OrchidSafari
Periodic and Seasonal Dormancy
Moderator: Marilyn Light
January 8, 2003

MarilyninOttawa
Many plants have evolved ways to avoid the stress of seasonal cold or drought. They become dormant. Some plants have also developed the ability to avoid depletion of reserves caused by copious fruiting or seed production. They either become dormant or cease flowering for one or more seasons.

What signals induce the dormant state and which control the length of dormancy and the eventual reversion back to active growth? Temperature, light (photoperiod) and level of hydration are all potential external signals. Controlling the change of state are gene expression, hormone levels, intercellular communication.

We know that various enzyme systems are sensitive to temperature shifts. Hormones are also known to influence dormancy, especially abscisic acid which is often involved in its onset. But the processes leading up to the onset of dormancy and to the release from the quiescent state remain elusive. There is likely a multi-gene mechanism involved.

While it is easy to determine that a terrestrial and/or deciduous orchid specimen is dormant by simply noting their absence above ground or the dropping of foliage, it is not so simple to establish the existence of the resting state in epiphytes.

We may not always know what signs should be looked for. Certainly, dormancy is a state to be respected if we are to succeed in growing an orchid. A misstep can prove fatal for an overly sensitive specimen so this is something that we should learn.

So how do we determine dormancy, how long it should last, what triggers its onset, and what signals its completion?

Orchids have evolved mechanisms to sense their environment, to exploit a favorable situation or to sidestep an extended unfavorable period. Those that did not develop such mechanisms are likely no longer with us, having succumbed to whatever stress should have been avoided. Our challenge as growers extends to knowing about where a particular orchid came from and not simply how it may behave.

Possibly, the simplest way for plants to avoid unexpected and excessive water loss through transpiration is to shed transpiring organs (leaves and flowers) before the stress becomes too taxing. You see this with overheated pleurothallids for example.

For regular seasonal droughts, there may be a signal monitored by the plant such as day length or minimum temperature which is used to set in motion a cascade of events culminating in leaf drop. Masdevallia lychniphora can be seasonally deciduous which is unusual for that genus.

With Pleione, plants of appropriate age will initiate leaf drop when exposed to near freezing temperatures. Leaf drop likely coincides with a physiological change of state from active to slowed metabolism such as would be expected with cooler weather.

Without the necessary cold exposure, the plants cannot become fully dormant and are likely to lose vigor and eventually die. Here, leaf drop signals that we have exposed the plants to the appropriate cold trigger. Without the visual signal, it would be difficult for a grower to be certain that a plant had entered dormancy.

Ed_in_Sat
We lost an FCC Vanda luzonica. After extended 40's exposure, it just stopped everything. We could never start it again. It stayed in perfect condition for 5 months, then suddenly fell apart.

MarilyninOttowa
With Habenaria carnea, too much cold will not only cause leaf drop but also the death of the plant. Dormancy generally coincides with a dry period during warm weather. Intimate knowledge of the plant and its natural behavior is needed if the grower is to provide appropriate signals to the plant that the time has come to enter dormancy.

With such orchids, it is often best to grow the plant as well as possible when it is actively growing, then observe and note the time course of events leading up to seasonal dormancy. Gradually reducing water and completely eliminating fertilizer is one good way to ease the plant into the desired dormant state.

In order to verify that a dormant organ is present, one has to examine the resting organs such as rhizomes or tubers. The presence of fat tubers together with leaf fading is a signal that the correct conditions have been achieved.

With epiphytes, it is more difficult to determine the state of old auxiliary buds along a rhizome. The only way to verify that such buds are merely dormant is to divide the rhizome such that the buds are released from any influence of the primary growing point.

Once residual hormone levels have been depleted, then buds can begin internal development and break dormancy. You might be surprised just how many possible growing points were lying in wait for an opportunity to grow. Often, there are complex biological clocks managing the growth and development of plants.

