- HIKING TRAILS AND WILDFLOWERS by Keith and Barbro McCree -
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Institute of Life Science, Texas A & M University, College Station, Texas 77843, USA
Abstract
The dependence of rate of respiration on rate of photosynthesis, dry weight and time, has been studied under controlled conditions, with the object of deriving equations for use in computer simulations of the net production rates of crops. Under the particular conditions studied, the following equation was found to hold: R = 0.25P + 0.015W, where R and P are the rates of respiration and gross photosynthesis of the whole plant, in g(CO2) day-1 m-2 (ground), and W is the dry weight of living material on the plant, in g (CO2 equivalents) m-2 (ground). Changes in R lagged several hours behind changes in P. Over periods of more than one day, changes in P, R and W were very dependent on the rates of death and regrowth of the various organs of the plants. These factors arc often neglected in computer simulations of crop production rates.
Introduction
In this paper I shall be considering respiration as one or the parameters used in computer simulations of whole crop response. Anyone who attempts to predict whole crop response is going to have to make some simplifications. The simplifications may be less drastic than they used to be, because computers are larger than they used to be, but they will still be simplifications. It is the duty of physiologists and biochemists to lead us in the direction of more realistic simplifications.
To the physiologist or biochemist, respiration is a process by which materials a, b and c are converted into materials x, y and z. To the systems analyst, it is a leak in a 'black box'. It is time we attempted to reconcile these two views. If we are to make any progress, the systems analyst will have to begin thinking of respiration as an essential process which happens to be accompanied by the evolution of CO2, while the physiologist will have to try to integrate his knowledge of mechanisms into generalizations about whole plant behaviour. Generalizations are, after all, the essence or the scientific method.
I shall present the results of some 'black box' type experiments made on real plants growing under controlled conditions, with the intention of stimulating discussion from both groups.
The background to these experiments is as follows. It has been conclusively demonstrated (McCree & Troughton, 1966a, b; Ludwig et al., 1965; King & Evans, 1967) that since the rate of respiration of a plant is not proportional to its size, the curve relating the net rate of photosynthesis of a crop to the total leaf area does not show the optimum which is predicted by many early models of crop photosynthesis. In our own early experiments with small clover plants (McCree & Troughton, l966a), the rate of respiration became proportional to the rate of photosynthesis, when this was varied by changing the light level. When the plant size was allowed to increase, at a constant light level (McCree & Troughton, 1966b), the rate of respiration increased with the rate of photosynthesis, but there was also a size effect. In the experiments described here, size and light level have been varied systematically in an attempt to clarify the interaction of size, photosynthetic rate and rate of respiration in this clover crop.
Materials and methods
In all of our experiments, New Zealand white clover (Trifolium repens L.) was grown at 20°C, 85% R.H., 320 ppm CO2, a modified Hoagland solution in Perlite, a 12-hour day to keep the plants in the vegetative state, and l00 W m-2 of photosynthetically active radiation (400-700 nm) from 400 W color-corrected mercury reflector lamps. Each plant was confined within a cylindrical wire frame, to make it grow in imitation of a section through a stand of clover. Leaf area indices of up to 10, dry weight densities of up to 1.4 kg m-2, and net production rates of up to 35 g(d.w.) day -1 m-2were attained.
At various points during the growth cycle, the CO2 exchange rates of sample plant were measured by enclosing the whole plant (including roots) in a transparent chamber in which the conditions were identical with those under which the plant had been growing, except that the irradiance was varied. The net rate of uptake of CO2 in the light and the rate of evolution of CO2 during the night were continuously monitored. The dark evolution rate during the photoperiod was also determined, by switching off the light for half an hour at 3-hourly intervals. The results are expressed in terms or 3 variables: the dark evolution rate, called R, the gross rate of uptake during the photoperiod (that is, the total light response of the plant), called P, and the total dry weight of living material on the plant, called W. All are calculated for unit area of ground.
Results
Fig. l shows the results of an experiment in which R was measured at 3-hourly intervals following a drastic reduction of irradiance. The curves are for plants of different size, increasing in order from the bottom of the chart. In all cases there was an exponential decay, followed by a steady rate of respiration which was obviously related to the size of the plant. The top set of 4 curves shows what happened when the light was reduced at different times of day, and the broken curve indicates how R varied throughout the daily cycle, when the irradiance was not reduced at all. The fall of R during a 'normal' night was much less than that caused by reducing the irradiance during the photoperiod. We also found that when the night period was continued beyond the usual 12 hours, R began to fall at the time when the light would normally have been switched on. In order to explain the decay of R as being due to the depletion of a reservoir of photosynthate, one would have to postulate the existence of two reservoirs and a photoperiod switch.
Under natural conditions, the irradiance would change continuously, producing complex changes of P and R with time. One can infer the possible pattern of behaviour from the results of an experiment (fig. 2) in which plants were put through a simplified cycle of light changes. The irradiance was held constant throughout each day, but at a different level each day. For each plant, the 24-hour totals of P and R cycled with the irradiance, in the direction of the arrows.
