Genotypic difference in yield response to far-red light in dwarf tomato: explained by fruit sink and source strength and predicted by seedlings’ height plasticity

Supplemental far-red (FR) light can enhance tomato yield and fruit soluble sugar content, yet its effects across genotypes remain poorly understood. The aims of this study were to determine whether genotypic differences in yield response to FR light are driven by dry matter production or partitioning and to identify an early predictive indicator (seed or seedling stage) of FR responsiveness in dwarf tomato genotypes. We studied 23 dwarf tomato genotypes for yield and fruit sugar responses to supplemental FR. To determine whether cultivars appearing non-responsive to FR exhibit stage-specific sensitivity, 2 non-responsive genotypes were exposed to FR during distinct developmental phases. In addition, seedling traits, including height, chlorophyll index, leaf morphology, and germination responses, were assessed to evaluate their potential to predict FR-induced yield responsiveness. We found that 10 of the 23 genotypes exhibited a significant increase in fruit fresh-weight yield under supplemental FR, ranging from 31% to 175% relative to the No FR treatment, whereas 13 genotypes showed no significant response. Similarly, 16 of the 23 genotypes exhibited a significant increase in total soluble sugar concentration, expressed on a dry-weight basis, under supplemental FR, ranging from 18% to 88% relative to No FR, whereas 7 genotypes showed no significant response. Enhanced yield responses were associated with increased fruit sink and source strength. Genotypes classified as non-responsive remained insensitive irrespective of the developmental stage of FR application, indicating true genotype-specific FR-insensitivity. Furthermore, seedling’s elongation response to FR was strongly correlated with the magnitude of yield enhancement under FR (r = 0.89). These findings demonstrate strong genotypic variation in tomato responses to supplemental FR and identify seedling height response as a promising early indicator of FR-induced yield responsiveness within the tested panel of dwarf tomato genotypes and controlled light conditions.