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Chemical Composition of Australian Mangroves I. Inorganic Ions and Organic Acids
Chemical Composition of Australian Mangroves I. Inorganic Ions and Organic Acids MARIANNE POPP
Institut fur Pflanzenphysiologie, Universitat Wien, Postfach 285, A-1091 Wien, Austria Received October 19, 1983 . Accepted November 18, 1983
Summary Young and old leaves from 22 mangrove species of Northern Queensland (Australia) were investigated for inorganic ions and organic acids. Na+ and Cl- concentrations expressed on plant water basis were close to sea water with the exception of Heritiera littoralis and Hibiscus tiliaceus, which are both regarded as brackish water species. Leaf age did not appear to effect Na+ and Cl- storage much. sol- and Mg2+ increased markedly in old leaves of salt-secreting species such as A vicennia marina, A vicennia eucalypti/olia, Aegialitis annulata, Aegiceras corni· culatum and Acanthus ilicifolius as well as in the members of the Rhizophoraceae. Free oxalate was found in all salt-secreting species and young leaves of Lumnitzera racemosa. Malate and citrate were present in all species, while quinate and shikimate occurred frequently. The contribution of organic acids to the anion content was important in only a few cases. Mechanisms of salt-regulation and problems in the classification of mangroves are discussed. Key words: Mangroves, Ca2+, CI-, K+, Ml+, Na+, organic acids,
sol-.
Introduction The osmotic adaptations of mangroves to the low and variable water potentials of their environment have long been a matter of interest (Walter and Steiner 1936; Scholander et al. 1962, 1966). The Na+ and Cl- concentrations reported in these previous studies, although high, accounted for only 50 to 70 % of the total reported leaf osmotic potentials (Scholander et al. 1966; Stewart and Ahmad 1983). Therefore, determinations of the most abundant inorganic ions and also of organic acids were included here in an effort to account for a greater percentage of total leaf osmotic potentials. The contributions of low molecular weight carbohydrates and nitrogencontaining compounds (proline, methylated onium compounds) to osmoregulation in mangroves will be the subjects of parts II and III in this series.
Materials and Methods Leaves of 22 mangrove species were collected at Hinchinbrook Island (Northern Queensland, 18°15' S), Chunda Bay (100 km south from Hinchinbrook Is1.), and in the case of Avicen· Abbreviations: pw = plant water; d.m.
=
dry matter. Z. Pjlanzenphysiol. Bd. 113. S. 395-409. 1984.
396
MARIANNE POPP
nia marina and Aegiceras comiculatum, also at Bateman's Bay (New South Wales, 35°42' S). Depending upon leaf size, 20 to 50 leaves of different ages were sampled per tree. The first and second leaves from the apex were termed «young»; those farthest removed from the apex (fifth to eighth leaf) were termed «old». The samples were put on dry ice immediately after harvest and subsequently freeze-dried. From the salt-secreting species, two parallel samples were harvested. Surface salt secretions were washed off from one of the samples by rinsing it quickly three times with distilled water. Hot water extracts, as well as 1 N HCl extracts, were prepared from the ground, freeze-dried leaves; concentrations of K+, Na+, Ca2+ and Mg2+ were subsequently determined by atomic absorption spectrophotometry (Varian AA6). The HCl extracts were used to determine total cation concentrations. This was of special interest in those species which contained free oxalate, because the Ca2+ was precipitated as sparingly soluble Ca-oxalate during the preparation of the hot-water extracts. Unfortunately, there was no simple or reliable method to assess which fraction of the total Ca2+ existed as free ions in these species in vivo. Concentrations of total anions, Cl- and sol- were determined using the hot-water extracts as described previously (Albert and Kinzel 1973; Albert 1975). Organic acid fractions and inorganic phosphate were obtained by fractionating the hot-water extracts using ion exchangers (Albert and Popp 1977) and then analyzed by two different gas-chromatographic methods (SE 52, Popp and Kinzel 1981; QF 1, Philipps and Jennings 1976). In the case of Rhi· zophora species and Hibiscus tiliaceus organic acids were extracted by boiling the plant material three times with 50 % ethanol. This was necessary because the hot-water extracts of these samples were so sticky that they could not be passed over ion exchange resins. Nitrate content was tested by gas-chromatography (Muller and Siepe 1979), but was under the limit of detection in all cases. Ion concentrations were mainly expressed in equ· m -3 plant water because this is a more meaningful basis for considering osmotic relations than the conventional dry matter basis (Cassidy 1970) and facilitates inter-species comparison (Storey and Wyn Jones 1977). But, the following limitation of this procedure should be kept in mind. Hot-water extracts were prepared by using 20 ml of extraction medium per g dry matter, while the plant water content of the leaves varied between 1 to 4 ml per g d· m. This 5 to 20-fold increase in solvent may extract a greater quantity of ions than would be present in the soluble phase of the cells in vivo. However, the results obtained by this method were similar to those when crushed leaf juices were used (Scholander et al. 1966; Downton 1982).
Results The differences between the water- and acid-soluble fractions were greatest in Ca2+ concentrations even in those species where no free oxalate was detectable (Fig. 1). This was true for all species sampled with the six species in Fig. 1 being representative for different types of ion composition. HCl-extracts also demonstrated the extent of Na+ deposition in water-insoluble complexes. The concentrations of HCl-soluble Na+ varied between 10 and 40% of the water-soluble fraction depending on the species, but without any marked effect of leaf-age (Fig. 1). The highest proportion of HCI-soluble Na+ was observed in the three Rhizophora species. It remains to be determined whether there is a connexion between the high HCI-soluble Na+ fraction and the very sticky nature of the extracts (see Materials and Methods,
Z. Pjlanzenphysiol. Ed. 113. S. 395-409. 1984.
Ions and organic acids in mangrove leaves
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Lumnitzera racemosa 200
Xylocarpus granatum
Heritiera Iittoralis
Avicennia marina
10
Aegiceras corniculatum
··· ··· ~: ··· ~:.. ··· ·· ~: ~: ~:~ :. g~:. r:~:
Rhizophora stylosa
~: ~:
/
/
Fig. 1: HCI- and water-soluble cation concentrations (/lequ), based on dry matter, of young and old mangrove leaves. The entire column height represents the HCI-soluble concentrations, the patterned areas represent the watersoluble fraction.
also Walter and Steiner 1936). Such a connexion would be suggested by the presence of high amounts of pectins and similar substances able to bind cations. Salt-secretion is a well established mechanism to control Na+ as well as Cl- content in the leaves. Deposits of Na+ and CI- on the leaf surface of A7.licennia marina were markedly higher in the subtropical habitat of Chunda Bay than at the temperate habitat of Bateman's Bay (Fig. 2). This was in accordance with the findings of Drennan and Parmenter (1982) that salt secretion is positively correlated to Z. Pjlanzenphysiol. Bd. 113. S.395-409. 1984.
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Avicennia marina
100
-
young
C III
old
a. .., 50 I
E
CI S04Na K Mg
Chunda Bay
Bateman's Bay
Fig. 2: Ion concentrations of both washed and unwashed leaves of Avicennia marina from two different habitats (equ' m- 3 plant water). The patterned areas represent the concentrations of washed leaf samples, the entire column height represents the concentrations in unwashed samples.
the amount of water transpired. However, ion concentrations in Avicennia marina leaves varied little between collecting sites of different climatic conditions (Fig. 2). Ion concentrations of salt-secreting mangrove species are from washed leaf samples in all cases (Fig. 3). These values, presenting the ion concentrations within the leaves, were more appropriate for comparison to the salt concentrations in the non-secreting species than non-washed samples. The Na+ and CI- concentrations of washed leaf samples from the salt-secreting mangroves were very close to sea water and similar to those of other species (Figs. 4, 5 and 6). There was also little change in these two ions between young and old leaves, whereas concentrations of as well as of Mg2+ increased with age. This agreed with the observation that the latter two ions are scarcely or not at all secreted by the salt glands (Fig. 2, Scholander et a1. 1962; Boon and Allaway 1982). Ionic composition of species in the Rhizophoraceae (Figs. 4, 5) resembled the ionic composition of the salt-secreting species (Fig. 3). Na+ and Cl- concentrations were close to sea water concentrations and not influenced by leaf age. The Mg2+ concenincreased markedly in old leaves. The concentrations were rather high, and
sol-
sol-
Z. Pjlanzenplrysiol. Bd. 113. S.395-409. 1984.
Ions and organic acids in mangrove leaves
399
other
anions
1000 old
.... c:
<11
Q.
