*3.5. Light Use Efficiency*

Mizuna had a higher maximum (1.26 g mol−<sup>1</sup> ) LUE than lettuce (0.74 g mol−<sup>1</sup> ) (*p* < 0.0001) (Figure 8). Mizuna had its maximum LUE at PPFDs from 50 to 200 µmol m−<sup>2</sup> s −1 , while

lettuce LUE was maximal at PPFDs of 200 to 275 µmol m−<sup>2</sup> s −1 . This indicates that mizuna can convert incident light into biomass more efficiently than lettuce, especially at lower PPFDs. The maximum lettuce LUE we observed (0.74 g mol−<sup>1</sup> ) is slightly higher than the LUE previously reported (0.61–0.65 g mol−<sup>1</sup> ) for 'Green Salad Bowl' lettuce [6]. Legendre and van Iersel [6] used the same method to calculate LUE, but, did not use CO<sup>2</sup> enrichment which may have reduced leaf photosynthesis and thus LUE.

Other studies reported substantially lower LUE values (<0.6 g mol−<sup>1</sup> ) for lettuce [23,30], but in those studies, LUE calculations were based on the amount of light provided to the growing space, rather than light reaching the canopy of the crop. Their LUE values are thus heavily dependent on the plant density of the growing spaces. Light use efficiency can also vary among cultivars [31,32]. Therefore, it is hard to compare our LUE values of lettuce with those studies. There are no prior reports for mizuna LUE.

Multiple factors contributed to the higher LUE of mizuna. The combined effects of more chlorophyll and higher ΦPSII of mizuna likely resulted in higher ETR and thus more photosynthesis than in lettuce. High photosynthetic rates can increase the relative growth rate, which in turn reduces the fraction of carbohydrates allocated to maintenance respiration and increases carbon use efficiency [33]. That in turn can increase LUE.

The LUE decreased at high PPFDs for both mizuna (PPFD <sup>≥</sup> <sup>200</sup> <sup>µ</sup>mol m−<sup>1</sup> s −1 ) and lettuce (PPFD <sup>≥</sup> <sup>350</sup> <sup>µ</sup>mol m−<sup>1</sup> s −1 ) (Figure 8). This reduction of LUE at high PPFD may be due to the decrease of ΦPSII with increasing PPFD (Figure 5).
