High Green Light Substitution Reduces Tipburn Incidence in Romaine Lettuce Grown in a Plant Factory with Artificial Lighting

Optimizing lighting in plant factory with artificial lighting (PFAL) is essential for balancing lettuce growth and quality. Rapid plant growth in PFAL often induces physiological disorders, especially tipburn, which is associated with calcium deficiency in newly emerging leaves. This study aimed to evaluate the effects of different proportions of green light substitutions on tipburn incidence and the growth of romaine lettuce cultivated in a PFAL. Plants were grown under different proportions of green light at a total light intensity of 200 µmol m^-2 s^-1, consisting of 40% (G40), 60% (G60), 80% (G80), and 100% (G100) green light. The results showed that increasing the proportion of green light significantly reduced tipburn incidence from 49% to 25%, while shoot fresh weight declined by 7% and 25% when green light substitution increased to 80% and 100%, respectively. Net CO₂ assimilation of outer leaves remained similar among G40, G60, and G80 but declined by approximately 13% under 100% green light (G100). Higher proportions of green light markedly increased calcium accumulation and chlorophyll content in the inner leaves. These results suggest that higher proportions of green light may improve the inner-leaf light environment, enhance inner-leaf physiological function, and thereby reduce tipburn incidence. Substituting 60% green light achieved a good balance between growth performance and tipburn reduction. This approach offers an effective method to mitigate tipburn and improve lettuce quality in PFALs.