Derek,
I think Nataraj's comment is related to the concept of characterizing genetic potentials of cultivars for modeling purposes. Many other factors may modify the actual behavior of the cultivar during a simulation. Because you are simulating water stress in your sensitivity analysis the effect of soil water availability on growth and development events should be expected.

regards
Kofikuma

Derek McNamara <[log in to unmask]> wrote:
> 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
> [log in to unmask]