792 



PLANT GROWTH 



(Boroughs and Bonner 1953). Also the experiments were of short duration, coleop- 

 tile growth usually lasting at least 30 h. 



Roots caused to elongate by auxin antagonists (see section Vllb) have also been 

 studied in this connection and it was concluded that no appreciable change in 

 protein took place (Burstrom, 195 1). Unfortunately the large increase in elongation 

 {ca. 90%) was accompanied by only a small increase (12-30%) in fresh weight, so 

 that much of it must have been due to a change in geometry. The modest increases 

 in fresh weight were in fact accompanied by increases in protein nitrogen of com- 

 parable magnitude (4-30% in different experiments) but it would be hardly 

 justified to draw a firm conclusion. A complication in the case of roots is that they 

 form two proteolytic enzymes during elongation (Robinson, 1956); these reach 

 a maximum at the end of the growth zone. They do not increase when isolated 

 root fragments are grown in sucrose, but they may be responsible for hydrolyzing 

 protein that was formed during elongation. 



Disks of potato tuber have given the most positive answer to this question at 

 present. It was first shown in studies of salt uptake that they synthesize protein 

 and it seems probable that some protein synthesis is necessary not only for the up- 

 take of salt but for its subsequent retention (Steward and Preston, 1941). Similar 

 phenomena occur in growth. When the disks are aerated as for growth experi- 

 ments, protein is synthesized rapidly for the first 2 days after cutting, and the 

 soluble nitrogen (aminoacids) decreases correspondingly. Auxin clearly and 

 reproducibly increases the protein synthesis, though only by some 15% (Thimann 

 and Loos, 1957). The water uptake was increased by 30-40%, in the same auxin 

 concentration. In artichoke tuber, which responds more strikingly to auxin, the 

 protein increase was also much larger, — 400% in one experiment — though follow- 

 ed by some hydrolysis later. In both tissues protein synthesis mainly precedes the 

 cell enlargement and cannot therefore be considered a result of growth. It may 

 tentatively be deduced that auxin causes the formation of new protein in these 

 tissues, and that this may well include one or more enzymes active in cell enlarge- 

 ment. Other enzymes not concerned with the process could well be synthesized 

 too {cf. above). If in some tissues the enzymes involved comprise only a small 

 fraction of the total protein this would explain the lack of net protein synthesis 

 therein. 



(/?) Osmotic inhibition 



Cell enlargement can be stopped not only by inhibiting metabolism (section Vlf ) 

 but also by reducing the availability of water. This was first shown in 1938 when 

 Thimann and Schneider grew Avena coleoptile sections in auxin in presence of 

 different concentrations of mannitol, which was chosen as being an essentially 

 inert and relatively impermeable solute. Elongation was reduced in linear propor- 

 tionality to the mannitol concentration ; at 0.3 1 M it was stopped altogether, though 

 there was a slight recovery after 24 h., no doubt because a small amount of manni- 

 tol enters the cells in time, reinstating an osmotic gradient. The suction force of 

 the tissues, necessary for growth, is evidently 0.31 X 22.4 or some 7 atmospheres. 



By making measurements every few h., Ordin et al. recently (1956) found a 

 marked shrinkage at first in 0.3 A/ mannitol or higher, followed by recovery and 