500
M
I
~~.. .. ::.. ....:: .:
E
Acanthus ilicifolius
Aegialitis annulata
..... .... .... .. ....
.. ......
Aegiceras corniculatum
CI
..:~
Na
::.: :. :~
Avicennia marina
Fig. 3: Ion concentrations (equ), based on plant water content, in young and old leaves of saltsecreting mangrove species.
trations of Na+, Cl- and Mg2+ agreed with those reported by Atkinson et al. (1967) and Ball and Cowan (unpublished results). Concentration of sOi- were not determined in these two studies, but since concentrations of up to 300 equ· m -3 pw are possible, the contribution of sOi- to the osmotic potential in these species should be considered. As reported for Ceriops tagal (Popp et al. in press), the ionic composition of members of the Rhizophoraceae did not change much between different collecting sites. Only in plants from inland habitats, without any tidal influence, were the ionic concentrations reduced (Bruguiera gymnorhiza, Fig. 5). While Cl- was decreased to about 74 to 82 % of the coastal site concentrations, N a+, Mg2+ and sOi- were markedly diminished at the inland site. Fig. 6 represents 5 species out of three different families (Combretaceae, Sonneratiaceae, Meliaceae) without salt glands and with rather high Na+ and Cl- accumulation (between 400 and 800 equ· m- 3 pw). Ion concentrations for Xylocarpus mekongensis were only determined for young leaves because this tree is deciduous, and only young leaves were available at the time of collection. Interestingly, the ion concentrations in these young leaves of Xylocarpus mekongensis were closer to those of the old rather than the young leaves of Xylocarpus granatum. This is possibly due to
z. Pjlanzenphysiol. Bd. 113. S.395-409.
1984.
400
MARIANNE POPP
equ. m- 3 plant water
100
50
Bruguiera exaristata
Ceriops taga/ var. taga/
Rhizophora apicu/ata
Rhizophora /amarckii
i
Rhizophora sty/osa
Fig. 4: Ion concentrations (equ), based on plant water content, in young and old leaves of Rhizophoraceae species.
the comparatively low water content of the young leaves of Xylocarpus granatum (only 0.98ml pw·g-1d·m). Old leaves of Xylocarpus granatum had 1.91ml pw· g-I d . m, while the estimates for X ylocarpus mekongensis were 2.5 ml pw· g-I d· m. Both Xylocarpus species had high concentrations of water-soluble Ca2+; total Ca2+ was even higher than HCI-soluble Na+ in both cases (Fig. 1). Another feature of these two species was a more pronounced accumulation of organic acids than in most of the other mangroves under investigation (Table 1). Comparison of the ion concentrations between old leaves of Lumnitzera racemosa and Xylocarpus granatum illustrates the different water relations in mangrove leaves. Ionic concentrations of Lumnitzera racemosa and Xylocarpus granatum appeared similar when expressed as pw (Fig. 6), they differed markedly when expressed in /Lequ· g-I d· m (Fig. 1). The reason for this was the succulent character of the Lumnitzera racemosa leaves (3.4 ml pw· g-I d· m respectively d· m corresponded to 22. 9% of fresh matter), while d· m made up for 34.4% of fresh matter in the more xeromorphic leaves of Xylocarpus granatum. Unlike other halophytes (Yeo and Flowers 1980; Munns et al. 1983), salts themselves contributed little to the d· m. Depending on the species Na+ made up for 1.2 to 5.6 % of d· m. Z. Pjlanzenphysiol. Bd. 113. S.395-409. 1984.
Ions and organic acids in mangrove leaves
401
1000
old
...c:
you n 9
1\1
other anions
a. <'> I
500
E
Mg
C.
:::I
C'
Q)
CI
K Na
Hinchbrook Island
near Chunda Bay Inland
Fig. 5: Ion concentrations (equ), based on plant water content, of leaves of Bruguiera gym. norhiza from two different habitats.
Fig. 7 presents a number of species with lower ion concentrations than in Figs. 3 to 6. These lower concentrations were associated with both habitat and physiological features. Heritiera littoralis as well as Hibiscus tiliaceus are regarded as brackish water species or mangal associates (Chapman 1976; Barth 1982; Dowling and McDonald 1982). But, even in brackish water, Na+ concentrations exceeded that of K+. The preferential storage of K+ and the above unity K+ INa+ ratios gave rise to the assumption that these two species have special ion uptake and transport characteristics. This is possibly similar to some monocotylous halophytes which are also able to maintain low Na+ concentrations in their shoots (Albert and Popp 1977; Gorham et al. 1980). The striking differences in ion content of young and old leaves of Heritiera littoralis as determined on pw basis was again associated with pronounced changes in water content (young leaves 1.0 ml . g-1 d . m, old leaves 2.9 mI· g-1 d· m, compare Fig. 1 and Fig. 7). The remaining three species in Fig. 7 occurred in normal mixed mangrove stands, for example Excoecaria agallocha close to Lumnitzera racemosa (Fig. 6) and Osbomea octodenta next to Ceriops tagal (Fig. 4). They also differed from their neighbouring species to some extent in their ion concentrations and ratios. The K+ INa + ratio in other mangrove species ranged between O.S to 0.1 (sea water 0.021) depending on age Z. Pjlanzenphysiol. Ed. 113. S.395-409. 1984.
402
MARIANNE POPP
other
anions
1000
young Mg old
....I:
Ca
a. M
I
E
500
I
Lumnitzera littorea
••• 000
Lumnitzera racemosa
Sonneratia alba
Xylocarpus granatum
~~
K
...
// // // // //
I
Xylocarpus mekongensis
Fig. 6: Ion concentrations (equ), based on plant water content, of young and old leaves from members of the Combretaceae, Sonneratiaceae and Meliaceae.
and species. Young Excoecaria agallocha leaves had a K+ INa + ratio of 0.7 and the old leaves of 0.6. Young leaves of Scyphiphora hydrophylacea had even a K+/Na+ ratio of 1.0. More studies like that of Rains and Epstein (1967) are necessary to elucidate the ways of K+ uptake in mangroves. A comparison of the concentrations of organic acids (Table 1) with those of the inorganic ions (Figs. 1-7) reveals that the contribution of organic ions to charge balance was important in only some cases (Xylocarpus spp., Lumnitzera littorea, some Rhizophoraceae). Even mangroves which contained free oxalate did not accumulate this acid to levels as high as found in some Chenopodiaceae (Osmond 1963; Albert and Popp 1977). Malate and citrate, ubiquitous in the plant kingdom (Nierhaus and Kinzel 1971), were present in all samples. Malate was highest in Xylocarpus granatum and Lumnitzera littorea, while citrate was above 100 equ -m- 3 pw in Avicennia eucalypti/olia, Bruguiera gymnorhiza, Heritiera littoralis and again Lumnitzera littorea. Shikimate and quinate are often found in woody species, possibly because they are intermediates of phenylpropane and its derivatives (tannins, lignin). Interestingly, quinate concentrations were always higher in young leaves.
z. Pjlanzenphysiol. Bd. 113. S. 395-409. 1984.