This may be observed even in flask where seedling orchids such as Grass Pinks (Calopogon sp), Diuris sp, and Catasetum lose leaves and rest for a time. Even seeds germinate to an internal clock. This is especially true of seasonally dormant species. Replating seedlings entering dormancy is risky. A preferred strategy is to re-plate or de-flask just before seedlings resume growth.

Letting the plant signal when it is ready to resume growth may be the best approach. The period of dormancy could be a few weeks to as much as six months. The period has likely evolved in harmony with the typical habitat of a particular species. In the tragic loss of the V. luzonica, it is quite possible that the re-start mechanism was damaged such that the plant could not resume physiological activity.

Grass Pinks may be exposed to as little as a few weeks to as much as six months of unfavorable winter conditions depending on the provenance yet seedlings in flask break dormancy after about two months. Continued unfavorable conditions in nature likely suppress premature development but a minimum rest is assured.

During the dormant period, all is not necessarily quiescent. There may not be apparent growth in terms of new leaves but internally, within developing buds, things are slowly but surely being readied for the next growing season. Many physiological activities precede flower development and leaf formation.

Roots are often the first organ to grow. With temperate terrestrial orchids, roots begin to extend just as the plants are losing their old leaves. The soil remains comfortably warm for plant roots long after the first frost. Water is often plentiful and roots take advantage of the favorable environment.

Picture Here we see a developing bud of Epipactis helleborine with developing roots. You can see the root bud emerging at the base of the white bud and to the left. It looks like a pearl.

Within the dormant buds, flower primordia remain relatively undifferentiated until about a month before blooming. Picture Here we see the floral primordia of Epipactis helleborine some 10 months before blooming. A lot can happen between now and then. All or none of these flowers may develop further and it depends upon watering (rainfall) in August and September summed the year before!

There is a rapid expansion of flower structures in the order of sepals, petals, style and stamens. If water is lacking then flower primordia may abort further development or some structures may fail to develop properly.

Short-term plant survival generally takes precedence over flowering in long-lived perennial orchids.

John_in_Arcadia_CA
Is that genetically determined or just the rule of nature?

MarilyninOttawa
To become long-lived, a species must have survived previous life challenges by way of one or several mechanisms. These would be genetic, either as nuclear or plastid mutations which permitted a plant to survive when others without that favorable mutation could not.

Flowering and fruiting can be a great burden on a plant even if it is well grown. Because of about a two-year lag between flowering and dormant response, it may be difficult to relate sudden plant inactivity or dormancy to a previous event.

Heavy blooming even without fruit set puts a considerable demand on a plant both in terms of water and sugar reserves. Heavy blooming plants need well developed, healthy root systems and abundant foliage if they are to maintain production.

Orchid fruit production, especially where fruits are carried for many months, can severely tax a plant, even cause its death.

More often, the exhausted plant either produces smaller growths (few to no flowers), many small growths (few to no flowers), or misses a growing season or two altogether. Flowers produced may or may not be fertile.

In the case of terrestrial orchids, a stressed plant may simply does not appear for one or more seasons. Beneath the soil, roots are produced annually but the shoot buds fail to extend above ground.

To conserve breeding plants, keep them growing vigorously and keep records of all breeding occasions. Limit fruit set although pollen can be used generously without concern. Watch for delayed negative response to fruit production. Limit breeding with plants that suffer a setback from carrying fruits.

I have been giving you a very brief overview of an extremely complex subject. The experts agree that there is not yet a clear answer to the mechanisms of dormancy. What we need to bear in mind is the consequence of dormancy in terms of keeping species in our collections healthy and productive.

N_Calif_Kathy
Many of the plants you mention are terrestrials, however I take it epiphytes are somewhat similar in starting dormancy (rest) and breaking dormancy(???)

MarilyninOttawa
My research experience is mostly with terrestrials but I have dealt with periodic dormancy, seed dormancy, the physiological impact of climate on dormancy, etc. My experience with epiphytes is more limited but apart from Crassulacean Acid Metabolism which is almost exclusively the domain of tropical epiphytes, I feel that there is a lot of dormancy issue in common with temperate and tropical orchids.

