Effects of synthetic growth substances in level of endogenous auxins in plants 



Plant material used consisted mainly of sunflower and garden pea seedlings, 

 2 to 3 weeks old, having been grown either in soil or in water culture at a 

 constant temperature of 25°C in a fourteen-hour day of an intensity of 

 150 foot-candles. 



2:4-D treatment took the form of painting the cotyledons or lower pair 

 of foliage leaves with a 0-1 per cent solution of the ammonium salt. When 

 growth responses had developed in the shoots (i.e. after about 24 hours) 

 extracts were made of them for assay. Another treatment was to grow 

 seedlings for a number of days in very dilute solutions (0- 1 to 0-5 p. p.m.) of the 

 ammonium salt. Considerable root growth inhibitions were thereby 

 produced. Both roots and shoots were subsequently extracted and assayed. 

 Similar root applications were made with the triethanolamine salt of 

 maleic hydrazide and TIBA at concentrations of 10 to 30 p. p.m. The 

 characteristic response to MH treatment was a marked suppression of lateral 

 root production. TIBA, in addition to inhibiting root growth, modified the 

 response of lateral roots to gravity so that many grew vertically upwards. 

 After treatments varying from 6 to 12 days roots were extracted for assay. 



Macerated material was extracted for 24 hours in ice-cold absolute 

 ethanol followed by transference to peroxide-free ether as described by 

 Kefford (1955). The volume of the extract was reduced by evaporation to 

 about 0-5 ml and this was spotted directly onto the starting line of a Whatman 

 No. 1 filter paper strip. The developing solvent for the chromatographic 

 separation was an zVopropyl alcohol-ammonia-water mixture (Kefford, 

 1955). After running, the paper was dried and then cut transversely into an 

 appropriate number of portions. The growth substance content of each 

 portion was then assayed by growing excised pea root segments laid directly 

 upon it in 0-75 ml of | per cent sucrose solution. Full details of this quantita- 

 tive assay have already been published (Audus and Thresh, 1953). The 

 position of indole-3-acetic acid (lAA) on the chromatogram was determined 

 by a simultaneous running of synthetic lAA on a similar paper strip and 

 identifying its position by spraying with ferric chloride-perchloric acid 

 reagent. From the assay calibration curves (see Audus and Thresh, 1953) 

 the quantities of lAA in the extracts could be directly calculated. Parallel 

 assays of aliquots from the same extract showed that errors arising during 

 running of the chromatogram and subsequent assay were of the order of 

 20-25 per cent. Assays of separate extracts of duplicate samples from the 

 same plant material showed that errors due to variable extraction losses and 

 plant sample differences gave variations between estimates of the order of 

 35-49 per cent. These latter errors necessitated a number of replicate 

 experiments for each treatment in order to establish the significance of any 

 differences observed. 



RESULTS 



(a) Chromatograms of normal plants 



In sunflower shoots, in addition to lAA [Rf 0-3-0-4) three other active 

 spots were consistently observed, a stimulatory spot at Rf0-\-Q-2 and 

 inhibitory spots at /?/o-65-0-8 and i?^ 0-9. The inhibitor at /?^0-65-0-8 

 corresponds closely with the inhibitor /5 of Bennet-Clark and Kefford (1953). 

 In pea shoots these two inhibitors have also been consistently found. Further 



V 249 