Ions and organic acids in mangrove leaves
403
Table 1: Organic acid concentration in young and old mangrove leaves (equ' m -3 plant water). Species
Family
Acanthus ilici/olius
Acanthaceae
Aegialitis annulata
Plumbaginaceae
Aegiceras corniculatum
M yrsinaceae
Leaf age
Oxalate
Malate
Citrate
y
13.1 12.1
8.2 1.6
8.6
69.8 56.6
7.0
4.6
9.7
5.0
57.1 74.5
5.1 5.6
26.0
0
y 0
y 0
Avicennia eucalypti/olia
A vicenniaceae
Avicennia manna
A vicenniaceae
Bruguiera exaristata
Rhizophoraceae
Bruguiera gymnorhiza
Rhizophoraceae
Ceriops tagal v. australis
Rhizophoraceae
Ceriops tagal v. tagal
Rhizophoraceae
Rhizophora apiculata
Rhizophoraceae
Rhizophora lamarckii
Rhizophoraceae
Rhizophora stylosa
Rhizophoraceae
Excoecaria agallocha
Euphorbiaceae
Heritiera littoralis
Sterculiaceae
Lumnitzera littorea
Combretaceae
Lumnitzera
Combretaceae
Sonneratia alba
Sonneratiaceae
Xylocarpus mekongensis
Meliaceae
Xylocarpus granatum
Meliaceae
Hibiscus tiliaceus
Malvaceae
2.6 2.7
90.8
2.9
42.0 201.0
1.2
86.3
y
41.2 73.7
2.1 12.1
16.1 45.2
y
29.0
3404
0
11.6
18.2
26.9 5.2
90.3 35.0
15.5 2.9
157.7 132.9
y
29.9
0
30.5
112.3 99.5
1.5
62.1
131.0
y
3004
28.6
304
33.5
0
16.3
2804
3.1
704
95.9 55.2
y
13.0 50.6
13.6 62.7
6.0
31.4 7.6
64.0 120.9
23.4 23.8
67.3
43.7 16.5
11704 107.6
y
50.3
y
4004
138.5
27.9
59.8 69.3
38.3
0
704
104.6
y
41.6 42.6
72.9 87.8
55.7 17.7
148.1
9.2 9.3
10.5
35.1
y
10.6
0
12.2
y
28.0
0
9.7
y
107.9
0
27.8
8.9
117.2 47.1
3004 145.2 56.8
116.8 7.8
33.5 1.6
3.8 11.8
7.3 25.9
2.6 33.2
4.2
0
42.9 43.7
12.0 904
y
81.4
29.8
y
248.3 143.8
25.2 31.3
lOA 13.9
18.0
2804
22.9
36.8
y y 0
y
0
y 0
10.7
15.5 2.3
258.2 37.2
51.9
0
13.7
4.8
170.2
35.1 51.5
y 0
Rubiaceae
84.0 75.0
9.8 18.3
0
Scyphiphora hydrophylacea
2.6 2.7
63.2
y
0
Myrtaceae
29.9 16.1
204
3.5
Total
0
0
Osbornea octodenta
Quinate
29.3 119.5
0
racemosa
Shikimate
7.3
116.2 64.5 31.1 6.2
49.5 43.9
16.8 6.8
44.2
1904 54.9 44.1
7.1 2.7
9.9
121.1
9004
371.0 180.3
2.5
Z. Pjlanzenphysiol. Bd. 113. S.395-409. 1984.
404
MARIANNE POPP
equ .m- 3 plant water
old
500
Excoecaria agal/ocha
Osbornea octodenta
Scyphiphora Hibiscus hydrophylacea tiliaceus
Heritiera littoralis
Fig. 7: Ion concentrations (equ) based on plant water content, of young and old leaves of mangroves from various families (Euphorbiaceae, Myrtaceae, Rubiaceae, Malvaceae and Sterculiaceae). Discussion Like all other halophytes (Waisel 1972; d. Albert 1982), mangroves have often been subjected to classification. Although the codification proposed Ooshi et al. 1975; Lear and Turner 1977; Saenger 1982) consists only of three groups: salt-secretors, salt-excluders and salt-accumulators, the terms are more confusing than helpful (Osmond et al. 1980). This is because the terms describe the salt-regulation mechanisms of different organs. «Salt-secreting» species (Scholander et al. 1962) remove salt from their leaves by means of salt-glands, while «salt-excluding» species (described by Scholander et al. 1962; named so by Atkinson et al. 1967) keep the salt content of their xylem sap low by «ultrafiltration» in the roots. Since little work has been done on mangrove roots (Kramer and Preston 1978; Lawton et al. 1981), it is not known, where this ultrafiltration system is located, and if the ions are excluded from the roots or from the xylem sap. However, final concentrations in the leaves of species from all groups are very similar (Figs. 3-6). Moreover, it cannot be said that in each species there is only one mechanism responsible for regulation of ion content in the leaves. For example, the xylem-sap of the salt-secreting species, Aegialitis annulata, contains 85 to 122 equ' m- 3 Cl- (Atkinson et al. 1967). This is higher than the xylem-sap concentration of non-secreting species, but still far below the concentrations of Cl- in the surrounding sea-water. Thus in a salt-secreting species some regulation at the root level has already appeared. A non-secreting species, like Rhizophora mucronata, is able to reduce its CI- concentration in the xylem-sap to much lower concentrations than secreting species: 17 Z. PjIanzenphysiol. Bd. 113. S.395-409. 1984.
Ions and organic acids in mangrove leaves
405
equ' m- 3 (Atkinson et al. 1967). But, even this low Cl- content in xylem-sap would be deleterious for these leaves due to their long life span (6-12 months, Gill and Tomlison 1971; 12-18 months, Christensen and Wium-Anderson 1977) if no other regulating mechanism intervened. Atkinson et al. (1967) suggested a «dilution by growth» effect because a remarkable constancy in the CI- concentration in leaves of different ages was observed. They estimated that a daily increase of 3 % in dry matter would maintain Cl- at the observed levels on a pw basis. Experiments and calculations by Ball and Cowan (unpublished results) on Rhizophora apiculata are at variance with this estimate because a leaf of Rhizophora apiculata would reach its final salt concentration within 56 days, a far shorter time than the observed life spans. In addition, shedding of senescent leaves is of limited importance for the salt balance in the Rhizophoraceae species (Ball and Cowan, unpublished results). Ball and Cowan suggest a retranslocation of Na+ and Cl- from leaves to other plant parts. This retranslocation was recently observed in the salt-tolerant herbaceous species Trifolium alexandrinum (Winter 1982) and previously observed in other plants (Lessani and Marschner 1978; Jacoby 1979; McNeil 1980). Results presented in this study support retranslocation of Na+ and Cl- since sOi- and Mg2+ concentrations increased markedly in old leaves of Rhizophoraceae, while Na+ and CIconcentrations did not increase (Fig. 4). An overall «dilutionary effect» does also not explain the observed changes in the ratios of CI- Isoi- and Na+ IMg2+ in young and old leaves (Fig.4). It seems more likely that the constant levels of Na+ and Cl- are due in part to a transport out of the leaves, while other ions such as sOi- and Mg2+ accumulate.
If the water content per unit d· m is accepted as a measure of succulence Gennings 1976), Lumnitzera racemosa may be taken as an example of this salt-regulation mode. Biebl and Kinzel (1965) described the significance of succulence for the new-world mangrove, Laguncularia racemosa, which belongs to the Combretaceae as does Lumnitzera racemosa. In Lumnitzera racemosa, the increase of water content appeared sufficient to maintain ion concentrations at the same levels in old and young leaves (Figs. 1, 6). However, in Heritiera littoralis and Xylocarpus granatum, increased water content caused a dilution of ions in the older leaves (Figs. 6, 7). It is evident from both our study and the literature that salt regulation in mangroves is accomplished by the same mechanisms described for other halophytes (Albert 1975, 1982; Flowers et al. 1977; Larcher 1980; Munns et al. 1983). Therefore, it would be advantageous to use the terms «salt-accumulator» and «salt-excluder» for mangrove species in the same way as for halophytes (Gorham et al. 1980; Popp 1983; Yeo 1983). Salt-accumulators store high concentrations of inorganic ions, primarily Na+ and Cl-, in their leaves. Salt-excluders maintain low Na+ and Cl- concentrations in their leaves and adjust osmotically, in part, by accumulation of organic solutes. The easily determined ion concentrations of leaves would provide a clear basis for classification. Assessment of which salt regulation mechanisms are involved Z. Pjlanzenphysiol. Bd. 113. S.395-409. 1984.
