Why do some plants bloom in summer (off topic)

ConroeJoe@aol.com ConroeJoe@aol.com
Fri, 27 Feb 2004 05:51:25 PST
It has been known for many years that some plants respond to day length to 
control certain activities such as flowering, dormancy, and general growth, etc. 
 The ability is called photoperiodism, and it is not limited to plants.  Many 
organisms take cues from the number of hours of daylight (or number of hours 
of darkness).  

In more recent years a number of proteins have been identified called 
photoreceptors; these molecules respond to different wavelengths of light and have 
been shown to effect some of the plant responses to light (e.g., shade avoidance 
and choloroplast movement within cells).  

One of the more obvious photoperiodic responses of plants is flowering.  How 
do plants know when to flower?  Many plants can grow in cool temperatures and 
even flower, but it would be folly to flower too early in the season—a late 
frost could destroy all of the flowers, which tend to be tender.  Thus, many 
plants have evolved a mechanism to delay flowering till days are longer, hence 
the season is more advanced and chances of a frost are reduced.  Of course, 
there are many other reasons plants bloom at a certain time, one major factor is 
the presence of a pollinator.  There are many plants that bloom only at the 
precise time their pollinating insect can be expected to be present.  In fact 
some insect-plant pollinator relationships are very fine tuned, and the flowers 
and insects are only active for a few weeks each year.  If they miss each other 
it might spell doom for both species; no seeds set for the plant and no 
pollen or nectar for the insects.  

Anyway, a molecular mechanism that controls flowering in Arabidopsis has been 
worked out in the past few years.  The mechanism turns out to be fairly 
simple.  Photoreceptors are activated by light, and when they are active they can 
prevent the degradation of a protein called CONSTANS.  In turn, CONSTANS can 
activate certain genes that promote flowering.  As with any fine tuned system 
there are opposing controls.  Other photoreceptors cause the degradation of 

So there is a balance between the gain of CONSTANS and the loss of CONSTANS. 
 Finally, it turns out that the two opposing mechanisms occur late in the day 
(stabilizing CONSTANS) or early in the day (degrading CONSTANS).  The effect 
is that levels of CONSTANS rise and fall each day.  However, the stabilizing 
effect is enhanced as days become longer and more CONSTANS is able to survive. 
 Finally, when the hours of light reach a critical value, enough CONSTANS 
accumulates to activate flowering genes.  

Plants are clever indeed.  From what we know about them it seems likely that 
CONSTANS is only one way plants regulate flowering by day length.  Probably we’
ll know  more in the next few years.  

LINK 1:  Photoreceptor Regulation of CONSTANS (Abstract)
LINK 2:  Science Update Article

LINK 3:  Photoreceptors in Arabidopsis


Joe , zone 9, Snowdrops emerged this week (Galanthus hybrid)

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