Dissecting seed dormancy and germination in Aquilegia barbaricina, through thermal kinetics of embryo growth.

Threshold-based thermal time models provide insight into the physiological switch from the dormant to the non-dormant germinating seed. This approach was used to quantify the different growth responses of the embryo of seeds purported to have morphophysiological dormancy (MPD) through the complex phases of dormancy release and germination. Aquilegia barbaricina seeds were incubated at constant temperatures (10-25 °C) and 25/10 °C, without pre-treatment, after warm+cold stratification (W+C) and GA3 treatment. Embryo growth was assessed and the time of testa and endosperm rupture scored. Base temperatures (Tb ) and thermal times for 50% (θ50 ) of embryo growth and seed germination were calculated. W+C enabled slow embryo growth. W+C and GA3 promoted rapid embryo growth and subsequent radicle emergence. The embryo internal growth base temperature (Tbe ) was ca. 5 °C for W+C and GA3 -treated seeds. GA3 treatment also resulted in similar Tb estimates for radicle emergence. The thermal times for embryo growth (θe50 ) and germination (θg50 ) were four- to six-fold longer in the presence of GA3 compared to W+C. A. barbaricina is characterised by a multi-step seed germination. The slow embryo growth during W+C reflects continuation of the maternal programme of development, whilst the thermal kinetics of both embryo and radicle growth after the removal of physiological dormancy are distinctly different. The effects of W+C on the multiphasic germination response in MPD seeds are only partially mimicked by 250 mg·l-1 GA3 . The thermal time approach could be a valid tool to model thermal kinetics of embryo growth and radicle protrusion.