The MINI-PAM-II combined with the newly developed digital leaf clip 2035-B measures simultaneously PAM fluorescence, photosynthetically active radiation (PAR), leaf temperature and air humidity.
The present experiment demonstrates how these parameters vary during a partly cloudy spring day. The photosynthetic sample was an attached leaf of Ilex aquifolia growing the garden of the Walz Company.
Figure 1 shows that, during the first half of the day, PAR increased occasionally to values around 500 μmol m-2 s-1 and the quantum yield of photosystem II, Y(II), varied in an opposite manner to PAR.
As expected, air temperature increased with PAR and relative humidity decreased with increasing temperature.
Using Y(II) and PAR (see Fig. 1), and standard parameters (84% of incident light absorbed by the leaf and equal partitioning of absorbed light quanta between photosystems I and II), electron transport rates (ETR) were calculated by the software of the MINI-PAM-II. These ETR data are plotted against the respective PAR values. Clearly, all ETR versus PAR data during the course of a day can be described by a single curve. Fitting the model of Jassby and Platt (1976, Limnol Oceanogr 21: 540-547) to the data resulted in the blue curved line of Fig. 3.
The three cardinal points of this best fit curve are indicated in Fig. 3: α, initial slope. ETRmax, maximum electron transport rate. IK, minimum saturating PAR. These cardinal points are useful to characterize the light acclimation status of a sample. Often, in high light acclimated leaves, the ETRmax and IK parameters are elevated.