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MARIANNE POpp
and to what extent they are responsible for the control of ion content is more difficult than ion concentration determinations and therefore, not a good basis for classification. Whatever the mechanism of salt-regulation may be, 20 out of the 22 mangrove species under investigation accumulate Na+ and CI- to such an extent that questions of ion compartmentation and of intracellular osmotic adjustment are raised. Recent investigations demonstrated a high Cl- demand by photosystem II activity in some mangrove chloroplasts (Critchley 1982, 1983; Critchley et al. 1982). However, it cannot be concluded from this that metabolic processes in mangroves are more salttolerant than in other halophytes. There are also reports on high Cl- concentrations in chloroplasts of herbaceous halophytes (d. Jennings 1976) and positive salt effects on photosynthetic electron transport (Wignarajah and Baker 1981), but the salt sensitivity of the metabolic machinery of various halophytes is well documented (d. Flowers et al. 1977; Albert 1982; Munns et al. 1983). Compartmental analysis on halophytes is scarce and nearly non-existent for mangroves (Eshel and Waisel 1979; Beigl 1981; Harvey et al. 1981; Stelzer 1981; Yeo 1981). Van Steveninck et al. (1976) described two different types of vacuoles, one rich in Cl- and the other low in Cl-, for Aegiceras comiculatum. It seems likely that also in other mangrove species Cl- and Na+ are not equally distributed within the cell, but rather are accumulated in the vacuole to a major extent. The degree of accumulation may vary between cell types as shown for Suaeda maritima (Harvey et al. 1981), but nevertheless creates a demand for compatible solutes in the cytoplasm. The organic solutes present in mangroves were shortly outlined previously (Popp et al. in press) and will be treated in more detail in the following two parts of this senes. Acknowledgements This study was supported by grants from the H. and E. Walter-Stiftung (West Germany, Schimper-Fellowship 1981) and by the Austrian Science Research Fund (project No. P 4051). The kind help and support from Dir. Dr. J. and E. Bunt as well as from Dr. B. Clough (AIMS, Townsville) during sample collection is gratefully acknowledged. Thanks are due to Mag. V. Langer for technical assistance, to Dr. E. Kandeler, M. Hinterleitner and 1. Holzapfel for help in preparing the final version of the manuscript and to Prof. Kinzel for reading the manuscript. The author is very grateful to A. Dickie for comments on the manuscript.
References ALBERT, R: Salt regulation in halophytes. Oecologia (Berl.) 21, 57-71 (1975). - Halophyten. In: H. KINZEL (ed.), Pflanzeniikologie und Mineralstoffwechsel. Eugen Ulmer, Stuttgart, p. 32-204, 1982. ALBERT, R. und.!"!. KINZEL: Unterscheidung von Physiotypen bei Halophyten des Neusiedlerseegebietes (Osterreich). Z. Pflanzenphysiol. 70, 138-158 (1973). ALBERT, R. and M. Popp: Chemical composition of halophytes from the Neusiedler Lake region in Austria. Oecologia (Berl.) 27, 157-170 (1977).
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Ions and organic acids in mangrove leaves
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ATKINSON, M. R., G. P. FINDLAY, A. B. HOPE, M. G. PITMAN, H. D. W. SADDLER, and K. R. WEST: Salt regulation in the mangroves Rhizophora mucronata Lam. and Aegialitis annu· lata R. Br. Aust. J. Biological Sciences 20, 589-599 {1967}. BARTH, H.: The biogeography of mangroves. In: D. N. SEN, K. S. RAJPUROHIT {eds.}, Contributions to the ecology of halophytes, Tasks for vegetation science 2. Dr. W. Junk, Publishers, The Hague, Boston, London, p. 35-60,1982. BEIGL, E.: Verteilung der Alkaliionen zwischen Cytoplasma und Vakuole in Zellen h6herer Pflanzen. Eine neuartige Untersuchungsmethode. Diss. Universitat Wien, 1981. BIEBL, R. und H. KINZEL: Blattbau und Salzhaushalt von Laguncularia racemosa {L.} GAERTN. f. und anderer Mangrovebaume auf Puerto Rico. Osterr. Bot. Zeitschrift 112, 56-93 (1965). BOON, P. I. and W. G. ALLAWAY: Assessment of leaf-washing techniques for measuring salt secretion in Avicennia marina {Forsk} Vieh. Aust. J. Plant Physiol. 9, 725-734 {1982}. CASSIDY, N. G.: The distribution of potassium in plants. Plant, Soil 32, 263-267 {1970}. CHAPMAN, V. J.: Mangrove Vegetation. J. Cramer, Vaduz, 1976. CHRISTENSEN, B. and S. WIUM-ANDERSEN: Seasonal growth of mangrove trees in Southern Thailand. The phenology of Rhizophora apiculata Bl. Aquat. Bot. 3,281-286 {1977}. CRITCHLEY, CH.: Stimulation of photosynthetic electron transport in a salt-tolerant plant by high chloride concentrations. Nature 298, 483-485 {1982}. - Further studies on the role of chloride in photosynthetic 02 evolution in higher plants. Biochimica et Biophysica Acta 724, 1-5 {1983}. CRITCHLEY, CH., I. C. BAIANU, GOVINDJEE, and H. S. GUTOWSKY: The role of chloride in O 2 evolution by thylakoids from salt-tolerant higher plants. Biochimica et Biophysica Acta 682,436-445 (1982). DOWLING, R. M. and T. J. Mc DONALD: Mangrove communities of Queensland. In: B. CLOUGH {ed.}, Mangrove ecosystems in Australia: structure, function and management. Australian National University Press, Canberra, p. 79-93, 1982. DOWNTON, W. J. S.: Growth and osmotic relations of the mangrove Avicennia marina, as influenced by salinity. Aust. J. Plant Physiol. 9, 519-528 {1982}. DRENNAN, P. and N. W. PAMMENTER: Physiology of salt excretion in the mangrove Avicennia marina {Forsk.} Vierh. New Phytol. 91, 597-606 {1982}. ESHEL, A. and Y. W AISEL: Distribution of sodium and chloride in leaves of Suaeda monoica. Physiol. Plant 46, 151-154 {1979}. FLOWERS, T. J.: Halophytes. In: D. A. BAKER, J. L. HALL {eds.} Ion transport in plant cells and tissues. North Holland, Amsterdam, p. 309-334, 1975. FLOWERS, T. J., P. F. TROKE, and A. R. YEO: The mechanism of salt tolerance in halophytes. Ann. Rev. Plant Physiol. 28, 89-121 {1977}. GILL, A. M. and P. B. TOMLINSON: Studies on the growth of red mangrove {Rhizophora mangle L.} 3. Phenology of the shoot. Biotropica 3, 109-124 {1971}. GORHAM, I., L. L. HUGHES, and R. G. WYN JONES: Chemical composition of salt-marsh plants from Ynys-Mon {Anglesey}. The concept of physiotypes. Plant, Cell and Environment 3, 309-319 {1980}. HARVEY, D. M. R., J. L. HALL, T. J. FLOWERS, and B. KENT: Quantitative ion localization with Suaeda maritima leaf mesophyll cells. Planta 151, 555-560 {1981}. JACOBY, B.: Sodium recirculation and loss from Phaseolus vulgaris L. Ann. Bot. 43, 741-744 (1979). JENNINGS, D. H.: The effects of sodium chloride on higher plants. BioI. Rev. 51, 453-486 {1976}. JOSHI, G. V., B. B. JAMALE, and L. BHOSALE: Ion regulation in mangroves. In: G. E. WALSH, S. C. SNEDAKER, and H. J. TEAS {eds.}, Proceedings of the international symposium on biology and management of mangroves. Institute of Food and Agricultural Sciences, University of Florida, Gainesville, Florida, Vol. II, p. 595-607, 1975.
