■782 PLANT GROWTH 10 



and Bonner, 1948, for review). Rye roots are exceptional in requiring lAA or a 

 related compound formed by autoclaving tryptophan (Roberts and Street, 1955). 



Sections of leaves differ from the above in that they require purines and amino 

 acids in addition to sucrose for appreciable cell enlargement. Adenine and guanine 

 are fairly effective, though kinetin brings about the same amount of growth at 

 much lower concentration (see section Vila). Arginine and asparagine have 

 small growth-promoting efTects in some cases. Leaves of higher plants do not grow 

 for long in culture, though embryonic rye leaves in a sucrose-salts medium have 

 enlarged to about 20 times their original size (De Ropp, 1945-47; see section Vila, 

 p. 798). Purines and vitamins had little effect on the growth. Fern leaves and young 

 leaf primordia of Helianthus, however, seem to grow indefinitely but slowly on a 

 complete nutrient medium in the light (Steeves and Sussex, 1957, Steeves et al., 

 1957), providing they are excised as young primordia. 



Potassium ions greatly promote cell enlargement in most isolated tissues. The 

 optimum concentration generally lies at about 0.005 A/. Growth of coleoptile sec- 

 tions in auxin plus sucrose may be increased up to 60% by optimum potassium 

 (Thimann and Schneider, 1938). Isolated roots in culture show a strong response 

 to potassium, principally in cell enlargement. In tissue cultures, too, potassium 

 has a large effect (Heller, 1953). 



Both manganese and cobalt further promote the growth of Avena coleoptile 

 sections, cobalt being optimal at about 3-io"-^Af and Mn at '^•lO'^M. Pea stems 

 respond equally well to cobalt but scarcely at all to manganese. There is consider- 

 able evidence that the two metals act in different ways (Thimann, 1956b). Leaf 

 tissue disks (Miller, 1952) and the tissue cultures of pea epicotyls mentioned also 

 respond markedly to cobalt. The action of these metals is rather curious, since 

 manganese is required for whole plants only in traces, and cobalt not at all. In 

 leaf disks the effect of cobalt is interrelated with that of light (Miller, 1952, 1954) 

 but this seems not to be the case in coleoptile and pea segments (Thimann, 1956b) ; 

 rather the cobalt appears to act by "channelling" the metabolism towards growth. 

 Thus its effect is exerted in presence of sucrose, but if acetate is supplied, cobalt 

 acts only as an inhibitor. There are other interrelations with the energy supply. 



Calcium ions strongly inhibit elongation, being at least 10 times more effective 

 than could be accounted for by their osmotic concentration (Thimann et al., 1950). 

 Magnesium, however, has little effect either in promoting or inhibiting elongation. 



(/) Specific inhibitors 



A great variety of more or less specific enzyme poisons have been shown to in- 

 hibit cell enlargement. From these effects it has been possible to dedvice some of the 

 enzyme systems participating in the process (see Thimann, 1956a, Chap. 6 for review). 



The largest class are those compounds which react with sulfhydryl groups. So 

 far, every such compound tested has proven to inhibit cell enlargement. Many 

 enzymes of great importance in carbohydrate metabolism, including the dehy- 

 drogenases of phosphoglyceraldehyde, succinic acid and alcohol contain -SH 

 groups which are essential for their action. Model reactions between cysteine on 

 the one hand, and iodoacetate, triiodobenzoate and coumarin on the other, have 

 been described by Leopold and Price (1957). Historically the first of these inhib- 



