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Differential salt deposition and excretion on leaves of Avicennia germinans mangroves

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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.
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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°32N,
75°50W). 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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Mangroves refer to an ecological group of evergreen woody plants distributed in a zone of tidal infl uence – both on sheltered coasts and at the banks of estuaries. They provide a variety of ecosystem goods and services to human society and also prevent coastal areas from hazards of hurricanes and Tsunamis. Mangroves usually grow in variable fl ooding regimes but salinity appears to be the most important factor affecting their growth and distribution. Best growth of mangroves is found in half strength seawater while a 50 % growth reduction is found in full strength seawater. However, survival of different mangrove species in hyper-saline conditions could vary with different morphological and physiological adaptations. This review is an attempt to gather information on distribution, growth dynamics and eco-physiology of mangroves in Pakistan. Information gathered with the help of past and present researches would help in the restoration and methodical care of the mangroves along the coast of Pakistan besides developing them as source of commercial products and spot for a burgeoning ecotourism industry.
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Salt glands—identified and described as early as the mid-nineteenth century—represent sophisticated anatomical devices involved in the removal of salts in excess from aerial parts of halophytes. Major types of secretory structures are being discussed in respect to the large diversity of taxonomical representatives: salt glands (typical to Plumbaginaceae, Frankeniaceae, Tamaricaceae, Primulaceae, and mangroves), salt (vezicular) bladders of Chenopodiaceae species, and epidermal bladder cells found in Mesembryanthemaceae.
Article
Sediment cores in mangrove forests (Kandelia obovata, Avicennia marina, and Sonneratia caseolaris) and adjacent mudflat were collected to quantify the distribution of acid-volatile sulfide (AVS) and simultaneously extracted metals (SEM) in Futian mangrove forest, South China. The influence of mangrove species on the ecological risk due to heavy metals was also evaluated based on SEM–AVS analysis. The results showed that the presence of mangrove plants improved the moisture content (MC), total organic carbon (TOC), electrical conductivity (EC), and reduced pH value compared to the mudflat. The levels of AVS in mangrove forest sediments were lower than in the mudflat, which may be related to their marine origin or the negative effects of root-secreted oxygen on AVS formation. In the mudflat, the peak level of AVS appeared deeper in the sediment profile compared to K. obovata and S. caseolaris sediment, which may be related to that the oxygen released from mangrove root could affect the reducing condition and affect AVS formation. Zn contributed to the main components of SEM in all sediments. Of all the factors investigated, pH had important influence on the distribution of AVS and SEM in the mudflat. The values of SEM/AVS and [SEM]-[AVS] in the mudflat were lower than in the mangrove forest sediment at 10–40 cm depth, with no ecological risk in all sediments. Taking into account the TOC concentration, there were no adverse effects due to heavy metals in the sediments of the mangrove forest or the mudflat.
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Mangroves refer to an ecological group of evergreen woody plants distributed in a zone of tidal influence – both on sheltered coasts and at the banks of estuaries. They provide a variety of ecosystem goods and services to human society and also prevent coastal areas from hazards of hurricanes and Tsunamis. Mangroves usually grow in variable flooding regimes but salinity appears to be the most important factor affecting their growth and distribution. Best growth of mangroves is found in half strength seawater while a 50 % growth reduction is found in full strength seawater. However, survival of different mangrove species in hyper-saline conditions could vary with different morphological and physiological adaptations. This review is an attempt to gather information on distribution, growth dynamics and eco-physiology of mangroves in Pakistan. Information gathered with the help of past and present researches would help in the restoration and methodical care of the mangroves along the coast of Pakistan besides developing them as source of commercial products and spot for a burgeoning ecotourism industry.
Article
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Mangrove forests provide valuable ecosystem services but are declining in many tropical locations. The abundance of mangrove species in coastal fringe forests is related to biotic processes such as species succession or competition and abiotic factors, including nutrient availability, physiochemical water quality, soil composition, and tidal inundation. We examined the abundance of Rhizophora mangle and Avicennia germinans relative to environmental factors, including porewater chemistry, soil substrate, and distance from the ocean, on Bahamas Island. In this system, R. mangle were primarily found in litter-dominated soils and abundances were positively related to distance from the ocean, while A. germinans was only found in sandy soils closest to the ocean. Although phosphate, alkalinity, and salinity in porewater did not explain the distribution of species, free chloride varied significantly with distance from the ocean. These results suggest that soil conditions and tidal inundation may help determine the distribution of mangrove species on Caribbean islands.
