*3.1. Projected Canopy Size and Incident Light*

Mizuna had a larger PCS than lettuce, starting from the early growth stages (Figure 1; Figure S1). One reason for this early difference in PCS is the faster germination and larger cotyledons of mizuna compared to lettuce. Due to their small canopy size, seedlings capture only a small fraction of the provided light. Therefore, increased PCS at early stages can increase light capture and growth of seedlings [11]. A previous study compared different lettuce cultivars and found that early PCS was a good predictor of final shoot biomass [10]. We observed the same pattern in our study, with a strong positive correlation (*R* = 0.91 for lettuce and *R* = 0.89 for mizuna, *p* < 0.0001) between early PCS (lettuce 10 d and mizuna 8 d after seeding) and the dry weight of both lettuce and mizuna (Figure S2).

This higher PCS of mizuna during the early part of the growing cycle (at PPFDs <sup>≥</sup> <sup>125</sup> <sup>µ</sup>mol m−<sup>2</sup> s −1 ) may have helped it to capture more light and grow faster than lettuce. A previous greenhouse study conducted with 'Little Gem' lettuce observed the same trend; plants with larger PCS in early growth stages absorbed more light, grew faster, and produced additional canopy faster than plants with a smaller PCS [4]. In that prior study, plants were grown with different photoperiods, but the same DLI. In contrast, plants in the current study were grown with the same photoperiod, but different PPFDs, thus resulting in different DLIs.

At a PPFD of 50 µmol m−<sup>2</sup> s −1 , both mizuna and lettuce grew slowly and had a low PCS at harvest compared to plants grown at higher PPFDs (Figure 2). At PPFDs ≥ 125 µmol m−<sup>2</sup> s −1 , the PCS of both crops was much greater than at a PPFD of 50 µmol m−<sup>2</sup> s −1 However, at PPFD <sup>≥</sup> <sup>200</sup> <sup>µ</sup>mol m−<sup>2</sup> s −1 , the PCS of mizuna was greater than that of lettuce (Figure 2) (*p* < 0.0001). The growth difference between the two crops is at least partly because of the observed differences in PCS and its impact on incident light. The incident light of mizuna increased more rapidly with increasing PPFD than that of lettuce, consistent with the increasing difference in dry weight between the two crops as PPFD increased (Figures 3 and 7). Other studies also mentioned a positive correlation between PCS and incident light [5,11,19]. Additionally, several other studies on leafy greens show a positive correlation between biomass gain and incident light integrated over the entire crop cycle [5,20,21].

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