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KRAMER, D. and J. PRESTON: A modified method of x-ray microanalysis of bulk-frozen plant tissue and its application to the problem of salt exclusion in mangrove roots. Microscopica acta, suppl. 2: Microprobe analysis in biology and medicine 193-200, 1978. LARCHER, W.: Physiological plant ecology. Springer, Berlin, Heidelberg, New York, 1980. LAWTON, J. R., A. TODD, and D. K. NAIDOO: Preliminary investigations into the structure of the roots of the mangroves, A vicennia marina and Bruguiera gymnorrhiza, in relation to ion uptake. New Phytol. 88, 713-722 (1981). LEAR, R. and T. TURNER: Mangroves of Australia. University of Queensland Press, St. Lucia, 1977. LESSANI, H. and H. MARSCHNER: Relation between salt tolerance and long-distance transport of sodium and chloride in various crop species. Aust. J. Plant. Physiol. 5, 27-37 (1978). Mc NEIL, D. L.: The role of the stem in phloem loading of minerals in Lupinus albus L. cv. ultra. Ann. Bot. 45, 329-338 (1980). MULLER, H. und V. SIEPE: Vergleichende Bestimmung der Nitratgehalte von Lebensmitteln mit Hilfe verschiedener Methoden. Deutsche Lebensmittel-Rundschau 75 (6), 175-183 (1979). MUNNS, R., H. GREENWAY, and G. O. KIRST: Halotolerant Eukaryotes. In: O. L. LANGE, P. S. NOBEL, C. B. OSMOND, and H. ZIEGLER (eds.), Encyclopedia of Plant Physiology New Series, Vol. 12C, Physiological Plant Ecology, III. Springer, Berlin, Heidelberg, New York, p. 59-135,1983. NIERHAUS, D. und H. KINZEL: Vergleichende Untersuchungen tiber die organischen S1iuren in Blattern hiiherer Pflanzen. Z. Pflanzenphysiol. 64, 107-123 (1971). OSMOND, C. B.: Oxalates and ionic equilibria in Australian salt bushes. Nature (London) 198, 503-504 (1963). OSMOND, C. B., O. BJ0RKMAN, and D. J. ANDERSON: Physiological processes in plant ecology. Towards a synthesis with A triplex. Ecological Studies 36. Springer, Berlin, Heidelberg, New York, 1980. PHILLIPS, R. D. and D. H. JENNINGS: The estimation of plant organic acids by gas-liquid chromatography. New Phytol. 77, 333-339 (1976). POPP, M.: Genotypic differences in the mineral metabolism of plants adapted to extrem habitats. Plant and Soil. 72,261-273 (1983). POPP, M. and H. KINZEL: Changes in the organic acid content of some cultivated plants induced by mineral ion deficiency. J. Exp. Bot. 32, 1-8 (1981). RAINS, D. W. and E. EpSTEIN: Preferential absorption of potassium by leaf tissue of the mangrove, Avicennia marina: an aspect of halophytic competence in coping with salt. Aust. J. bioI. Sci. 20, 847-857 (1967). SAENGER, P.: Morphological, anatomical and reproductive adaption of Australian mangroves. In: B. F. CLOUGH (ed.), Mangrove ecosystems in Australia. Australian National University Press, Canberra, p. 153-191, 1982. SCHOLANDER, P.F., H. T. HAMMEL, E. HEMMINGSEN, and W. GAREY: Salt balance in mangroves. Plant Physiol. 37, 722-729 (1962). SCHOLANDER, P. F., E. D. BRADSTREET, H. T. HAMMEL, and E. A. HEMMINGSEN: Sap concentrations in halophytes and some other plants. Plant Physiol. 41, 529-532 (1966). STELZER, R.: Ion localization in the leaves of Puccinellia peisonis. Z. Pflanzenphysiol. 103, 27-36 (1981). STEWART, G. R. and I. AHMAD: Adaptation to salinity in angiosperm halophytes. In: D. A. ROBBS, W. S. PIERPOINT (eds.), Metals and Micronutrients. Academic Press, London, New York, p. 33-50,1983. STOREY, E., N. AHMAD, and R. G. WYN JONES: Taxonomic and ecological aspects of the distribution of glycinebetaine and related compounds in plants. Oecologia (Berl.) 27, 319-332 (1977).
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VAN STEVENINCK, R. F. M., W. D. ARMSTRONG, P. D. PETERS, and T. A. HALL: Ultrastructural localization of ions. III. Distribution of chloride in mesophyll cells of mangrove {Aegiceras comiculatum Blanco}. Aust. J. Plant. Physiol. 3, 367-376 {1976}. WAISEL, Y.: Biology of halophytes. Academic Press New York, London, 1972. WALTER, H. und M. STEINER: Die Okologie der Ost-Afrikanischen Mangroven. Z. f. Botanik 30,65-193 {1936}. WIGNARJAH, K. and N. R. BAKER: Salt induced responses of chloroplast activities in species of differing salt tolerance. Photosynthetic electron transport in Aster tripolium and Pisum sati· vum. Physiol. Plant 51, 387-393 {1981}. WINTER, E. : Salt tolerance of Trifolium alexandrinum L. II. Ion balance in reaction to its salt tolerance. Aust. J. Plant Physiol. 9, 227-237 {1982}. YEO, A. R.: Salt tolerance in the halophyte Suaeda maritima L. Dum.: Intracellular compartmentation of ions. J. Exp. Bot. 32, 487-497 {1981}. - Salinity resistance: Physiologies and prices. Physiol. Plant. 58, 214-222 {1983}. YEO, A. R. and T. J. FLOWERS: Salt tolerance in the halophyte Suaeda maritima L. Dum.: Evaluation of the effect of salinity upon growth. J. Exp. Bot. 31, 1171-1183 {1980}.
Z. Pjlanzenphysiol. Bd. 113. S.395-409. 1984.
Institut fur Pflanzenphysiologie, Universitat Wien, Postfach 285, A-1091 Wien, Austria Received October 19, 1983 . Accepted November 18, 1983
Summary Young and old leaves from 22 mangrove species of Northern Queensland (Australia) were investigated for inorganic ions and organic acids. Na+ and Cl- concentrations expressed on plant water basis were close to sea water with the exception of Heritiera littoralis and Hibiscus tiliaceus, which are both regarded as brackish water species. Leaf age did not appear to effect Na+ and Cl- storage much. sol- and Mg2+ increased markedly in old leaves of salt-secreting species such as A vicennia marina, A vicennia eucalypti/olia, Aegialitis annulata, Aegiceras corni· culatum and Acanthus ilicifolius as well as in the members of the Rhizophoraceae. Free oxalate was found in all salt-secreting species and young leaves of Lumnitzera racemosa. Malate and citrate were present in all species, while quinate and shikimate occurred frequently. The contribution of organic acids to the anion content was important in only a few cases. Mechanisms of salt-regulation and problems in the classification of mangroves are discussed. Key words: Mangroves, Ca2+, CI-, K+, Ml+, Na+, organic acids,
sol-.
Introduction The osmotic adaptations of mangroves to the low and variable water potentials of their environment have long been a matter of interest (Walter and Steiner 1936; Scholander et al. 1962, 1966). The Na+ and Cl- concentrations reported in these previous studies, although high, accounted for only 50 to 70 % of the total reported leaf osmotic potentials (Scholander et al. 1966; Stewart and Ahmad 1983). Therefore, determinations of the most abundant inorganic ions and also of organic acids were included here in an effort to account for a greater percentage of total leaf osmotic potentials. The contributions of low molecular weight carbohydrates and nitrogencontaining compounds (proline, methylated onium compounds) to osmoregulation in mangroves will be the subjects of parts II and III in this series.
Materials and Methods Leaves of 22 mangrove species were collected at Hinchinbrook Island (Northern Queensland, 18°15' S), Chunda Bay (100 km south from Hinchinbrook Is1.), and in the case of Avicen· Abbreviations: pw = plant water; d.m.
=
dry matter. Z. Pjlanzenphysiol. Bd. 113. S. 395-409. 1984.
