OrchidSafari ARCHIVES* Flower Induction in Orchids

OrchidSafari
Topic: Flower Induction in Orchids
Moderator: Marilyn Light
November 13, 2002

[The prechat handout appears at the end of this discussion]

MarilyninOttawa
Today, we will be discussing, by popular request, flower induction in orchids.

Plants use light to control many life processes including flowering but the induction of flower production in plants is incredibly complex and parts of the puzzle still remain a mystery.

Flower induction was once thought to be regulated via plant nutrition. When nitrogen levels were high compared to carbohydrate production, the plant remained vegetative but when the ratio was reversed, this led to flowering. This theory fell by the wayside with the discovery of photoperiodism, the process whereby plants controlled the onset of flowering according the length of day.

Actually, it was the length of uninterrupted dark period that was implicated and the sensing mechanism was found to be in the leaves. Plants can sense the day length through a complex pigment-based system effective in the red-far red parts of the light spectrum. The process is very sensitive and plants can be manipulated to bloom in time for specific events through manipulation of the day length.

Nowadays, the genetic approach has been used to elucidate the role of genes in flowering induction in non-orchids such as Arabidopsis. This is proving useful in unraveling the mysteries of the process but there is clearly much for us still to learn and understand.

The entire shoot-to-be is encapsulated in the shoot bud. Parts may not yet be fully constituted but they are formed months before they expand into mature parts. Here we see the tiny 8 mm long perennating bud of a terrestrial orchid, Epipactis helleborine. To locate the bud, follow the root up to the top. picture

The perennating bud is pink and located in the upper lefthand corner of the image.

We found in this species that one could predict the likelihood of blooming by measuring the length of the bud in August, a year before the next blooming season.

If you slice open a tulip bulb before fall planting, you will find the flower bud and leaf initials waiting to be released from their sleep. The same goes for a Cypripedium overwintering bud or even a shoot bud found at the base of a Cymbidium pseudobulb. Slice the bud vertically and you will find the structures that will later expand and grow when the bud begins to expand.

Here we see a vertical slice through the overwintering belowground bud of Epipactis helleborine. This 14 mm long bud contains an entire embryonic inflorescence yet this picture was taken a full year before the flowers might eventually open. In fact, the plant bearing this bud was still in flower! picture

What this means is that months before flowers even appear, the process of flower induction has already happened. If conditions have not been appropriate at that time, flowers will not be produced no matter what fertilizers, light or temperature treatment is provided later on. However, even if flower primordia have been produced, a grower misstep could cause the buds to cease development. The presence of flower buds in the shoot bud does not necessarily guarantee future blooms.

Ed_in_Sat
I certainly endorse this thinking!!!

MarilyninOttawa
As you know Ed, there can be suppression of expression in Phalaenopsis. High temperatures can bring about the suppression of flower bud formation in the inflorescences of susceptible species and hybrids

Ed_in_Sat
You've seen Gavino Rotor's early work in this area?

MarilyninOttawa
Not his work specifically, but from both personal experience with Phal. schilleriana, Phal. lueddemaniana and Phal amboinensis specis and hybrids grown in the West Indes and later here in Canada. What Yin-Tung Wang discussed during the ASHS 2000 symposium summarizes what I have observed.

MarilyninOttawa
There are several factors controlling flowering in orchids. These are plant nutrition, photoperiod and temperature. Not all mature orchids are affected similarly by these factors so we will generalize.

Age of plant - Plants are termed juvenile and adult. There can be a short transition between these stages as in Psygmorchis pusilla where flowering can happen within a year of seed germination, or there can be a very extended juvenile period as we see in some Cattleyas and Cypripediums where ten or more years before reaching blooming size is not uncommon.

Nutritional status - Plants have evolved mechanisms whereby they can control reproduction until they have sufficient resources to support flowering and fruiting. Nutrient concentration of particularly nitrogen and phosphorus in roots and pseudobulbs may have to be in a particular ratio in order to cause one or more hormone signals to initiate flower induction. Generally speaking, nitrogen concentration must be low in comparison to phosphate, which must be high. But, the relative supply of carbohydrate reserve may also be important in certain orchids such as Cymbidium.

Light and Photoperiod - For an excellent presentation of this subject, I suggest the following: button Not all orchids are sensitive to day length and this is irrespective of where there natural range may be. Day neutral species bloom year round without any clear flowering season but they may bloom in harmony with weather conditions and seasons. There are short day species such as Cattleya bowringiana which bloom as the day length shortens (the dark period lengthens). picture Other genera in this category include Oncidium, Miltonia, and some Paphiopedilums. Healthy, mature short day specimens will not flower if they are not provided with 12 hours of uninterrupted darkness over several weeks to months before the usual blooming season. A daylength signal is perceived by a receptor system found in the foliage. The signal is in the red-far red range. Moonlight will not affect short day plants but streetlight or even a short bout of flashlight every night can disrupt the sensing mechanism.

pecteilis_in_KY
Well! I though it had to do with the sugar starch balance.

