Salinity tolerance in some mangrove species from Pakistan

Growth, ionic and water relations of three mangrove species viz. Avicennia marina, Ceriops tagal and Rhizophora mucronata werestudied in different seawater concentrations (0, 25, 50, 75 and 100%).All mangrove species showed optimal growth at 50% seawater. Relativelymore biomass was accumulated by R. mucronata while C. tagalhad the tallest individuals. Tissue water potential became more negativewith the increase in salinity and stomatal conductance was decreased in allplants. Higher stomatal conductance was noted in R. mucronata,followed by A. marina and C. tagal. Sodium and chloride ionsincreased with the increase in salinity and this accumulation was muchhigher in A. marina.     

Effects of experimental sedimentation on the phenological dynamics and leaf traits of replanted mangroves at Gazi bay,Kenya

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Effects of pretreated domestic wastewater supplies on leaf pigment content, photosynthesis rate and growth of mangrove trees: A field study from Mayotte Island, SW Indian Ocean

After 12 and 18 months of daily wastewater discharge into mangrove plots in Mayotte Island, SW Indian Ocean, leaf pigment content, photosynthesis rate and growth of Rhizophora mucronata and Ceriops tagal mangrove trees were evaluated and compared with similar individuals from control plots. Chlorophyll and carotenoid contents, measured using an HPLC analyser, were significantly higher in leaves of mangrove trees receiving wastewater discharges. Photosynthesis and transpiration rates, analysed using an LCi portable system, increased significantly for mangrove trees in impacted plots. Measurements of leaf areas, young branch length and propagule length showed significant increases in plots receiving wastewater. These results suggest a beneficial effect of domestic wastewater on R. mucronata and C. tagal mangrove tree functioning. Analyses and observations on mangrove ecosystems as a whole - taking into account water and sediment compartments, crab populations and nitrogen and phosphorus cycles - are nevertheless necessary for evaluation of bioremediation capacities of mangrove ecosystems.     

Tidal exchange of macrolitter between a mangrove forest and adjacent seagrass beds (Gazi Bay,Kenya)

In the present study the tidal transport of macrolitter between the mangrove forest in Gazi bay (Kenya) and the adjacent seagrass meadows in the bay was investigated, by deploying large standing nets, which extended over the entire height of the water column, in the transition zone between both ecosystems. In addition, the presence of macrolitter on the floor ofRhizophora mucronata andCeriops tagal stands was studied. The macromaterial (>2 mm) that was collected with the nets consisted of mangrove material (26%, mostly leaf material), seagrass leaves (60%) and macroalgae (14%). Transport was bidirectional, indicating shuttle movements of the litter, driven by the opposite flow direction of flood and ebb tides. Litter from the mangrove species consisted mainly of leaves from species occurring at the outer zone of the forest,i.e., Rhizophora mucronata andSonneratia alba. This finding suggests that the complex spatial structure of the forest hampers outflow of macrolitter from the more inner parts. Consequently, this material remains trapped within the forest. The dominant presence of seagrass litter in the macromaterial transported with the tidal water, and the conspicuous and persistent presence of seagrass litter in the low lying, peripheralR. mucronata plots (but not in the more elevatedC. tagal plots) suggest that the mangrove forest of Gazi bay is the recipient of carbon and nutrients from the seagrass system. It is hypothesized that the element cycling of the inner parts of the mangrove forest proceeds as that of a rather closed system, whereas element cycling in the outer parts has conspicuous reciprocal connections with the adjacent seagrass meadows.     