396
MARIANNE POPP
nia marina and Aegiceras comiculatum, also at Bateman's Bay (New South Wales, 35°42' S). Depending upon leaf size, 20 to 50 leaves of different ages were sampled per tree. The first and second leaves from the apex were termed «young»; those farthest removed from the apex (fifth to eighth leaf) were termed «old». The samples were put on dry ice immediately after harvest and subsequently freeze-dried. From the salt-secreting species, two parallel samples were harvested. Surface salt secretions were washed off from one of the samples by rinsing it quickly three times with distilled water. Hot water extracts, as well as 1 N HCl extracts, were prepared from the ground, freeze-dried leaves; concentrations of K+, Na+, Ca2+ and Mg2+ were subsequently determined by atomic absorption spectrophotometry (Varian AA6). The HCl extracts were used to determine total cation concentrations. This was of special interest in those species which contained free oxalate, because the Ca2+ was precipitated as sparingly soluble Ca-oxalate during the preparation of the hot-water extracts. Unfortunately, there was no simple or reliable method to assess which fraction of the total Ca2+ existed as free ions in these species in vivo. Concentrations of total anions, Cl- and sol- were determined using the hot-water extracts as described previously (Albert and Kinzel 1973; Albert 1975). Organic acid fractions and inorganic phosphate were obtained by fractionating the hot-water extracts using ion exchangers (Albert and Popp 1977) and then analyzed by two different gas-chromatographic methods (SE 52, Popp and Kinzel 1981; QF 1, Philipps and Jennings 1976). In the case of Rhi· zophora species and Hibiscus tiliaceus organic acids were extracted by boiling the plant material three times with 50 % ethanol. This was necessary because the hot-water extracts of these samples were so sticky that they could not be passed over ion exchange resins. Nitrate content was tested by gas-chromatography (Muller and Siepe 1979), but was under the limit of detection in all cases. Ion concentrations were mainly expressed in equ· m -3 plant water because this is a more meaningful basis for considering osmotic relations than the conventional dry matter basis (Cassidy 1970) and facilitates inter-species comparison (Storey and Wyn Jones 1977). But, the following limitation of this procedure should be kept in mind. Hot-water extracts were prepared by using 20 ml of extraction medium per g dry matter, while the plant water content of the leaves varied between 1 to 4 ml per g d· m. This 5 to 20-fold increase in solvent may extract a greater quantity of ions than would be present in the soluble phase of the cells in vivo. However, the results obtained by this method were similar to those when crushed leaf juices were used (Scholander et al. 1966; Downton 1982).
Results The differences between the water- and acid-soluble fractions were greatest in Ca2+ concentrations even in those species where no free oxalate was detectable (Fig. 1). This was true for all species sampled with the six species in Fig. 1 being representative for different types of ion composition. HCl-extracts also demonstrated the extent of Na+ deposition in water-insoluble complexes. The concentrations of HCl-soluble Na+ varied between 10 and 40% of the water-soluble fraction depending on the species, but without any marked effect of leaf-age (Fig. 1). The highest proportion of HCI-soluble Na+ was observed in the three Rhizophora species. It remains to be determined whether there is a connexion between the high HCI-soluble Na+ fraction and the very sticky nature of the extracts (see Materials and Methods,
Z. Pjlanzenphysiol. Ed. 113. S. 395-409. 1984.
Ions and organic acids in mangrove leaves
397
Lumnitzera racemosa 200
Xylocarpus granatum
Heritiera Iittoralis
Avicennia marina
10
Aegiceras corniculatum
··· ··· ~: ··· ~:.. ··· ·· ~: ~: ~:~ :. g~:. r:~:
Rhizophora stylosa
~: ~:
/
/
Fig. 1: HCI- and water-soluble cation concentrations (/lequ), based on dry matter, of young and old mangrove leaves. The entire column height represents the HCI-soluble concentrations, the patterned areas represent the watersoluble fraction.
also Walter and Steiner 1936). Such a connexion would be suggested by the presence of high amounts of pectins and similar substances able to bind cations. Salt-secretion is a well established mechanism to control Na+ as well as Cl- content in the leaves. Deposits of Na+ and CI- on the leaf surface of A7.licennia marina were markedly higher in the subtropical habitat of Chunda Bay than at the temperate habitat of Bateman's Bay (Fig. 2). This was in accordance with the findings of Drennan and Parmenter (1982) that salt secretion is positively correlated to Z. Pjlanzenphysiol. Bd. 113. S.395-409. 1984.
398
MARIANNE POPP
Avicennia marina
100
-
young
C III
old
a. .., 50 I
E
CI S04Na K Mg
Chunda Bay
Bateman's Bay
Fig. 2: Ion concentrations of both washed and unwashed leaves of Avicennia marina from two different habitats (equ' m- 3 plant water). The patterned areas represent the concentrations of washed leaf samples, the entire column height represents the concentrations in unwashed samples.
the amount of water transpired. However, ion concentrations in Avicennia marina leaves varied little between collecting sites of different climatic conditions (Fig. 2). Ion concentrations of salt-secreting mangrove species are from washed leaf samples in all cases (Fig. 3). These values, presenting the ion concentrations within the leaves, were more appropriate for comparison to the salt concentrations in the non-secreting species than non-washed samples. The Na+ and CI- concentrations of washed leaf samples from the salt-secreting mangroves were very close to sea water and similar to those of other species (Figs. 4, 5 and 6). There was also little change in these two ions between young and old leaves, whereas concentrations of as well as of Mg2+ increased with age. This agreed with the observation that the latter two ions are scarcely or not at all secreted by the salt glands (Fig. 2, Scholander et a1. 1962; Boon and Allaway 1982). Ionic composition of species in the Rhizophoraceae (Figs. 4, 5) resembled the ionic composition of the salt-secreting species (Fig. 3). Na+ and Cl- concentrations were close to sea water concentrations and not influenced by leaf age. The Mg2+ concenincreased markedly in old leaves. The concentrations were rather high, and
sol-
sol-
Z. Pjlanzenplrysiol. Bd. 113. S.395-409. 1984.
Ions and organic acids in mangrove leaves
399
other
anions
1000 old
.... c:
<11
Q.
500
M
I
~~.. .. ::.. ....:: .:
E
Acanthus ilicifolius
Aegialitis annulata
..... .... .... .. ....
.. ......
Aegiceras corniculatum
CI
..:~
Na
::.: :. :~
Avicennia marina
Fig. 3: Ion concentrations (equ), based on plant water content, in young and old leaves of saltsecreting mangrove species.
trations of Na+, Cl- and Mg2+ agreed with those reported by Atkinson et al. (1967) and Ball and Cowan (unpublished results). Concentration of sOi- were not determined in these two studies, but since concentrations of up to 300 equ· m -3 pw are possible, the contribution of sOi- to the osmotic potential in these species should be considered. As reported for Ceriops tagal (Popp et al. in press), the ionic composition of members of the Rhizophoraceae did not change much between different collecting sites. Only in plants from inland habitats, without any tidal influence, were the ionic concentrations reduced (Bruguiera gymnorhiza, Fig. 5). While Cl- was decreased to about 74 to 82 % of the coastal site concentrations, N a+, Mg2+ and sOi- were markedly diminished at the inland site. Fig. 6 represents 5 species out of three different families (Combretaceae, Sonneratiaceae, Meliaceae) without salt glands and with rather high Na+ and Cl- accumulation (between 400 and 800 equ· m- 3 pw). Ion concentrations for Xylocarpus mekongensis were only determined for young leaves because this tree is deciduous, and only young leaves were available at the time of collection. Interestingly, the ion concentrations in these young leaves of Xylocarpus mekongensis were closer to those of the old rather than the young leaves of Xylocarpus granatum. This is possibly due to
z. Pjlanzenphysiol. Bd. 113. S.395-409.
1984.