John_in_Arcadia_CA
If the flashlight has a blue lens on it would it still affect the sensor?

MarilyninOttawa
There must be no red/far red light in the emitted spectrum.

art,simpsonville_sc
Does it vary from plant to plant? Why do some seedlings bloom early others bloom later?

MarilyninOttawa
Which genus are we discussing Art? Or orchids in general?

art,simpsonville_sc
Cattleyas primarily but I would think it would hold for most orchids

MarilyninOttawa
I will digress momentarily to answer Art as best I can. The question was - why do some seedling Cattleyas (hybrids?) bloom before others? There could be many reasons including the genetic makeup of each individual, especially if they are hybrids. Temperature could have an effect but it is likely also having to do with sensitivity to day length and individual nutritional status. We can go into this in more detail later.

Ed_in_Sat
I suspect distribution within the flask may have a great deal to do with this.

Orchidglade
There is always statistical randomness

art,simpsonville_sc
Are these tendencies typically passed on the hybrids or is it something you can breed for or against?

MarilyninOttawa
I suspect that there are inherited tendencies. One can certainly breed for vigor and can choose parents less sensitive to day length. These traits do pass into the hybrid population. One of my hybrids (C. Fruit Salad) bloomed in two groups several years apart. The earliest to bloom were splash petalled while the later were mostly concolor.

Ed_in_Sat
We have aurantiacas that bloom twice a year every year.

MarilyninOttawa
There are orchids such as C. aurantiaca, C. and C. skinneri that bloom on the lengthening days of spring. Here we see the magnificent clone Hettie Jacobs in flower in May. picture C. Fruit Salad is aurantiaca 'Mac' x C. Cherry Chip. Cherry Chip blooms 2-3 times a year while the aurantiaca blooms only in spring -but - I agree that one might encounter aurantiacas that could bloom on each new growth. Perhaps the plants you have grow continuously so as to have growths in bloom that often?

Ed_in_Sat
Yes, that seems generally true. Our aurantiacas are generally ever growing and bloom at least 2x per year. Two AM's have come from this group so quality is not affected by the blooming frequency.

MarilyninOttawa
It would be interesting to remake Fruit Salad with your line.

John_in_Arcadia_CA
In hybrids, will a plant with parents that normally bloom at different seasons cause the seedlings to bloom in 2 different time periods? I have a cross that seems to do that.

MarilyninOttawa
The traits could segregate that way

MarilyninOttawa
Temperature - Temperature, particularly at night, can influence flowering in many orchids. In Phalaenopsis schilleriana, too high a night temperature will override the flower induction process. picture

In Phal. schilleriana and similar heat-sensitive, spring-blooming phals, inflorescences may form but no flower buds will appear. Instead, vegetative offsets or keikis will be produced either at the tip of the spike or elsewhere in place of flowers. A night temperature below 20C (68F) is needed to ensure reliable blooming. We see similar keiki formation in Dendrobium which may also be related to mixed signals arising from both inappropriate temperature conditions and carbohydrate status.

MarilyninOttawa
Recently, investigators working with Cymbidium niveo-marginatum have looked at the various mechanisms controlling flower induction in this species. The goal is to see if we can speed up flowering such that plants could be propagated long before they would normally bloom for the first time. Where the usual interval is 4 to 7 years for this species, manipulation by various physical and chemical means induce early flowering in 90 days.

The investigators found that a combined treatment of hormones, restricted nitrogen supply with phosphate enrichment, and root removal forced the Cymbidium plantlets to change from vegetative growth to flowering.

To read more about this interesting work, look for this article in a local university library. Kostenyuk, I. et al. (1999) Induction of early flowering in Cymbidium niveo-marginatum Mak in vitro. Plant Cell Reports 19: 1-5.

For those of faced with reluctant bloomers, there are several possible options. If the plant is an adult, grow it well, ensuring that there are adequate reserves to support eventual flowers.

Manipulate nutrient levels in the months prior to expected blooming. Restrict nitrogen while increasing light and phosphate fertilization.

You will not want to prune roots but you can manipulate the system by restricting water supply. When roots are no longer actively growing, movement of plant reserves will be limited.

Ensure that you vary the daylength exposure over the year. 12 hours continual darkness in winter and 8 hours continual darkness in summer is a reasonable goal.

