The Effects of Gibberellins on the Growth of Excised Tomato Roots

1. At appropriate concentrations both gibberellic acid (GA) and1-naphthalene-acetic acid (NAA) enhance the main axis growthof excised tomato roots grown in culture media containing sucroseat concentrations below 1 per cent. Lateral root extension growthis enhanced by GA at all sucrose concentrations tested; onlyat the lower sucrose concentrations is this effect observedwith NAA. Both GA and NAA increase the number of emergent lateralroots and this effect is most marked in media of low sucrosecontent. Both GA and NAA at higher concentrations inhibit rootgrowth but NAA exhibits its full range of growth effects overa much narrower concentration range than GA.
2. GA, like NAA,speeds up the loss of meristematic activity whichoccurs whenindividual meristems are repeatedly subculturedin media containing1 per cent, or higher concentrations ofsucrose.
3. The promotionof main axis growth by both GA and NAA involvesenhanced cellelongation and cell division. At a moderatelyinhibitory concentrationGA reduces both cell elongation andcell division; this is notthe case with NAA.
4. Gibberellins A₁, A₂, and A₄ resemble GA(gibberellin A₃) intheir growth effects. Allogibberic acidlike G A promotes lateralroot extension growth but causes markedinhibition of root growthat a much lower concentration thanGA.

     

Effects of 3-Indolylacetic Acid and 3-Indolylacetonitrile on the Growth of Excised Tomato Roots

1. The inhibition by IAA (3-indolylacetic acid) and by IAN (3-indolylacetonitrile)of the growth of excised tomato roots cultured for 7 days at27 C. in a modified White's medium is described. 510^–9g./ml, IAA or 510^–6 g./ml, IAN cause approx, 50 per cent,inhibition of the linear growth of the main axis. With IAA decreasein number of laterals closely parallels the decrease in lineargrowth of the main axis; with IAN reduction in linear growthof the main axis occurs at concentrations above 10^–8 whereasnumber of laterals does not decrease until the concentrationexceeds 10^–6.
2. Study of the course of cell elongationin the exodermal cellsshowed that in the standard medium andin media containing 510^–9IAA or 510^–6 IAN theprocess takes about 7 hours; thefinal cell lengths in IAA andIAN media are lower than in standardmedium owing to a slowerrate of elongation. The decrease inlinear growth of the mainaxis in presence of IAA could be accountedfor by the decreasein cell length; this was not the case withIAN. The implicationsof this are considered.
3. Determinations of the distance (mm.)between, and of the numberof exodermal cells separating, theadjacent laterals in oneorthostichy showed that IAN enhancesthe frequency of lateralswhereas this is either unaffectedor decreased by IAA. The enhancementof lateral frequency inIAN arises from shortening of the cellsof the main axis anddecrease in the number of cells separatingadjacent laterals.
4. The results are considered to support the view that IAN haseffects on root growth different from those of IAA. Study ofthe degree of inhibition of main axis growth and of alterationsin lateral frequency resulting from treatment with mixturesof IAA and IAN provided data which could also be most easilyexplained on this hypothesis.

     

The Presence of Gibberellins in Excised Tomato Roots

Substances having similar physiological properties to the gibberellins(located by the ‘Meteor’ dwarf pea bio-assay) havebeen detected in extracts from excised tomato root. The chromatographicbehaviour of the most active zone is similar to that of gibberellinA₁. Experiments using the d-1 and d-5 mutants of mazie did not indicatethe presence of substances with differential effects on thesetwo mutants.     

The Carbohydrate Nutrition of Tomato Roots: III. The Effects of. External Sucrose Concentration on the Growth and Anatomy of Excised Roots

Variation of the concentration of sucrose in White‘s mediummarkedly affects the increase in length of the main axis, andthe number and total length of the laterals developed by excisedtomato roots. Highest values for these three features are obtainedwith medium containing 1.5 per cent, sucrose. Root materialhas been analysed anatomically. Root diameter, cortical thickness,number of xylem vessels, and development of the cortical air-spacesystem increase with increase in sucrose concentration. Thedegree of vacuolation of extra-stelar meristematic cells increases,and protophloem differentiates further from the promeristemwith rising sucrose concentrations. The interpretation of theseresults is discussed.     

