*3.4. Quantum Yield of Photosystem II and CO<sup>2</sup> Assimilation*

With increasing PPFD, ΦPSII of both crops decreased at a similar rate (Figure 5A). At higher PPFD, a larger proportion of the PSII reaction centers are closed and more of the absorbed light energy is dissipated as heat to minimize photoinhibition [15,28]. This rise in heat dissipation results in a smaller fraction of the excitation energy being directed towards the PSII reaction centers, reducing ΦPSII [17,29]. Many previous studies have observed a decrease in ΦPSII, but increasing ETR, in response to increasing PPFD [15–17]. We observed the same pattern in ΦPSII with increasing PPFD. We did not calculate the ETR because leaf absorptance was not measured. However, the higher CCI of mizuna suggests higher leaf absorptance compared to lettuce [12], in which case differences in electron transport rate between the two species would have been larger than the differences in ΦPSII. Even though we observed a reduction in ΦPSII of both crops with increasing PPFD, the ΦPSII of mizuna was always higher (~0.05 mol mol−<sup>1</sup> ) than that of lettuce (Figure 5A) (*p* < 0.0001). A study conducted to understand the effects of different PPFDs on the photochemistry of three species adapted to different light levels, found a higher ΦPSII and higher ETR in high-light adapted species compared to the species adapted to moderate or low-light [16]. This suggests that mizuna is better adapted to high light levels than lettuce, and therefore has a higher ΦPSII.

With increasing PPFD, the net CO<sup>2</sup> assimilation rate of mizuna tended to increase more rapidly than that of lettuce (Figure 5B, *p* = 0.08). This is consistent with the higher CCI and ΦPSII of mizuna. The lower SLA of mizuna suggests thicker leaves compared to lettuce. The higher CCI associated with thicker leaves can increase the light absorptance and CO<sup>2</sup> assimilation rate per unit leaf area [23].
