Lecture XI — 155 — Theories 



pines acquired P with difficulty." "It is suggested that the stimulating 

 effect of mycorrhizal fungi on conifer seedlings is due to heightened 

 metabolism, associated in this instance with transfer of phosphorus 

 and growth stimulators from fungus to seedling" (McComb & 

 Griffith, Plant Physiol. 21:11-17, 1946). Young (1940) attributes 

 the benefit to the fungus, for "With the presence of adequate P for 

 fungus growth and the availability of a phosphatide supply, the 

 fungus is enabled to carry out the breakdown of raw organic matter 

 and transfer the products to the higher plant." Young presents a 

 diagram to show the effect of phosphate applications on the nutritional 

 cycle of the mycorrhizal fungus and the pine tree. Intake of P is 

 important to the higher symbiont in formation of phosphoric com- 

 plexes as seen in the next lecture. 



Stahllan Hypothesis: — "The mineral hypothesis was first defi- 

 nitely formulated by Stahl (1900). He considered Frank''s con- 

 ception of a close relationship between mycotrophy and N-supply to 

 be false and claimed that, in soils poor in mineral nutrients, trees are 

 brought into competition with fungi and bacteria; and through my- 

 corrhizal symbiosis the tree is benefitted. Stahl^s hypothesis has not 

 met with favour amongst modern investigators except with Hatch, 

 who goes beyond Stahl in claiming the mycorrhizal relation to be 

 chiefiy a 'physical relationship', i.e., the chief significance of the 

 mycorrhiza is in its increase in absorbing surface." 



Hatchian Hypothesis: — Hatch (1937), however, said his 

 hypothesis was "tentative" and applied only to "ectotrophic" mycor- 

 rhizae, and stated : "The mycotrophic relationship in pine, and pre- 

 sumably in other plants possessing ectotrophic mycorrhizae, is a 

 symbiotic mechanism which increases, chiefly by physical and therefore 

 by relatively non-selective means, the absorption of soil nutrients. . . . 

 The greater absorption capacity of mycorrhizal seedlings is brought 

 about by, and is proportional to, increases in the effective absorbing 

 surface areas of short-roots resulting from fungal invasion. . . . Trees 

 are dependent on symbiotic association with mycorrhizal fungi for 

 their soil nutrients and therefore for their existence in all but the 

 most fertile agricultural soils." 



RouTiEN & Dawson (1943) sought to amplify the Hatchian 

 hypothesis, and after experimenting concluded that mycorrhizae 

 increase the salt absorbing capacity of the roots primarily by adding 

 to the supply of exchangeable H-ion derived in part at least from 

 carbonic acid. They found that development of mycorrhizae increased 

 the average rate of aerobic COg production of each short root from 



