Targeted lighting facilitates redirection of transpiration stream toward young leaves

The rapid growth of lettuce in plant factories with artificial lighting (PFALs) under optimal conditions frequently leads to tipburn. Conventional strategies to mitigate tipburn often involve reducing growth rates, which compromises productivity. Tipburn mainly manifests in inner leaves, which are enclosed by outer leaves and exhibit low transpirational calcium translocation. In this study, we hypothesized that applying partial blue lighting (PBL) to inner leaves of butterhead lettuce (Lactuca sativa var. capitata) would enhance transpirational calcium translocation and reduce tipburn while promoting growth. This was regarded as the alternative hypothesis to the null hypothesis that PBL does not significantly affect calcium translocation, tipburn, or growth. To test this hypothesis, we conducted two experiments varying the timing and duration of PBL application in relation to tipburn severity and growth. In the first experiment, we compared PBL applied during the photoperiods or dark periods. PBL at 20 μmol m−2 s−1 during dark periods efficiently enhanced calcium translocation and highly improved growth without exacerbating tipburn. In the second experiment, we further examined continuous and intermittent PBL during dark periods. Continuous PBL for 8 h demonstrated the greatest growth improvement without exacerbating tipburn. Overall, PBL treatment enhanced calcium translocation and growth. Although we failed to reject the null hypothesis, as tipburn severity remained very low in both control and the PBL treatment with no significant difference, it is noteworthy that tipburn did not exacerbate despite enhanced growth. This study provides a novel strategy to enhance productivity without exacerbating tipburn in butterhead lettuce cultivated in PFALs.