The Effects of Gibberellins on the Growth of Excised Tomato Roots

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Abstract
At appropriate concentrations both gibberellic acid (GA) and 1-naphthalene-acetic acid (NAA) enhance the main axis growth of excised tomato roots grown in culture media containing sucrose at concentrations below 1 per cent. Lateral root extension growth is enhanced by GA at all sucrose concentrations tested; only at the lower sucrose concentrations is this effect observed with NAA. Both GA and NAA increase the number of emergent lateral roots and this effect is most marked in media of low sucrose content. Both GA and NAA at higher concentrations inhibit root growth but NAA exhibits its full range of growth effects over a much narrower concentration range than GA. GA, like NAA, speeds up the loss of meristematic activity which occurs when individual meristems are repeatedly subcultured in media containing 1 per cent, or higher concentrations of sucrose. The promotion of main axis growth by both GA and NAA involves enhanced cell elongation and cell division. At a moderately inhibitory concentration GA reduces both cell elongation and cell division; this is not the case with NAA. Gibberellins A1, A2, and A4 resemble GA (gibberellin A3) in their growth effects. Allogibberic acid like G A promotes lateral root extension growth but causes marked inhibition of root growth at a much lower concentration than GA.

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