Three white clover cultivars of differing geographical origin,
namely AberHerald, Grasslands Huia
and Sandra, were grown in binary mixtures with perennial ryegrass
(cv. Preference) for a period of
3 years at Uppsala (59° 49′ N, 17° 39′ E)
where the length of the main growing season is c. 200 days.
Changes in growing point numbers, stolon length and stolon
weight from autumn until late spring
were followed by four to six destructive harvests each season.
Changes in carbohydrate content
(starch and water-soluble carbohydrates) were also determined
on six or seven occasions during the
same time. Cold hardening and dehardening were followed by
artificial freezing tests on stolons
removed from the plots on seven occasions during the last study
season. The rate of node production
was followed in the last spring. Temperatures in the stolon environment
were recorded during the last two study seasons.
All cultivars suffered a loss of stolon length during the winters,
with Huia being most affected. The
extent of recovery during spring varied between years. Late
spring frosts checked the growth of both
AberHerald and Huia more than that of Sandra. The accumulation
of starch appeared to depend on
autumn temperatures and there was a decline in starch content
during the winter. Sandra maintained
a higher content of water-soluble carbohydrates during the
winter and the spring than the other
cultivars. Sandra also achieved a higher degree of freezing
tolerance and dehardened later than both
AberHerald and Huia. However, temperatures in the stolon
environment always exceeded the degree
of freezing tolerance attained irrespective of cultivar.
It is concluded that the accumulation and utilization of
non-structural carbohydrates are important
in determining the extent to which a white clover cultivar survives
the winter and spring in a harsh
climate. Freezing tolerance per se does not seem to be the most
limiting trait, but in a climate where
frequent spring frosts occur, a late dehardening seems to be valuable.