400
MARIANNE POPP
equ. m- 3 plant water
100
50
Bruguiera exaristata
Ceriops taga/ var. taga/
Rhizophora apicu/ata
Rhizophora /amarckii
i
Rhizophora sty/osa
Fig. 4: Ion concentrations (equ), based on plant water content, in young and old leaves of Rhizophoraceae species.
the comparatively low water content of the young leaves of Xylocarpus granatum (only 0.98ml pw·g-1d·m). Old leaves of Xylocarpus granatum had 1.91ml pw· g-I d . m, while the estimates for X ylocarpus mekongensis were 2.5 ml pw· g-I d· m. Both Xylocarpus species had high concentrations of water-soluble Ca2+; total Ca2+ was even higher than HCI-soluble Na+ in both cases (Fig. 1). Another feature of these two species was a more pronounced accumulation of organic acids than in most of the other mangroves under investigation (Table 1). Comparison of the ion concentrations between old leaves of Lumnitzera racemosa and Xylocarpus granatum illustrates the different water relations in mangrove leaves. Ionic concentrations of Lumnitzera racemosa and Xylocarpus granatum appeared similar when expressed as pw (Fig. 6), they differed markedly when expressed in /Lequ· g-I d· m (Fig. 1). The reason for this was the succulent character of the Lumnitzera racemosa leaves (3.4 ml pw· g-I d· m respectively d· m corresponded to 22. 9% of fresh matter), while d· m made up for 34.4% of fresh matter in the more xeromorphic leaves of Xylocarpus granatum. Unlike other halophytes (Yeo and Flowers 1980; Munns et al. 1983), salts themselves contributed little to the d· m. Depending on the species Na+ made up for 1.2 to 5.6 % of d· m. Z. Pjlanzenphysiol. Bd. 113. S.395-409. 1984.
Ions and organic acids in mangrove leaves
401
1000
old
...c:
you n 9
1\1
other anions
a. <'> I
500
E
Mg
C.
:::I
C'
Q)
CI
K Na
Hinchbrook Island
near Chunda Bay Inland
Fig. 5: Ion concentrations (equ), based on plant water content, of leaves of Bruguiera gym. norhiza from two different habitats.
Fig. 7 presents a number of species with lower ion concentrations than in Figs. 3 to 6. These lower concentrations were associated with both habitat and physiological features. Heritiera littoralis as well as Hibiscus tiliaceus are regarded as brackish water species or mangal associates (Chapman 1976; Barth 1982; Dowling and McDonald 1982). But, even in brackish water, Na+ concentrations exceeded that of K+. The preferential storage of K+ and the above unity K+ INa+ ratios gave rise to the assumption that these two species have special ion uptake and transport characteristics. This is possibly similar to some monocotylous halophytes which are also able to maintain low Na+ concentrations in their shoots (Albert and Popp 1977; Gorham et al. 1980). The striking differences in ion content of young and old leaves of Heritiera littoralis as determined on pw basis was again associated with pronounced changes in water content (young leaves 1.0 ml . g-1 d . m, old leaves 2.9 mI· g-1 d· m, compare Fig. 1 and Fig. 7). The remaining three species in Fig. 7 occurred in normal mixed mangrove stands, for example Excoecaria agallocha close to Lumnitzera racemosa (Fig. 6) and Osbomea octodenta next to Ceriops tagal (Fig. 4). They also differed from their neighbouring species to some extent in their ion concentrations and ratios. The K+ INa + ratio in other mangrove species ranged between O.S to 0.1 (sea water 0.021) depending on age Z. Pjlanzenphysiol. Ed. 113. S.395-409. 1984.
402
MARIANNE POPP
other
anions
1000
young Mg old
....I:
Ca
a. M
I
E
500
I
Lumnitzera littorea
••• 000
Lumnitzera racemosa
Sonneratia alba
Xylocarpus granatum
~~
K
...
// // // // //
I
Xylocarpus mekongensis
Fig. 6: Ion concentrations (equ), based on plant water content, of young and old leaves from members of the Combretaceae, Sonneratiaceae and Meliaceae.
and species. Young Excoecaria agallocha leaves had a K+ INa + ratio of 0.7 and the old leaves of 0.6. Young leaves of Scyphiphora hydrophylacea had even a K+/Na+ ratio of 1.0. More studies like that of Rains and Epstein (1967) are necessary to elucidate the ways of K+ uptake in mangroves. A comparison of the concentrations of organic acids (Table 1) with those of the inorganic ions (Figs. 1-7) reveals that the contribution of organic ions to charge balance was important in only some cases (Xylocarpus spp., Lumnitzera littorea, some Rhizophoraceae). Even mangroves which contained free oxalate did not accumulate this acid to levels as high as found in some Chenopodiaceae (Osmond 1963; Albert and Popp 1977). Malate and citrate, ubiquitous in the plant kingdom (Nierhaus and Kinzel 1971), were present in all samples. Malate was highest in Xylocarpus granatum and Lumnitzera littorea, while citrate was above 100 equ -m- 3 pw in Avicennia eucalypti/olia, Bruguiera gymnorhiza, Heritiera littoralis and again Lumnitzera littorea. Shikimate and quinate are often found in woody species, possibly because they are intermediates of phenylpropane and its derivatives (tannins, lignin). Interestingly, quinate concentrations were always higher in young leaves.
z. Pjlanzenphysiol. Bd. 113. S. 395-409. 1984.
Ions and organic acids in mangrove leaves
403
Table 1: Organic acid concentration in young and old mangrove leaves (equ' m -3 plant water). Species
Family
Acanthus ilici/olius
Acanthaceae
Aegialitis annulata
Plumbaginaceae
Aegiceras corniculatum
M yrsinaceae
Leaf age
Oxalate
Malate
Citrate
y
13.1 12.1
8.2 1.6
8.6
69.8 56.6
7.0
4.6
9.7
5.0
57.1 74.5
5.1 5.6
26.0
0
y 0
y 0
Avicennia eucalypti/olia
A vicenniaceae
Avicennia manna
A vicenniaceae
Bruguiera exaristata
Rhizophoraceae
Bruguiera gymnorhiza
Rhizophoraceae
Ceriops tagal v. australis
Rhizophoraceae
Ceriops tagal v. tagal
Rhizophoraceae
Rhizophora apiculata
Rhizophoraceae
Rhizophora lamarckii
Rhizophoraceae
Rhizophora stylosa
Rhizophoraceae
Excoecaria agallocha
Euphorbiaceae
Heritiera littoralis
Sterculiaceae
Lumnitzera littorea
Combretaceae
Lumnitzera
Combretaceae
Sonneratia alba
Sonneratiaceae
Xylocarpus mekongensis
Meliaceae
Xylocarpus granatum
Meliaceae
Hibiscus tiliaceus
Malvaceae
2.6 2.7
90.8
2.9
42.0 201.0
1.2
86.3
y
41.2 73.7
2.1 12.1
16.1 45.2
y
29.0
3404
0
11.6
18.2
26.9 5.2
90.3 35.0
15.5 2.9
157.7 132.9
y
29.9
0
30.5
112.3 99.5
1.5
62.1
131.0
y
3004
28.6
304
33.5
0
16.3
2804
3.1
704
95.9 55.2
y
13.0 50.6
13.6 62.7
6.0
31.4 7.6
64.0 120.9
23.4 23.8
67.3
43.7 16.5
11704 107.6
y
50.3
y
4004
138.5
27.9
59.8 69.3
38.3
0
704
104.6
y
41.6 42.6
72.9 87.8
55.7 17.7
148.1
9.2 9.3
10.5
35.1
y
10.6
0
12.2
y
28.0
0
9.7
y
107.9
0
27.8
8.9
117.2 47.1
3004 145.2 56.8
116.8 7.8
33.5 1.6
3.8 11.8
7.3 25.9
2.6 33.2
4.2
0
42.9 43.7
12.0 904
y
81.4
29.8
y
248.3 143.8
25.2 31.3
lOA 13.9
18.0
2804
22.9
36.8
y y 0
y
0
y 0
10.7
15.5 2.3
258.2 37.2
51.9
0
13.7
4.8
170.2
35.1 51.5
y 0
Rubiaceae
84.0 75.0
9.8 18.3
0
Scyphiphora hydrophylacea
2.6 2.7
63.2
y
0
Myrtaceae
29.9 16.1
204
3.5
Total
0
0
Osbornea octodenta
Quinate
29.3 119.5
0
racemosa
Shikimate
7.3
116.2 64.5 31.1 6.2
49.5 43.9
16.8 6.8
44.2
1904 54.9 44.1
7.1 2.7
9.9
121.1
9004
371.0 180.3
2.5
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equ .m- 3 plant water
old
500
Excoecaria agal/ocha
Osbornea octodenta
Scyphiphora Hibiscus hydrophylacea tiliaceus
Heritiera littoralis
