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
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