Role of Mechanical Loading in Growth of Sunflower (Helianthus annuus) Seedlings

Seedlings of the herbaceous annual Helianthus annuus L. were grown under three regimens of daily bending. Bending wasadjusted during ontogeny so that the deflexion of the stem tip divided by the stem height was constant (elastic similarity). After 6weeks, mechanically–treated plants exhibited a significantly greater ratio of stem diameter to stem height, flexural stiffness, stemgrowth rate, and proportion of collenchymatous tissue in the stem. Treated plants were also significantly smaller in height,showed a greater proportion of stem tissue as cortex, and greater initial stem growth rate. No significant differences wereobserved in the elastic modulus of the tissue composing the stem, above–ground biomass, and stem diameter. The degree ofmechanical loading also had a significant effect on most parameters. The most highly stressed plants were thicker, shorter, and elastically stiffer with more collenchyma and less cortex in the stem. Since plants were loaded for only 60 s d–1 it is concludedthat mechanical effects early in life can have profound effects on the form and hence ecology of seedlings.