I am studying the flower initiation and the genetic reasons behind it in orchids.I have noticed certain dendrobes like dendrobium aphyllum specially leaves are totally gone when flowers bloom. What can be the possible reason? How can I study the gene expression patterns controling such activities?
Hi Paromik,
I have a large orchid collection and grow several of the deciduous species in genus Dendrobium section Dendrobium, including Den. aphyllum. My guess is that the genetic basis for their deciduous character is the same as that for other deciduous plants (i.e stress cues such as lack of moisture, change in temperature, light levels and day length will induce senescence of the leaves and then leaf drop via their abscission layer, which is clearly defined in Dendrobium leaves). BTW, I'm a structural biologist and not a plant scientist, but a quick search showed me that your question doesn't seem to have been clearly answered for even well-studied plant species (but don't take my word for this, I'm not an expert in this field and I did not carry out an in-depth search for papers on the subject).
I guess that you would need to collect leaf samples during different seasons and study their mRNA and microRNA pools that might indicate changes in gene expression.
Here's what I know from growing these plants for many years. The deciduous species in this section live in areas with clearly defined seasons and they shed their leaves during the dry season, which coincides with winter in the northern hemisphere. A combination of environmental conditions results in the loss of leaves:
- in winter they don't receive rain for 2-3 months (they store water in their succulent stems to survive this period and reduce water loss by shedding all their leaves),
- the temperature drops considerably in winter (sometimes to near freezing at night) compared to the height of the growth season (30-40 degrees C at noon in summer),
- the species in this section of the genus Dendronbium come from subtropical rather than equatorial regions and therefore are still subject to changes in daylength (a well-known cue for senescence or flowering in many plants),
- yet they actually receive more light in winter than in summer (even though they days are shorter) because the trees that they grow on also shed their leaves in winter, so they get full sun in winter and shade from the surrounding canopy for the rest of the year.
The most important cue for loosing their leaves seems to be the lack of rain. Even if you provide all other cues, they will not drop their leaves as long as they have enough moisture. Flowering is only triggered by a combination of all of the above factors followed by an increase in day length and temperature in spring. Lack of any of the aforementioned factors during the rest period results in very poor or no flowering. In particular, excess moisture that inhibits leaf loss during the dry season will result in lush growth in spring and in the production of keikis (daughter plantlets) instead of flowers from the nodes where the flowers should emerge. Even a plant that has been provided with all the right environmental cues during the dry season and lost all its leaves will not flower properly if it receives too much water in early spring just before the flowers start forming. This seems to be a survival mechanism whereby these plants prefer to use the extra moisture to grow into larger plants instead of for producing flowers. I assume that the reason for this is that larger plants stand a better chance of surviving a really harsh dry season in the future.
I hope this helps.
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