Stable soil pH of thai mangroves in dry and rainy seasons and its relation to zonal distribution of mangroves

Soil characteristics of mangrove forests at Amphur Laemngob, Trat Province, east Thailand were investigated in both dry andrainy seasons. Sonneratia alba, Avicennia alba andRhizophora mucronata mainly grew as seaward mangroves.Rhizophora apiculata, Ceriops tagal, Excoecaria agallocha andBruguiera gymnorrhiza grew as meso mangroves in most cases.Lumnitzera racemosa was found as a landward mangrove. The salinity of the soil in which these 8 species grew was significantly higher in the dry season than in the rainy season. Soil pH did not change irrespective of the season. Each species seemed to grow at inherent soil pH. Bruguiera gymnorrhiza, Sonneratia alba andAvicennia marina were found both in Japan and Thailand. The soil pH of each mangrove species was similar in Japan and Thailand. The soil salinity of each mangrove species in Japan was similar to that in Thailand in the dry season. It was suggested that soil pH and salinity in the dry season are the important factors governing the zonal distribution of mangroves.     

Mangrove rehabilitation dynamics and soil organic carbon changes as a result of full hydraulic restoration and re-grading of a previously intensively managed shrimp pond

Hydraulic restoration by opening the shrimp pond banks facilitated the establishment of planted mangroves and colonisation by non-planted mangrove species and was shown to be an effective method of mangrove rehabilitation. Planted Rhizophora apiculata and Rhizophora mucronata had grown significantly in 6 years, to 300 and 350 cm, respectively. However, the growth rate of Bruguiera cylindrica was merely 150 cm in the same period despite vigorous growth in the initial stage. About 15 non-planted mangrove species had colonised within 6 years after reopening the banks, with the dominant species being Avicennia marina (46.9%) followed by B. cylindrica (27.0%) and Ceriops tagal (14.9%). After the enhancement, soil organic carbon increased considerably from 110 to 160 tonC ha⁻¹ in 2 years at the lower elevation, indicating that hydraulic restoration could stimulate carbon recovery through enhancement of mangrove growth. However, soil organic carbon decreased by almost half in the higher ground, suggesting that carbon decomposition was accelerated due to drying of soils.     

Photosynthetic gas exchange of the mangrove,Rhizophora stylosa Griff., in its natural environment

Photosynthetic gas exchange properties of leaves of the mangrove, Rhizophora stylosa Griff., were investigated in order to assess its productivity and gain some insight into the constraints set upon it by the saline habitat. Mature trees of this dominant species were studied in their natural, tidal-forest environment at Hinchinbrook Is., North Queensland for two periods during the dry season. Individual leaves were enclosed in a chamber wherein environmental conditions were varied. CO₂ assimilation, transpiration and environmental parameters were monitored during daylight hours by instrumentation housed in a mobile laboratory mounted on a barge. Analysis of the daily course of leaf gas exchange revealed a CO₂ assimilation capacity comparable with that of many glycophytic trees. Photosynthesis was strongly influenced by leaf temperature as well as photon flux density. There was a strong and steadily increasing inhibition of gas exchange as leaf temperatures and, consequently, the leaf to air VPD increased. CO₂ assimilation rates and leaf conductances to water vapour diffusion were strongly correlated, resulting in nearly constant internal CO₂ concentrations in the leaves under the full range of conditions. The effect of leaf orientation in minimizing the leaf-to-air temperature difference was striking. The close coordination between stomatal conductance and CO₂ assimilation rate in this mangrove results in high water use efficiency. This sparing use of water may be an important factor underlying the high salinity tolerance of mangroves.Contribution No. 254 from the Australian Institute of Marine Science     

Relation of water transport to leaf gas exchange properties in three mangrove species

Mangrove species more tolerant to salinity may function with less efficient water transport, which may be related to more conservative water use. To test the hypothesis, we investigate the gas exchange and hydraulic properties of three mangrove species: Rhizophora mangle L., Laguncularia racemosa Gaert and Avicennia germinans (L.)L. Experiments were performed with adult plants growing naturally in the field under a salinity of 35‰. Gas exchange parameters showed that A. germinans had significantly higher photosynthetic rates, and lower stomatal conductance and transpiration rates, compared to the other two mangroves. In concert with this, instantaneous water use efficiency was significantly high in A. germinans, intermediate in L. racemosa and lowest in R. mangle. The hydraulic parameters of the three mangrove species were in the lowest end of the range reported for tropical trees. However, the three mangrove species exhibited measurable differences in hydraulic parameters related to the control of water requirements for maintenance of carbon gain. L. racemosa and A. germinans showed less efficient water transport at shoot level but were the more efficient species in water use at the leaf level in comparison to R. mangle. Received: 7 April 1999 / Accepted: 25 July 1999     

