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.