Effects of Gibberellic Acid and Temperature on the Growth of Young Seedlings of Syringa Vulgaris L.

The application of gibberellic acid (0, 5, 20 and 80 µg) to seedlings of Syringa vulgaris L. about two weeks after germination increased significantly the total length, the length of internodes and the dry weight (d.w.) of shoots and the net assimilation rate. GA₃ also had a small but significant positive effect on the number of pairs of leaves, especially at high temperatures; it increased the girth, but this effect was not significant.

GA₃ reduced significantly the d.w. of roots and leaves but did not affect the leaf/root ratio. GA₃ had no effect on the total plant d.w. or the relative growth rate.

The effect of GA₃ on shoot growth was dependent on temperature and on the stage of growth. One and two weeks after its application it had the maximum relative effect at high temperatures (21–24 °C) but at the end of the experiment (8.5 weeks) the maximum effect was reached at 12 °C; it decreased with increasing temperature and was not significant at 24 °C. By this stage there were, however, no statistical interactions between temperature and GA₃ for total length and for d.w. of shoots, roots, leaves and of the whole plant.

Increasing temperatures over the range 12–24 °C resulted in increases in the following characteristics: the number of pairs of leaves; length of internodes, diameter and total length of the shoot; the d.w. of shoots, roots, leaves and of the whole plant; the d.w. ratios of leaves/roots and shoots/roots; and the relative growth rate and net assimilation rate. High temperatures reduced the root/whole plant dry weight ratio. The effect of temperature on the number of pairs of leaves was linear, and results at alternating temperatures (24°/18° and 21°/15 °C, 8 hr/16 hr) did not deviate significantly from values expected on the basis of mean daily temperature.