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g r o w e r t o o l s
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Fundamentals Of Flowering
Day Length Effects on Bedding Plant Flowering I
by John Erwin, Ryan Warner, and Neil Mattson
Department of Horticultural Science
University of Minnesota
This article is the first of a series presenting new research results from University of Minnesota researchers on effects of daylength and supplemental lighting on flowering of seed-propagated bedding plants.
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Figure 1a.
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Figure 1b.
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Figures 1a and 1b. The effect of providing a single week of continuous lighting (long-days induce flowering in petunia) to Petunia x hybrida 'White Storm' (a) and 'Purple Wave' (b). Note that 'White Storm' was sensitive to getting the single week of continuous lighting during the first week (weeks 0-1) and 'Purple Wave' was not sensitive to the week of continuous lighting until weeks 2-3 after germination. This showed that the juvenile period for 'White Storm' was shorter than that for 'Purple Wave'.
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INFORMATION on what makes bedding plants flower could allow you to control flowering to schedule flowering accurately, as we do with many potted plant crops. Because of the importance of this information to the North American greenhouse industry, we started a long-term research study examining how daylength, supplemental lighting, and temperature affect flowering of many bedding plant species and how to precisely control flowering of bedding plants.
This article and the next will focus on how light intensity, or irradiance, affects flowering of bedding plant species. The following articles will show how daylength affects flowering of common and uncommon bedding plants. The results have tremendous applications for the industry, especially for those who grow bedding plants from seed.
Juvenility Vs. Maturity
Plant flowering is controlled by internal and external signals. With any seed-propagated bedding plant there is an internal signal that determines when a plant is mature and capable of flowering or responding to a flowering stimulus such as daylength. For instance, there is a period with animals after birth when they are incapable of reproducing. Similarly, plants are often unable to flower immediately after germinating. The period after germination when a seedling is not capable of flowering or responding to a flowering stimulus is called the 'juvenile period.' When a seedling is capable of responding to a flowering stimulus we say it is 'mature.'
How do we know when a seedling has changed from the juvenile to the mature phase? Unfortunately, with most seed-propagated annuals, there is no visible change in how the plant looks when it changes from a juvenile to a mature plant. Probably the best indicator we have is the number of leaves that a plant has.
Much work has been conducted using leaf number as a measure of plant age. Fortunately, we find that the leaf number when a plant changes from a juvenile to an adult plant can be the same over time but will change with the species and environment. Recent articles on perennials by researchers at Michigan State University have shown that seed-propagated perennials must have a certain number of leaves on a plant before they are able to respond to a cooling treatment (vernalization), which is necessary for flowering with many perennials. The same is the case for many seed-propagated annuals; they must have a certain number of leaves before they can respond to an external treatment.
Breeding of seed-propagated flowering crops has emphasized earliness of flowering over the years to reduce the time to flower. Reduced time to flower means reduced production costs to the grower. Because of this, there has been progressive selection for reduced juvenile period length by breeding companies. In other words, many of the traditional bedding plants have few leaves they must unfold when they change from juvenile to adult plants and can flower.
For this reason, many traditional types of bedding plants need to unfold few leaves before they can sense a stimulus. Often new types of bedding plants such as violas and Wave type petunias have not been heavily bred and can have a longer juvenile period. For instance, the new petunia type 'Purple Wave' is much closer to the wild germplasm and has a longer juvenile period. Note in Figure 1 that the plants did not perceive the one week of long-days (stimulates flowering in petunia) until after two weeks, for example, 'Purple Wave' petunia has a juvenile period that lasts about two weeks, compared to the very short juvenile period for 'White Storm.'
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Figure 2a.
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Figure 2b.
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Figures 2a and 2b. The effect of increasing irradiance from 0 to +150 umol m-2 s-1 (+750 footcandles) supplemental high pressure sodium lighting on time to flower (a) and leaf number below the first flower (b) of the facultative irradiance response plants lathyrus and tweedia and the irradiance indifferent plants thunbergia and tithonia. Ambient light integral was 12 to 13 moles per day with an average irradiance of 500 to 606 μmol m-2 s-1 (2,500 to 3030 footcandles) during the course of this experiment.
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Irradiance On Early Flowering
Many environmental factors affect how long the juvenile period is in seed-propagated plants. Perhaps the most important environmental factor that affects juvenile period length is light. When we talk about light, we can talk about light color (light quality), duration (photoperiod or daylength), light intensity (irradiance), or total amount per day (light integral). Together, light intensity and amount of light per day (irradiance and light integral) quantify how much light is available for photosynthesis instantaneously (irradiance) or over a 24-hour period (light integral).
We have known for some time that the amount of light a plant gets per day (light integral) affects how soon some seed-propagated annuals flower. For instance, there is a rule-of-thumb that every day that you provide supplemental lighting to a seed geranium crop early after germination reduces time to flower by one day. There has been incredibly little information on how supplemental lighting affects flowering of other bedding plant species, even though plug growers commonly use supplemental lighting to improve plug quality. Often this supplemental lighting does increase plant mass, which is a determinant of plant quality, but we do not know the ultimate effect of these lights on flowering. Some growers just assume that adding lights results in earlier flower. This is not the case for some crops.