Spatial variations in macrobenthic fauna recolonisation in a tropical mangrove bay

Recolonisation by crab species and sediment-infauna taxa (at class level) in artificially regenerated mangrove stands of Avicennia marina, Rhizophora mucronata and Sonneratia alba (5 yr old) were studied using respective bare sites (open without mangroves or denuded) and natural sites (relatively undisturbed) as controls. The controls were chosen based on site history, physical proximity and tidal inundation class in reference to the particular reforested mangrove stand and samples randomly taken. A number of environmental variables were measured; interstitial water salinity and temperature (measured at low tide) were lower, whereas sediment organic matter content was higher in the areas with mangrove cover, with the natural sites having the highest content. The bare sites were generally sandier, whereas the areas with mangrove cover had higher proportions of clay and silt. Generally, there was a higher crab density in the reforested sites than in the bare sites, whereas crab species diversity (Shannon diversity index) did not vary from one site to another for any of the mangrove species. In terms of crab species composition, the reforested sites were more similar (Sørensen similarity coefficient) to the natural sites and less to the bare controls. For sediment-infauna, the reforested sites had a significantly higher density than the respective bare controls, while the natural sites had the highest density. The number of sediment-infauna taxa in both the reforested and natural sites of all the mangrove species was similar and higher than in the comparable bare sites. The results suggest that the reforested sites are supporting more faunal recolonisation, and therefore becoming more akin to the natural mangrove sites in terms of the investigated functional indicators. The findings seem to support the use of artificial mangrove regeneration (in areas where natural regeneration has been impeded by physical conditions or otherwise) as an effective management tool in the restoration and conservation of the functional integrity of degraded mangrove habitats.Key words: Crabs, Environmental variables, Kenya, Recolonisation, Restored mangroves, Sediment-infauna     

Interaction between Water and Wind as a Driver of Passive Dispersal in Mangroves

Although knowledge on dispersal patterns is essential for predicting long-term population dynamics, critical information on the modalities of passive dispersal and potential interactions between vectors is often missing. Here, we use mangrove propagules with a wide variety of morphologies to investigate the interaction between water and wind as a driver of passive dispersal. We imposed 16 combinations of wind and hydrodynamic conditions in a flume tank, using propagules of six important mangrove species (and genera), resulting in a set of dispersal morphologies that covers most variation present in mangrove propagules worldwide. Additionally, we discussed the broader implications of the outcome of this flume study on the potential of long distance dispersal for mangrove propagules in nature, applying a conceptual model to a natural mangrove system in Gazi Bay (Kenya). Overall, the effect of wind on dispersal depended on propagule density (g l^-1). The low-density Heritiera littoralis propagules were most affected by wind, while the high-density vertically floating propagules of Ceriops tagal and Bruguiera gymnorrhiza were least affected. Avicennia marina, and horizontally floating Rhizophora mucronata and C. tagal propagules behaved similarly. Morphological propagule traits, such as the dorsal sail of H. littoralis, explained another part of the interspecific differences. Within species, differences in dispersal velocities can be explained by differences in density and for H. littoralis also by variations in the shape of the dorsal sail. Our conceptual model illustrates that different propagule types have a different likelihood of reaching the open ocean depending on prevailing water and wind currents. Results suggest that in open water, propagule traits (density, morphology, and floating orientation) appear to determine the effect of water and wind currents on dispersal dynamics. This has important implications for inter- and intraspecific variation in dispersal patterns and the likelihood of reaching suitable habitat patches within a propagule''s viable period.     

Comparative study of water uptake and photosynthetic gas exchange between scrub and fringe red mangroves,Rhizophora mangle L.