Assess the night temperature range in your growing area. Use a minimum/maximum recording thermometer. Some orchids will not flower unless the night temperature is below 68F (20C) or even lower for several weeks to months before expected flowering.

Control of flowering in orchids is a fine balance of many factors. Once you have found a winning formula for a particular plant, take careful note for future reference and share your findings with others..

That is the end to the formal presentation. The floor is open to discussion and questions.

Orchidglade
We got down to 68 last night for the first time since last spring. I can hardly wait to see what will be induced to bloom

MarilyninOttawa
To start the ball rolling: some comments this session had to do with the variability between
related individual plants. This can be a challenge, especially with hybrids. I have had Phals which have never flowered in the perennially hot West Indian climate, bloom within a couple of months of bringing them to Canada. Here it could be a problem with continued exposure to high temperatures. Summer blooming phals can be especially susceptible to high temperatures.

N_Calif_Kathy
Which ones are the summer blooming ones? Just to name a few... is that like venosa hybrids? Peopel get asked about temps and phal blooms all the time. I always thought it was the diurnal fluctuation more than a week or two's drop in temps. Especially where indoor growers are concerned. Am I wrong?

MarilyninOttawa
Summer-blooming phals include hybrids with Phal amboinensis and many yellow/art shades. These are not all summer-bloomers of course but they tend to be adversely affected by exposure to high temperature once a spike has been initiated.

John_in_Arcadia_CA
I have noticed that some plants bloom with no trouble and others rarely if ever bloom even when sitting next to each other

Steve_in_the_Adirondacks
I think we all have our favorite names for plants like that.

Carole
What are some examples of day neutral species?

pecteilis_in_KY
Day neutral plants will grow on the equator. Any equatorial orchid will be day neutral.

MarilyninOttawa
Equatorial plants are not necessarily day neutral plants. Any orchid species which blooms more or less year round irrespective of 8 hours, 12 hours or 16 hours of darkness could be termed day neutral. In my experience, Masd. infracta and Masd. naranjapatae are day neutral. Jacquiniella equitantifolia is another year round bloomer. Psychopsis papilio is as well.

Ed_in_Sat
Tom, I'm not sure that is categorically true. We find in Cattleya that dowiana and ilk will bloom in warm weather, gaskelliana blooms when it feels like it and as often as it likes, percivaliana blooms several times a year, warneri as well. Certainly Laelia flava is almost random as is pumila. In the brassos, both digbyana and glauca bloom frequently thru the year.

pecteilis_in_KY
Marilyn, what is the native condition of those plants? Ed in the country both digbyana and glauca are generally controlled by wet-dry seasons. So if you give it continual wet it will flower. If it doesn't rot. *G*

Ed_in_Sat
Put it in a basket and turn on the hose!

MarilyninOttawa
The same holds true for Catasetums. [re:wet dry seasons]

I hope that you all see that the flower induction process is complex. I first became fascinated with the blooming of Xmas Cactus. You could either chill it or dry it out or put it in a cupboard (shorten the day) and it bloomed. In the wild, these plants respond to the most available trigger but they likely evolved through other triggering situations and so respond equally to several.

Jade
No wonder my cactus blooms so well. It got all three triggers.

MarilyninOttawa
Does that help Carole? Orchids which bloom sucessively over extended periods are likely included in the day neutral category. These include Onc. graminifolium. We can disrupt the successive blooming pattern by neutralizing the suppressor of the flowering expression. The hybrid, Onc. Vera Arthurs (graminifolium x gheisbrectianum) flowers all at once with a magnificent display of over 60 blooms.

Someone asked about the native habitat of some of the day neutral species I mentioned. Masd. nidifica has a wide distribution from Central America through Ecuador. All ecotypes behave similarly in that they bloom year round despite day length variation. If there is no selection pressure favoring a particular behaviour like blooming in response to day length which ensures reproduction then there will likely be no response to day length variation.

Thank you to everyone for being such a curious group. Lots of good questions too! I will consider doing something on the subject of periodic dormancy in orchids for January.

Control of Flowering in Orchids

Marilyn H. S. Light

Plants use light to control many life processes. Blue light is implicated in the control of plant growth and red light in the control of flowering. But the induction of flower production in plants is incredibly complex and parts of the puzzle still remain a mystery. Flower induction was once thought to be regulated via plant nutrition. When nitrogen levels were high compared to carbohydrate production, the plant remained vegetative but when the ratio was reversed, this led to flowering. This theory fell by the wayside with the discovery of photoperiodism, the process whereby plants controlled the onset of flowering according the length of day. Actually, it was the length of uninterrupted dark period which was implicated and the sensing mechanism was found to be in the leaves. Plants can sense the day length through a complex pigment-based system effective in the red-far red parts of the light spectrum. The process is very sensitive and plants can be manipulated to bloom in time for specific events through manipulation of the day length. Nowadays, the genetic approach has been used to elucidate the role of genes in flowering induction in non-orchids such as Arabidopsis. This is proving useful in unravelling the mysteries of the process but there is clearly much for us still to learn and understand.