Article
Using stereological procedures, a detailed analysis was made from thin section electron micrographs of secreting and nonsecreting salt glands of Frankenia grandifolia (Cham. and Schlecht) and Avicennia germinans (L.) Stem. In F. grandifolia secretory cells, vacuolar volume significantly decreased, while the volume of endoplasmic reticulum increased in secreting glands. Numerous minivacuoles were predominantly located along the periphery of secreting secretory cells, some in apparent fusion with the plasma membrane. No difference was found in mitochondrial volume in the secretory cells between secreting and nonsecreting glands. In A. germinans, there was a significant decrease in vacuolar volume in secreting secretory cells. The volume of the endoplasmic reticulum and mitochondria also increased in these cells. However, no evidence of mini-vacuolar fusion with the plasma membrane was observed. These results indicate that the physical process of secretion may differ between F. grandifolia and A. germinans; in both, however, the ultrastructural observations support the contention that specific structural parameters are correlated with the process of secretion.
Article
Avicennia marina salt glands were studied by scanning electron microscopy (SEM), in order to relate their ultrastructure to the extracellular salt secretion process. It was found that a multiphase asynchronous process regulates the salt secretion. In the light of the present study and previous evidence we describe two mechanisms of salt secretion. The first mechanism is a merocrine one, consisting of the formation of a vesicle that expands until it reaches a maximal size, when it bursts, releasing the salt solution. Later the burst vesicle disintegrates and a new one begins to form. The second suggested mechanism is an holocrine one and begins with the accumulation of the secreted solution in the subcuticular space. This results in the tearing of the cuticle and the release of the salt solution as droplets. The first suggested mode of secretion operates regularly, while the second one takes place only occasionally.
Article
The relative changes in osmolality of leaf tissue, xylem sap and leaf secretion, as well as leaf gas exchange characteristics of the mangrove A. germinans, cultivated under moderate salinity (0 to 428 mol NaCl m-3) and hypersaline conditions (856 mol NaCl m-3), were examined. Water content and net amount of solutes per unit leaf area increased at moderate salinity, and then declined under hypersalinity. Osmolality of xylem sap increased with salinity treatment in actively transpiring plants as well as at night when transpiration was minimized. However, osmolality values of salt-treated plants were higher at night than during the day, which indicated the dependence of this parameter on water flow. At moderate salinity, salt secretion increased with salinity treatment. This allowed plants to reduce water uptake slightly and to maintain relatively high carbon assimilation. However, under hypersaline conditions, salt secretion tended to be limited, which may be the result of a saturated process. Salt secretion is a highly active mechanism and involves several metabolically controlled steps. Therefore, in hypersaline conditions, decreases in the solutes carried to leaves were more important. Thus. stomatal conductance of A. germinans was severely reduced. However, this lead also to a drastic reduction in carbon assimilation rates.
Article
Adaxial peels were made from fully expanded leaves of Avicennia germinans (L). Stearn. The peels consisted of the epidermis, epidermal salt glands, and underlying hypodermal cells. Electron microscopic examinations showed that the structural integrity (except for the innermost hypodermal cells) of all cell types was not altered in making the peels. When the peels were floated on salt solutions, the glands were shown to be functionally competent in secretion. Secretion also occurred when the peels were floated on distilled water, presumably from salts stored in the hypodermis. Secretion occurred in the dark and since all cell types in the peels lacked chloroplasts, glandular function could not be directly coupled energetically to photosynthesis. However, secretion was shown to be temperature dependent and inhibited by azide and dinitrophenol, which indicates that the energy cost underlying secretion is mitochondrial and most likely coming from the mitochondria-enriched gland cells. Inhibitors known to affect membrane H+ ATPase activity and membrane transport also inhibited secretion, indicating that membrane transport is probably the primary mechanism underlying secretion. Lanthanum, a membrane calcium antagonist, also inhibited secretion.
