Differential salt deposition and excretion on leaves of Avicennia germinans mangroves

Abstract

We examined the control of salt ex-cretion by leaves of the mangrove Avicennia germi-nans. Endogenously secreted salt on intact leaves was compared to exogenously deposited salt spray on excised leaves. Lower salt excretion was observed on seaward leaves with high salt deposition, and higher salt excretion was found on landward leaves with low salt deposition. Similar total salt loads ac-cumulated on leaves located throughout the tree crown, suggesting that Avicennia germinans controls salt excretion at the foliar level by responding to variation in salt deposition. We propose that the salt excretion rate in leaves is a response to the gradient of salt between the xylem and leaf surface.

Full text

Caribbean Journal of Science, Vol. 44, No. 2, 267-271
Copyright 2008 College of Arts and Sciences
University of Puerto Rico, Mayagu¨ez
Differential salt deposition and
excretion on leaves of Avicennia
germinans mangroves
MEGAN E. GRIFFITHS
1,
*, RANDI D. ROTJAN
2
,
and GEORGE S. ELLMORE Department of Biol-
ogy, Tufts University, Medford, MA 02155,
U.S.A.,
1
Current address: School of Biological
and Conservation Sciences, University of Kwa-
Zulu-Natal, Private Bag X01, Scottsville, 3209,
Pietermaritzburg, South Africa,
2
Current ad-
dress: Department of Organismic and Evolu-
tionary Biology, Harvard University, Cam-
bridge, MA 02138, U.S.A., *Corresponding au-
thor: Griffithsme@ukzn.ac.za
ABSTRACT.—We examined the control of salt ex-
cretion by leaves of the mangrove Avicennia germi-
nans. Endogenously secreted salt on intact leaves
was compared to exogenously deposited salt spray
on excised leaves. Lower salt excretion was observed
on seaward leaves with high salt deposition, and
higher salt excretion was found on landward leaves
with low salt deposition. Similar total salt loads ac-
cumulated on leaves located throughout the tree
crown, suggesting that Avicennia germinans controls
salt excretion at the foliar level by responding to
variation in salt deposition. We propose that the salt
excretion rate in leaves is a response to the gradient
of salt between the xylem and leaf surface.
KEYWORDS.—mangrove, salt balance, salt excretion,
salt spray, Hummingbird Cay, Bahamas
Plants that grow in tidal areas are sub-
jected to high salinity both at the root level
through seawater inundation (Morrow and
Nickerson 1973) and at the leaf level
through the deposition of airborne salt
spray (Boyce 1954). While many halophytic
species rely on ions from seawater to main-
tain negative water potential (Scholander et
al. 1962; Paliyavuth et al. 2004), the total
concentration of salts within the growing
tissues needs to be regulated because so-
dium and chlorine are toxic at high levels
(Hasegawa and Bressan 2000; Zhu 2001). In
the mangrove genus Avicennia, high salin-
ity in leaf cells can inhibit ATPase activity
(Cherian et al. 1999), leaf production
(Clough 1985) and photosynthesis (Cherian
et al. 1999; Sobrado 1999). To limit ion con-
centration in the transpiration stream, the
roots of these plants filter over 90 percent of
salt from the water they uptake (Scholan-
der et al. 1962; Drennan and Pammenter
1982). Additionally, Avicennia spp. decrease
salt accumulation in leaves by storing ions
in the leaf hypodermis, followed by active
excretion through glands on the leaf sur-
face (Waisel et al. 1986; Smith et al. 1989;
Dschida et al. 1992; Balsamo and Thomson
1995).
As a result of excretion, salt crystals build
up on the leaf surface. This salt layer re-
mains until it is removed by rain or high
winds (Ish-Shalom-Gordon and Dubinsky
1990). Plants that actively secrete salt pre-
sumably have mechanisms to prevent salt
from re-entering the leaves. However,
breaks in the leaf cuticle and stomata can
serve as entry points for salt to penetrate
the leaf tissue (Boyce 1954). Mangroves
growing in coastal areas also accumulate
salts on the leaf surface deposited by salt
spray (Smith et al. 1989). While it is recog-
nized that mangrove salt management (fil-
tration, storage, and excretion) responds to
salt content within the leaf (Boon and Alla-
way 1986; Dschida et al. 1992; Balsamo and
Thomson 1993), it is not known whether
this process is affected by salt load on the
leaf surface. Total salt load is the product of
salt spray deposited on the leaf surface by
exogenous processes, plus endogenous salt
deposited by excretion from leaves. If en-
dogenous excretion can detect and respond
to surface salt deposited by exogenous
wind and wave action or removed by rain-
fall, salt-secreting mangroves such as A.
germinans may be able to balance salt load
throughout the crown to compensate for
differences in crown exposure to salt spray
or rain.
The objective of this study was to deter-
mine whether A. germinans controls exter-
nal concentrations of salt on leaf surfaces
throughout the crown. We measured the
267

