The following is a response from Dr. Ritchie to a question posted by Elvin
Roman Paoli about the CERES-Maize model.
 
Gerrit
 
 
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>Date: Mon, 23 Jun 1997 18:04:38 -0400
>To: Elvin Roman Paoli <[log in to unmask]>
>From: "Joe T. Ritchie" <[log in to unmask]>
>Subject: Re: kernel number function (GPP)
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>Dear Elvin:
>
>Gerrit asked me to reply to your question:
>
>>>We want to know what is the current function to determine kernel number
>in the CERES-Maize model. The original function is suppose to be
>>>
>>>        GPP=G2*(PSKER-195.0)/(1213.2+PSKERSP-195.0).
>>>
>>>We have a different vertion in our source code, which is:
>>>
>>>        G2*PSKER/7200.0 + 50.0
>>>
>>>Please, could somebody explain us what these two constants (7200, 50) mean?
>>>Is there any publication about this change?
>
>I must answer your question with great uncertainty about this MOST
>IMPORTANT subject.  We have published nothing on it.  In the CERES-Maize
>book you will see a reference to the first equation used from Edmeades and
>Daynard, 1979, Can. J. Plant Sci. 59:585-601.  Jim Kiniry and I developed
>that equation after much debate and with little uncertainty about it.  Both
>are based on logic and an assumption that the average photosynthesis rate
>during a stage when the cob and ear are growing in volume rapidly, will
>determine the number of grains that will fill.  There are always more
>primordia than will fill, so we make no effort to calculate the potential
>grain numbers.  As I recall, the first equation was giving us little
>response to variations in the calculated value of PSKER
>so it was changed to the linear equation.  The value of 7200 probably
>represents a high number for PSKER so that the number of kernels per plant
>comes out to near the input G2 value for high (clear weather) PSKER values
>and considerably lower for less clear conditions or cases where intercepted
>light interception is low (low LAI).  I think the 50 is a logical intercept
>for zero PSKER, based on the fact that there would be some kernels even if
>there was no photosynthesis because of stored assimilate for that purpose.
>These both are highly speculative and could possibly be improved upon.  Any
>suggestions?
>
>A debate Jim Kiniry and I had was whether the grain numbers should be
>sensitive to the total photosynthesis during the uncertain critical period
>of ear structure growth (my argument) or whether it was related to the
>average rate of photosynthesis (Jim's argument based on the Edmeades and
>Daynard argument).  I saw Greg Edmeades at CYMMIT recently and the two of
>us agreed that this problem would be a good topic for a symposium at the
>1998 ASA meetings.  I am going to bring it up at the crop modeling business
>meeting.
>
>You have asked an important question.  Thanks for letting me think about it.
>
>
>
>Joe T. Ritchie, Professor            (517) 353-4702 (voice)
>Homer Nowlin Chair                   (517) 355-0270 (FAX)
>MSU Crop and Soil Sciences
>A570 Plant and Soil Sciences Bldg.
>East Lansing, MI 48824-1325
>
>
>
>
 
 =======================================================================
Gerrit Hoogenboom
Associate Professor
Department of Biological and Agricultural Engineering
The University of Georgia
Griffin, Georgia 30223-1797, USA
 
Phone:  770-229-3438 (New !)
FAX:    770-228-7218
E-mail:  [log in to unmask]
 =======================================================================