Article
Seedlings of A. marina and R. stylosa were grown for 12 months in nutrient solutions containing 0, 25, 50, 75 and 100% seawater. Plants of both species grew poorly without sodium chloride in the culture medium. Growth was greatly stimulated in 25% seawater which, within the range of treatments used, was the optimum salinity for growth by both species. Dry matter production by R. stylosa fell sharply between 25 and 50% seawater, whereas A. marina displayed an extended growth response in up to 75% seawater. Plants of A. marina grown on solutions of increasing salinity accumulated increasing amounts of sodium and chloride in all tissues. As a result, all tissues had osmotic potentials that were considerably more negative than that of the solution on which they were grown. This implied that the reduction in growth displayed by plants of A. marina grown at high salinity was due to inhibition of growth by high concentrations of sodium and/or chloride, rather than to an effect of water stress. By contrast, the leaves and stem of R. stylosa contained relatively low concentrations of sodium and chloride, with the result that the osmotic potentials of these tissues were close to, or more positive than, that of the solution on which the plants were grown. This suggests that the poor shoot growth of this species at high salinities was due to water stress. The results of this study support the view that A. marina, which has salt-secreting glands in its leaves, is more salt-tolerant than R. stylosa, which does not possess salt-secreting glands.
Article
Leaves of A. marina are unsuitable for measurements of salt secretion because the lower leaf surface, where most secretion occurs, is densely tomentose. Washing of leaves with distilled water, although effective in removing secreted salt, was followed by a period of apparently increased salt secretion. Washing of leaves with strong osmotica was not followed by such a large increase in the rate of salt secretion as was washing with distilled water, suggesting that this acceleration was due to an osmotic flow of water into the salt-gland complex from the washing liquid. The apparent overestimation of secretion rates over short periods was probably not due to incomplete removal of pre-secreted salt by the initial wash nor, on the basis of a comparison of leaf washings with salt contents of the leaf, to the leaching of salt from the leaf interior into the solution used to wash the leaf. Subtraction of the amount of salt secreted in the first 2 h from the total amount secreted over periods of up to 96 h resulted in roughly constant calculated rates of secretion, so that in this species the steady rate of salt secretion, not accelerated by washing with distilled water may be calculated by using a duplicate set of leaves to measure the salt secreted in the first 2 h, and subtracting this from the total secreted over a longer period. Rates of Cl- secretion, so corrected, were about 0.2 µmol m-² s-¹. Unless this allowance is made, secretion rates based on washing with distilled water are overestimates, although the degree of overestimation is reduced as the length of secretion period is increased.
Article
Freeze-fracture electron microscopy was used to investigate intramembranous particle (IMP) densities and particle distributions in the plasma membrane and tonoplast of the cells of secreting and nonsecreting leaves of Avicennia germinans (L.) Steam. Intramembranous particle densities of the protoplasmic (P) and exoplasmic (E) face of the plasma membrane and tonoplast were significantly higher in hypodermal cells of secreting leaves than of nonsecreting leaves. In contrast, no significant differences in the frequency of intramembranous particles were found in any membrane faces of secreting or nonsecreting mesophyll cells. However, particle densities were higher in the plasma membrane and tonoplast of the mesophyll cells, compared to the hypodermal cells, with the exception of the P-face of hypodermal plasma membranes of secreting tissue, which had the highest particle density measured. Particle distributions were dispersed and no discernible patterns such as paracrystalline arrays or other multi-IMP structures were observed. Results support the hypothesis that secretion is coupled to changes in membrane ultrastructure, and the possibility that salt secretion is an active process driven by integral membrane proteins such as the H+/ATPase. Additionally, the hypodermal cells of the leaf may function as storage reservoirs for salt as well as water, suggesting a regulatory role in salt secretion.
Article
Diurnal and long-term excretion by leaves of Avicennia marina seedlings growing in aqueous culture was correlated with substrate salinity and transpiration. Excretion was greater in 100% than 50% seawater but the reverse was true for transpiration. The diurnal excretion pattern, with exudation minimal during the day and maximal during the night, showed a negative correlation with the daily transpiration pattern. The total amount of salt excreted, however, showed a positive correlation with the total amount of water transpired. Root and xylem sap salinities were linearly related to substrate salinity but leaf Na+ increased to a maximum, indicating that control of leaf salt content is at the foliar, rather than the root level.