rates of salt spray deposition and of salt
excretion on the leaves throughout the
crown. We hypothesized that leaves on the
side of the plant facing the ocean would
accumulate more salt spray as a function of
exposure to salt-laden winds. Drennan and
Pammenter (1982) have documented fluc-
tuations in salt excretion by leaves of A.
marina, suggesting control of excretion at
the foliar level. We further hypothesized
that plants would secrete higher levels of
salt on leaves that accumulated less salt
spray than on leaves already laden with ex-
ogenously deposited salt.
Our field experiment was carried out at
Hummingbird Cay Field Station, located in
the Jewfish Chain west of Georgetown,
Great Exuma Island, Bahamas (23°32⬘N,
75°50⬘W). Focal plants were located along
the low-energy northwestern coast of
Hummingbird Cay in an area with moder-
ate wave action and salt spray deposition
(seaward = 0.11 mg/cm
2
/d
1
and land-
ward = 0.08 mg/cm
2
/d
1
). Rooting sub-
strate was oolitic mud, submerged every 12
h by seawater.
Avicennia germinans L. tolerates fluctuat-
ing salinity in coastal regions throughout
the tropical and subtropical Atlantic (Tom-
linson 1986; Dodd et al. 2000). Previous
studies of A. germinans on Hummingbird
Cay showed that the species thrives in la-
goons where salinities consistently exceed
that of seawater (Morrow and Nickerson
1973), indicating a high level of salt toler-
ance. Our samples were collected from five
mature trees of A. germinans equivalent in
size and growing at equal distances from
the high tide mark. Avicennia germinans has
opposite leaves. Twenty leaf pairs were se-
lected on each plant used in the experi-
ment, with ten leaf pairs located on the sea-
ward side of the crown, and ten on the
landward side. For each leaf pair, one leaf
was excised and wired in the same place on
the stem, while a second intact leaf re-
mained attached to the plant. This allowed
us to differentiate between salt load from
excretion and deposition; the attached leaf
accumulated salt on the leaf surface as a
result of both excretion and salt spray de-
position, whereas the excised leaf, cut off
from its water supply, only accumulated
salt through salt spray deposition. There
had been no rain for at least 5 d before the
experiment.
Prior to the start of the experiment, total
salt load on individual leaves was deter-
mined by rinsing the leaf surfaces with 20
ml of fresh water and measuring electrical
conductivity of the rinsing solutions with
Oakton TDSTestrs 3 and 4
TM
(Oakton In-
struments, Vernon Hills, Illinois). The fresh
rinse ensured that focal leaves began with
no surface salt at the onset of our experi-
ment. Washing leaves can effectively re-
move secreted salt (Boon and Allaway
1982), although other studies have demon-
strated that low levels of salt excretion are
possible after excision has occurred (So-
brado 2001). We therefore monitored salt
excretion on control leaves taken to the
laboratory and found that the excised
leaves produced no measurable salt excre-
tion.
Following 72 h of exposure, leaves were
collected and salt load was determined by
re-rinsing the leaf surface to measure the
resulting conductivity. To convert conduc-
tivity to salt deposition, we used a standard
curve from 70 samples of NaCl eluted in 20
ml of deionized water. The relationship be-
tween conductivity and salt spray was: salt
[mg] = (conductivity [␮S] + 4.333 [␮S]) /
115.667 [␮S/mg]. The outline of each leaf
was traced onto a sheet of paper and traces
were scanned with NIH Image software to
calculate leaf area. Each leaf area was mea-
sured in cm
2
so that salt load could be ex-
pressed per unit leaf area (mg cm
−2
). Total
salt load on excised leaves demonstrated
the rate of salt spray deposition for a leaf
pair. Salt excretion was determined by sub-
tracting the salt spray deposition value for
the excised leaf from the total salt load (de-
position + excretion) on the attached leaf in
the pair.
We used 2-way ANOVAs to look for dif-
ferences due to location within the crown
(landward and seaward) and each indi-
vidual tree (n = 5) for variables of total salt
load, endogenous salt excretion and exog-
enous salt deposition. We used a paired t-
test to compare leaf area within leaf pairs,
and a 2-sample t-test to compare leaf area
on landward and seaward sides of the
NOTES268

