gradient" or "moisture continuum" (Lindsey 

 et a1. 1961; Gemborys and Hodgkins 1971; 

 Bedinger 1978; Richardson et al. 1978; 

 Fredrickson 1979; Huffman 1979; Whitlow 

 and Harris 1979; Huffman and Forsythe 

 1981). These terms may be misleading; it 

 is not the availability of water, but the 

 inavai lability of oxygen due to the pres- 

 ence of water. The emphasis on the anae- 

 robic aspect of this gradient generates a 

 clearer picture of the actual effects of 

 flooding and saturated soils on plant sur- 

 vival; hence, its use in this report. 



PLANT RESPONSES TO ANOXIA-RELATED STRESSES 



Stresses Generated by Anaerobic Conditions 



The effects of periodic or perma- 

 nent flooding are the crucial selective 

 stresses on bottomland hardwood plants and 

 are responsible for the sorting of species 

 into broad community types (Huffman and 

 Forsythe 1981). The plant growing in a 

 saturated substrate must respond to sev- 

 eral physical and chemical changes, among 

 them: 



(1) depletion of available oxygen in 

 soil water, in a period as short 

 as 3 days (Nuritdinov and Varta- 

 petyan 1976; Phung and KnipTing 

 1976; Teskey and Hinckley 1977); 



(2) shifts in soil pH— variable, 

 though in general a convergence 

 toward neutrality, with acidic 

 soils becoming more alkaline and 

 calcareous soils becoming more 

 acidic (Grable 1966; Kennedy 

 1970; Rahmatullah et al. 1976; 

 Teskey and Hinckley 1977); 



(3) accumulation of potentially tox- 

 ic compounds in the plant, the 

 rhizosphere, and in the larger 

 soil solution; examples are car- 

 bon dioxide, ethanol, sulfides, 

 nitrites, aluminum, iron, and 

 manganese (Teskey and Hinckley 

 1977); 



(4) shifts in the redox states of 

 chemical species, including es- 

 sential nutrients, generally 

 from more oxidized to more 

 reduced; the reduced forms are 

 considered generally less desir- 



able for plant uptake and assim- 

 ilation (Brady 1974; Teskey and 

 Hinckley 1977); and 



(5) shifts in nutrient availabil- 

 ities, partially due to item 

 (4) (Teskey and Hinckley 1977). 



The responses of plants to these and other 

 flood stresses were reviewed by Teskey and 

 Hinckley (1977), who emphasized that the 

 key to plant survival in flooded condi- 

 tions is the adaptability of the root 

 system. 



The cessation of uptake and exchange 

 functions through root dormancy or death 

 during flooding affects plant metabolism 

 in several ways. The immediate losses of 

 these root processes is due to the lack of 

 oxygen. The root system has access to 

 free oxygen, necessary for normal respira- 

 tion, through only two routes: (1) absorp- 

 tion from the soil -air-water complex by 

 the roots themselves, or (2) transport 

 from aboveground plant tissues through the 

 vascular system or intercellular spaces to 

 the roots. Although all plants probably 

 have a shoot-to-root intercellular space 

 network through which oxygen can diffuse 

 to the root system (Salisbury and Ross 

 1978), this system is well developed in 

 only a few plants (rice, for example). 

 Thus the depletion of soil oxygen by the 

 roots eventually shuts down respiration in 

 root cells. As respiration ceases, water 

 and ion uptake is inhibited (1) by chang- 

 ing membrane permeabilities in root cells, 

 affecting movement of both water and ions, 

 and (2) by reducing the amount of energy 

 available for membrane transport, affect- 

 ing primarily ion movement. 



The inability of flood-intolerant 

 species to absorb and use water and nutri- 

 ents leads to foliar water deficits, sto- 

 matal closure, and reduced gas exchange. 

 Consequently, transpiration and photosyn- 

 thetic rates are slowed, cellular synthe- 

 sis requiring unavailable nutrients is 

 curtailed, and overall plant growth is 

 impeded (Teskey and Hinckley 1977). The 

 plants literally die of dehydration in 

 standing water. 



Plant Adaptations to Flood Stresses 



Plant adaptations to flood stresses 

 may be categorized as physical or meta- 



32 



