Changes in Ultrastructure and Abscisic Acid Level, and Response to Applied Gibberellins inTaxus maireiSeeds Treated With Warm and Cold Stratification

Abstract

Seeds ofTaxus maireiare known for their deep dormancy which can only be broken by a procedure involving warm stratification followed by cold stratification. Treatments with alternating temperatures of 25/15 or 23/11 °C (12 h light) for 6 months followed by 5 °C for 3 months were successful in overcoming seed dormancy. After 6 months of warm stratification, cytological changes observed included: enlargement of the embryo; a decrease in the number of lipid bodies; appearance of ER; and increases in mitochondria, plastids, dictyosomes, vacuoles and microbodies in the shoot apical meristem. Cold stratification following the warm treatment induced cell division, and one or two distinct nucleoli in the shoot apical meristem cells were observed. Both warm and cold stratification reduced endogenous ABA concentrations from the original 8888 pg per freshly harvested seed to 392 and 536 pg, respectively. Treatment with exogenous gibberellins after seeds had been warm-stratified showed that GA4and GA7were effective at promoting seed germination, but GA3was not. These results suggest that the strong seed dormancy ofT. maireicould be caused by a high ABA content and underdevelopment of the embryos in freshly shed seeds. We conclude that warm stratification with alternating temperatures increases the growth of embryos by cell expansion and enlargement and decreases ABA content, but seeds still remain ungerminated. Cold stratification may induce the response to GAs and initiate cell division resulting in release from physiological dormancy and subsequent germination ofT. maireiseeds.