Bahram,

High temperature is well known to reduce both grain number (from reduced
fertility) and grain size (reduced cell numbers per grain).  It is not
surprising to me that simple RUE models that have a final HI to compute
grain yield will fail to work.  I would expect them to fail under that
circumstance.  The question is whether you can improve on this by
modeling grain number, grain filling rate, and grain filling duration.
I can not help you on the issue of modeling grain number for whea (maybe
Tony Hunt can help there), but there are some good references that
document shorter grain filling duration and reduced grain size (and
maybe reduced filling rate as well).  See Spiertz.  1974.  Neth. J.
Agric. Sci. 22:207.  Also Sofield et al. 1974.  Royal Society of New
Zealand Bulletin 12:909.  There must also be more recent literature, but
that is what I had handy.

Good luck,
Ken Boote

Bahram Andarzian wrote:

>   Dear all my friends
>  Hi
>
>  I would be thanking in advance if anybody can help me.
>  I develop a model, which can simulate wheat photosynthesis; respiration, growth and grain yield in non-stressed condition (potential). In order to simulation of grain yield, harvest index insert in the model and model accurately predict grain yield in optimum condition but it can not accurately predict grain yield in late sowing date. Because at such a circumstance high temperature result in decreasing harvest index and grain yield. Then HI, which is genotype dependent variable, should be changed in this state.
>  Therefore I am going to model the effect of high temperatures on the number of grains, grain filling rate and grain filling duration.
>  Are there model or mathematical relations between temperature and grain yield components?
>  How can calculate the grain number as a fraction of spike dry weight before grain filling?
>
>  Bahram Andaryan
>  Ph.D student in crop physiology
>  Ahvaz University
>  Ahvaz-Iran
>
>
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