IN VITRO PROPAGATION

 OF PLEIONE ORCHIDS FROM SEED 

Introduction

In horticulture Pleione orchids are most commonly propagated asexually through annual pseudobulb division and bulbils. These methods produce exact clones of the parent plant and, possibly together with tissue culture, are the only methods for propagating named cultivars. Sexual propagation via seed is a viable alternative for large scale production of Pleione species and the only method for breeding new hybrids and cultivars.

In nature Pleione seed, like that of most orchids, need to form a symbiotic relationship with certain fungi to germinate and/or to survive after germination. The fungi provide the nutrients required for growth and development. We can provide these artificially using sterile in vitro (in glass) techniques  by sowing and incubating the disinfected seed on gel media containing all the required nutrients in the correct forms and balanced concentrations.  This also provides an opportunity  to incubate using optimized light and  temperature conditions.

Pleiones are among the easiest orchids to grow from seed because the seed coats are insubstantial so that, unlike many ground orchids, very little inhibition to germination is present in fresh Pleione seed. Also there is no requirement for dark and/or cold periods to initiate activity.

Pollination and production of seed is very easy. The in vitro germination and subsequent stages to full size flowering plants is more taxing and takes several years but is very satisfying. No laboratory experience or scientific knowledge is necessary although both are undoubtedly useful.

Pollination and seed production

First decide on the parent plant or plants. If you want to propagate a species, P. forrestii for example, then you could self pollinate using just one flower or take pollen from one flower to pollinate another. The former may be desirable if the particular flower has special characteristics. The latter may be preferable for preserving genetic diversity as long as the two flowers are not genetic clones. If you wish to cross two different species or hybrids then you must decide which is the pollen donor (male) and which is the pollen receiver (female). This must be carefully recorded and the female plant duly labeled.

The flower should not be pollinated until 5 days after fully opening but then as soon as possible. Too early and the pollen will not stick. Too late and the pollen may have degraded or the stigma may no longer be receptive.

The following series of numbered photographs illustrates Pleione pollination as a step by step process:

1.  A Pleione Hekla flower has been cut open to show the main reproductive structure called the column.

2.   A column has been removed from a flower so the procedure can be better demonstrated. In practice the following steps are performed in situ.

3.  The white pollen cap is shown at the tip of the column.

4.  Use a thin pointed (but not sharp)implement such as a tooth pick or slightly blunted cocktail stick to gently lever the pollen cap from the column.

5.  Pick the pollinia from the cap removed from the column in photo 4. There should be 4 pollinia arranged as two joined pairs. The point where each pair are joined is very sticky and will attach to the point of the pick when touched.

6.  A flap on the underside of the column separates the pollen cap area from the rest of the column. The area just behind the flap is also very sticky. The pollinia pair should be raised with the pick to this area where it should stick. Detach and withdraw the pick by scraping it against the flap. The photograph shows the pollinia successfully attached to the sticky area on the underside of the column immediately behind the flap.

7.  If pollination has been successful the column will drop and close with the lower part of the flower within a day or two. In nature this would protect the fertilized area of the column from further interference. The flower will then wither much faster than if it was left unpollinated.

8.  Over the next few weeks the ovary behind the flower will elongate and swell forming the seed pod. The pod will not be ready to harvest for several months as the seeds mature very slowly inside.

The seed pod should be harvested when it turns distinctly yellow or brown or shows any sign of splitting open and placed in a paper bag somewhere cool and dry to equilibrate for a few days.

When the pod is hard and brittle carefully break it open to release the seeds onto a clean white sheet of paper. Remove any bits of pod debris.  Store clean seed in small labeled paper envelopes in the refrigerator (not freezer). For longer term storage it is best to further wrap the air dry seed in aluminium foil to prevent desiccation.

For best results sow seed as soon as possible after harvesting. The difference in germination rates and subsequent vigor between fresh seed and seed stored for several months can be dramatic. This may be because Pleione seed coats are very thin resulting in desiccation. Also it is well known that seed drying and aging can initiate dormancy and germination inhibitors.

Growing from seed part 2 (Flasking) - Click here

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