EFFECTS OF TEMPERATURE AND OXYGEN ON SEED-GERMINATION AND SEEDLING GROWTH IN SUNFLOWER (HELIANTHUS-ANNUUS L)

After long dry storage (9 months at least), sunflower (Helianthus annuus L. cv. Mirasol) seeds are not dormant. They germinate in a wide temperature range (5-40-degrees-C) with a thermal optimum at ca 25-degrees-C. High temperatures (45-degrees-C) totally prevent germination. Pericarp and seed coat inhibit germination at high temperatures or in atmospheres deprived of oxygen. The optimal temperature for seedling growth is also ca 25-degrees-C. Hypocotyl growth seems to be less vigorous at low temperatures than root growth. The hypocotyl elongates faster in hypoxia than in air, whereas root elongation is inhibited when seedlings are deprived of oxygen. Seeds can germinate at 40-degrees-C, but the growth of seedlings is inhibited even when later transferred to 25-degrees-C. Incubation at 45-degrees-C induces loss of ability of seeds to germinate and of seedlings to grow at 25-degrees-C.