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The mangrove Avicennia marina adjusts internal salt concentrations by foliar salt secretion. Deliquescence of accumulated salt causes leaf wetting that may provide a water source for salt-secreting plants in arid coastal wetlands where high nocturnal humidity can usually support deliquescence whereas rainfall events are rare. We tested the hypotheses that salt deliquescence on leaf surfaces can drive top-down rehydration, and that such absorption of moisture from unsaturated atmospheres makes a functional contribution to dry season shoot water balances. Sap flow and water relations were monitored to assess the uptake of atmospheric water by branches during shoot wetting events under natural and manipulated microclimatic conditions. Reverse sap flow rates increased with increasing relative humidity from 70% to 89%, consistent with function of salt deliquescence in harvesting moisture from unsaturated atmospheres. Top-down rehydration elevated branch water potentials above those possible from root water uptake, subsidising transpiration rates and reducing branch vulnerability to hydraulic failure in the subsequent photoperiod. Absorption of atmospheric moisture harvested through deliquescence of salt on leaf surfaces enhances water balances of Avicennia marina growing in hypersaline wetlands under arid climatic conditions. Top-down rehydration from these frequent, low intensity wetting events contributes to prevention of carbon starvation and hydraulic failure during drought.

Leaf structure and water relations were studied in a temperate population of Avicennia marina subsp. australasica along a natural salinity gradient [28 to 49 parts per thousand (ppt)] and compared with two subspecies grown naturally in similar soil salinities to those of subsp. australasica but under different climates: subsp. eucalyptifolia (salinity 30 ppt, wet tropics) and subsp. marina (salinity 46 ppt, arid tropics). Leaf thickness, leaf dry mass per area and water content increased with salinity and aridity. Turgor loss point declined with increase in soil salinity, driven mainly by differences in osmotic potential at full turgor. Nevertheless, a high modulus of elasticity (e) contributed to maintenance of high cell hydration at turgor loss point. Despite similarity among leaves in leaf water storage capacitance, total leaf water storage increased with increasing salinity and aridity. The time that stored water alone could sustain an evaporation rate of 1 mmol m-2 s-1 ranged from 77 to 126 min from subspecies eucalyptifolia to ssp. marina, respectively. Achieving full leaf hydration or turgor would require water from sources other than the roots, emphasizing the importance of multiple water sources to growth and survival of Avicennia marina across gradients in salinity and aridity.

A three-domain pressure – volume relationship (PV curve) was studied in relation to leaf anatomical structure during dehydration in the grey mangrove, Avicennia marina. In domain 1, relative water content (RWC) declined 13% with 0.85 MPa decrease in leaf water potential, reflecting a decrease in extracellular water stored primarily in trichomes and petiolar cisternae. In domain 2, RWC decreased by another 12% with further reduction in leaf water potential to -5.1 MPa, the turgor loss point. Given the osmotic potential at full turgor (-4.2 MPa) and the effective modulus of elasticity (~40 MPa), domain 2 emphasized the role of cell wall elasticity in conserving cellular hydration during leaf water loss. Domain 3 was dominated by osmotic effects and characterized by plasmolysis in most tissues and cell types without cell wall collapse. Extracellular and cellular water storage could support an evaporation rate of 1 mmol m-2s-1 for up to 54 and 50 min, respectively, before turgor loss was reached. This study emphasized the importance of leaf anatomy for the interpretation of PV curves, and identified extracellular water storage sites that enable transient water use without substantive turgor loss when other factors, such as high soil salinity, constrain rates of water transport.The pressure-volume relationships in field grown leaves of the mangrove, Avicennia marina, exhibited three domains dominated successively by 1) the presence and consumption of extracellular water, 2) variable turgor and loss of intracellular water, and 3) osmotic behavior of flaccid cells and plasmolysis. Multiple sites of extracellular water storage were identified, including hollow trichomes and novel structures named “cisternae”. When fully charged, extracellular and cellular water storage could support a typical evaporation rate of 1 mmol m-2s-1 for 54 and 50 min, respectively, before turgor loss was reached. This study emphasizes the importance of leaf anatomy for the interpretation of PV curves, and identifies extracellular water storage sites that enable transient water use without substantive turgor loss when other factors, such as high soil salinity, constrain rates of water transport.

The particulate methane monooxygenase (pMMO) from Methylococcus capsulatus (Bath) is a multi-copper protein that hydroxylates methane and other small n-alkanes to their corresponding alcohols with high regiospecificity and stereoselectivity. The copper ions appear to be arranged into ˜5 trinuclear copper clusters. Two of these clusters are thought to be involved in the dioxygen chemistry and alkane hydroxylation mediated by the enzyme. Accordingly, these catalytic clusters (C-clusters) are typically oxidized, as the protein is isolated. The remaining copper ions are normally reduced, and it has been suggested that they provide a reservoir of reducing equivalents needed for the turnover of the enzyme. In this study, we have oxidized the protein to different levels of oxidation of the copper ions using dioxygen, hydrogen peroxide, ferricyanide, and dioxygen in the presence of the suicide substrate acetylene, and have characterized the oxidized copper centers using low temperature electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy. The results are consistent with the grouping of the copper ions into catalytic clusters and electron transfer clusters (E-clusters). Quantification indicates that there are, indeed, two C-clusters. A model in which the two C-clusters are activated by dioxygen starting from their fully reduced states and mediate the hydroxylation chemistry of methane will be presented and discussed.