Seed studies show complex story in shrub survival
Several years ago my interests in the plant cell division cycle
took my group into research on seed dormancy. We soon learned that dormancy is
a complex topic, as is well illustrated by the work of Karlsson et al.
(Linköping, Sweden
and Murfreesboro, Tennessee, pp. 323–330). This group has studied dormancy and germination in a
Mediterranean shrub, Viburnum tinus. The authors showed that removal of
the pericarp (or at least the exocarp and mesocarp) is essential for
germination, and thus the normal consumption of the fruit by birds plays a
major role in seed dispersal. The seeds were exposed to an artificial annual
Mediterranean-type temperature cycle or to one of three constant diurnal cycles
with no seasonal variation. Post-harvest embryo growth and emergence of radicle
and shoot were monitored for 18 months, generating a complex data set. If the
artificial annual cycle started with ‘winter’ (as in nature), post-harvest
embryo growth started at about 30 weeks; root emergence was first seen at 38
weeks, with maximum germination (defined by root emergence) at 50 weeks. Shoot
emergence was not detected until approx. week 53, with the plateau value being
attained at about week 60. If, however, the annual regime had started with
‘summer’, all these timings came forward by up to 20 weeks. With seeds not
exposed to annual fluctuations, the 5/15 °C and the 15/25 °C regimes inhibited root and shoot emergence (although there was
embryo growth at 5/15 °C), whereas the seeds exposed to constant 10/20 °C were somewhat similar to those in the
‘summer-first’ annual cycle. From their data, the authors conclude that a weak
physiological dormancy is broken, normally over a period of several weeks (with
no particular temperature requirement), that roots emerge in the first spring
after seed/fruit ripening and continue to grow, and that seedling establishment
is completed in the second spring when the shoot emerges – a complex situation
indeed.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
