Microbiol i6 



CJoming, lastly, to individual plants within the same variety of 

 species — these also are known to have differing root microfloras, 

 particularly if the plants are of different ages, or if they are 

 grown in dissimilar environments (2). Much of the information that 

 does exist concerning rhizosphere differences betvreen individual 

 plants is in connection with the ix)ot-destroying fungi. Knowledge, 

 insofar as the bacterial saprophytes are concerned, is very limited. 



Why Do Root Microfloras Differ ? 



Why do root microfloras differ? This constitutes a basic question 

 in any discussion of the rhizosphere. We may start with some very 

 broad generalizations. One is that certain microbes are present on 

 root siirfaces because those roots provide some special nutritional 

 or growth-promoting factors. For many microbial species on roots, 

 this may simply be a matter of luxurious food supply and an ability 

 to compete for that supply. Other microbial species may fail to 

 colonize these same roots, even though they are nominally capable of 

 using the food supply present, because they are unable to grow in the 

 presence of toxic or inhibitory substances produced by the roots. 

 Finally, microbial interactions may influence the quality of the micro- 

 flora, in that the growth or absence of growth of some microbes may 

 encourage the growth of others, or inhibit them. V/e will return in a 

 moment to some further discussion of these synergistic and antibiotic 

 factors. 



At the present, let us consider the possibility that substances pro- 

 duced by plant roots can be toxic to certain microorganisms and harm- 

 less to others. Many higher plants produce substances that are 

 variously inhxbitory to other plants. Some developments in this field 

 have been reviewed by Bonner (21). He suggested that these specific 

 toxic substances could be responsible for ecological phenomena such as 

 the composition of plant comm\inities or the sequence of particular 

 species in a succession. 



Phytopathologists also have recognized the importance of plant- produced 

 materials as determining factors in the varietal susceptibility of 

 crop plants to fungal root pathogens. That chemicals produced by the 

 host plant influence the host's resistance to disease was first shown 

 by Link and associates (22,23,2U). The resistance of colored onions 

 to smudge and neck rot diseases caused by fungal parasites was found 

 due to toxic phenolic substances identified as protocatechuic acid 

 and catechol. 



Timonin (20), noting that wilt-resistant and wilt-susceptible varieties 

 of flax showed striking differences in the fungal flora of their rhizo- 

 spheres, studied the two varieties under sterile culture conditions. 

 The nutrient solution in which the resistant variety had grown contained 

 appreciable amounts of cyanide, ^./#ien he allowed the solutions in which 

 the plants had grown to diffuse from colloidon sacs into the surround- 

 ing soil, thus creating artificial rhizospheres, differences in the 



