Shade avoidance responses in seedlings of maize hybrids representing 100 years of breeding history.

Understanding how the spectral quality of light affects maize physiology is critical, as it mediates an early response to competition. We measured the effects of light quality on physiological traits at the seedling stage in 27 maize genotypes released from 1920 to 2022. High and low red/far-red ratios light conditions were simulated in a growth chamber. Low R/FR ratio caused shade avoidance responses, including increased plant height and decreased stem diameter. We hypothesized that long term selection for yield would reduce phenotypic response to low R/FR light conditions in seedling stages because more modern hybrids have been selected under progressively higher planting densities. While changes in phenotype, including reduced leaf area, root length, total stomatal pore area and increased leaf SPAD index, were observed over past 100 years breeding time, for most traits, no significant genotype x light treatment interaction (p > 0.05; F = 0.64-1.00) for any trait. Selection has likely improved resource use efficiency for other factors affected by plant-to-plant competition, such as water or nitrogen.