D. spicata 



c 

 E 



o 



CO 



O 



o 



(A 



a> 



3 

 z 

 g 



a 

 cc 

 O 



CO 

 OQ 



< 

 cc 



NaCI (mM/l) 



Figure 46. Effects of NaCl concentration 

 in the root medium on the rate of Rb 

 absorption by excised root tissue of S. 

 alterniflora and D. spicata (1 mM Rb; 2 

 mM Ca; reprinted from Bot. Gazette, 1981, 

 by R.T. Parrando, J.G. Gosselink, and C.S. 

 Hopkinson with permission of The Univer- 

 sity of Chicago) . 



the concentration of other ions such as 

 K (Smart and Barko 1978). Finally, the 

 plant leaves have secretory glands called 

 hydathodes which selectively secrete cer- 

 tain ions. All this regulatory activity 

 requires extra energy expenditure by the 

 plant. It is not surprising then that the 

 growth rate decreases as the external salt 

 concentration increases. 



The problem of anoxia is complex 

 because it affects not only the plant 

 Itself but also the microblally mediated 

 biochemical reactions that occur in the 

 soil around the roots. Oxygen is required 

 as an electron acceptor in aerobic cell 

 respiration. Its presence allows the 

 efficient oxidation of organic sugars to 

 carbon dioxide and water to produce high 

 energy-reduced organic compounds and the 



cell's ready energy currency adenosine 

 triphosphate (ATP). 



In the absence of oxygen, cell 

 metabolism is incomplete; less energy is 

 released from an equivalent amount of 

 sugar (1 mole of glucose yields 2 moles 

 of ATP under anaerobic conditions compared 

 to 36 moles under aerobic conditions); and 

 organic "waste products" like ethanol and 

 lactic acid accumulate because they cannot 

 be oxidized to carbon dioxide (Figure 47). 



In the surrounding root medium, when 

 oxygen is depleted, other materials act as 

 electron acceptors, almost always through 

 some microbial intermediary rather than 

 through strictly inorganic chemical 

 transfonnations. Many ionic species are 

 reduced. The reduced form of metallic 

 ions such as manganese and iron is more 

 soluble than the oxidized form, and the 

 ions can accumulate to toxic levels. At 

 very low reduction potentials, sulfate is 

 reduced to the highly toxic sulfide. 

 Since the substrate is largely organic and 

 micro-organisms are active, organic toxins 

 such as ethylene can also potentially be 

 produced. 



Marsh plant species have developed a 

 number of adaptations to cope with anoxia, 

 but even with these the plants are 

 stressed by sublethal effects of 

 anaerobiosis (Mendelssohn and McKee 1982). 

 One of the main adaptations of nearly all 

 wetland plant species is the extensive 

 development of aerenchyna tissues in the 

 leaves, stems, and roots, which allow the 

 diffusion of oxygen from aerial plant 

 parts into the roots 

 Teal and Kanwisher 

 evidence that this 

 nomially enough to 

 metabolic requirements 

 In addi tion, diffusion of oxygen out 

 roots can buffer the effect of soil 



(Etherington 1975, 

 1966). There is 

 oxygen source is 

 satisfy the root 

 of wetland plants, 

 of the 

 anoxia 



by creating a thin, oxidized layer in the 

 rhizosphere. Mendelssohn and Postek 

 (1982) eloquently denonstrated through 

 scanning electron microscopy and x-ray 

 microanalysis that the brown precipitate 

 often seen surrounding S_. al terni flora 

 roots is indeed highly enriched in 

 oxidized iron (Fe) and manganese (Mn). 



to 



Another adaptation of wetland plants 

 anoxia is the evolution of the ability 



51 



