The present work deals with the effect of wind on sunflower (Helianthus annuus L.) plants at early stages of the reproductive development on the rate of floral differentiation.

Plants were grown under artificial lighting in 2L pots containing soil appropriately fertilized, with a light/dark regime of 18+6 h. Fifteen days after emergence (DAE) three treatments were performed. Wind, where free plants were exposed for 4 h/day during the light interval, 1h after irrigation, to a continuous air current of 5 m.s ^-1(WS-1) or 8 m.s^-1 (WS-2) at the apical level and control (C, staked plants) were plant were kept still during the whole experimental period. Starting at 20 DAE reproductive development was studied. At harvest (48 DAE) leaf area, aerial (stem+leaves) dry weight, stem length and the flexural stiffness of first internode (FSFI) were determined.

Plants from treatment WS-1 showed reductions in leaf area (2.5 %), stem height (10.3 %) and aerial dry weight (16.1 %) compared to control plants. Plants from treatment WS-2 showed reductions in leaf area (26.9 %), stem height (20.6 %) and increase in aerial dry weight (11.6 %) compared to control plants. The FSFI was similar in WS-1 and WS-2, with values of 0.022 and 0.025 N.m2, respectively but significantly higher (p < 0.05) than in control plants (0.015 N.m2 ). In wind-treated plants the rate of reproductive development (floral stage or FS) was significantly delayed (P< 0.05), decreasing from 0.26 units of FS.day^-1 in C to 0.23 units of FS.day^-1 in WS-1 and 0.16 units of FS.day^-1 in WS-2, extending in the latter cases the attainment of FS 5 in 2.9 days and 7.8 days, respectively compared to control plants. A numerical model of the young sunflower stem under dynamic lateral action was studied. Based on its external dimensions and distribution and mechanical properties of its constitutive tissues, a 3-D finite element model of the stem was built. Bending due to lateral wind load was simulated. Stresses and strains generated in the surface and inside the stem were calculated.

Morphological changes observed during the present study can be attributed to thigmomorphogenetic responses and internal hormonal regulation caused by stem bending and leaf movement by wind action. Stem bending in the simulation generates a significant mechanical stress at vascular level. These observations could help to explain why a mechanical perturbation is highly effective to induce physiological and growth changes at early stages of plant development.