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COORDINATING PHOTOSYNTHETIC ACTIVITY: CIRCADIAN RHYTHMS
source: Louis J. Gross
Professor of Ecology and Evolutionary Biology and Mathematics
Director, National Institute for Mathematical and Biological Synthesis
Director, The Institute for Environmental Modeling
Department of Ecology and Evolutionary Biology
Mathematics Department
University of Tennessee
500 mmol per m2 per second photon flux density (PPFD) is most certainly exceeded by anyone that uses 400+ watt MH and HPS lamps...As for lower wattage HID, T5 and CFL, I cannot say for sure.
source: Louis J. Gross
Professor of Ecology and Evolutionary Biology and Mathematics
Director, National Institute for Mathematical and Biological Synthesis
Director, The Institute for Environmental Modeling
Department of Ecology and Evolutionary Biology
Mathematics Department
University of Tennessee
Under normal conditions, photosynthesis and stomatal opening have circadian rhythms. Here, photosynthesis is measured as carbon assimilation and stomatal opening is measured as conductance to water vapor. For the red kidney bean plant, data shows carbon assimilation varies between 5.8 and 8.2 mmol CO2 per m2 per second, with an amplitude is approximately 2.4 (A = 2.4). The period is approximately 24 hours (p = 24). We can approximate the normal circadian rhythm of photosynthesis as
We can graph the circadian rhythm of photosynthesis over time. The graph of stomatal conductance, the lower curve, is also given, with coordinates on the second axis.
Interpretation: Under a normal 24 hour day-night cycle, both carbon assimilation and stomatal conductance have circadian rhythms. Both tend to be highest at noon, when light energy is highly available, and lowest at midnight.
If changes in light are driving the circadian rhythms, we might expect the sine waves to flatten when a plant is moved to constant light. However, when red kidney bean plants are moved from normal light (day and night every 24 hours) to constant moderate light, the circadian rhythm persists for several days with the same amplitude and period as the above curves. This indicates that changes in light and dark are not driving the circadian rhythm. Instead the rhythm is driven by an internal clock.
The opening and closing of stomata control the concentration of carbon dioxide inside the leaf. When stomatal conductance is high, a larger amount of CO2 can enter the leaf and be available for photosynthesis. Consequently, we might hypothesize that the circadian rhythm in stomatal conductance is causing the rhythm in carbon assimilation. If we manipulate a leaf to have constant levels of CO2, we might expect the rhythms of stomatal conductance and carbon assimilation to disappear. However, under constant moderate light (200 mmol per m2 per second photon flux density) and constant intercellular CO2 (28 Pa), the circadian rhythms still persist for several days with the same amplitude and period as under normal conditions (the curves in the previous graph).
Since photosynthesis is dependent on light energy, we might expect to see changes in circadian rhythm depending on the intensity of light. Under conditions of high light and high CO2, photosynthetic activity is very high. We can determine how such environmental conditions affect the circadian rhythm.
The graph below shows data from a single kidney bean leaflet for the circadian rhythms in carbon assimilation and stomatal conductance under constant high light (500 mmol per m2 per second photon flux density) and constant ambient CO2 (35 Pa).
Under conditions of constant high light, the circadian rhythms in carbon assimilation and stomatal conductance quickly flatten out after a couple of days. The period is slightly longer than 24 hours. Notice that on the first day, the values of carbon assimilation (sy = 17 mmol CO2 per m2 per second) and stomatal conductance (sy = 0.52 mol H2O per m2 per second) were much higher than the values under moderate light conditions (sy = 7 and sy = 0.15, respectively). Under high light conditions, these values decrease over time, along with a decreasing amplitude. This is quite different from constant levels of moderate light, where the circadian rhythms persist for several days with the same amplitude and period as a 24 hour night-day cycle.
Under high light conditions, photosynthetic activity is very high. Therefore it is not suprising the initial values of carbon assimilation and stomatal conductance were much higher than leaflets in moderate light. Under conditions favorable to high photosynthetic activity, carbohydrate quickly accumulates in the leaves. The resulting decline in carbon assimilation and stomatal conductance over time may have occurred as starch accumulated in the leaves, disrupting chloroplast structure.
Conclusions: The cellular clock of plant cells is well adapted to the 24 hour day-night cycle of the planet. Circadian rhythms arise in various aspects of photosynthesis, including carbon assimilation, stomatal conductance, and in levels of essential elements of photosynthesis, such as Photosystem II and RuBP. Changes in light intensity and carbon dioxide concentration can affect circadian rhythms of photosynthesis.
Circadian rhythms are essentially sine waves with periods of 24 hours and variable amplitudes. Quantifying properties of circadian rhythms, such as amplitude and period, can help us understand the mechanisms controlling rhythms.
500 mmol per m2 per second photon flux density (PPFD) is most certainly exceeded by anyone that uses 400+ watt MH and HPS lamps...As for lower wattage HID, T5 and CFL, I cannot say for sure.