Entering and breaking dormancy in epiphytes is multi-fold. We have apical dominance where the hormonal influence of a lead growth keeps dormant back buds dormant.

We have what is called the annual rest ... often after blooming. This type of dormancy is a little (or a lot more complex). We find this often with orchids coming from regions of seasonal climate - colder, drier, wetter, hotter. Orchids coming from moderate regions such as the lowland tropics are usually free of the demands of seasonal dormancy.

Susan-from-Oregon
So if a plant is slow to come out of dormancy, I should take an axe to it?

MarilyninOttawa
Depends what the plant is, Susan. If the plant's internal clock has run down, ie, it is capable of exiting dormancy, then a gentle axe or warmth or higher humidity or brighter light may do the trick. But the plant must be ready to break dormancy. The challenge is to know exactly what influences the clock to keep ticking. Exposure to a critical temperature (ie no higher than x) may be necessary for the system to function.

The rub is that we often have only one plant of a kind and read about what to do from another's experience with one plant of that kind but these individuals, although they share the same name, may have come from completely different habitats. They may be different ecotypes. And then there are hybrids...

The one benefit of hybridization with, say, nobile Dendrobiums, is that many of the more challenging traits have been cancelled out through selective breeding. Astute breeders select for ease of blooming which is a selection for a less rigorous resting state.

Jade
That is a welcome change. The nobile definitely have their own agenda.

N_Calif_Kathy
Plants sense their environment via changes in temperatures, which affects the biochemical reaction rates and cause 'stuff; to happen, like leaf drop etc? It isn't 'seasonal' in terms of light? Moon phases (kidding I know that's disproven)

MarilyninOttawa
Daylength can also play a role, certainly, but it seems that plants can respond to several triggers, not necessarily only one.

pecteilis_in_KY
B. digbyana to heat and dryness. Nobile to light and cold. Cyms to day length and cold.

Susan-from-Oregon
I have a Lycaste that's been more or less dormant for 5 years. The most growing it did was when it spent one hot summer in a trash can in my garage. It tries to make a weak growth every few years. It sometimes succeeds in making a new pseudobulb. Each bulb far smaller than the last. I think when it finally dies. It will have died from wrongful dormancy.

Jade
I have a couple like that Susan, also a catasetum. I guess I should stick to phrags and the occasional giant catt. species.

Michael.Exler
Sounds like the trash can method worked for a while?

MarilyninOttawa
There could bve a problem with getting the plant into dormancy OR the plant could be otherwise unhealthy. Which species is it?

Susan-from-Oregon
Lyc. cruenta. Its not healthy. No roots at all.

MarilyninOttawa
Susan, Lyc. cruenta should not give you such problems. I suggest you start over with a healthier plant.

This is not a story about an orchid but a Christmas Cactus. I remember being puzzled by my grandmother's ability to flower her plant every year merely by placing it in the den where it was bright and cool. Others put the plants in a dark cupboard, or dried them out, or chilled the dickens out of them, all with the same result...flowers. So this plant responds to 3 different triggers, day length, temperature and dryness. Through its evolution, the plant has developed mechanisms to respond to any of these signals and so, nowadays, any signal will do. I tell you this because it could be the same with some orchids.

Jade
Often those put in cupboards didn't get much water as well. Dry periods send lots of plants into dormancy.

MarilyninOttawa
Take a look at the Rhyncolaelia digbyana and it cries, hot and dry for at least some of its growing period. Hard, waxy foliage is a good indication of expected expose to very bright light (perhaps when trees are deciduous).

Randy,_MI
I've noticed more problems getting divisions of old-old cactuses to bloom than the newer hybrids, which are not nearly so finicky.

MarilyninOttawa
Some of the newer ones are Zygocactus rather than the old-time Schlumbergera. Maybe this is the reason.

The end.