Summary The red mangrove (Rhizophora mangle L.) occurs frequently in both scrub and fringe mangrove forests. Our previous study demonstrated that individuals of this mangrove species growing in scrub and fringe forests differ significantly in both morphological and physiological characteristics. To further characterize physiological differences between scrub and fringe mangroves, we compared their differences in water uptake and photosynthetic gas exchange during different seasons. In the wet season (June–October, 1990), scrub mangroves showed lower D and ¹⁸O values of stem water than fringe mangroves, indicating more usage of rain-derived freshwater. In the dry season (Jan–April, 1991), however, scrub mangroves utilized the same water source as fringe mangroves, reflected by their similar D and ¹⁸O values of stem water. Consistently, there were significant differences in predawn water potentials between scrub and fringe mangroves in the wet season (October 1990) with higher values for scrub mangroves, but no significant differences in the dry season (January 1991). Higher elevation in the scrub forest seems to be the major factor responsible for the shift of water sources in scrub mangroves. On Apr. 27 and Aug. 8, 1990, scrub mangroves showed lower CO₂ assimilation rate, stomatal conductance, and intercellular CO₂ concentration than fringe mangroves. There were no differences in these gas exchange characteristics on the other two measuring dates: Oct. 17, 1990 and Jan. 11, 1991. Instantaneous water use efficiency was significantly higher for scrub mangroves than for fringe mangroves on three of the four sampling dates. Similarly, leaf carbon isotope discrimination of scrub mangroves was always significantly lower than that of fringe mangroves, indicating higher long-term water use efficiency. Higher water use efficiency in scrub mangroves is a result of stomatal limitation on photosynthesis, which may entail considerable carbon cost to the plants.     

Litter Fall Dynamics of Restored Mangroves (Rhizophora mucronata Lamk. and Sonneratia alba Sm.) in Kenya

Mangrove forests are active carbon sinks and important for nutrient cycling in coastal ecosystems. Restoration of degraded mangrove habitats enhances return of ecosystem goods and services, including carbon sequestration. Our objective was to assess the restoration of primary productivity of reforested mangrove stands in comparison with natural reference stands in Gazi Bay, Kenya. Litter fall data were collected in nine Rhizophora mucronata and Sonneratia alba monospecific stands by use of litter traps over 2 years. Litter was emptied monthly, dried, sorted, and weighed. The reforested and natural stands showed seasonality patterns only in the production of reproductive material. Leaves constituted the highest percentage to total litter fall. Litter productivity rates for the R. mucronata stands were not significantly different and ranged from 6.61–10.15 to 8.36–11.02 t ha^?1 yr^?1 for the restored and natural stands, respectively. The productivity of 5 years R. mucronata stands reached 5.22 t ha^?1 yr^?1 and was significantly different from other stands. Litter productivity rates for S. alba stands was 7.77–7.85 for the restored stands and 10.15 t ha^?1 yr^?1 for the natural stand but differences were not significant. Our results indicate that plantations of at least 11 years have attained litter productivity rates comparable to the natural forests. This suggests that productivity of replanted mangroves is likely to reach complete recovery by this age under the prevailing environmental conditions.     

Annual cyclicity in high-resolution stable carbon and oxygen isotope ratios in the wood of the mangrove tree Rhizophora mucronata

In the present study, the high‐resolution stable carbon (¹³C/¹²C) and oxygen (¹⁸O/¹⁶O) isotope ratio profiles in the wood of the mangrove Rhizophora mucronata Lam., a tropical tree species lacking distinct growth rings, were investigated. Variations of both isotope ratios revealed a remarkable annual cyclicity with lowest values occurring at the latewood/earlywood boundary (April–May) and highest values during the transition from earlywood to latewood (October–November). Based on the current knowledge of the physiology of this mangrove species, as well as on the current literature available on high‐resolution profiles of stable isotope ratios in tree rings, possible driving forces responsible for this seasonal pattern are discussed. The annual cyclicity, together with a conspicuous isotope pattern appearing in the El‐Niño year 1997, promises great potential for tropical dendrochronology.     