Because there is no term to describe how irradiance or light integral affects flowering, we made two new terms to describe these two responses: facultative irradiance and irradiance indifferent response groups.
When a plant has a facultative irradiance response, extra lighting reduces the leaf number below the first flower, i.e. flowering occurs earlier developmentally.
When a plant has an irradiance indifferent response, extra lighting has no effect on the leaf number below the first flower, i.e. extra lighting does not hasten flowering developmentally.
There is some confusion with how lighting affects earliness in flowering with many crops. Remember that when you add lights, you heat the plants because of the infrared light emitted by most lamps. With many plants, the earlier flowering after adding lights is simply because plants are warmer and grow faster and therefore, flower earlier. This can be the case with earlier flowering after adding lights over irradiance indifferent plants where leaf number below the flower is unaffected. For instance, in Figure 2a and 2b you can see that although leaf number was not greatly affected by adding supplemental lighting, time to flower decreased on the irradiance in different plants Black-Eyed-Susan vine (Thunbergia) and Mexican sunflower (Tithonia) as light intensity increased from 0 to +150 umol m-2 s-1 (+750 footcandles) supplemental high pressure sodium lighting. In contrast, with facultative irradiance, plants flower much quicker because they have fewer leaves below the flower and they are growing faster (leaves unfolding faster) because of heating from the lamps. For instance, in Figures 2a and 2b time to flower was decreased from 78 to 57 days and 92 to 75 days on the facultative irradiance response plants sweet pea (lathyrus) and tweedia as light intensity increased from 0 to +150 umol m-2 s-1 (+750 footcandles) supplemental high pressure sodium lighting.
In most cases, heating with lamps is pretty expensive and not a very cost-efficient way to speed up plant development. Therefore, unless you need more plant mass on a seedling, it may not be economical to add supplemental lighting to irradiance indifferent bedding plant seedlings.
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Table 1.
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Faculative Irradiance Response
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Irradiance Indifferent Response
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Catananche caerula 'Blue'
(Catananche)
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Ageratum houstonianum 'Blue Danube'
(Ageratum)
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Lathyrus odoratus 'Royal White'
(Sweet Pea)
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Amaranthus hybridus 'Pygmy Torch'
(Amaranthus)
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Linum perenne
(Flax)
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Ammi majus
(Ammi)
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Tweedia caerula 'Blue Star'
(Tweedia)
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Asperula arvensis 'Blue Mist'
(Asperula)
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Convolvulus tricolor 'Blue Enchantment'
(Dwarf Morning Glory)
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Centaurea cyanus 'Blue Boy'
(Bachelor's Buttons)
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Cosmos bipinnatus 'White Sensation'
(Cosmos)
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Cobea scandens 'White'
(Cup and Saucer Vine)
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Gazania rigens 'Daybreak Red Stripe'
(Gazania)
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Collinsia heterophylla
(Collinsia)
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Limnanthes douglasii
(Fried Eggs, Meadow Foam)
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Dolichos lablab
(Hyacinth Bean Vine)
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Nemophila maculata 'Pennie Black'
(Five-spot)
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Eschscholzia californica 'Sundew'
(California poppy)
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Nicotiana alata 'Domino White'
(Flowering Tobacco)
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Ipomopsis rubra 'Hummingbird Mix'
(Standing Cypress)
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Origanum vulgare
(Oregano)
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Limonium sinuata 'Heavenly Blue'
(Statice)
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Silene armeria
(Sweet William Catchfly)
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Mathiola longipetala 'Starlight Scentsation'
(Stock)
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Mina lobata
(Mina Vine)
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Oenothera pallida 'Wedding Bells'
(Sundrops)
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Phacelia campanularia
(California Bluebell)
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Sanvitalia procumbens
(Creeping Zinnia)
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Thunbergia alata
(Black-Eyed-Susan Vine)
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Tithonia rotundifolia 'Sundance'
(Mexican Sunflower)
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Table 1. Lighting classification of different seed-propagated bedding plant species with respect to flowering
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About the authors: John Erwin is an associate professor of greenhouse crop physiology and extension, and Neil Mattson and Ryan Warner are graduate research assistants, Department of Horticultural Science, University of Minnesota. The authors wish to thank the Richard E. Widmer Research Fund, FIRST, and the Fred C. Gloeckner Foundation for their generous support of this research.
Table 1 shows what irradiance response group many bedding plants fit into. The next article will feature a similar table with many more bedding plants that we have studied. With irradiance indifferent plants, extra lighting did not affect leaf number below the flower. In contrast, with facultative irradiance group plants, extra lighting reduced the leaf number below the first flower on these plants, which reduced the time to flower dramatically.
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