GROWTH AND PHYSIOLOGICAL-RESPONSES OF NEOTROPICAL MANGROVE SEEDLINGS TO ROOT-ZONE HYPOXIA

Seedlings of Rhizophora mangle L., Avicennia germinans (L.) Steam., an d Laguncularia racemosa (L.) Gaertn. f. were cultured in aerated or N- 2-purged solution for 12 weeks to assess their relative responses to l ow oxygen tensions. All three species responded to low oxygen treatmen t by modifying physiological and morphological patterns to decrease ca rbon loss by root respiration. However, the extent to which seedling p hysiology and morphology were altered by low oxygen treatment differed among species. Maintenance of root oxygen concentrations, root respir ation rates and root extension rates by R. mangle demonstrated an abil ity to avoid low oxygen stress with minimal changes in root morphology and physiology. In contrast, oxygen concentrations in A. germinans an d L. racemosa roots declined from 16 to 5% or lower within 6 h of trea tment. Root hypoxia led to significant decreases in respiration rates of intact root systems (31 and 53% below controls) and root extension rates (38 and 76% below controls) by A. germinans and L. racemosa, res pectively, indicating a greater vulnerability of these species to low oxygen tensions in the root zone compared with R. mangle. I conclude t hat the relative performance of mangrove seedlings growing in anaerobi c soils is influenced by interspecific differences in root aeration an d concomitant effects on root morphology and physiology.