Ultrastructural analysis of the loss of desiccation tolerance in germinating Eremanthus erythropappus seeds

Eremanthus erythropappus(DC.) MacLeish is an important forest species native to the Brazilian savanna biome,an environment with well-defined rainy and dry seasons.Its seeds are desiccation tolerant,non-dormant and dispersed at the end of dry season.This exposes them to the first sporadic rains that can trigger germination but the subsequent dry days can compromise survival of the newly germinated seed.This study evaluates if the damages caused by drying of germinating E.erythropappus seeds,at the stage when they have lost desiccation tolerance,are lethal.The percentage of normal seedlings was evaluated when seeds with different imbibition times were dried to their initial water content and then pre-humidified and rehydrated.Desiccation tolerance was fully lost after 0.5 mm of hypocotyl-radicle protrusion(approximately 72 h) when the radicle had root hairs,a possible indicator of sensitivity to desiccation.Disintegration of cell contents and ultrastructural damage to the seeds subjected to drying were observed by electron scanning microscope.Root hairs disappeared after drying,reappearing after pre-humidification,possibly an attempt to increase the surface area of the radicle for water uptake.However,the hypocotylradicle axis became darkened,probably by antioxidant system failure,causing seed death.