium. Both of these
sites have been found to be active in nutrient Uptake in many aquatic
plants (Denny 1972). Bristow (1975) found that the level of phosphate
in the foliage compartment medium was unrelated to rate of translocation

of 32 32

P. He concluded that the uptake of P by one compartment was

40

41
independent of the supply of 32F in the other compartment. In the

Vallisneria studies, phosphate levels in the foliage medium significant-

 

ly affected 33P release from the plant. Autoradiographs further con-
firmed that phosphorus uptake could occur through the foliage Or through
the roots. Acropetal and basipetal translocation occurred within the
plants as seen from the autoradiography. Bidirectional movement of phos-

phorus has been found in other macrophytes including Zostera marina (Mc-

 

Roy and Barsdate 1970), Myriophyllum exalbescens (DeMarte and Hartman

 

1974), and Nuphar luteum (Twilley et a1. 1977). Nichols and Keeney

 

(1976) found some bidirectional flux of nitrogen in Myriophyllum spica-

 

gym, In Vallisneria, basipetal movement did occur; however, plants ex-
33

 

posed to P for one hour appeared to incorporate as much 33P in their
roots as did plant foliage exposed for 10 hours to 33P (Figure 4). Rate
of translocation from leaves to roots was not measured. Schults and Ma—

lueg (1971) working with Vallisneria americana found translocation of

 

phosphorus occurring in both directions with more translocation from roots
to the foliage than downward from foliage to roots.

Autoradiographs supported findings that suggested initial rapid up-
take and translocation from the roots (Figure 2). That 51.7% of 33P in-
jected into the rooting medium was found in the foliage with 0 mg/l P04-P
added and only 5.8% 33P was found in the foliage with medium at 0.01 mg/l
PO4-P suggested that total phosphorus was limiting in the tissue. This
data could explain why release of 33P into the fOliage medium relative to
the amount taken up by roots was relatively low (Table 2). Only when the
plants were given an excess of nutrients, as was the case with the Lake
Lansing sediment (200 mg total P/l) or the addition of the 10 mg PO4-P/1

treatment, was the percent release of 33P much greater. Gerloff and

42
Krombholz (1966) found the critical phosphorus tissue content of

Vallisneria americana, a closely related species of V, spiralis, to be

 

0.13 percent by weight. Above this level, addition of phosphorus would
result in luxury consumption. However, the biomass of the roots and

foliage may also have influenced phosphorus uptake. The dry weight of
roots of plants given 0.01 mg/l PO4-P was 18.4 mg and 51.5 mg for those

33P, respectively. The

plants absorbing 55.5 percent and 66.5 percent
dry weight of the root given 0 mg/l PO4-P was only 2.1 mg for that plant
absorbing 1.8 percent labelled phosphate. Plants with greater root

biomass may be able to absorb and translocate more phosphorus then those

plants with less root biomass.
Those plants given 0.1 mg/l PO4-P to the root compartment did not
absorb as much 33P as those plants given less inorganic phosphate (Table

3). Dry weights indicated that on the average, plants that had the

33

least root biomass took up less P. Plants given 0.1 mg/l PO4-P had an

average root weight of 6.68 mg, while they absorbed an average of 17.1

percent 33P. Plants given 0.01 mg/l had roots weighing 16.7 mg, and
they absorbed 40 percent 33P. The latter set of plant roots weighed
2.4 times more than the former and absorbed 2.8 times more 33P. It is

likely that phosphorus absorption is a function of root biomass and ion
concentration. Bristow gt_al, (1971) found a correlation between root

mass and the proportion of phosphate taken up by roots in Myriophyllum

 

brasiliense.

 

Macnaphytc-aigaz Lnteaactiond
A direct transfer of phosphorus from Vallisneria to the epiphytes
colonizing the plant and to algae in the foliage compartment was found.

Epiphytic and planktonic algal populations absorbed up to 31.6 percent

43

of the 33

P released to the medium from the plant. Since epiphytes
could not be completely scraped from the containers or plant, this is
likely an underestimate of the amount of 33P in algal cells.

Allen (1971) studied the release of dissolved organic carbon from

Najas flexilis to its epiphytic community. His data suggested that an

 

epiphyte-macrophyte interaction occurred in relation to carbon release.
A similar interaction might have occurred with phosphorus release in

Vallisneria. Although uptake and translocation occurred immediately

33P, release of phosphorus fluctuated, and usually

33

upon injection of
significant differences between P04-P concentration and P release were
not seen for several days. Where plants began to senesce before epiphy—
tic colonization occurred on the glass or plant, only 2.02 percent of
33P translocated was released for the treatment using 0.01 PO4-P (Table
3). Where colonization did occur, 37.7 percent of the 33P translocated
was released for the same concentration. However, for PO4-P concentra-
tions of 0 mg/l, percent release for colonized and uncolonized contain-
ers was similar. Hutchinson (1975) suggests that the excretion of dis-
solved organic compounds leading to the increase in epiphytic coloniza-
tion may be an adaptive mechanism for plants since invertebrates of
freshwaters generally feed on epiphytic communities rather than on the
higher plants. This same adaptive mechanism may be true for phosphorus

release from plants.

