Rate and time of GA
4/7treatment affect vegetative
growth and flowering in a genetic line
of
Aquilegia
hybrida
Sims
T.J. Gianfagna
*, R.H. Merritt
Plant Science Department, Rutgers University, 59 Dudley Road, New Brunswick, NJ 08901-8520, USA
Accepted 17 May 1999
Abstract
Aquilegia`Rose-White' flowered from seed in about five months without vernalization. GA4/7
treatment increased stem height, induced earlier flowering, and increased the number of flowers/ plant. Each of these responses, however, was dependent upon both rate (10, 25, or 50 ppm) and timing of application (4-, 8-, or 12-leaf stage). Treatment at the 4-leaf stage resulted in earlier flowering at all rates, had no effect on stem height, but reduced the number of flowers/plant and total leaf area. Treatment at the 8-leaf stage induced earlier flowering at all rates, and increased both the number of flowers/plant and plant height (50 ppm), but had no effect on total leaf area. Treatment at the 12-leaf stage had no effect on time of flowering, but increased the number of flowers/plant, plant height (50 ppm) and total leaf area, at all rates. A rate of 50 ppm at the 8-leaf stage is the optimal treatment resulting in earlier flowering with increased plant height and number of flowers/plant.#2000 Elsevier Science B.V. All rights reserved.
Keywords: GA4/7;Aquilegiahybrida; Flowering; Vernalization
1. Introduction
There is considerable interest in the development of herbaceous perennial plants capable of flowering from seed in the first season or at the time of sale in the spring. Most herbaceous perennials require either long days and/or
* Corresponding author. Fax: +1-732-932-9441.
vernalization to flower and these conditions can be provided in the greenhouse by supplemental illumination and low night temperature (Armitage, 1994). Unfortunately, most commercial cultivars of aquilegia (Aquilegiahybrida Sims) cannot be induced to flower by these treatments. White et al. (1990) grew 13 cultivars under 8, 12, 16 and 20 h photoperiods for five months and found no evidence of floral induction. Shedron and Weiler (1982) were only able to induce flowering with an 8-week 48C vernalization treatment after the plants had produced 12±15 leaves. Most aquilegia cultivars apparently have an extended juvenile phase in which they are insensitive to flower-inducing treatments. The `Rose-White' genetic line, however, has a very brief juvenile phase and will flower from seed within five months (Merritt et al., 1997). `Rose-White' can be grown successfully at relatively cool temperature conditions (16±198C/9±138C, d/n), but little else is known about its cultural requirements. Treatment with gibberellins (GA4/7) induced earlier flowering, and increased both plant height
and number of flowers/plant, and was much more effective than GA3(Gianfagna
and Merritt, 1998). Timing of GA4/7treatment may affect flowering response. If
plants are treated too early, flowering may be advanced, but the plant may not produce as many flowers. If plants are treated late, flower numbers may be increased, but flowering date may be unaffected. Given the value of an early flowering aquilegia and the benefits of GA4/7treatment, the following work was
designed to determine the effects of rate and time of application on flowering and plant height.
2. Materials and methods
Seeds of the genetic line `Rose-White' a soon-to-be-released commercial cultivar (Goldsmith Seeds, PO Box 1349, Gilroy, CA 95021, USA) were germinated 6 October 1997 in vermiculite at 248C with light (150mmoles/m2/s).
Seedlings were transplanted 27 October 1997 in Pro Mix in 12.7 cm square plastic pots (1 l) and grown at 168C/98C (9 h day/15 h night) in the greenhouse at New Brunswick, NJ. Natural light was augmented with high pressure sodium lamps (80mmoles/m2/s) from 0800 to 1700 hours. Plants were treated with GA4/7
comparison test. Statistical significance was declared if the two-sidedp-value was 0.05.
3. Results and discussion
Plant height, leaf area, flowering date and number of flowers/plant were affected by GA4/7, but the response was dependent upon both rate and stage
of plant development at the time of application. When plants were treated at the 4-leaf stage with 10 ppm GA4/7, they flowered 8 days earlier. Higher rates
also caused earlier flowering, but no earlier than the 10 ppm rate (Fig. 1). The earlier flowering, however, was accompanied by a significant reduction in the number of flowers/plant (Fig. 2). Plants treated with 50 ppm GA4/7at the 4-leaf
stage had 40% fewer flowers. Total leaf area was significantly reduced by GA4/7treatment at all rates. Plants treated with 50 ppm GA4/7had about 80% of
the leaf area of untreated plants (Fig. 3). Stem elongation was not effected by treatment (Fig. 4).
GA4/7 also induced earlier flowering when treatment was delayed to the
8-leaf stage (Fig. 1). Plants treated with 50 ppm GA4/7 flowered 12 days
Fig. 1. Effect of rate and time of GA4/7application on time of flowering in aquilegia cv.
Fig. 2. Effect of rate and time of GA4/7application on flower number/plant in aquilegia cv.
Rose-White.
earlier. In contrast to the results from the 4-leaf stage treatment, the number of flowers/plant significantly increased (Fig. 2). This was accompanied by an increase in stem elongation at both the 25 and 50 ppm rate (Fig. 4). Leaf area was not affected by GA4/7 at any treatment rate when made at the 8-leaf
stage (Fig. 3).
GA4/7 had no effect on time of flowering when applied at the 12-leaf stage
(Fig. 1); however, at the 50 ppm rate there was an increase in flower number/ plant (Fig. 2). The largest plants were also produced by the 50 ppm treatment rate. Leaf area and plant height increased about 20% when treatment was delayed to the 12-leaf stage (Figs. 3 and 4).
were no differences in the total number of leaves produced for the 12-leaf treatments compared to untreated plants (data not shown), therefore the increase in total leaf area with these treatments (Fig. 3) is a result of increased leaf expansion. GA4/7 at a rate of 50 ppm applied at the 8-leaf stage was the most
useful treatment. Plants flowered early, produced the greatest number of flowers/ plants, and the inflorescence was long enough to position the flowers well above the leaves.
In most cultivars of aquilegia, vernalization is a prerequisite for stem elongation and flowering. In other plants such as spinach, which respond in a similar manner to vernalization, GA4/7 has clearly been implicated in stem
elongation (Metzger, 1995), but not necessarily in flowering. In statice (Limonium sinuatum Mill.), however, GA4/7 treatment can induce early flowering (Wilfret
and Raulston, 1975) and increase the number of flowering stems (Garner and Armitage, 1996). Timing of application is important in statice as well. Treatment with GA3one week after transplanting most effectively increased the number of
flowering stems, while treatment four weeks after transplanting advanced flowering date the most (Garner and Armitage, 1996). In `Rose-White', stem elongation and flowering have been partially uncoupled. Although all untreated plants flowered, the short inflorescence and the limited number of flowers may reflect low levels of GA4/7production and form the basis for the beneficial effect
of exogenous GA4/7treatment. How GA4/7induces earlier flowering is unclear.
Perhaps this hormone increases the sensitivity of the shoot apex to the floral stimulus or production of the floral stimulus itself is increased. In our next study we will determine the effect of GA4/7treatment on stem growth and flowering in
a number of the now recently released aquilegia cultivars that flower without vernalization (Garner and Armitage, 1998).
Acknowledgements
We would like to thank Dr. J.R. Trout for help with the statistical analysis and Mr. J. Messeroll for technical assistance.
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