crown. All statistical analyses were con-
ducted using Systat 10 (SPSS Inc.), and data
met the assumptions of each statistical test.
We found that the total salt load accumu-
lating on attached leaves was equivalent on
the seaward and landward sides of the
same plants (Figure 1a; Table 1). However
the source of the accumulated salt differed
according to leaf location within the crown.
Leaves from the seaward sides of plants
had significantly higher levels of salt spray
deposition than did leaves from the land-
ward sides of the same plant (Figure 1b;
Table 1). On the other hand, landward
leaves had higher levels of salt excretion
than did leaves from the seaward side of
the same plants (Figure 1c; Table 1). Leaf
area did not differ significantly within leaf
pair (paired t=−1.65, df = 98, P= 0.103) or
location in the crown (2-sample t=−0.35,
df = 196, P= 0.726).
Our work indicates that two mechanisms
contribute to salt load on the surface of A.
germinans leaves: one exogenous process
(deposition of salt spray) and one endog-
enous process (excretion of salt). We found
that salt spray deposition on A. germinans
differs among leaves from the seaward and
FIG. 1. Comparison of seaward versus landward
salt concentrations on leaves of Avicennia germinans on
Hummingbird Cay, Bahamas. Salt concentrations
were determined by leaf pairs consisting of one living
attached leaf and one opposite leaf excised, but wired
onto the node (n = 20 leaf pairs per plant, 5 plants
examined). The upper figure (a) shows total salt load
leaf pairs prior to manipulation (salt spray deposition
+ salt excretion), the middle figure (b) shows salt
spray deposition after 72 h (measured from excised
leaves), and the lower figure (c) shows salt excretion
(determined by salt levels on attached leaves).
TABLE 1. Two-way analysis of variance on the ori-
entation (landward, seaward) of Avicennia germinans
(n = 5 trees, labeled as Plant ID) on Hummingbird
Cay, Bahamas, df = degrees of freedom; SS = sum
of squares.
Source df SS FP
(a) Total salt load
Orientation 1 0.70 2.65 0.107
Plant ID 4 3.44 3.26 0.015
Orientation * Plant ID 4 0.25 0.24 0.917
Error 89 23.52
Total 98 27.91
(b) Salt spray deposition
Orientation 1 0.33 9.52 0.003
Plant ID 4 0.28 2.08 0.092
Orientation * Plant ID 4 0.10 0.73 0.577
Error 78 2.67
Total 87 3.37
(c) Salt excretion
Orientation 1 0.60 12.36 <0.001
Plant ID 4 0.68 3.53 0.010
Orientation * Plant ID 4 0.21 1.09 0.366
Error 89 4.32
Total 98 5.82
NOTES 269

landward sides of the same plant, suggest-
ing that location within the crown is impor-
tant in determining how much salt spray
accumulates on a particular leaf. However,
the total salt load on the surface of each
individual A. germinans leaf is similar, indi-
cating that the plants may compensate for
higher salt spray deposition on the seaward
side of the crown by differentially secreting
salt on leaves on the landward side. This
ensures that leaves have a similar overall
salt load on each leaf, regardless of differ-
ences in salt spray deposition. We propose
that the excretion rate in leaves is a re-
sponse to the gradient of salt between the
xylem and leaf surface. The sharpest gradi-
ent (promoting the highest excretion rates)
would be on leaves with the least salt de-
position already on them. If there is asym-
metry in salt deposition, there would be
complimentary asymmetry in salt excre-
tion, as we detected here. A mechanism for
the perception of this gradient has yet to be
found but this hypothesis could be tested
experimentally by applying exogenous salt
onto leaves and seeing if excretion rates
drop, and/or by rinsing salty leaves off and
seeing if excretion rates increase.
Internal salt-load balance is an important
strategy for mangroves; external salt bal-
ance might be equally important if there are
negative fitness consequences for plants
bearing too much salt per unit leaf area. The
ramifications of salt overload and the flex-
ibility of salt-compensatory mechanisms
during extreme conditions have not been
examined. However, this study provides
important evidence of external salt-balance
control and we suggest that maintaining
equivalent salt loads on leaf surfaces
throughout the crown is an important ad-
aptation for tidal plants.
Acknowledgments.—We thank J. Baldwin,
E. Beal, S. Beals, G. Campillo-Bermudo, L.
Phan, S. Przyjemski, E. D. Schulz, A. Siegel,
R. Struwe, and A. Vo for help with data
collection and A. V. Bernhard for his in-
sightful comments and useful discussions.
This research was funded though the Hum-
mingbird Cay Foundation and the Biology
Department at Tufts University.
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