Increased net productivity of Scenedesmus cultures indicated that

 

the algae could utilize the phosphorus released from the macrophyte.
All algal cultures showed an increase in net productivity through the
first four days after being given aliquot samples from phosphorus-en-

riched and phosphorus starved Vallisneria. Since algae have been found

 

44
capable of luxury consumption Of phosphate, an increase in net produc-
tion over a short time could occur for both phosphorus-starved and
phosphorus-enriched cultures. Allen (1971) suggested that organic
materials released from Ngja§_first underwent microbial degradation be-
fore being absorbed by algae. The form of phosphorus released from Val;
lisneria was not determined. Since axenic cultures were not used, bac-
terial degradation prior to algal uptake may have occurred. Lean and
Nalewajko (1976) discussed the relationship between algal uptake of phos-
phorus and phytOplankton productivity. Algal cell volume and phosphorus

uptake were not directly correlated. In their cultures of Scenedesmus,

 

almost all 32? was absorbed by the algae before cell volume increased.
In the present study, it was not analyzed if phosphorus uptake and net
productivity were directly correlated; however, an increase in net pro-
ductivity was attributed to the addition of phosphorus to the cultures,

derived from Vallisneria foliage.

 

The contribution of released phosphorus from aquatic macrophytes to
surrounding algal and epiphytic populations could be significant, espec-
ially in phosphorus-limited waters. In an estuary, McRoy and Goering
(1974) studied the transfer of nitrogen from sediments through macrophytes
to an epiphytic community and offered this as an explanation for high
stocks of epiphytic algae in low nutrient waters. In phosphorus-limited
waters, in the littoral area of lakes, rooted macrophytes may contribute
to an increase in algal productivity. Phosphorus lost to the sediments
could be absorbed by rooted aquatic plants and released from their leaves
into the water making available a previously unobtainable source of phos-
phorus for epiphytic and planktonic algal populations. Phosphate release

from Elodea is currently being studied jg_situ (G. Lie, in preparation).

45
Further studies should examine the rates of phosphorus release from
other rooted aquatic plants and the impact on biota in natural waters.
A knowledge of the form of phosphorus released is needed as is the
extent to which phosphorus becomes available to biota. This may be im-

portant in predicting the trophic status of freshwater lakes.

CONCLUSIONS

Phosphorus released from the foliage of Vallisneria was partially

 

incorporated into epiphytic and algal populations. Net productivity of
algae was enhanced when supplied with the foliage medium in which the

phosphorus was released from the macrophyte. The total phosphorus re-

leased from the foliage was relatively low and may be attributed to se-
veral factors. The enclosed root compartment could have created a very
different environment than that in which the plant had been previously
grown. Acclimation to the nutrient medium and partitioned environment
may not have been successful within the three days prior to treatment.

It was also indicated that plants were phosphorus limited in many cases.
Phosphorus limitation could be a major cause Of a small percentage of

released phosphorus into the foliage medium.

46

APPENDIX

APPENDIX

Al. Nutrient composition of modified Gaudet's medium (Gaudet 1963)

Compound

Calcium nitrate (Ca(N03)2.4H20)
Potassium nitrate (KNO3)
Potassium chloride (KCl)
Magnesium sulfate (M9504.7H20)
Sodium sulfate (NaZSO4)
Manganese sulfate (MnSO4.H20)
Zinc sulfate (ZnSO4.7H20)

Boric acid (H3803)

Potassium iodide (KI)

Copper sulfate (C0504.5H20)
Ammonium molybdate ((NH4)6M07024)
Sodium-Iron EDTA

Sodium bicarbonate (NaZHCO3)

Concentration
(mg/l)

30

8

6.5
75

20
0.34
0.30
0.15
0.075
0.004
0.008
0.5
30

47

48

A2. Nutrient composition of modified Bold's Basic Medium (James 1974)

Compound Concentration
(mg/l)
Sodium nitrate (NaNO3) 250
Magnesium sulfate (M9504.7H20) 50
Dipotassium phosphate (KZHPO4) 100
Potassium monophosphate (KH2P04) 150
Calcium chloride (CaClz) 25
Sodium chloride (NaCl) 25
Sodium bicarbonate (NaZHCO3) 100
Zinc sulfate (ZnSO4) 8.8
Molybdenum tetraoxide (M004) 0.71
Cobalt nitrate (Co(N03)2.6H20) 0.49
Manganese chloride (MnClZ) 1.44
Copper sulfate (CUSO4.5H20) 1.57
Boric acid (H3803) 11
EDTA stock 50

H-Fe stock 4.98

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