Nitrogen enrichment during decomposition of mangrove leaf litter in an east African coastal lagoon (Kenya): Relative importance of biological nitrogen fixation

In situ decomposition of senescent leaves of twoabundant mangrove species (Rhizophora mucronataLamarck and Ceriops tagal (Perr) C.B. Rob),enrichment of nitrogen and activity of dinitrogenfixing bacteria during decomposition were investigatedduring both rainy and dry seasons in a tropicalcoastal lagoon (Gazi, Kenya). Rates of leafdecomposition were higher for R. mucronata thanfor C. tagal and were highest, for both species,during rainy season. Rates of decomposition, expressedas percentage dry mass loss, over a decompositionperiod of 50 days was: C. tagal (rainy season),69%; C. tagal (dry season), 27%; R.mucronata (rainy season), 98%; and R.mucronata (dry season), 48%. High rainfall anddiurnal tidal inundation appear to enhance the leafdecomposition process. Maximum rates of nitrogenfixation were 380 nmol N₂ h^-1 g^-1 dw forC. tagal (rainy season), 78 nmolN₂ h^-1 g^-1 dw for C. tagal (dryseason), 390 nmol N₂ h^-1 g^-1 dw for R. mucronata (rainy season) and 189 nmolN₂ h^-1 g^-1 dw for R. mucronata (dry season). Although N₂ fixation rates werehighest during rainy season, total nitrogenimmobilised in the leaves was highest during the dryseason. Biological nitrogen fixation can account forbetween 13 to 21% of the maximum nitrogen immobilisedin the decaying mangrove leaves. Nitrogen fixation, asa source of allochthonous nitrogen, sustains anitrogen input to the mangrove ecosystem, which addssignificantly to the nitrogen input through leaflitterfall.     

Mangrove Tree Specificity and Conservation Implications of the Arboreal Crab <Emphasis Type="Italic">Parasesarma leptosoma</Emphasis> at Mngazana,a Mangrove Estuary in the Eastern Cape,South Africa

The arboreal crab Parasesarma leptosoma has been recently discovered at Mngazana, a southerly mangrove system in southern Africa, where crab tree preferences were studied using an indirect (browse leaf damage) and a direct (tree traps) method. The extent of crab induced leaf damage was compared for three mangrove species at two sites, one next to a tidal creek and one away from the creek. Using ANOVA, significant differences were found between tree species (P < 0.001) at different distances from the creek (P < 0.022). Crabs were found to occur on Rhizophora mucronata and Brugueira gymnorrhiza, but not on Avicennia marina. This reflected a gradient in browsing, from well-browsed R. mucronata (100% near the creek and 25.7% away from the creek), to medium browsing of B. gymnorrhiza (51.5% near and 0% away) and no browsing on A. marina (near or away). These differences could be explained in terms of palatability, as both R. mucronata and B. gymnorrhiza are salt excluders, while A. marina secretes salt from its leaves. Leaf consumption levels averaged between 1.73% and 2.6% of leaf area for R. mucronata and 0–1.76% for B. gymnorrhiza. For both R. mucronata and B. gymnorrhiza there was a significant correlation between the number of crabs caught directly and the amount of browse leaf damage (P < 0.01). Crab number was also significantly correlated with tree circumference for R. mucronata (r ² = 0.67) and B. gymnorrhiza (r ² = 0.76, P < 0.05), with crabs more prevalent on the former tree species and no crabs trapped on A. marina (91.7%, 38.3% and 0% catches, respectively), thus reflecting the results obtained by the indirect method. Total Nitrogen and Phosphate were measured for both sediment and leaves of the three mangrove species at the two sites. Leaf comparisons showed significant differences (P < 0.01) for both Total Nitrogen and Phosphate with R. mucronata having the highest values, followed by A. marina and lastly B. gymnorrhiza. Total Nitrogen was significantly higher for both B. gymnorrhiza and R. mucronata compared with A. marina, while leaf phosphate was significantly lower for B. gymnorrhiza when compared with both R. mucronata and A. marina. No significant differences were found for leaf nutrients between sites, with the exception of A. marina and R. mucronata Total Nitrogen, which was significantly higher at the near creek sites (P < 0.05). Sediment analysis showed no significant differences (P > 0.05) in either nutrients or median particle size. Thus, R. mucronata, especially near the creek, had higher nutrient value and was probably more palatable and could explain observed differences in crab distribution. Very little browse damage was encountered in saplings below 10 cm. Most poles chopped by the local communities are R. mucronata in the 15–20 cm category, which coincides with peak crab frequencies in the 15–25 cm size classes for R. mucronata and B. gymnorrhiza, so that this selective harvesting is affecting this crab population maximally. Predictions were made as to the effect of crab loss, tree replacement rate and alternatives to chopping, which would boost community socio-economic levels and reduce the anthropogenic pressure on this biodiverse southerly mangrove system.     

