500 



Plan Biomass 

 Plant Heighl 



5 ID 15 20 



DISTANCE FROM STREAM (m) 



70 



50 



I 

 O 



z 

 < 



-30 



10 



Figure 44. Variation in total aboveground 

 biomass and height of Spartina 

 a1 terni flora with distance inland from the 

 marsh edge in a Barataria basin salt marsh 

 (Buresh 1978). 



INLAND 



POSITION ON TRANSECT 



GULF 



Figure 45. Gulf-inland variations in live 

 and total biomass in Spartina alterniflora 

 marshes (Gosselink et al . 1977y] 



The third example shows the increase 

 in biomass from the coast inland. The 

 first two examples illustrate complex 

 gradients in the physiological sense; the 

 last may be due simply to a gradient of 

 decreasing salinity. 



Physiologically a plant growing in a 

 marsh has to solve one or both of two 

 problems. All marsh plants are 



periodically exposed to high salt 

 concentrations and to anoxic soil 

 conditions and accompanying sediment 

 chemical changes. 



As indicated earlier, the dominant 

 salt and brackish marsh plants are 

 salt tolerant rather than salt requiring. 

 Generally, growth is depressed as salt 

 concentration increases (Parrondo et al . 

 1978). One reason for this is that the 

 high concentration of salt surrounding the 

 roots makes it osmotically difficult for 

 plant cells to absorb water. 



The plant could get around this 

 problem by simply absorbing salt to 

 decrease the internal osmotic potential. 

 But this leads to biochemical problems 

 because the Na and CI ions interfere with 

 the activity of many enzymes, probably 

 through steric effects. For example, the 



enzyme-mediated absorption of the radio- 

 tracer, rubidium (Rb) by excised roots of 

 S^. al terniflora and D. spicata is 

 strongly inhibited by salt in the root 

 medium (Figure 46). This may occur be- 

 cause Na replaces Ca, which has been shown 

 to stimulate ion uptake, on the cell 

 membranes. 



Plants have adapted to the problems 

 posed by salt in a number of ways. These 

 all involve mechanisms to exclude or 

 selectively absorb only certain ions, to 

 raise the osmotic concentration of the 

 plant cells to overcome the water uptake 

 problem, and/ or to secrete unwanted ions. 

 S_. al terni flora has apparently evolved 

 all three mechanisms. The osmotic 

 concentration of its cells is always 

 slightly higher than the substrate 

 concentration, creating a favorable 

 gradient for water flow into the plant. 

 This is accomplished both by absorption of 

 salts from the external medium and by 

 production of osmotically active organic 

 compounds. 



The absorption of salt is not a 

 passive process. The relative 



concentrations of different ions within 

 the plant cells indicate that absorption 

 is selective, with the exclusion of Na and 



50 



