Preferential freezing avoidance localized in anthers and embryo sacs in
wintering Daphne kamtschatica var. jezoensis flower buds
visualized by MRI
Abstract
To explore diversity in cold hardiness mechanisms, high resolution
magnetic resonance imaging (MRI) was used to visualize freezing
behaviors in wintering flower buds of Daphne kamtschatica var.
jezoensis, which have no bud scales surrounding well-developed
florets. MRI images showed that anthers remained stably supercooled to
-14 ∼ -21°C or lower whilst most other tissues froze by -7°C. Freezing
of some anthers detected in MRI images at ∼ -21°C corresponded with
numerous low temperature exotherms and also with the “all-or-nothing”
type of anther injuries. In ovules/pistils, only embryo sacs remained
supercooled at -7°C or lower, but slowly dehydrated during further
cooling. Cryomicroscopic observation revealed ice formation in the
cavities of calyx tubes and pistils but detected no ice in embryo sacs
or in anthers. The distribution of ice nucleation activity in floral
tissues corroborated the tissue freezing behaviors. Filaments likely
work as the ice blocking barrier that prevents ice intrusion from
extracellularly frozen calyx tubes to connecting unfrozen anthers.
Unique freezing behaviors were demonstrated in Daphne flower
buds: preferential freezing avoidance in male and female gametophytes
and their surrounding tissues (by stable supercooling in anthers and by
supercooling with slow dehydration in embryo sacs) whilst the remaining
tissues tolerate extracellular freezing.