bolic. The former includes the provision 

 of oxygen to the roots or the restoration 

 of proper root function, or both. Meta- 

 bolic mechanisms adjust plant biochemistry 

 to decrease the potentially harmful ef- 

 fects of anaerobic respiration. The most 

 successful species in saturated conditions 

 are those that possess both physical and 

 metabolic adaptations (Teskey and Hinckley 

 1977). 



The abilities of plant species to 

 restore and maintain the stressed root 

 system lie on a continuum (Teskey and 

 Hinckley 1977): 



(1) very tolerant--primary root 

 maintenance, secondary and ad- 

 ventitious root growth, 



(2) moderately tolerant--primary 

 root deterioration, adventitious 

 root growth, and 



(3) intolerant--primary root deteri- 

 oration, no adventitious root 

 growth. 



Adventitious and secondary roots pro- 

 duced under flooded conditions are anatom- 

 ically different from primary roots in 

 ways that enhance root function in satu- 

 rated soils. They are more porous, facil- 

 itating (1) oxygen diffusion from the 

 aerial shoots (Luxmoore et al. 1973), (2) 

 gaseous exchange between root cells and 

 soil solution, and (3) perhaps better 

 movement of water and ions into the root 

 (Jat et al. 1975). They are also more 

 tolerant to elevated carbon dioxide con- 

 centrations and exhibit increased anae- 

 robic respiration (Hook and Brown 1973). 



Some tree species produce special 

 root structures other than secondary and 

 adventitious roots. The classic example 

 is the pneumatophores of baldcypress and 

 pond cypress (knees) (Figure 18) and water 

 tupelo and swamp tupelo (arched roots). 

 Aerial roots may supply additional oxygen 

 to the root system (Teskey and Hinckley 

 1977). Buttress formation (Figure 19) 

 and "stooling" not only provide stabler 

 anchoring in the less firm floodplain 

 soils but also may help aerate the root 

 system. 



Similar functions are provided by the 

 characteristically wide, shallow, matted 



root systems (Figure 20) of bottomland 

 trees which (1) provide support, (2) 

 increase oxygen use efficiency in satu- 

 rated conditions by their proximity to 

 more highly oxygenated surfact sediments, 

 (3) reduce losses of nutrients from the 

 system through rapid uptake, and (4) pro- 

 tect the floodplain from erosion. 



The primary metabolic mechanism in 

 flood-tolerant species is a shift in the 

 end-products of glycolysis. Normal glu- 

 cose metabolism and energy (ATP) produc- 

 tion in the cell proceeds via three steps: 

 (1) glycolysis (anaerobic), (2) Kreb's 

 citric acid cycle (aerobic), and (3) oxi- 

 dative phosphorylation (aerobic). In the 

 absence of free oxygen, only glycolysis is 

 completed, and ethanol normally accumu- 

 lates as an undesirable end product. 

 Flood-tolerant species can generate 

 organic acids instead of ethanol as pro- 

 ducts of glycolysis (Crawford and Tyler 

 1969) and thus avoid ethanol toxicity. 

 Furthermore, the organic acids may be 

 transported to the stem and leaves 

 (Chirkova and Gutman 1972; Vester 1972) 

 and used in cellular synthesis (Crawford 

 1976). 



A second metabolic adaptation has 

 been described for some tolerant trees by 

 Hook et al. (1970). The roots of these 

 species oxidize the rhizosphere, prevent- 

 ing root deterioration and enhancing nu- 

 trient uptake. 



Finally, there is some evidence that 

 flood-tolerant species can substitute ni- 

 trate for free oxygen as a terminal elec- 

 tron acceptor in cellular reactions 

 (Crawford 1976). The reduction of nitrate 

 to ammonium (denitrif ication) then would 

 help maintain cellular energy production 

 and biosynthesis in roots. This benefit 

 could occur only if excess nitrate were 

 available in an environment where denitri- 

 fication is the prevalent process. 



F actors Affecting Plant 

 Response to Flooding 



Of the many factors that influence 

 plant survival during flooded conditions, 

 the timing, depth, and duration of flood- 

 waters are the most critical (Teskey and 

 Hinckley 1977; Huffman and Forsythe 1981). 

 These characteristics are themselves func- 

 tions of regional precipitation and local 



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