628 S. Housley and B. J. Deverall 



tained small elongations. However, Brian and co-workers appear to 

 have used different techniques in these studies, and it has been re- 

 cently shown by Purves and Hillman (25) that, at least for dark-grown 

 plants, the size of sections used and position on the stem from which 

 they are excised are of importance in determining response. 



The results of Brian and Hemming (4) are of interest in that they 

 show a synergism between GA and indole-3-acetic acid (lAA). Such 

 synergisms have not always been obtained (15, 25). The less recent 

 literature has been summarized by Stowe and Yamaki (26). In Brian 

 and Hemming's experiment, section growth in light was examined in 

 a basal medium of 2 per cent sucrose -f- phosphate buffer, medium -\~ 

 lAA, medium -|- GA, and medium -f GA and lAA. ^Vhile GA alone 

 had little or no effect, when in the presence of lAA it increased sec- 

 tion elongation over that obtained in lAA alone. Brian and Hem- 

 ming offered no hypothesis to account for these results, but it seemed 

 to the present authors that the synergistic enhancement of IA.\-in- 

 duced growth by GA could result from an lAA-sparing action brought 

 about, for example, by the blocking of an lAA-destroying system with 

 GA. This possibility has been examined in the experiments reported 

 in the present paper by observing the rate of disappearance of lAA 

 from solutions containing excised pea stem tissues or pea breis oE 

 lAA-oxidase enzyme in the presence and absence of GA. Growth of 

 stem tissues was not measured in this study. 



MATERIALS AND METHODS 



Pisum sativum, Trogress No. 9,' (Sharpe Seed Co.) ^vas used ex- 

 cept when stated to the contrary. GA (Imperial Chemical Industries, 

 Ltd.) and lAA (British Drug Houses, Ltd.) were used at a concentra- 

 tion of 10 mg/1 in all experiments. 



Estimation of lAA was made colorimetrically using the Salkowski 

 reaction technique of Gordon and Weber (10) to develop the red Fe- 

 lAA complex which was measured with a Hilger photoelectric ab- 

 sorptiometer (1 cm. cell and Ilford 604 filter). GA with Salkowski 

 reagent gave no absorption (against reagent as a blank in the ma- 

 chine), and no interference occurred with lAA-color development 

 when GA and lAA solutions were mixed and allowed to stand for 5 

 to 120 min. before addition of reagent. The relationship between ab- 

 sorption and concentration of lAA is linear, and absorptions of four 

 lAA concentrations are given below: 



lAA Concn. (mg/1) 2.5 5 10 20 



Absorption Units 0.044 0.122 0.267 0.528 



Enzyme breis for lAA-oxidase experiments were prepared from 7- 

 day-old plants with third internodes extending. Growth was in the 

 dark at 27° C. with occasional brief irradiation with weak green 



