Microbiol; 10 



surface microflora can be altered by experimental or agronomic pro- 

 cedures. Before discussing these individually, I wish to emphasize 

 that the root microflora is known to vary according to age and con- 

 ditioQ of plant growth (Ul); therefore, any treatment that produces 

 in net effect a dissimilar plant will undoubtedly affect the root 

 microflora. I mention this point now in order that I need not con- 

 tinually repeat it later when speaking of various soil and plant 

 treatments. 



Firstly, manipulation of the physical environment — light, moistirre, 

 pH, temperature — influences the root microflora. Harley and Waid 

 (39) have shown that different levels of daylight radiation affect 

 the growth and nature of the mycrrhizal and rhizoplanal microfloras 

 of beech roots. They concluded that the condition of the host plant 

 was the major factor in determining the nature of the root population 

 Also, it has recently been shown that length of photoperiod affects 

 the nodulation response of legume roots (U2). 



Moisture, as one might expect, markedly influences the number of 

 microorganisms in the rhizosphere. Both Timonin (20,U3) and myself 

 (UU) have shown that drier soils have larger microbial populations in 

 the rhizosphere than do wet soils. I have found that this is largely 

 a sampling effect, due to decreased adherence of soil particles to 

 roots in the drier soils. 



Liming of soils was reported, by Obraztzova (li5), to increase the 

 density of microorganisms in the rhizosphere; and Pohlman (U6) noted 

 that in soil initially of pH 5, nodules on roots of alfalfa were 

 largely concentrated in the soil layer receiving high-lime treatment, 

 regardless of whether this layer was 8 to 16 or 16 to 2U inches deep. 



Temperature also influences the root microflora, at least insofar as 

 the root-invading pathogens are concerned. The effect of temperature 

 differs with the crop and the parasite. Sugar beet seedlings are less 

 susceptible to damping-off at soil temperatures below 60° F. than at 

 75 or 80°, but tomato seedlings are more susceptible. In common bunt 

 of wheat, the greatest infection occurs at soil temperatures of UO to 

 60° F. If fall wheat in the northern states is planted early while 

 warm soil temperatures still prevail, the wheat seedlings largely 

 escape infection. 



Plant mutilation treatments may be used to modify thR root microflora. 

 I have noted (1) that stem and root girdling procedures affect the 

 incidence of saprophytic fungi in the rhizosphere of cotton; and 

 Mitchell and myself (37) have shown that with cotton, cutting half 

 the leaves from flowering plants and deflowering of fully-leafed plants 

 provide plants whose roots differ in their bacterial floras and in 

 their susceptibility to the cotton root-rot fungus. 



Soil treatment, either with inorganic fertilizers or with organic 

 manures, does not appear directly to affect the rhizosphere microflora. 



