Salinity Effects on Photosynthesis and Growth in Alternanthera philoxeroides (Mart.) Griseb.

Alternanthera philoxeroides, alligator weed, was grown at five different NaCl concentrations to determine the effect of salinity on factors related to the net rate of CO(2) uptake (P(n)). Over the range of 0 to 400 millimolar NaCl, P(n) declined 51%. Stomatal conductance declined in parallel with P(n) and as a result there was no reduction in intercellular CO(2) concentration and therefore no reduction in the amount of CO(2) available for photosynthesis. The CO(2) compensation point did not change with salt stress. Increases in leaf thickness tended to compensate slightly for the negative effects of salinity on leaf cell metabolism, at least in relation to P(n). On a mesophyll cell area basis, soluble protein was relatively constant in leaves developed at 100 to 400 millimolar NaCl while total chlorophyll decreased at all salinities. Dry weight production and P(n) were closely correlated in alligator weed grown at different salinities. Plants produced less leaf area per unit dry weight as salinity increased, which may aid in water conservation.