Seed development, arrest and germination Seed –Embryo –Seed coat Thick walled cells, waxy –Nutritive tissue Endospermic vs. non-endospermic

Seed development, arrest and germination Multiple hormone controls (Fig ) GA and IAA correlated with rapid cell division and differentiation ABA promotes arrest and dormancy –Via dehydration & inactivates many receptors At maturation arrest occurs– embryo inactive and seed dehydrated

Seed development, arrest and germination Germination = > imbibition of water > resumption of metabolism - hydrolitic enzymes mobilize ‘food’ reserves - Carbohydrate respired &/or gluconeogenesis of fats - resumption of embryo expansion > seed coat ruptures – radicle (first root) emerges Seeds time germination w/ sensors E.g. cryptochrome, phytochrome

GA promotes germination In corn and other grasses, 5. GA also promotes also lipase and protease production IAA stimulates seedling growth

Rest vs. dormancy Rest = germinates w/ water, warmth (25-45 o C) and O 2 Dormancy = seeds req’ more developmental signals –Maturation of embryo req’d =After-ripening –Seed coat impermeable to water and O 2 Scarification req’d –Leaching of inhibitors such as ABA, phenolics, coumarin, etc. –E.g. many desert annuals –Chilling Stratification req’d –Light Sensor is phytochrome