Structural and functional plasticity of herbaceous plants stems under illumination gradients

The stem provides mechanical support, transports water and assimilates, and is involved in regulating shoot architecture and adaptive plasticity. This study experimentally tested how changes in illuminance intensity affect stem morphometry (height and diameter) and lignification index (lignin mass fraction) in Brassica rapa L. var. rapa, a rapid-cycling line Wisconsin Fast Plants a fast-growing herbaceous species under controlled conditions. Measurements were taken on the 28^th day of cultivation.

A single-factor laboratory experiment (n = 90) was conducted with three photon flux levels (60, 180, and 420 μmol m⁻² s⁻¹) under identical temperature, humidity, photoperiod, and watering regimes. Differences were tested using analysis of variance. It was shown that with increasing illumination, stem height decreased, while diameter and lignin content increased (ANOVA: F(2.87) = 79.44; 44.11; 33.96; p < 0.001). Post-hoc comparisons confirmed gradient differences between all pairs of light levels. Additional analysis revealed a negative relationship between height, diameter, and lignification, and a positive relationship between diameter and lignin. The results are interpreted as an adaptive shift from an elongation strategy to a shoot strengthening strategy.