PHOTOPERIODIC EFFECTS IN WOODY PLANTS 237 



chromatographed in isopropanol (80) + ammonia ( 10) + water (10) 

 (V/V), and assayed with oat coleoptiles according to Nitsch's method 

 (1955). The results (Fig. 9) showed drastic differences: the extract 

 of long-day plants contained several growth-promoting substances and 

 practically no inhibitory ones, whereas the extract of short-day plants 

 had less auxins and more inhibitors. Thus, it became apparent at once 

 that the story of dormancy might be a complex one, involving both an 

 increase in inhibitors and a decrease in growth-promoting substances. 

 However, in the case of Waxman's experiment, the short-day tissues 

 had been taken from plants which had been dormant for 1 2 weeks and 

 which, therefore, were considerably older than the long-day tissues. A 

 more detailed study was necessary. 



A detailed study was made with sumac tissues, in this laboratory 

 also. The sumac seedlings of which the growth behavior has been 

 shown in Fig. 6, were harvested at various times and lyophilized. The 

 upper leaves and the tips were extracted separately. The extracts were 

 chromatographed on paper strips and assayed biologically for growth- 

 regulating substances. Figure 10 gives the results obtained with the 

 first internode bioassay (Nitsch and Nitsch, 1956). This bioassay is 

 very sensitive (one-thousandth of a microgram of indole-3 -acetic acid 

 (lAA) produces a significant increase in growth), but responds little 

 to inhibitors. Consequently, the results given in Fig. 10 reflect changes 

 in the level of growth-promoting substances of the auxin type without 

 giving much information about the inhibitor picture. As shown on the 

 left part of Fig. 10, the tips of plants grown under long days contain a 

 growth-promoting substance which moves to the same Rf position as 

 lAA. After 1 week of short days, when the growth rate is declining 

 (Fig. 6), there is still a large amount of this substance in the tips, an 

 amount slightly larger, on a concentration basis, but slightly smaller, 

 on a per plant basis, than that of long-day seedlings (Table I). After 

 2 weeks of short days, the level of growth-promoting substances drops. 

 At that time, the growth of the sumac seedlings has stopped com- 

 pletely (Fig. 6). A first result of this investigation is, therefore, the 

 observation that as the length of the short-day treatment increases, the 

 level of growth-promoting substances decreases. However, from the 

 results presented here, it is not possible to determine whether the lower- 

 ing of the auxin level is a cause or a consequence of the decreased 

 growth rate produced by short days. 



