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Re: HYB: daylength vs temperature & bloom initiation

From: "David G. Holm" <sherlock@amigo.net>

On 1 Apr 99, at 9:11, Linda Mann wrote:

> What's the interaction between daylength and temperature & effects on
> bloom initiation?

Linda - my comments will mostly come from my background working with
potatoes.  I do not claim to have an understanding of the physiological 
processes associated with growth and development in irises and how these 
processes are influenced by daylength (photoperiod), other environmental 
factors (temperature, stresses, etc.), and cultural management.  However I 
would suggest that similar processes are very likely going on in many 
different plant species.  The main difference between plant species is how 
they respond to the various stimuli and this is all governed by the genetic 
makeup of the species.

Where shall I start?  Changes in daylength "triggers"  various physiological
processes in the plant.  Other factors such as temperature and cultural
management factors (soil fertility, etc.) and stresses have modifying
influences.  All growth and development processes including bloom initiation
are affected by these factors.  In fact what happens to a plant one year can 
influence subsequent years growth and development.  Plants have good 
"memories".  Plants are like computers - they remember all of the 
environmental inputs and process that information.  The output is what we 
see in growth.
> I'd love to hear more about this - you mentioned
> initiator hormones as well as inhibitors - could there be a daylength
> 'motivator' in addition to the dormancy period or is it more likely to be
> varying dormancy (inhibitor?) requirements?   What's going on with these
> hormones while the plant is dormant?   What's it up to?  What does it have
> to finish with before it can start growing again?  Are there cultural
> things I can do to "trick" it into taking longer?  Are there temperature
> thresholds or is it a cumulative amount of warmth?

All growth results from a balance of growth promotor and inhibitor 
hormones.  These hormones are naturally occuring and always present in 
the plant.  Depending on this balance we either have plant growth or the 
lack of it (dormancy).  Environmental influences (including photoperiod) 
trigger the production of these hormones.

I am not aware of any cultural practices that you may employ to, for 
instance, overcome dormancy to allow rebloom on a normally none 
reblooming cultivar.  We can employ cultural practices to push the genetic 
potential of a cultivar.  For instance how we fertilize a plant may influence 
the length of the bloom period and/or the plant and flower quality.  
Reblooming cultivars apparently have the genetic programming that allows 
it to circumvent the "normal" growth cycle observed in non-reblooming 
cultivars.  This genetic programming affects the balance of growth promotor 
and inhibitor hormones.

In many plants bloom can be force by altering photoperiod or by applying 
plant growth substances.  These plant growth substances are synthesized. 
Unless research has been already conducted in these areas, one would 
have to conduct many controlled experiments over time to study it in depth 
before reliable recommendations could ever be made.  If these studies were 
undertaken we could perhaps trick non-reblooming cultivars to rebloom or 
to even extend the bloom season.  This is only speculation however.



Dave Holm
Professor of Horticulture (Potato Breeding)
Colorado State University

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