With orchids as with related monocotyledonous plants such as tulips and lilies, growth is initiated in special tissue (meristem) located at the base of the shoot bud. It is important to note that in monocots, unlike dicots, what forms in the shoot bud is what later expands to form the entire shoot including leaves, flowers and pseudobulb. The entire shoot-to-be is encapsulated in the shoot bud. Parts may not yet be fully constituted but they are formed months before they expand into mature parts. Thus, if you slice open a tulip bulb before fall planting, you will find the flower bud and leaf initials waiting to be released from their sleep. The same goes for a Cypripedium overwintering bud or even a shoot bud found at the base of a Cymbidium pseudobulb. Slice the bud vertically and you will find the structures that will later expand and grow when the bud begins to expand. What this means is that months before flowers even appear, the process of flower induction has already happened. If conditions have not been appropriate at that time, flowers will not be produced no matter what fertilizers, light or temperature treatment is provided. However, even if flower primordia have been produced, a growers misstep could cause them to cease development. The presence of flower buds in the shoot bud does not necessarily guarantee future blooms.

There are several factors controlling flowering in orchids. These are plant nutrition, photoperiod and temperature. Not all mature orchids are affected similarly by these factors so we will generalize.

Age of plant - Plants are termed juvenile and adult. There can be a short transition between these stages as in Psygmorchis pusilla where flowering can happen within a year of seed germination, or there can be a very extended juvenile period as we see in some Cattleyas where ten years before reaching blooming size is not uncommon.

Nutritional status - Plants have evolved mechanisms whereby they can control reproduction until they have sufficient resources to support flowering and fruiting. Nutrient concentration of particularly nitrogen and phosphorus in roots and pseudobulbs may have to be in a particular ratio in order to cause one or more hormone signals to initiate flower induction. Generally speaking, nitrogen concentration must be low in comparison to phosphate which must be high. But, the relative supply of carbohydrate reserve may also be important in certain orchids such as Cymbidium.

Photoperiod - Not all orchids are sensitive to day length. Such day neutral species bloom year round without any clear flowering season. There are short day species such as Cattleya bowringiana which bloom as the day length shortens (the dark period lengthens). Short day species will not flower if they are not provided with 12 hours of uninterrupted darkness over several weeks to months. A day length signal is perceived by a receptor system found in the foliage. The signal is in the red-far red range. Moonlight will not affect short day plants but street light or even a short bout of flashlight every night can disrupt the sensing mechanism. There are orchids such as C. aurantiaca and C. skinneri that bloom on the lengthening days of spring.

Temperature - Temperature, particularly at night, can influence flowering in many orchids. In Phalaenopsis schilleriana, too high a night temperature will over ride flowering. Inflorescences will form but no flower buds will appear. Instead, vegetative offsets or keikis will appear. We see such keiki formation in Dendrobium which may also be related to mixed signals arising from inappropriate temperature conditions.

Recently, investigators working with Cymbidium niveo-marginatum have looked at the various mechanisms controlling flower induction in this species. The goal is to see if we can speed up flowering such that plants could be propagated long before they would normally bloom for the first time. Where the usual interval is 4 to 7 years for this species, manipulation by various physical and chemical means induce early flowering in 90 days. The investigators found that a combined treatment of hormones, restricted nitrogen supply with phosphate enrichment, and root removal forced the Cymbidium plantlets to change from vegetative growth to flowering.
See Kostenyuk, I. et al. (1999) Induction of early flowering in Cymbidium niveo-marginatum Mak in vitro. Plant Cell Reports 19: 1-5.

For those of faced with reluctant bloomers, there are several possible options. If the plant is an adult, grow it well, ensuring that there are adequate reserves to support eventual flowers. Manipulate nutrient levels in the months prior to expected blooming. Restrict nitrogen while increasing light and phosphate fertilization. You will not want to prune roots but you can manipulate the system by restricting water supply. When roots are no longer actively growing, movement of plant reserves will be limited. Ensure that you vary the day length exposure over the year. Examine the night temperature range. Some orchids will not flower unless the night temperature is below 68F (20C) or even lower.

Control of flowering in orchids is a fine balance of many factors. Once you have found a winning formula for a particular plant, take careful note for future reference.

-end