SIMED,a crop simulationmodel,as a tool
for teachingcrop physiology’
2D. A. Holt, G. E. Miles, R. J. Bula, M. M. Schreiber,
and R. M. Peart
ABSTRACT
Thetremendous
quantityof detailed information
on cropphysiologicalprocesses
posesa problem
for
agronomy
teachers.Presentingthis informationso
that students
canassimilate
andintegrateit effectively is a challenging
task. A cropsimulation
model
is a
usefultool for accomplishing
this objective.SIMED
(Simulationof MEDicago)
is a computer
sin~ulator
of dry matteraccumulation
in alfalfa (Medicago
sativa
L.). Majorphysiological
processes
of the alfalfa crop
systemare summarized
in a simple materials flow
diagram,
whichcanbe usedas a basisfor modeling
almostanycrop. In a beginning
course,the flow diagramcanbe usedto illustrate howa cropfixes, processes,translocates,
andrespirescarbon
andhowthis
leadsto dry matteraccumulation.
At an intermediate
level, factorsaffectingeachof thephysiological
rates
depictedin the diagram
canbe presented
at anydesired level of detail. Thediagram,enriched
withdetails of environmental
andfeedback
controls,helps
illustrate the interrelationships
between
physiological
processes
andshows
howthe individualprocesses
are
integratedinto wholeplant responses.
If computer
facilities are available,actualsimulations
canbe conducted,usingthe originalfunctionalrelationships
of
SIMED
or othersdevisedby instructorsor students.
Graduate
studentsmaybe askedto developtheir own
crop models,usingSIMEDas a pattern. Literature
search,thoughtfulanalysis,andpropermanipulation
of units are requiredto accomplish
this task. SIMED
canserve-as a conceptual
framework
to help students
learnandretain details. It makes
possiblemany
useful andinformativelaboratoryexercises.It provides
an easilyremembered
courseoutlineandis potentially usefulas a termprojectto replacesuchexercises
as
termpapers.
Additionalindexwords:Courseoutlines, Concept
development.
CROP physiology
is a very complex subject.
The relevant textbooks and professional journals are full of detailed information derived from
1journal Paper No. 6344, Indiana Agric. Exp. Stn.,
Lafayette, IN 47907.
2Associate professor of agronomy, Purdue Univ.; Assistant professor of agricultural engineering, ClemsonUniv.;
area director, ARS-USDA,
Lafayette, Ind.; research agronomist, ARS-USDA,
Purdue Univ.; and professor of agricultural engineering, Dep. of Agricultural Engineering, Purdue
Univ., respectively.
53
research on this subject. It is easy to bury students
with interesting but isolated facts about physiological processes in plants, thus frustrating any inclination they may have to try to integrate this detailed
plant physiology information into some systematic,
coherent scheme. We believe this frustration,
felt
by both students and faculty,
led to the recent
popularity of "whole plant physiology," a concept
promoted as being more useful and more palatable
to "crop physiologists"
than the basic physiology
which is the foundation of "plant physiology".
Whether "whole plant physiology" actually exists
or if, in fact, it disappears when scrutinized closely, is another question. Techniques do exist, however, for teaching crop physiology in a rigorous and
detailed manner and yet in such a way as to encourage and help the student to see relationships
and integrate his knowledge effectively.
The use of
SIMED(Simulation of MEDicago) or a similar crop
simulation model as a teaching tool is one such
technique.
SIMEDis basically a flow model, depicting the
flow of carbon compounds into, through, and out
of the alfalfa (Medicago sativa L.) crop, the conversion of nonstructual
carbohydrates
to more
permanent plant constituents such as cellulose and
protein, and the resulting accumulation of dry matter (Holt et al., 1975). As such, it deals with the
basic plant physiological
processes of photosynthesis, photorespiration, dark respiration, translocation, and growth, and the important environmental
and feedback mechanisms that control the rates of
these processes. While SIMEDis a model of alfalfa,
its basic schematic and many of its functional relationships would be appropriate for other crops. It
deals in some way with much of the subject matter
which would normally be covered in a crop physiology course or the physiology portion of some
other agronomy course.
In this article, we briefly describe SIMEDand its
use as a teaching tool. Webelieve that SIMEDis, as
models go, relatively
simple. It is much too complex, however, to describe all its features in this
article. Readers interested in a detailed account of
the philosophy, the methods of calculation,
the
54
JOURNAL
OF AGRONOMIC
EDUCATION
PHOTOSYNTHESIS
~" *-
LEAF
CARBOHYDRATE
~
,
LEAFcELL
MASS
STEMCELL
MASS
CARBOHYDRATE
/ ROOTTRANSLOCATION
RATE
Fig. 1--SIMEDis based on this diagramof material flow in the alfalfa
functional relationships, and the predictive ability
of SIMED
are referred to Holt et al. (1975).
