induction, while lower NPQ does not
5.2 Model description and parameterization
5.3.2 Chlorophyll fluorescence
Simulations of steady-state and dynamic PSII quantum yield (.//) and Stern-Volmer NPQ coefficient (XGL∫ƒ) were used to test simulations of PSII activity with published measurements on A. thaliana Col-0. The following types of measurements were used: 1. Steady-state response of .// to irradiance.
2. Steady-state response of .// to ]S.
3. Dynamic changes in .// after an increase in irradiance. 4. Dynamic changes in XGL∫ƒ after an increase in irradiance.
Simulations of steady-state PSII quantum yield (.//) as a function of irradiance were compared to measurements by van Rooijen et al. (2015) at ambient CO2 and Hald et al.
(2008) at a CO2 mole fraction of 2000 μmol mol−1. Simulations of the steady-state
irradiance of 1500 μmol m−2 s−1 and from Kaiser et al. (2016) at a constant irradiance of
1000 μmol m−2 s−1. In all cases, ambient O2 was used for the measurements. The
simulations were performed with simulation protocols analogous to the ones described in the previous section, with the exception that rectangular flashes, with intensities taken from the descriptions of experiments (always between 6500 μmol m−2 s−1 and 8000 μmol m−2 s−1) were added at the end of each CO2 and irradiance step to calculate . // and XGL∫ƒ. An apparent, simulated chlorophyll fluorescence yield of the virtual leaf was calculated as the rate of fluorescence emission of the chlorophyll molecules associated to PSIIac, divided by the irradiance incident on the leaf. .// was computed as 123 − 13 1 23, where 13 was the chlorophyll fluorescence yield before applying a flash and 123 was the maximum chlorophyll fluorescence yield achieved during the flash. The model predicted accurately the effects of CO2 and irradiance on .// (Figure 5.10), although in most cases, simulated .// was higher than those of measurements. No information on measurement errors was available, so it is not clear whether the differences between the simulations and measurements are significant. The model is expected to underestimate measurements XGL∫ƒ as neither photoinhibition nor state transitions are being simulated and these processes contribute to XGL∫ƒ in A. thaliana (Kasahara et al., 2002; Nilkens et al., 2010; Dall'Osto et al., 2014). Given the large uncertainty in XGL∫ƒ values from different sources and the fact that the model does not take into account the effect of photoinhibition, XGL∫ƒ simulations were tested only with measurements by Kaiser et al. (2016) and Herdean et al. (2016) for the first 10 minutes after an increase in irradiance from a dark adapted state, assuming that, under such conditions, XGL∫ƒ is mostly determined by the qE mechanism and chloroplast movement. The same experiments were also used to test dynamic simulations of .//.
Steady-state responses of XGL∫ƒ were not used for testing the model, as the results obtained from different sources (Hald et al., 2008; van Rooijen et al., 2015; Kaiser et al., 2016) were not consistent. Possible factors explaining the lack of consistency across experiments would be the light history of the plants being measured, the rates of photoinhibition achieved during the measurements, the spectral distribution of the source of irradiance, and [CO2].
From the data reported by Herdean et al. (2016) and Kaiser et al. (2016), three photosynthetic induction curves from darkness to 70 μmol m−2 s−1, 600 μmol m−2 s−1 and
1000 μmol m−2 s−1 were chosen, all measured at ambient CO2, and O2. The simulation
protocol was adapted to the experimental conditions and flashes were simulated at the same timepoints as in the experiments and with the same flash intensities. In all cases, .// decreased rapidly after the increase in irradiance followed by a slow recovery (Figure 5.11A). The simulations reproduced this behaviour qualitatively, and the agreement was best for the measurements by Kaiser et al. (2016), with a slight overestimation as expected from Figure 5.10.
The increase in XGL∫ƒ during the first 10 minutes of induction was also reproduced by the model in the case of the measurements by Kaiser et al. (2016). However, the agreement with some of the measurements was poorer, although the model still reproduced the dynamic patterns qualitatively. In the induction curve using 70 μmol m−2
s−1 as irradiance, measured XGL∫ƒ displayed a clear overshoot during the first three
minutes. The model capture this correctly, although it underestimated the magnitude and extension of the overshoot. In the case of the induction curve at 650 μmol m−2 s−1 of
irradiance, the model predicted an increase in steady-state .// (to be expected based on Figure 5.10) which was not apparent in the measurements (despite the decrease in measured XGL∫ƒ). Herdean et al. (2016) did not report measurements of An or gs and thus it is not possible to assess whether the simulations correctly reproduced metabolic demand, which would have an effect of XGL∫ƒ and .// during induction.
Figure 5.8: Measured (symbols) and simulated (lines) steady-state net CO2 assimilation as a function
of irradiance (A) and CO2 mole fraction (B). The CO2 response was measured with irradiances of 1000
μmol m−2 s−1 by Kaiser et al. (2016) and 1500 μmol m−2 s−1 by Flexas et al. (2007b). The irradiance
response was measured at a CO2 mole fraction of 400 μmol mol−1 by Kaiser et al. (2016).
Figure 5.9: Measured (symbols) and simulated (lines) relative net CO2 assimilation (An) as a function
of time after an increase in irradiance level, from 0 μmol m−2 s−1 to 1000 μmol m−2 s−1 (black line and
circles in panel A) and 70 μmol m−2 s−1 to 800 μmol m−2 s−1 (red line and triangles in panel A) and
after a decrease in irradiance level from 800 μmol m−2 s−1 to 130 μmol m−2 s−1 (black line and circles
in panel B) and from 600 μmol m−2 s−1 to 200 μmol m−2 s−1 (red line and triangles in panel B). Relative
An was calculated by subtracting An at time = 0 and dividing by the difference between maximum and
Figure 5.10: Measured (symbols) and simulated (lines) steady-state quantum yield of PSII (ΦII) as a
function of irradiance (a) and CO2 mole fraction (b). The response to irradiance was measured and
simulated for different CO2 mole fraction (see legend). The response to CO2 mole fraction was
measured at two irradiances (see legend). The response to irradiance at an air CO2 mole fraction of
2000 μmol mol−1 and the CO2 response at an irradiance of 1500 μmol m−2 s−1 were taken from Hald
et al. (2008). The irradiance response at 400 μmol m−2 s−1 was taken from van Rooijen et al. (2015).
The CO2 response at 1000 μmol m−2 s−1 was taken from Kaiser et al. (2016).
Figure 5.11: Measured (symbols) and simulated (lines) quantum yield of PSII (A) and XGL∫ƒ (B) as
a function of time after an increase in irradiance from 0 to 1000 μmol m−2 s−1 (Kaiser et al., 2016),
650 μmol m−2 s−1 (Herdean et al., 2016) and 70 μmol m−2 s−1 (Herdean et al., 2016).
The mismatch may also be explained by an overestimation of the kinetics of the component of XGL∫ƒ associated with protonation of the PsbS protein, although this would not explain the low measured .// in the 0 – 650 μmol m−2 s−1 transient.