Influence of salinity on leaf structure and water relations in the mangrove, Avicennia marina (Forssk.) Vierh.
Creators: name: Nguyen, Hoa Thi
Publisher: name: The Australian National University
Year: 2016
Subjects: Mangroves
Avicennia marina
salinity
obligate halophyte
water relations
leaf anatomy
PV curve
leaf water storage

description: Physiological and anatomical analyses were used to identify processes underlying growth responses of the grey mangrove, Avicennia marina subsp. australasica, to salinities ranging from fresh to seawater conditions. Avicennia marina was revealed as an obligate halophyte, as seedlings failed to grow in 0 to 5% seawater, whereas maximal growth occurred in 50 to 75% seawater. Relative growth rates were affected by changes in leaf area ratios (LAR) and net assimilation rates (NAR) along the salinity gradient, with NAR generally being more important. Gas exchange characteristics followed the same trends as plant growth. However, water use efficiency was maintained nearly constant across all salinities, consistent with carbon isotopic signatures. Development and composition of hydraulic tissues were consistent with salinity-dependent patterns in water use and growth, and revealed a structural explanation for low stomatal conductance and growth in low salinity. As carbon cannot be gained without water loss, experiments were designed to explore roles of leaf anatomy in leaf water relations. A three-domain pressure – volume relationship (PV curve) was studied in relation to leaf anatomical structure during dehydration in 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. These results 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. Coordination between leaf anatomy and water relations was studied in a temperate population of A. marina subsp. australasica in response to growth along a natural salinity gradient and compared with two subspecies that grew naturally in similar soil salinities to those of subsp. australasica but under different climatic conditions: subsp. eucalyptifolia (salinity 30 ppt, wet tropics) and subsp. marina (salinity 46 ppt, arid tropics). The turgor loss point declined with increase in soil salinity, driven primarily by the osmotic potentials at full turgor as cell wall elasticity was insensitive to growth salinity. Leaf water storage capacitance was similar across subspecies, but leaf water storage increased with increase in salinity and evaporative demand. However, achieving full hydration or full turgor of leaves required sources of water other than the roots. Thus, integration of leaf anatomy and water relations is central to understanding adaptive responses of Avicennia marina to complex gradients in salinity and aridity.