There are other crop models with similarities to
SIMEDthat might also have a useful teaching function. The schematic representation of SIMEDhas
features in commonwith the basic plant model
ELCROS,developed by Brouwer and DeWit (1969)
and is similar in some ways to the model presented
by Thornley (1971).
A CROP MODEL
Our visualization of the alfalfa crop as a system
is represented by systems dynamics symbols in Fig.
1. The six rectangles represent categories of plant
constituents, namely leaf, stem, and root carbohydrates, and leaf, stem, and root "cell masses," which
include all constituents except nonstructural carbohydrates. Material exchange between compart-
crop.
ments is represented by solid lines with "valve-like"
controllers. Arrows at the ends of these lines (or
"pipes") indicate the possible direction of flow.
The "valves" labeled "translocation" control an
actual flow of materials in the plant system. Other
"valves" represent chemical change of one category
of constituents into another.
Figure 1 shows the carbon of CO2 entering the crop and
becoming nonstructural
leaf carbohydrates
via the process
of photosynthesis.
Leaf carbohydrates
may remain in the
leaf carbohydrate
pool, may become leaf cell mass, may be
lost by dark respiration
or photorespiration,
or may be
translocated
to the stem carbohydrate
pool. The carbohydrates of this pool can be used for stem cell growth, consumed by dark respiration,
translocated,
or remain in the
stem carbohydrate
pool. The carbohydrates
translocated
to
the roots may become root tissue,
may be lost by respiration,
or may remain in the root carbohydrate
pool. The
root carbohydrate
pool may also provide a source of structural material
and energy for leaf and stem development
during regrowth following winter or after harvest of the
aboveground dry matter.
HOLTET AL.:
SIMED IN TEACHING CROP PHYSIOLOGY
From Model to Simulation
Each "valve" in Fig. 1 represents a physiological process. The rate of each process depends on
physiological
conditions within the plant and on
the physical
environment.
The summation and
integration over time of these rates for a particular
compartment is the net growth of that plant component, expressed as dry matter accumulation or
loss. If all compartments are considered simultaneously and continuously, the result is a simulation
of alfalfa growth.
Each of the physiological rates in SIMEDis defined as a maximum possible rate multipled by a
series of factors.
The maximum possible rate is
expressed in relative terms, i.e., grams of dry matter
produced, translocated,
converted or lost per gram
of leaf, stem or root "cell mass" during a period of
time. This maximum rate is multiplied
by the
grams of leaf, stem, or root "cell mass" per square
meter of land surface so that rates are ultimately
expressed as quantities
of dry matter produced,
translocated,
converted, or lost per time unit per
unit area of land on which the crop is growing.
A given physiological
process proceeds at the
maximumrate only when all controlling factors are
at optimum levels for that particular process. When
any controlling factor is at sub or supra-optimal
levels, the rate of the process is reduced. This situation is depicted in SIMED by multiplying
each
maximumrate by a series of factors, each representing our estimate of the effect of some environmental or physiological variable on the rate of the process. The rate of photosynthesis,
for example, is
computed as follows:
Rate of Photosynthesis = (Maximum
rate/unit leaf weight)
X(leaf weight]unitland area) X (radiation factor) X (leaf
maturity factor) X (leaf area factor) X (leaf carbohydrate
factor) X (leaf water potential factor).
Whena controlling variable, such as temperature
or carbohydrate concentration,
is at an optimal
level for a given physiological process, the associated factor is assigned a value 1.0. At other levels,
the factor is assigned a value between 0 and 1.0,
representing our estimate of the independent effect
of that variable on the rate of the process. An example is shown in Fig. 2, which depicts the relationship between leaf water potential and rate of photosynthesis in SIMED. Simulated photosynthesis
is
not limited by leaf water potential until leaf water
potential
falls below -20 bars. Below -25 bars,
simulated photosynthetic
rate is 0.0 because the
leaf water potential factor is 0.0. The details of
rate calculations
in SIMEDare explained by Holt
et al. (1975).
