title: Growth responses of the mangrove Avicennia marina to salinity: development and function of shoot hydraulic systems require saline conditions.
abstract: OBJECTIVE
  Halophytic eudicots are characterized by enhanced growth under saline conditions. This study combines physiological and anatomical analyses to identify processes underlying growth responses of the mangrove Avicennia marina to salinities ranging from fresh- to seawater conditions.
  
  
  METHODS
  Following pre-exhaustion of cotyledonary reserves under optimal conditions (i.e. 50% seawater), seedlings of A. marina were grown hydroponically in dilutions of seawater amended with nutrients. Whole-plant growth characteristics were analysed in relation to dry mass accumulation and its allocation to different plant parts. Gas exchange characteristics and stable carbon isotopic composition of leaves were measured to evaluate water use in relation to carbon gain. Stem and leaf hydraulic anatomy were measured in relation to plant water use and growth.
  
  
  RESULTS
  Avicennia marina seedlings failed to grow in 0-5% seawater, whereas maximal growth occurred in 50-75% seawater. Relative growth rates were affected by changes in leaf area ratio (LAR) and net assimilation rate (NAR) along the salinity gradient, with NAR generally being more important. Gas exchange characteristics followed the same trends as plant growth, with assimilation rates and stomatal conductance being greatest in leaves grown in 50-75% seawater. However, water use efficiency was maintained nearly constant across all salinities, consistent with carbon isotopic signatures. Anatomical studies revealed variation in rates of development and composition of hydraulic tissues that were consistent with salinity-dependent patterns in water use and growth, including a structural explanation for low stomatal conductance and growth under low salinity.
  
  
  CONCLUSIONS
  The results identified stem and leaf transport systems as central to understanding the integrated growth responses to variation in salinity from fresh- to seawater conditions. Avicennia marina was revealed as an obligate halophyte, requiring saline conditions for development of the transport systems needed to sustain water use and carbon gain.
authors.affiliation: Plant Science Division, Research School of Biology, The Australian National University, Canberra, ACT 0200, Australia and Institute of Botany, University of Natural Resources and Life Sciences (BOKU), Vienna, Austria hoa-thi.nguyen@anu.edu.au.
authors.forename: Hoa T
authors.initials: HT
authors.lastname: Nguyen
authors.affiliation: Plant Science Division, Research School of Biology, The Australian National University, Canberra, ACT 0200, Australia and Institute of Botany, University of Natural Resources and Life Sciences (BOKU), Vienna, Austria.
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Stanton
authors.affiliation: Plant Science Division, Research School of Biology, The Australian National University, Canberra, ACT 0200, Australia and Institute of Botany, University of Natural Resources and Life Sciences (BOKU), Vienna, Austria.
authors.forename: Nele
authors.initials: N
authors.lastname: Schmitz
authors.affiliation: Plant Science Division, Research School of Biology, The Australian National University, Canberra, ACT 0200, Australia and Institute of Botany, University of Natural Resources and Life Sciences (BOKU), Vienna, Austria.
authors.forename: Graham D
authors.initials: GD
authors.lastname: Farquhar
authors.affiliation: Plant Science Division, Research School of Biology, The Australian National University, Canberra, ACT 0200, Australia and Institute of Botany, University of Natural Resources and Life Sciences (BOKU), Vienna, Austria.
authors.forename: Marilyn C
authors.initials: MC
authors.lastname: Ball
mesh_terms: D032407:Avicennia / Q000033:anatomy & histology / Q000254:growth & development* / Q000502:physiology; D001692:Biological Transport; D018520:Plant Shoots / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology; D054712:Salinity; D055049:Salt Tolerance; D055051:Salt-Tolerant Plants / Q000033:anatomy & histology / Q000254:growth & development* / Q000502:physiology; D014867:Water / Q000378:metabolism*
chemical_list: D014867:Water
keywords: Avicennia marina; hydraulic anatomy; mangrove; obligate halophyte; plant growth; salinity