Stimulation of Auxin Action by Lip ides 



423 



earlier, and were active at even slightly lower concentrations (19). The 

 absolute growth, however, remained no greater than before. The 

 other substances were inactive. 



Vitamins E and K^ possess a bicyclic ring with an isoprenoitl 

 side chain. In animals the side chain is not essential for vitamin K 

 activity and, accordingly, the side-chainless vitamin K5 and menadione 

 analogues were tested on pea sections (19). No growth promotion was 

 noted, but when vitamin K/s side chain, phytol, was tested by itself, 

 growth promotion was found. A less exact analogue of the vitamin E 

 side chain, farnesol, was ineffective. However, the activity of phytol 

 makes it seem likely that the potency of both vitamin E and K^ is 

 due to their possession of a long chain hydrocarbon substituent. 



Some common property of a wide variety of long-chain hydro- 

 carbon compounds is thus responsible for the greater elongation of 

 the pea stem sections. Tests of a number of components of lipide 

 metabolism, such as coenzyme A, thioctic acid, acetate, pyruvate, 

 cytidine, choline, etc., and of compounds involved in isoprenoid 

 synthesis, such as mevalonic acid and its lactone, as well as adenine, 

 niacinamide, and ribose, have not revealed any connection of this 

 phenomenon with well-known biochemical pathways. Nor have 

 physical factors such as surface action of these compounds or in- 

 creased uptake of auxin been found to be implicated in any tests 

 yet carried out. The details of these largely negative findings have 

 therefore been left to other publications (18, 19). 



In this connection, one of the most significant features of the 

 lipide effect is the fact that the optimal amounts required are much 

 too small to meet any major nutritive requirement of the sections. 

 Table 3 shows that a 4 //if concentration of vitamin Ki - scarcely 

 more than twice the concentration of lAA used - suffices for a 



Table 3. The dependence on auxin of the response to Upides of 10 mm. 'Laxton's 

 Progress' pea epicotyl sections. Percentage increase in length with standard devia- 

 tions after 24 hrs. (17). 



■ Sucrose, 1.25 per cent + 50 fiM C0CI2 + 5 mM KH2PO4 (pH 5.5) + 0.004 per 

 cent Pluronic F-68. 



