2.3.1. Blue Light Response Function of Internodes

Based on the simulated *BPFDper*, the relative elongation response of the second and third internode were closer to each other compared to using the emitted BPFD (Figure 3). Especially at high BPFD levels, the response of the two internodes was close, implying a common response function to *BPFDper*.

**Figure 3.** Least square mean of length of second (point) and third (triangle) internode relative to B310 in response to BPFD emitted by the LED modules (**A**) or simulated BPFD perceived by the internode (**B**). Dashed line showing the function fitted to relative internode lengths higher than one and dotted lines showing the interception of the function with 1. Black line showing the final response function to BPFD. Error bars indicate standard error of the LS-mean (*n* = 8).

Due to the relatively small differences between the parameters *t<sup>e</sup>* and *t<sup>m</sup>* of the beta-function and no clear tendencies in their response to BPFD, only differences in the final length of the internode *(Lmax)* were considered in the response function. Internodes with a relative length to B310 below one were considered to have no elongation response to *BPFDper* and a common function for final internode length was fitted to the *BPFDper* of the internodes under treatments with a relative length to B310 higher than one (Figure 3B).

The common function for the relative length of the second and third internode was:

$$L\_{\rm rel} = -0.01 \, \* \, \text{BPFD}\_{\rm per} + 1.91 \, \text{r} \tag{1}$$

The interception of this function with one was at 79.38 µmol m−<sup>2</sup> s <sup>−</sup><sup>1</sup> *BPFDper*, which was hereby the minimum amount of BPFD that an internode should perceive to express no elongation response to *BPFDper*.

This resulted in the BPFD response function:

$$\text{Intermediate length} = \text{L}\_{\text{min}} \left( 1 + (79.38 - \text{BPFD}\_{\text{per}}) \* 0.01 \right), \text{ 79.38 - } \text{BPFD}\_{\text{per}} > 0,\tag{2}$$

where *Lmin* is the final internode length with no elongation response to *BPFDper* and *BPFDper* is the BPFD perceived by the internode. The black line in Figure 3B shows the response function in the range from the minimum (16.78 µmol m−<sup>2</sup> s −1 ) to maximum (108.13 µmol m−<sup>2</sup> s −1 ) perceived light during the simulations.
