The mechanisms of rapid seed germination in Caragana species adapted to low mean annual precipitation

Key message The rapid seed germination of Caragana species adapted to arid environments is closely related to their high seed coat permeability, mist absorption, high GA promotion, and reduced ABA inhibition. Abstract Caragana species is adapted to a low-precipitation environment by rapid seed germination, and this trait has been widely used for afforestation in arid and semiarid regions of China. In this study, seed coat structure, water permeability, mist/dew absorption, gibberellin (GA), and abscisic acid (ABA) biosynthesis inhibitor on seed germination were measured and compared among six Caragana species growing in different mean annual precipitation (MAP) environments. The results showed that three Caragana species from low-MAP environments had a relatively loose seed epidermal cuticle structure, distinct tubular structure of the palisade layer between the epidermal cuticle and osteosclereids layer, high water permeability of the seed coat, rapid mist/dew absorption, rapid seed germination, and higher final germination rate. However, seeds of the three Caragana species from high-MAP environments had dense seed epidermal cuticle structures and lamellar tubular structures of the palisade layer between the epidermal cuticle and osteosclereids layer, and exhibited poor or scarce water permeability of the seed coat. Uniconazole, GA biosynthesis inhibitor, inhibited seed germination in C. korshinskii from low-MAP environments, and nordihydroguaiaretic acid and fluorenone, ABA synthesis inhibitors, prompted seed germination in C. stipitata from high-MAP environments. The findings suggest that the rapid seed germination of Caragana species is closely related to their high seed coat permeability, mist absorption, high GA promotion, and reduced ABA inhibition to adapt to low-MAP environments.