REST IN BUDS OF WOODY PLANTS 205 



untreated shoots of apple trees only somewhat over 12% of the 

 buds opened. With a mineral oil spray, even when it was 

 completely saturated, we obtained an increase in sprouting of 

 about 50%, although we could not expect from a saturated oil 

 any chemical action. It would seem probable that the inert oil 

 film interferes with the oxygen supply of the cells, again a situa- 

 tion leading towards anaerobiosis. We also investigated the 

 effect of mineral oils with different degrees of unsaturation. Our 

 results show a considerable increase of the rest-breaking action 

 with an increasing proportion of the unsaturated compounds in 

 the oil (lower U.M.R.). Furthermore, we dissolved dinitro- 

 compounds in the mineral oil. All three which we tried — and 

 dinitrophenol which we studied in another series of tests — 

 increased the rest-breaking action of the spray, thus doubling 

 the number of growing buds of the control. Today this dinitro- 

 cresol-mineral oil spray^ is used widely by growers in Israel 

 and in certain other subtropical countries. 



The physiological action of dinitrophenol has been reviewed 

 by Simon^. Its uncoupling action is caused by inactivation of 

 enzymes concerned with oxidative phosphorylation. Since so 

 fundamental a process may affect cell metabolism at several 

 points, we cannot point to a definite reaction bringing about the 

 breaking of rest. But we do find a common denominator with 

 previously mentioned methods of rest breaking: dinitro- 

 compounds lead cell respiration into fermentative pathways. 

 Bahgat' in the laboratory of Bennett broke the rest of pear 

 shoots by holding them under nitrogen and found subsequently 

 in their tissues both ethyl alcohol and acetaldehyde. When he 

 treated dormant pear shoots with alcohol or acetaldehyde, he 

 was able to break their rest. Thus, we see that products of 

 anaerobic respiration are effective in terminating the rest period. 



The entry of buds into the resting stage is affected by the 

 length of the daylight period, as originally shown by Garner 

 and Allard^ and investigated further in the laboratories of 

 Borthwick^, Wareing^o ^nd Nitsch^^. They found with very 

 different woody plants, that short days induce rest, while long 



References p. 208 