Fig. 7: Ion concentrations (equ) based on plant water content, of young and old leaves of mangroves from various families (Euphorbiaceae, Myrtaceae, Rubiaceae, Malvaceae and Sterculiaceae). Discussion Like all other halophytes (Waisel 1972; d. Albert 1982), mangroves have often been subjected to classification. Although the codification proposed Ooshi et al. 1975; Lear and Turner 1977; Saenger 1982) consists only of three groups: salt-secretors, salt-excluders and salt-accumulators, the terms are more confusing than helpful (Osmond et al. 1980). This is because the terms describe the salt-regulation mechanisms of different organs. «Salt-secreting» species (Scholander et al. 1962) remove salt from their leaves by means of salt-glands, while «salt-excluding» species (described by Scholander et al. 1962; named so by Atkinson et al. 1967) keep the salt content of their xylem sap low by «ultrafiltration» in the roots. Since little work has been done on mangrove roots (Kramer and Preston 1978; Lawton et al. 1981), it is not known, where this ultrafiltration system is located, and if the ions are excluded from the roots or from the xylem sap. However, final concentrations in the leaves of species from all groups are very similar (Figs. 3-6). Moreover, it cannot be said that in each species there is only one mechanism responsible for regulation of ion content in the leaves. For example, the xylem-sap of the salt-secreting species, Aegialitis annulata, contains 85 to 122 equ' m- 3 Cl- (Atkinson et al. 1967). This is higher than the xylem-sap concentration of non-secreting species, but still far below the concentrations of Cl- in the surrounding sea-water. Thus in a salt-secreting species some regulation at the root level has already appeared. A non-secreting species, like Rhizophora mucronata, is able to reduce its CI- concentration in the xylem-sap to much lower concentrations than secreting species: 17 Z. PjIanzenphysiol. Bd. 113. S.395-409. 1984.
Ions and organic acids in mangrove leaves
405
equ' m- 3 (Atkinson et al. 1967). But, even this low Cl- content in xylem-sap would be deleterious for these leaves due to their long life span (6-12 months, Gill and Tomlison 1971; 12-18 months, Christensen and Wium-Anderson 1977) if no other regulating mechanism intervened. Atkinson et al. (1967) suggested a «dilution by growth» effect because a remarkable constancy in the CI- concentration in leaves of different ages was observed. They estimated that a daily increase of 3 % in dry matter would maintain Cl- at the observed levels on a pw basis. Experiments and calculations by Ball and Cowan (unpublished results) on Rhizophora apiculata are at variance with this estimate because a leaf of Rhizophora apiculata would reach its final salt concentration within 56 days, a far shorter time than the observed life spans. In addition, shedding of senescent leaves is of limited importance for the salt balance in the Rhizophoraceae species (Ball and Cowan, unpublished results). Ball and Cowan suggest a retranslocation of Na+ and Cl- from leaves to other plant parts. This retranslocation was recently observed in the salt-tolerant herbaceous species Trifolium alexandrinum (Winter 1982) and previously observed in other plants (Lessani and Marschner 1978; Jacoby 1979; McNeil 1980). Results presented in this study support retranslocation of Na+ and Cl- since sOi- and Mg2+ concentrations increased markedly in old leaves of Rhizophoraceae, while Na+ and CIconcentrations did not increase (Fig. 4). An overall «dilutionary effect» does also not explain the observed changes in the ratios of CI- Isoi- and Na+ IMg2+ in young and old leaves (Fig.4). It seems more likely that the constant levels of Na+ and Cl- are due in part to a transport out of the leaves, while other ions such as sOi- and Mg2+ accumulate.
If the water content per unit d· m is accepted as a measure of succulence Gennings 1976), Lumnitzera racemosa may be taken as an example of this salt-regulation mode. Biebl and Kinzel (1965) described the significance of succulence for the new-world mangrove, Laguncularia racemosa, which belongs to the Combretaceae as does Lumnitzera racemosa. In Lumnitzera racemosa, the increase of water content appeared sufficient to maintain ion concentrations at the same levels in old and young leaves (Figs. 1, 6). However, in Heritiera littoralis and Xylocarpus granatum, increased water content caused a dilution of ions in the older leaves (Figs. 6, 7). It is evident from both our study and the literature that salt regulation in mangroves is accomplished by the same mechanisms described for other halophytes (Albert 1975, 1982; Flowers et al. 1977; Larcher 1980; Munns et al. 1983). Therefore, it would be advantageous to use the terms «salt-accumulator» and «salt-excluder» for mangrove species in the same way as for halophytes (Gorham et al. 1980; Popp 1983; Yeo 1983). Salt-accumulators store high concentrations of inorganic ions, primarily Na+ and Cl-, in their leaves. Salt-excluders maintain low Na+ and Cl- concentrations in their leaves and adjust osmotically, in part, by accumulation of organic solutes. The easily determined ion concentrations of leaves would provide a clear basis for classification. Assessment of which salt regulation mechanisms are involved Z. Pjlanzenphysiol. Bd. 113. S.395-409. 1984.
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and to what extent they are responsible for the control of ion content is more difficult than ion concentration determinations and therefore, not a good basis for classification. Whatever the mechanism of salt-regulation may be, 20 out of the 22 mangrove species under investigation accumulate Na+ and CI- to such an extent that questions of ion compartmentation and of intracellular osmotic adjustment are raised. Recent investigations demonstrated a high Cl- demand by photosystem II activity in some mangrove chloroplasts (Critchley 1982, 1983; Critchley et al. 1982). However, it cannot be concluded from this that metabolic processes in mangroves are more salttolerant than in other halophytes. There are also reports on high Cl- concentrations in chloroplasts of herbaceous halophytes (d. Jennings 1976) and positive salt effects on photosynthetic electron transport (Wignarajah and Baker 1981), but the salt sensitivity of the metabolic machinery of various halophytes is well documented (d. Flowers et al. 1977; Albert 1982; Munns et al. 1983). Compartmental analysis on halophytes is scarce and nearly non-existent for mangroves (Eshel and Waisel 1979; Beigl 1981; Harvey et al. 1981; Stelzer 1981; Yeo 1981). Van Steveninck et al. (1976) described two different types of vacuoles, one rich in Cl- and the other low in Cl-, for Aegiceras comiculatum. It seems likely that also in other mangrove species Cl- and Na+ are not equally distributed within the cell, but rather are accumulated in the vacuole to a major extent. The degree of accumulation may vary between cell types as shown for Suaeda maritima (Harvey et al. 1981), but nevertheless creates a demand for compatible solutes in the cytoplasm. The organic solutes present in mangroves were shortly outlined previously (Popp et al. in press) and will be treated in more detail in the following two parts of this senes. Acknowledgements This study was supported by grants from the H. and E. Walter-Stiftung (West Germany, Schimper-Fellowship 1981) and by the Austrian Science Research Fund (project No. P 4051). The kind help and support from Dir. Dr. J. and E. Bunt as well as from Dr. B. Clough (AIMS, Townsville) during sample collection is gratefully acknowledged. Thanks are due to Mag. V. Langer for technical assistance, to Dr. E. Kandeler, M. Hinterleitner and 1. Holzapfel for help in preparing the final version of the manuscript and to Prof. Kinzel for reading the manuscript. The author is very grateful to A. Dickie for comments on the manuscript.
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