清澜港红树林湿地典型群落类型沉积物活性有机碳组分分布特征

滩涂海岸红树林生态系统通常具有较高的土壤养分,尤其是沉积物有机碳含量。海南岛红树林种类丰富且生长较好,通过对红树林沉积物有机碳组分的基础研究有利于提高对红树林湿地固碳能力的评估精度,加深对海洋蓝碳的认识。以清澜港红树林5种典型群落类型为对象,比较分析表层土壤(0—10 cm)总有机碳(TOC)、微生物生物量碳(MBC)、易氧化有机碳(EOC)、可溶性有机碳(DOC)含量差异及其与土壤因子之间的相关性。结果表明：(1)不同群落类型间,土壤TOC、MBC、DOC和EOC含量均值分别为66.76 g/kg、177.08 mg/kg、25.49 mg/kg和2.34 g/kg。对比发现,土壤TOC在角果木群落中含量最高,但各群落间无显著差异;土壤MBC在不同群落间存在显著差异,其中角果木群落和杯萼海桑群落显著高于榄李群落;土壤DOC在不同群落间存在显著差异,其中海莲群落和角果木群落显著高于其余群落;土壤EOC在不同群落间存在显著差异,其中角果木群落显著高于海莲群落和正红树群落。(2)活性有机碳各个组分占总有机碳的比例均值大小依次为EOC>MBC>DOC。土壤EOC、MBC、DOC的...     

Early growth responses of mangroves to different rates of nitrogen and phosphorus supply

Experiments were conducted in an outdoor facility to quantify growth responses of six mangrove species to rates of dissolved inorganic nitrogen and phosphorus supply mimicking the range of N and P mineralization rates in natural soils. Growth of all six species on nitrogen was nonlinear. Stem extension rates of Rhizophora apiculata and Xylocarpus granatum were enhanced to the highest rate of N supply (50 mmol m^− 2 d^− 1); Bruguiera gymnorrhiza, Avicennia marina, and Xylocarpus moluccensis stem growth leveled off by 10 mmol m^− 2 d^− 1. Stem growth of Ceriops tagal peaked at 24-26 mmol N m^− 2 d^− 1. Except for A. marina and C. tagal, rates of biomass increase declined at the highest supply rate, indicating NH₄⁺ toxicity. At different rates of P supply, stem extension rates and rates of biomass increase of R. apiculata and C. tagal best-fit Gaussian curves and B. gymnorrhiza stem growth and biomass increase best-fit sigmoidal and Gaussian curves, respectively; X. moluccensis stem and biomass growth increased linearly, but stem and biomass growth rates of A. marina did not vary in relation to P supply. Stem growth of X. granatum was Gaussian but rates of biomass increase best-fit a quadratic equation. Changes in leaf and root N and P content mirrored the growth responses. As rates of N and P mineralization in natural mangrove soils overlap with the lowest rates of N and P supplied in these experiments, the growth responses imply that mangroves are intrinsically nutrient-limited at mineralization rates often encountered in nature. Such species specificity may have significant implications for recruitment success and the establishment of species gradients within mangrove forests.     

