*3.3. Chlorophyll Content Index and Anthocyanin Content Index*

We observed a higher CCI in mizuna leaves compared to lettuce at all PPFDs (Figure 4A). Leaf light absorptance is positively associated with the CCI [12]. Therefore, mizuna likely had a higher leaf light absorptance in lettuce. However, we do not have the actual light absorptance data for this study.

With increasing PPFD, CCI increased in both species (Figure 4A). In previous studies, CCI increased with lower PPFD [4,18]. In those studies, lower PPFDs were combined with longer photoperiods to maintain the same DLI. Chlorophyll production is a light-regulated process [22]. Therefore, the CCI increase in low PPFD treatments in prior studies was associated with longer photoperiods, which increases the amount of time available for plants to produce chlorophyll [18]. In our study, photoperiod was the same in all treatments and therefore the daily light integral was higher at higher PPFDs.

In response to increasing PPFD, the CCI increase in mizuna was about four times greater than in lettuce, indicating that mizuna acclimates more strongly to different PPFDs than lettuce (Figure 4A). The greater CCI increase in mizuna may be partly the result of a lower SLA at PPFDs <sup>≥</sup> <sup>125</sup> <sup>µ</sup>mol m−<sup>2</sup> s −1 , compared to lettuce (Figure 7B). A higher SLA is associated with thinner leaves, which typically have low chlorophyll content per unit leaf area [23]. In our study, increasing PPFD decreased the SLA of both crops (Figure 7B). Such a decrease in SLA can increase the CCI, due to increased leaf thickness. We indeed observed strong negative correlations between the SLA and CCI of mizuna (*R* = −0.76, *p* = 0.0003) and lettuce (*R* = −0.79, *p* = 0.0001; Figure S3), but this relationship differed greatly between the two species. Mizuna's CCI decreased much more quickly with increasing SLA than that of lettuce. At mizuna's highest SLA (~375 cm<sup>2</sup> g −1 ), its CCI was similar (~5) to the lettuce CCI at its lowest SLA (~325 cm<sup>2</sup> g −1 )

The anthocyanin content index (ACI) was measured to identify whether the two crops differ in anthocyanin accumulation in response to increasing PPFD. Anthocyanins in leaves have a protective role against intense light and help dissipate excess excitation energy [24,25]. The ACI of both species did indeed increase with increasing PPFD (Figure 4B). However, the ACI of both crops was much lower than those previously reported for red leaf basil (ACI of 28–81) [26] and pak choi (ACI of 35–65) [27]. Consistent with the low ACI, we did not observe any red coloration on leaves of mizuna or lettuce, with increasing PPFD.
