Lettuce growth, morphology and critical leaf trait responses to far-red light during cultivation are low fluence and obey the reciprocity law

The mediating role of far-red (FR) light at the end of day (EOD) during cultivation on plant growth, morphology and critical leaf traits was analyzed. Three experiments were conducted on lettuce. Firstly, it was clarified whether or not the FR light effect depends on the presence of photosynthetic active radiation (PAR). Secondly, dose-response curves were obtained by applying six FR light intensities. Eventually, three FR light intensities were applied for different durations in a way that daily light integral (DLI) remained constant. Assessments included specific leaf area, dry mass partitioning, leaf optical properties, net photosynthesis rate (Pn), as well as leaf photosynthetic pigment, soluble sugar, starch, cellulose, soluble protein, nitrate, total nitrogen and total carbon contents. Pn was not significantly affected by FR light during growth, despite decreased leaf light absorption and photosynthetic pigment content (2 and 16%, respectively). Starch and nitrate contents were not affected by FR light, whereas cellulose, soluble protein and total nitrogen contents were generally decreased. When applied alone, FR light better promoted growth, radiation use efficiency (similar to 22%) and soluble sugar content (similar to 34%). The FR light-induced effect was intensity-dependent, while both qualitative and quantitative threshold responses were apparent depending on the trait. Five minutes of daily exposure to FR light (90 mu mol m(-2) s(-1)) were adequate in eliciting the photomorphogenic responses. Notably, no significant differences were observed in all traits between FR light intensities under the same DLI. In conclusion, FR light at the EOD better stimulates growth when applied alone, through improved light interception. FR light response is low fluence and depends on the DLI, obeying the reciprocity low.