Occurrence and Role of Growth Hormones 75 



the dark would suggest that a diurnal periodicity of auxin content 

 may be present in a normal growth situation. Evidence for such 

 periodicity in tomatoes has been produced by Went (1944). He found 

 a generally higher auxin concentration during the day followed by a 

 generally lower level at night, though secondary rises and drops oc- 

 curred too. 



Quite a different role of light has been suggested by Liverman 

 and Bonner (1953). They observed small increases in growth rate of 

 Avena coleoptiles in response to red light (600-800 m^x), which could be 

 reversed by far-red light (700-800 m/x). Kinetic considerations in the 

 manner described in chapter VIII led them to suggest that red light 

 increases the effectiveness of auxin by increasing the availability of 

 the hypothetical substance with which auxin reacts to cause growth. 

 It was further suggested that far-red light reduces growth by decreas- 

 ing the availability of the receptor substance. 



Differences in day length or photoperiods will cause differences in 

 auxin levels. Russian workers (Cajlachjan and Zdanova, 1938) have 

 found consistently higher auxin levels under long-day conditions than 

 under short-day conditions, no matter what the photoperiod classifica- 

 tion of the plants tested. Their observations were made on diffusible 

 auxin from stems, and similar results for diffusible auxin from leaves 

 have been obtained (Leopold, 1949). 



The formation of auxin in response to light appears to be a func- 

 tion of leaves. Defoliation of expanding ginkgo twigs reduced the 

 diffusible auxin in the apex 80 per cent in two days. This is partic- 

 ularly striking since Gunckel and Thimann (1949) obtained no dif- 

 fusible auxin from the leaves themselves. It is suggestive, at least, that 

 the leaves may be supplying a precursor from which auxin is produced 

 in the tip. 



The capacity of excised tissues growing on organic media to pro- 

 duce auxin in the dark has been demonstrated in tobacco callus (Skoog, 

 1944) and in excised roots (van Overbeek and Bonner, 1938). In both 

 of these studies the auxin was obtained with ether extraction tech- 

 niques. 



Zinc appears to be essential for auxin formation. The usual symp- 

 toms of zinc deficiency are primarily a general failure of elongation. 

 Skoog (1940) considered this lack of growth to be associated with a 

 low auxin content. Supplying deficient plants with zinc restored the 

 auxin supply. Likewise the addition of auxin or tryptophan relieved 

 the deficiency symptoms (Tsui, 1948). It would appear then that the 

 absence of zinc prevents the normal production of tryptophan, a pre- 

 cursor of indoleacetic acid. Other evidence (Elliott, 1952) indicates that 



