Role of Auxins in Control of Leaf Senescence 331 



Respiration Measurements 



Oxygen consumption was measured by the Warburg teclinique. In 

 the samples from the 2,4-D-treated areas it was essential that residual 

 ester remaining on the surface of the discs should not penetrate the 

 tissues through the cut edges. To avoid this, the six leaf discs were 

 balanced on edge in an upright position on a thin layer of 2 per cent 

 agar which covered the bottom of the flask. Water was added to the 

 side arm and potassium hydroxide to the center well. Each flask was 

 wrapped in a black bag and equilibrated at 20° C. for one hr.; 

 measurements of oxygen consumption were made at half-hour in- 

 tervals up to six hrs. The discs were removed from the flasks and stored 

 in 80 per cent ethanol and later used for the estimation of the nitro- 

 gen fractions. 



Nitrogen Measurements 



Total nitrogen, alcohol-insoluble and alcohol-soluble nitrogen 

 Avere estimated separately. Each sample of six discs was macerated 

 in 80 per cent (v/v) ethanol in a VirTis homogenizer and the suspen- 

 sion made up to volume. After removal of an aliquot of the suspen- 

 sion for the estimation of total nitrogen, the remaining suspension 

 was centrifuged and an aliquot of the supernatant liquid used for 

 the determination of alcohol-soluble nitrogen and the whole of the 

 remaining precipitate used for the determination of the alcohol- 

 insoluble fraction. Each sample was digested with selenium catalyst, 

 made alkaline with alkaline metaborate, and the ammonia distilled 

 off in vacuo into borate buffer which was titrated against HCl in the 

 usual way. 



RESULTS 



There is a measurable effect of 2,4-D on attached Euonymits leaves 

 as shown, within 24 hrs., by an increase in the rate of oxygen con- 

 sumption in discs of leaf blade cut from the treated areas (Figure 1). 

 The values for the rate of oxygen uptake of subsequent samples con- 

 tinue to rise until the thirteenth day after treatment, when the 2,4-D- 

 treated tissues are green and the remainder of the leaf is yellowing. 

 Discs from the treated spots are then respiring oxygen over three 

 times faster than discs from the control leaves. The rate of oxygen 

 consumption of discs from the untreated halves of 2,4-D-treated leaves 

 shows an initial, but less marked stimulation, but this rate falls off 

 again after the sixth day and then remains 30 per cent higher than 

 that of the controls. 