55
The Crop Simulation Model as a Conceptual
Framework
The basic SIMEDdiagram (Fig. 1), while devised
to represent symbolically the vegetative development of the alfalfa crop, is applicable in its simple
form to any field crop. Additional compartments
and rates can be added to depict the development
of reproductive organs. The schematic itself,
if
properly introduced, should be intelligible to undergraduate students and even perceptive high school
students.
The diagram, with perhaps some elaboration
on
the physiological processes shown, should serve to
introduce basic concepts of crop physiology in a
beginning agronomy course. Adding more detail
about the factors affecting the physiological rates
in the model and perhaps computing rates under
various conditions would be appropriate in an advanced undergraduate crop physiology or crop production course. In a dual level course, an instructor might go into considerable detail on the environmental and feedback mechanisms controlling
physiological rates. The instructor could describe
the difference between crops in the rate calculations and functional
relations
involved in the
model, and conduct some simulations
for demonstration.
In an advanced graduate level course, the students can be asked to modify the schematic, rate
calculations and]or functional relationships to fit
specific crops or to explore genetic potentials,
to
revise particular functions towards more detail or
better prediction, and to conduct simulations under
various sequences of conditions.
Thus SIMEDcan be useful at any level of plant
science teaching if the teacher will adjust the rigor
with which the rate processes and functional relationships are presented to the level of background
and motivation of the students.
O
~
~
~1~.4
I
I
I -I-~
......
~
water potential factor usedto computethe rate of
photosynthesis
in SlMED.
56
JOURNAL OF AGRONOMIC EDUCATION
SIMED as a Laboratory Exercise
SIMED or a similar model may be used as a
laboratory exercise if the instructor and/or students have access to a computer. Ideally, the simulation program and weather files would be on disk
storage at the computer, and simulations could be
initiated from a terminal located in the teaching
laboratory. Demonstrations and experiments can
be run in this manner. By perusing the output, the
student can literally "see" the plant grow and can
observe the different responses induced by different environmental conditions or different assumptions about the rates of the physiological processes.
Our present simulations are conducted in batch
process, but the computer could also be programmed to operate interactively, i.e., to query the
operator (instructor or student) concerning initial
conditions.
Again, the student need not understand all the
details of the model and its operation to use it and
to learn from it. Certain basic notions can be
demonstrated effectively by carefully planned simulations. For example, the different relationships
between temperature and photosynthesis in C3 and
C4 plants can be demonstrated by running parallel
simulations, altering only the functional relationship between temperature and rate of photosynthesis. At a more advanced level, the standard
growth analysis parameters can be calculated for
different simulation runs to demonstrate or investigate the effects of different environments on such
values as net assimulation rate (NAR), relative
growth rate (RGR), etc. Obviously, the number of
experiments or demonstrations that can be conducted is limited only by the imagination of the
instructor.
SIMED as a Course Outline
The SIMED schematic is essentially the analysis
of a crop as a system, depicting the organizational
structure of the system. It can serve as the basis
for course organization.
An instructor might
choose to follow the path of carbon through the
crop, and he might discuss the various physiological
processes involved as they are encountered. Alternatively, the teacher can discuss physiological
rate processes in any order, in each case presenting
the major controlling factors and showing how the
processes may differ in different organs. A morphological approach would be used to discuss all
the physiological processes as they occur in one
organ, say leaves, and then proceed to stems and
roots. This would be particularly appropriate in a
course where structure-function relationships are
emphasized.
Presumably the SIMED schematic is easier to
remember than an outline of printed words, so the
student should be better able to keep the course
organization in mind and to orient himself to the
subject matter.
A Simulation Model as a Term Project
A simulation model is a powerful tool for organizing information. Advanced students can use a
simulation model very effectively for this purpose
when they are required to develop a model for a
term project. Before the simulation approach was
employed in the senior author's graduate level
course in environmental physiology of crops, students were required to write a term paper on the
environmental physiology of a crop of their choice.
They searched the literature for information on
how that particular crop responded physiologically
and morphologically to various environmental factors. While most students made a reasonable effort
to organize the information, it was often transferred
from the journal to the term paper with relatively
little evaluation or integration. When the students
were required to seek the same information but to
organize it into the form of a crop simulation model
with which simulations could be conducted, the information had to be carefully evaluated, dealt with
quantitatively, and logically integrated into the
model. The students found this a much more
interesting, informative, and challenging project.
In our situation the students succeeded in producing adequate, even innovative, models. We were
able to conduct only limited calibration simulations
because of the difficulty in getting these models
computerized. A critical need is a very simple
technique for computerizing flow models of the
SIMED type. The CROPS simulation language
(Miles et al., 1976) is an important step in this
direction.