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DISCUSSION 



Dr. Galston: I remember some recent papers by Dr. Reinhokl who 

 found that the application of auxin to sunflower hypocotyl sections 

 causes a rapid egress of ninhydrin-positive nitrogenous materials, 

 largely ammonia, I believe. Yet, in your experiments, there was, if 

 anything, a competitive advantage conferred by the feeding of auxin 

 to the treated cells. I wonder if you can resolve this apparent dis- 

 crepancy? 



Dr. Osborne: I can only give you a suggestion. In Dr. Reinhold's 

 tissue there was no available surplus of substrates upon which the 

 rapidly metabolizing tissues could draw, whereas in these leaf tissues 

 we have substrates in the surrounding area of untreated leaf as a 

 source of supply for the 2,4-D-treated cells. The auxin-treated area 

 does not then have to break down nitrogenous materials inside its 

 own piece of tissue to keep pace with the high metabolic rate. In Dr. 

 Reinhold's experiments the loss of nitrogenous materials to the ex- 

 ternal solution was depressed by the addition of sucrose or succinate. 



Dr. Galston: Do you imply that there is an increased senescence of 

 the neighboring cells caused by a local application of 2,4-D? 



Dr. Osborne: Yes, and may I add just a few comments on some 

 work I did in the tropics on leaves of Combretiiui (Jour. Trop. 

 Agric. 35: 145. 1958). Spot applications of 2,4-D were made on leaves 

 and within 7 days the blade had yellowed, leaving the 2,4-D-treated 

 area as a green spot on a yellow background. The leaves then ab- 

 scised. II one collected leaves on the first two da)s after treatment 

 and diffused the petioles into blocks of agar and tested the blocks 

 in an abscission test, the petiolar diflusate was abscission-retarding. 

 This might be expected following applic:ition of a stibstance such 

 as 2,4-D. After about 3 days, when yellowing of the leaves was ap- 

 parent, the petiolar diffusate had little or no activity in the abscission 

 test. After 4, 5, and 6 days, the leaves became increasingly yellow and 

 petiolar diffusate became increasingly abscission-accelerating. Since 

 petiolar diffusates from naturally senescing leaves are abscission-ac- 

 celerating, I suggest that these results give a further indication that 



