k. Oxygen pathway to roots. Some species (e.g. Spartina alterni- 

 flora) have a specialized cellular arrangement that facilitates 

 diffusion of gaseous oxygen from leaves and stems to the root 

 system. 



Physiological adaptations 



4. Most, if not all, hydrophytic species are thought to possess physio- 

 logical adaptations for occurrence in areas that have prolonged periods of 

 anaerobic soil conditions. However, relatively few species have actually been 

 proven to possess such adaptations, primarily due to the limited research that 

 has been conducted. Nevertheless, several types of physiological adaptations 

 known to occur in hydrophytic species are discussed below, and a list of spe- 

 cies having one or more of these adaptations is presented in Table C2. NOTE: 

 Since it is impossible to detect these adaptations in the field, use of this 

 indicator will be limited to observing the species in the field and checking 

 the list in Table C2 to determine whether the species is known to have a 

 physiological adaptation for occurrence in areas having anaerobic soil 

 conditions) : 



a. Accumulation of malate. Malate, a nontoxic metabolite, accumu- 

 lates in roots of many hydrophytic species (e.g. Glyceria 

 maxima, Nyssa sylvatica var. bi flora) . Nonwetland species con- 

 centrate ethanol, a toxic by-product of anaerobic respiration, 

 when growing in anaerobic soil conditions. Under such condi- 

 tions, many hydrophytic species produce high concentrations of 

 malate and unchanged concentrations of ethanol, thereby avoiding 

 accumulation of toxic materials. Thus, species having the 

 ability to concentrate malate instead of ethanol in the root 

 system under anaerobic soil conditions are adapted for life in 

 such conditions, while species that concentrate ethanol are 

 poorly adapted for life in anaerobic soil conditions. 



b. Increased levels of nitrate reductase. Nitrate reductase is an 

 enzyme involved in conversion of nitrate nitrogen to nitrite 

 nitrogen, an intermediate step in ammonium production. Ammonium 

 ions can accept electrons as a replacement for gaseous oxygen in 

 some species, thereby allowing continued functioning of 

 metabolic processes under low soil oxygen conditions. Species 

 that produce high levels of nitrate reductase (e.g. Larix 

 laricina) are adapted for life in anaerobic soil conditions. 



c. Slight increases In metabolic rates. Anaerobic soil conditions 

 effect short-term increases in metabolic rates in most species. 

 However, the rate of metabolism often increases only slightly in 

 wetland species, while metabolic rates increase significantly in 

 nonwetland species. Species exhibiting only slight increases in 

 metabolic rates (e.g. Larix laricina, Senecio vulgaris) are 

 adapted for life in anaerobic soil conditions. 



C6 



