VI CELL ENLARGEMENT 79 1 



(Jensen, 1955). Here the stem or root tissue was incubated in lAA for a few h. and 

 then tested, either after homogenizing or histochemically in the tissue. The expo- 

 sure to lAA markedly increased peroxidase activity. The optimum lAA concentra- 

 tion for pea roots was io"^A/, for Vicia roots 10^ A/, both of which are high 

 enough to inhibit elongation {cf. section \TIb). The cells of the root cap, which 

 are high in peroxidase, and those of the meristem, which are low in it, showed 

 little or no change in the enzyme, but it was the cells of the elongating zone and 

 those beginning to differentiate which responded, their peroxidase level being 

 increased about 3 times. However, since the thin-walled parenchyma cells 

 showed almost no peroxidase either before or after, while the protoxylem cells 

 showed the biggest increase, the effect may be linked with differentiation rather 

 than with cell enlargement (see Galston, 1956). A similar increase of enzyme was 

 earlier noted for "lAA-oxidase," the enzyme causing oxidative destruction of 

 lAA (Galston and Dalberg, 1954). Here, however, other auxins not attacked by 

 the enzyme, and even auxin antagonists, also increased the amount of enzyme, so 

 that the effect is not specific. The destruction of lAA would of course not be 

 expected to parallel the growth, and indeed Pilet and Galston (1955) found more 

 of the enzyme in the non-growing than in the growing regions, of both pea stem 

 and lentil root. In the older regions of the root it was the peroxide-generating 

 capacity which increased, not the peroxidase itself; indeed the peroxidase itself 

 seems to decrease with age of the cell. Whether this auxin-induced increase in 

 auxin-destroying capacity is important in the control of growth remains to be 

 seen, but is certainly possible. Unfortunately, the data showing the increase in 

 oxidase in the lower parts of the pea stem are inconclusive, since they are presented 

 per mg of nitrogen, and it is obvious that the nitrogen content would be highest 

 in the young growing parts and relatively low in the mature and differentiated 

 stem, with its secondary wall. 



All in all, it may be said that some enzymes show an increase due to auxin, 

 which may be related to growth, while others show increases which likely are 

 quite unrelated. Such changes do raise the question of whether the action of auxin 

 on growth involves a synthesis of enzymes or not. The broader, but closely related, 

 question as to whether protein synthesis as a whole accompanies cell enlargement 

 has been vmder discussion for some time. Whole coleoptiles, attached to the seed, 

 do increase in protein content during growth, and so do the stems of whole plants 

 swelling under the influence of high auxin concentrations. Protein is synthesized in 

 root cells during elongation and far more than in the meristem (Clowes, 1958). 

 Invertase shows an increase of twenty-fold (Brown et al., 1952). On the other hand 

 soluble nitrogen is of course flowing into these tissues and some of the changes may 

 thus represent little more than nitrogen accumulation, unless the synthesis of pro- 

 tein and enzymes is clearly shown to take place at the expense of soluble nitrogen. 

 Isolated pea stem sections, after growth in auxin, provide a more critical case, but 

 here protein synthesis was found in early experiments and could not be repeated 

 later (see Thimann and Loos, 1957, for review). Isolated corn coleoptile sections, 

 when growing for 3 to 6 h. in auxin solution, did not show any increase over 

 auxin-free controls in their incorporation of labeled glycine or alanine into protein, 

 but there was some incorporation, due to protein turnover, in both groups 



Literature f). Ht6 



