Carbon assimilation and biomass partitioning in Avicennia germinans and Rhizophora mangle seedlings in response to soil redox conditions

Under controlled environmental conditions, seedlings of black mangrove (Avicennia germinans (L.) Stearn) and red mangrove (Rhizophora mangle L.) obtained from the Florida coastal zone were subjected to a range of soil redox conditions (Eh) plus 20 p.p.t. salinity to assess the effects of reducing soil Eh condition on selected plant functions. The hypotheses tested were: (1) the two species differ in their tolerance of reduced soil Eh conditions and (2) such differences are reflected in gas exchange and carbon allocation patterns. Plant gas exchange, growth and biomass partitioning were studied under +400, +210, 0 and -180 mV Eh treatments. Net photosynthesis (P-n) was reduced significantly in the lowest Eh treatment (-180 mV) in both species averaging 31.1 and 39.7% as compared to control (+400 mV) treatment in A. germinans and R. mangle, respectively. In addition, P-n was significantly (P<0.05) greater in R. mangle than A. germinans under most treatments, averaging 36.2% greater in R. mangle than A. germinans across treatments. Total dry weights (shoot and root) in A. germinans were reduced significantly to 61.6, 37, and 48.3% of control in +210, 0 and -180 mV treatments, respectively. In contrast, total dry weight in R. mangle was not reduced significantly (except in the +210 mV treatment) averaging 76.2, 102, and 83.5% of the control in +210, 0 and -180 mV treatments, respectively. Overall, seedlings of A. germinans showed a greater sensitivity to low Eh conditions than R. mangle seedlings and the level of sensitivity was closely related to the intensity of soil reduction. The present study demonstrated that seedlings of the two species differ in their responses to low soil Eh conditions and such differences may be partially explained by the differences in net photosynthetic rates and by the patterns of biomass partitioning, i.e. under low soil Eh conditions, there is a greater photosynthetic rate and a shift in biomass allocation to the roots in R. mangle compared to A. germinans. (C) 1997 Elsevier Science B.V.