This Demonstration simulates the response of net photosynthetic carbon assimilation () over the diurnal time course for either a sunny or overcast day. The equations are based on the leaf model of photosynthesis  that predicts based on the most limiting of three processes: (1) the kinetics of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (or rubisco) for fixing , (2) the rate of RuBP regeneration associated with electron transport rates on the thylakoid membrane (RuBP), and (3) the rate at which inorganic phosphate, required for regenerating ATP from ADP, is released during the utilization of triose phosphate (TPU). The most limiting process will determine the rate of ; the process limiting, rubisco, RuBP, or TPU is indicated by the color of the line. Adjusting any of the parameters will influence the pattern of throughout the day; two modeled lines, a solid (left controls) and a dashed (right controls), are included to allow for direct comparison to parameter adjustments. The equations from the original model  have been corrected with the temperature functions from  for rubisco-limited photosynthesis, [3, 4, 5] for RuBP-limited photosynthesis, and  for TPU-limited photosynthesis. Changes in the stomatal aperture in response to environmental factors, which influences , are not simulated here.
The leaf's environmental conditions include leaf intercellular concentration (), oxygen concentration (), and photosynthetic photon flux density PPFD (). The meteorological data is based on measured air temperatures and PPFD for a sunny and an overcast day in Champaign, IL. The daily minimum, mean, and maximum temperatures for the sunny day were 16.2, 24.2, and 31.7 °C for the sunny day and 18.5, 22.8, and 28.3 °C for the overcast day. The diurnal pattern of the meteorological data for these two days is presented in Figure 1 from . The temperature data can be adjusted up or down by 10 °C and PPFD can be altered from 50% to 150% of measured values.
The photosynthetic parameters include the maximum rate of carboxylation (), maximum rate of electron transport through photosystem II (), rate of triose phosphate utilization (), and rate of mitochondrial respiration (). These are all based on values at 25 °C.
Rubisco specificity (unitless) is a function of the enzyme kinetics of rubisco for and . A higher leads to a lower catalytic rate . The influence of changes in on can be determined using the slider that adjusts the kinetics for carboxylation and oxygenation of rubisco, and the consequent influence on catalytic rate occurs through altering . Details associated with these changes are presented in  and .
The temperature dependence of is derived using the data from  but with the temperature response of  modified as in  to normalize values to 25 °C. Under the heading "RuBP-Limited Adjustments", you can vary the temperature optimum and the temperature at which falls to of its value at (). The default parameterization for these values are from  for tobacco grown at 25 °C, but values for a range of species are provided elsewhere . Changes in and only affect when . You can also vary the leaf absorbance (unitless), the ratio of photosystem II to photosystem I (unitless), the curvature of the PPFD response curve (unitless), and the quantum efficiency of photosystem II ΦPSII (unitless).
Snapshot 1: effect of 5 °C warming on diurnal photosynthesis
Snapshot 2: doubling for an overcast day
Snapshot 3: doubling of for both scenarios on a sunny day but with a 10% down regulation in and for scenario 2
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