112 PLANT GROWTH SUBSTANCES 



uptake and respiratory activity measured by oxygen uptake or dry 

 weiglit loss. Higher than optimum levels of growth substances usually, 

 though not always, have caused parallel depression in over-all respiration 

 and the associated physiological responses. Table i summarizes some of 

 the data comparing these responses at optimum and inhibitive or toxic 

 levels. Both lAA and 2,4-D begin to inhibit coleoptile or stem elongation 

 and oxygen uptake in the range of 10 to 100 ppm. (13,15,18). Root 

 elongation is much more sensitive but so far there is little if any evidence 

 of an accompanying change in respiration (12). Protoplasmic streaming 

 in Avena coleoptile has a range of sensitivity similar to elongation 

 (33,34). In roots, however, while the optimum level for streaming is 

 similar to that for elongation the former is strongly inhibited only at 

 10 ppm. or more. Active or nonosmotic water uptake and respiration 

 in potato tuber slices (14,28) and Avena coleoptiles (17) are both 

 inhibited in the 10 to 100 ppm. range. In wheat roots (25), however, 

 2,4-D inhibited nitrate uptake down to o.i ppm. with an actual increase 

 in oxygen uptake at the upper range of 5 ppm. It appears that the corre- 

 lation in stem and coleoptile between the inhibition of respiration and 

 of other physiological effects does not hold in roots. 



Respiratory Changes in Germination 



Respiratory changes have also been observed in the toxic action of 

 growth substances in the germination and growth of whole seedlings. 

 Here in addition to elongation, water uptake and streaming, cell division, 

 and other physiological processes must be considered. Table 2 summarizes 

 this work. Pratt (26) first showed that wheat-seedling growth by dry 

 weight was inhibited beginning as low as o.oi ppm. of lAA while oxygen 

 uptake increased to about 50 ppm. and was strongly inhibited only at 

 150 ppm. This suggests that root tissue accounted for the major part of 

 the seedling response at this stage. Hsueh and Lou (16) have reported 

 some interesting though brief data comparing the nature of the res- 

 piratory changes in barley and rice after 2,4-D treatment. Rice, charac- 

 teristically an anaerobic seed, was relatively resistant even to 1,000 

 ppm. of 2,4-D while barley, an aerobic seed, was completely inhibited 

 between 140 and 700 ppm. Furthermore, there was a clear indication 

 that carbon dioxide evolution was less inhibited than oxygen uptake. 

 This was especially true of rice and was most marked at the two and 

 three-day stage where control samples of barley and rice, respectively, 



