Hello,
I was just wondering about your comment that light and temperature are the
only factors influencing the timing of physiological events.  I have been
doing a sensitivity analysis of CERES-Wheats soil inputs and it appears
that both the PAW limits and initial soil water content have large
influences on the anthesis date as well as maturity.  Am I missing
something or doing something wrong?  Just wondering.

Derek McNamara
At 01:55 PM 9/10/02 +0200, you wrote:
>Nataraj
>
>I share your doubts on the concept of thermal time.
>It looks "ugly" from a scientific point of view,
>because it has no physiological basis.
>
>Yet, the problem is, it seems to work ! And this has been
>proven in numerous studies. In our case, we tested the
>development routine of CERES-Wheat under a wide range
>of environmental conditions in Northern Germany
>and found close matches between measured and modelled
>data.
>
>Nevertheless, despite its wide application, there is
>still confusion, what actually determines the rate of
>development (in case of cereals the plastochron or
>the phyllochron ?).
>
>Other than temperature, a second environmental factor
>influences development - light. You are thus absolutely
>wright in pointing to the fact, that both factors (temperature
>and light) have an interactive influence on plant development.
>
>Light is not a simple stimulus, because it can vary and have
>influence in several ways, including its spectral distribution,
>quantity, direction, duration and periodicity. Add the fact,
>that different plant organs respond differently to these
>stimuli.
>
>Other than temperature and light, there does not appear to be
>another decisive factor influencing the timing of physiological
>events. Doubling of air CO2- concentrations, for example, does
>not influence phasic development or the rate of leaf appearance
>in wheat.
>
>Much work needs to be done, until we are actually able
>to formulate mathematical equations, which provide
>accurate representations of the real system.
>
>Until then, we need to stick to the "old" concept,
>which seems to work fine under practical conditions,
>despite its scientific "ugliness" (which poses the
>question, whether we should waste our time in finding
>deeper scientific meanings of the concept).
>
>In the case of CERES, four out of seven genetic
>coefficients are dealing with the influences
>of light and temperature on development
>(i.e. P1V, P1D, P5 and PHINT). If you work with
>this model, I would recommend to study the manual
>and literature on the topic. Proper genetic
>parameterization is important to achieve good
>results with the model.
>
>By the way, the concept of thermal time is
>really very old : It was first mentioned in
>1735.
>
>
>Cheers
>
>Matthias
>
>--
>Matthias Langensiepen, PhD
>Hannover , Germany
>
>Internet : www.langensiepen.net
>Mailbox : [log in to unmask]

Derek McNamara
Graduate Research Assistant
Mountain Research Center, Montana State University
P.O. Box 173490, 106 AJM Johnson Hall
Bozeman, MT 59717-3490, USA
406-994-5073
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