XVI International Botanical Congess

Salt entering mangrove roots is disposed of by (1) re-secretion from roots, (2) accumulation in expanding vacuoles, and (3) excretion by leaf salt glands. Proposal (4) - loss of old salt-filled leaves - cannot help to dispose of salt currently flowing into the plant, but accumulation of additional salt as leaves age could do so. To investigate this hypothesis, developmental sequences from early expanding to yellow pre-abscission leaves were collected from individual branches of Bruguiera cylindrica (no salt glands) and Avicennia rumphiana and Avicennia marina (with salt glands). Leaf thickness (succulence) increases 3-4% per node in B. cylindrica and A. rumphiana, but not in A. marina. Na+ content per unit leaf area increases progressively though to senescence in B. cylindrica (130%) and A. rumphiana (45%) but falls by 45% in A. marina. Calculations using also leaf demographic data show that these Na+ changes contribute significantly to salt disposal in Bruguiera. There is no evidence that senescence and abscission add to this. Nutrient scavenging from ageing mangrove leaves is also considered.