There are two reasons for the differences between the downward and upward parts of the cycles. Firstly, the 24 hours included a period of several hours during which R was adjusting to the new P. This biased the results upward when the light was decreasing, and downward when the light was increasing. Secondly, the plants were continuously gaining or losing weight, Large plants lost weight during the cycle because, at the lower light levels, R exceeded P, and leaves were dying (of natural causes) faster than they were being replaced. Small plants gained weight, because P exceeded R, and because the leaves were not sufficiently old to die during the cycle.
Despite the fact that the cycles could not be closed, there is a clear
pattern to the results. The cycles all have a similar slope to the P axis,
but when extrapolated they intercept the R axis at different heights. The
equation which fits this pattern is:
R = k1P + R0
where k1 is the mean slope of a cycle, and R0 is the
intercept at zero P. R0 is a function of plant size. In fact
according to fig. 3, in which R0 is
plotted against the dry weight of living material W, it appears that
R0= k2+ cW
The constant k2 was found to represent the mean rate of respiration
of unwanted organisms in the Perlite. Omitting this, and combining the two
equations, we obtain the following equation for the behavior of the plant
itself:
R = k1P +cW
where k1 is a dimensionless constant, but c has the dimensions
time-1. The best values for these constants were found to be
0.25, and 0.015 per day, respectively, when R and P were expressed in g(CO2)
day-1 m-2, and W in g(CO2 equivalents of
dry weight) m-2. In other words, in any one day, these plants
lost by respiration 25 % of their gain that day by photosynthesis, plus
1.5 % of their dry weight at that time.
In fig.4, values of R calculated from this equation are compared with those measured during the growth of two plants under constant conditions. Fig.5 shows a similar comparison for the decay in size which followed a reduction in irradiance from a high to a very low level. Most of this decay was due to plant organs dying faster than they were being replaced, and very little to the rate of respiration exceeding the rate of photosynthesis.
Discussion
The data obtained during these experiments indicate that the following principles govern the CO2 exchange rates of these plants. Firstly, the rate of respiration has two components, one of which is proportional to the rate of photosynthesis and the other to the total dry weight of living material on the plant. I believe that this principle will seem reasonable to a physiologist; the idea of a 'basal metabolism' has been used in animal physiology for many years. However, I doubt that an equation to describe the behavior of a whole plant could have been derived from present knowledge of cellular physiology.
It would be foolish to claim that the equation has any general validity. Only a very limited range of conditions and plant materials has been tested. For example, that very important environmental variable, temperature, has been kept constant. Here is an opportunity for physiologists and biochemists to predict what would happen to the two components of respiration if the temperature were allowed to vary in a manner similar to that found in nature, where the night temperature is always lower than the day temperature.
From a simulation point of view, perhaps the most important point to emerge is that all three variables in the equation are functions of time. In most of the published models, only P has been treated as time-dependent, although there has been a recent tendency to relate R to P (Tooming, 1967; Duncan et al., 1967). Also, since most models treat only a small part of the life cycle of a plant, the rate of death of organs has not generally been included, although it is just as important as the production rate in determining P, R and W (Ross, 1966). In our material, the average life span of a leaf, for example, was only 20 days, and it was immediately obvious that one could not simulate the behavior of the plant for more than one day without some information on both the production rates and the life spans of at least these organs. A complete model would have to include information on the way on which photosynthate was divided among the various organs, and on the life span of each organ, under the conditions being simulated.
I shall close on a philosophical note, concerning the proper place of behavioral experiments such as these in the 'systems analysis' of crop behaviour. It seems to me that while it might be possible to simulate the behavior of the whole crop from masses of physiological information obtained at the cellular level, it would be simpler, and perhaps more scientific, to base the simulations on principles. Experimentation on whole plants under controlled conditions then becomes a tool which is used to translate the principles of plant physiology into equations suitable for computer simulation of the response under natural conditions. I believe that in this combination of two of our most powerful technical aids, the phytotron and the computer, lies our best hope of synthesizing our knowledge about the countless mechanisms of plant response into a coherent picture of plant behaviour in the real world.
References
Duncan, W. G., R. S. Loomis, W. A. Williams & R. Hanau, 1967. A model for simulating photosynthesis in plant communities. Hilgardia 38: 181-205.
King, R. W. & L. T. Evans, 1967. Photosynthesis in artificial communities of wheat, lucerne, and subterranean clover plants. Austr. J. biol. Sci. 20: 623-635.
Ludwig, L. J., T. Saeki &L.T. Evans, 1965. Photosynthesis in artificial communities of cotton plants in relation to leaf area. Austr. J. bioi. Sci, 18: 1103-1118.
McCree, K. J. & J. H. Troughton, 1966a. Prediction of growth rate at different light levels from measured photosynthesis and respiration rates. Pl. Physiol. 41 : 559-566.
McCree, K. J. & J. H. Troughton, l966b. Non-existence of an optimum leaf area index for the production rate of white clover grown under constant conditions. Pl. Physiol. 41: 1615-1622.
Ross, Y. 1966, A mathematical description of the growth of plants. Dokl. Akad. Nauk. SSSR 171: 481-483. Tooming, H., 1967, Mathematical model of plant photosynthesis considering adaptation. Photosynthetica 1: 233-240.
From: Prediction and Measurement of Photosynthetic Productivity. Proceedings of the IBP/PP Technical Meeting, Trebon, 14 - 21 September 1969. Centre for Agricultural Publishing and Documentation, Wageningen, 1970.