Costs to Flowering of the Production of a Mechanically Hardened Phenotype in Brassica napus L.

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To examine potential costs of mechanical hardening in plants, dose‐dependent effects of mechanical perturbation on the growth and development of Brassica napus were investigated in a greenhouse experiment. Mechanical perturbation (stem and leaf flexure) was imposed by exposing seedlings to 0, 10, 20, 30, or 40 flexures with a bamboo rod once per day over a period of 35 d. In general, mechanical perturbation reduced plant height and root mass but increased stem diameter while decreasing pith diameter in a nearly linear dose‐dependent manner. In addition, mechanical perturbation delayed both cotyledon senescence and date of anthesis and also reduced flower number in a similar linear dose‐dependent manner. In contrast, effects of mechanical perturbation on final leaf area, leaf mass, and stem mass were not linearly related to dose, although means of these variables were smaller in plants subjected to 40 flexures per day than in all other treatments. Dose‐dependent effects of mechanical perturbation on B. napus were most noticeable as plastic changes in growth form, allometry, and development; i.e., mechanical perturbation produced increasingly shorter, radially thickened, and more compact and stronger plants with increasing dose. However, dose‐dependent delays in anthesis and reductions in flower number indicated that fitness of these plants in the field could be increasingly reduced by increasing exposure to mechanical perturbation, unless benefits gained through the development of a mechanically hardened phenotype counterbalanced costs to flower production.

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