Relationships between height and girth of mangroves and soil-water conditions in the Mary and Hawkesbury River estuaries,eastern Australia

Abstract The height and diameter at breast height (DBH) of mangroves on the Hawkesbury River, New South Wales and the Mary River, Queensland were related to soil-water salinity, soil-water content and distance from the mouth of the estuary. On the Hawkesbury River, both Avicennia marina and Aegiceras corniculatum declined in height and DBH with increasing soil-water salinity and soil-water content, and increased in height with distance from the mouth of the estuary. Both species showed an extensive range of all variables. On the Mary River, where species diversity was higher, no relationship was found between the height and DBH of A. marina and A. corniculatum and soil-water salinity and soil-water content. Both species increased in height with increasing distance from the mouth of the estuary, a characteristic shared with all other species studied (Excoecaria agallocha, Ceriops tagal var. australis, Rhizophora stylosa). The results suggest that growth characteristics of mangroves are not a simple response to salinity gradients in diverse systems and that other variables such as nutrient availability may be important controls on mangrove growth.     

An improved method for the simultaneous determination of photosynthetic O2 evolution and CO2 consumption in Rhizophora mucronata leaves

The photosynthetic gas-exchange has been assessed traditionally either as O₂ evolution or CO₂ consumption. In this study, we used a liquid-phase O₂ electrode combined with CO₂ optodes to examine simultaneously photosynthesis in intact leaves of mangrove Rhizophora mucronata. We verified suitable conditions for leaf photosynthetic rates by assessing pH levels and NaHCO₃ concentrations and compared these to the gas-exchange method at various PAR levels. The photosynthetic rate in response to pH exhibited a similar pattern both for O₂ evolution and CO₂ consumption, and higher rates were associated with intermediate pH compared with low and high pH values. The net photosynthetic quotient (PQ) of R. mucronata leaves ranged from 1.04–1.28. The PQ values, which were never lesser than 1, suggested that photorespiration did not occur in R. mucronata leaves under aqueous conditions. The similar maximum photosynthetic rates suggested that all measurements had a high capacity to adjust the photosynthetic apparatus under a light saturation condition. The simultaneous measurements of O₂ evolution and CO₂ consumption using the Clark oxygen electrode polarographic sensor with the CO₂ optode sensor provided a simple, stable, and precise measurement of PQ under aqueous and saturated light conditions.     

Changes in gas exchange characteristics and water use efficiency of mangroves in response to salinity and vapour pressure deficit

Summary Measurements were made of the photosynthetic gas exchange properties and water use efficiency of 19 species of mangrove in 9 estuaries with different salinity and climatic regimes in north eastern Australia and Papua New Guinea. Stomatal conductance and CO₂ assimilation rates differed significantly between species at the same locality, with the salt-secreting species, Avicennia marina, consistently having the highest CO₂ assimilation rates and stomatal conductances. Proportional changes in stomatal conductance and CO₂ assimilation rate resulted in constant and similar intercellular CO₂ concentrations for leaves exposed to photon flux densities above 800 mol·m^-2·s^-1 in all species at a particular locality. In consequence, all species at the same locality had similar water use efficiencies. There were, however, significant differences in gas exchange properties between different localities. Stomatal conductance and CO₂ assimilation rate both decreased with increasing salinity and with increasing leaf to air vapour pressure deficit (VPD). Furthermore, the slope of the relationship between assimilation rate and stomatal conductance increased, while intercellular CO₂ concentration decreased, with increasing salinity and with decreasing ambient relative humidity. It is concluded from these results that the water use efficiency of mangroves increases with increasing environmental stress, in this case aridity, thereby maximising photosynthetic carbon fixation while minimising water loss.Contribution No. 459 from the Australian Institute of Marine Science