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  • Author or Editor: Will Neily x
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Stem elongation rates (SER) in snapdragon end zinnia were recorded in 3 DIF regimes (+5, -5, and 0; Daily average: 18C) using both high resolution (linear transducers), and low resolution techniques. Three developmental stages were chosen for study: Stage 1 was vegetative growth, preceding the formation of a flower bud. Stage 2 was the period from bud formation to preliminary expansion. Stage 3 was the period just before anthesis.

Low resolution measurements showed a decrease in snapdragon height in response to a negative DIF. A negative DIF was less effective in reducing zinnia height especially after the third developmental stage. Final plant height for both species was not affected by placing plants in the 3 DIF regimes for 1 week during the growth cycle.

Snapdragon and zinnia displayed unique diurnal SER patterns. Snapdragon showed a large peak in SER at the start of the dark period followed by a gradual decline. SER increased again during the light period. Most growth in vegetative zinnias occurred around the light/dark transition. This peak growth tended to shift to the night period as buds were formed and flowering proceeded. High resolution measurements revealed a reduction in SER for both species at negative DIP; greatest decreases occurred during the night.

DIF exerts an influence on diurnal SER in both snapdragon and zinnia, despite well defined differences in SER patterns. Negative DIF suppresses the SER of both species at all 3 developmental stages, but must be applied consistently in order to produce significant differences in final plant height.

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Experiments were conducted to determine the effects of treatment with gibberellic acid (GA) on changes in diurnal growth rhythms caused by maturation and day/night temperature differential (DIF) in zinnia (Zinnia elegans Jacq. `Pompon'). Plants were treated with GA3 or with the GA biosynthesis inhibitor daminozide under three DIF regimes (+5 DIF: 21 °C DT/16 °C NT; 0 DIF: 18.7 °C constant; –5 DIF: 16.5 °C DT/21.5 °C NT), each with a daily average temperature of 18.7 °C, at two developmental stages: stage 1, the period of vegetative growth before flower bud formation; and stage 3, growth just before anthesis. Instantaneous stem elongation rates (SER) were measured using linear voltage displacement transducers. The DIF regime, as has been previously shown, influenced stem elongation primarily by altering the size of an early morning peak in SER; peak height increased as DIF became more positive. GA3 increased SER throughout the diurnal period with a proportionately larger effect on nighttime growth. Conversely, daminozide decreased SER more or less equally throughout the diurnal period. Neither GA3 or daminozide transformed growth patterns to match those of positive or negative DIF plants, but instead simply increased or decreased growth amplitude. Furthermore, neither growth regulator altered the basic diurnal SER pattern at any DIF, or influenced the observed shift to greater nighttime growth as plants matured from stage 1 to stage 3. The results suggest that neither the effects of DIF, or the age-related shift in diurnal growth distribution can be explained by changes in total availability of GA in the plant. Chemical name used: mono (2,2-dimethylhydrazide) butanedioic acid (daminozide).

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