GAS-EXCHANGE RESPONSES OF A MANGROVE SPECIES, AVICENNIA-MARINA, TO WATERLOGGED AND DRAINED CONDITIONS

This study was undertaken in summer on fully expanded leaves of Avicen nia marina trees in the Beachwood Mangroves Nature Reserve, Durban, So uth Africa. Data sets were obtained over 5-7 days of relatively dry co nditions and over two periods of 5 days during which the swamp was con tinuously inundated with dilute seawater (<150 mol m(-3) NaCl). Gas ex change responses were strongly influenced by photosynthetic photon flu x density (PPFD), leaf temperature and leaf to air vapour pressure def icit law). Carbon dioxide exchange was saturated at a PPFD of about 80 0 mu mol m(-2) s(-1). Maximal CO2 exchange rates ranged from 8.5 to 9. 9 mu mol m(-2) s(-1) with no differences between drained and waterlogg ed conditions. Under drained conditions, leaf conductance, transpirati on and internal CO2 concentrations were generally lower, and water use efficiencies higher, than during waterlogging. Continuous waterloggin g for 5 days had no adverse effect on CO2 exchange. Xylem water potent ials ranged from -1.32 to -3.53 MPa during drained and from -1.02 to - 2.65 MPa during waterlogged conditions. These results are discussed in relation to anatomical and metabolic adaptations of A. marina to wate rlogging stress.