Metabolism of Some Indole Auxins in Excised Tomato Roots

Indolylacetylaspartic acid (IAAsp) and possibly indolylacetylglutamicacid (IAG) are formed by exposure of excised tomato roots toIAA. Little ‘free’ IAA accumulates in the tissue.An unidentified substance reacting pink with nitricnitrite reagentis also formed. These substances are metabolized when IAA-treatedroots are transferred to auxin-free medium. IAAsp and IAA aresimilarly inhibitory to the growth of excised tomato roots.Excised tomato roots do not interconvert IAA and IAN. IAN-feedingleads to IAN accumulation and the appearance of indolylcarboxylicacid (ICA); transference to auxin-free medium causes a declinein the IAN activity but the ICA spot persists. The inhibitoryactivity of IAN is not due to its conversion to ICA. Excisedtomato and wheat roots respond very differently to externall-tryptophane but in neither case is there evidence of the conversionof tryptophane to ethyl acetate-soluble auxins.     

Stimulation of the Growth of Excised Cultured Roots of Soya Bean by Abscisic Acid

Excised root cultures of soya bean cultured at 30 °C inWhite's medium (1943) supplemented with 0•1 per cent yeastextract have been serially sub-cultured over 13 culture passagesof 7 days although a decline in the linear growth and lateralformation begins in the third passage and the roots are devoidof laterals from the 8th passage onwards. Applications of abscisicacid within the concentration range 0•01–0•001mg l^–1 and during the first two culture passages enhancedthe linear growth of the main axis, the number of emergent lateralsand the total length of laterals. This effect has been shownfor two cultivars, with roots derived from seed in both itsfirst and second year of storage and under different conditionsof culture and culture pH.     

Sugar Absorption and Sucrose Inversion by Excised Tomato Roots

The growth of excised tomato roots in sucrose is accompaniedby the appearance in the medium of glucose and fructose. Thequantitative relations between this appearance of glucose andfructose in the medium, total sugar absorption by the roots,and the decrease in the sucrose concentration of the mediumdo not suggest any causal relationship between sucrose uptakeand glucose and fructose appearance in the medium. Excised tomato roots exhibit surface invertase activity witha pH optimum at 4.0–4.4. Alterations of external pH, whichdid not affect sucrose absorption, drastically altered the levelsof glucose and fructose appearing in the medium. Glucose is preferentially absorbed from mixtures of glucoseand fructose, and by adjustment of the ratio of the two sugars,a mixture can be obtained from which equimolar absorption ofthe two sugars occurs. Root growth in this mixture is, however,very poor compared with that occurring in presence of sucrose. The results are discussed in the light of earlier studies onsucrose uptake by cultured tomato roots.     

Effects of Growth Retardants on the Growth of Excised Roots of Dolichos lablab L. in Culture

The effects of various growth-retarding chemicals on the growthof excised roots of Dolichos lablab in sterile culture are investigated.Application of a range of concentrations of CCC, phosfon, andB-995 inhibited growth in length of the roots by reducing thefrequency of cell division, with little or no effect on cellelongation. Treated roots had generally lower DNA, RNA, andprotein contents than controls, although CCC-treatment significantlyenhanced the soluble nitrogen content of the roots. The inhibitoryeffects of growth retardants were not reversed by IAA or GA,but substances like choline chloride and pyridoxine hydrochloridewere partially effective in reinstating normal growth in rootsgrowing in concentrations of CCC producing about 50 percentinhibition of growth in length.     

Studies of the Growth in Culture of Excised Wheat Roots

Evidence is presented that the growth of cultured wheat roots is enhanced in closed culture systems not only by carbon dioxide but by a further and apparently very volatile product of root metabolism. The formation of this second growth-stimulating substance is not considered to depend upon a reduction of oxygen tension in the culture system. Unlike ethanol the activity of the volatile promoter does not depend upon illumination of the cultures. The volatile promoter cannot be effectively absorbed by a number of gas absorbents. Attempts to demonstrate ethylene production by the roots have been unsuccessful.     

Studies of the Growth in Culture of Excised Wheat Roots

Aeration of root culture medium with air containing 5 % or 10 % CO₂ enhances growth, particularly lateral initiation and growth, compared with aeration wilh air or CO₂-free air. This beneficial effect of enhanced CO₂ pressure on the growth of cultured roots of Atson Elite 56 wheat ean also be demonstrated in presence of a high (40 %) and inhibiting level of oxygen. ¹⁴C labelling from a carbonate–bicarbonate solution of pH 7.0 by cultured wheat roots is enhanced by light and this labelling is not inhibited by the presence of 1 % ethyl urethane.     

Studies of the Growth in Culture of Excised Wheat Roots

Excised wheat roots in culture release a substance inhibitory to their growth and which can be absorbed on activated charcoal. This growth inhibitor can be isolated in pure form by diethylether extraction followed by DEAE cellulose and thin-layer chromatography. At very low concentration it inhibits lateral root development. The significance of this inhibitor to the problem of the continuous growth of cultured wheat roots is discussed.