Читать книгу Horticultural Reviews, Volume 49 - Группа авторов - Страница 24

Cold hardiness is the major limiting factor determining where oakleaf hydrangea can be cultivated, and therefore is a breeding priority in order to expand the cultivated range. Current hardiness estimates indicate USDA zone 5a may be the extent of cold hardiness for the species (Dirr et al. 1993; Halcomb and Reed 2012), although screening wild germplasm from the northern extent of the latitudinal cline may identify variation in cold tolerance (Hurme et al. 1997; Friedman et al. 2008; Pagter et al. 2010). Indeed, a latitudinal cline for midwinter cold hardiness is found in wild collected oakleaf hydrangea seedlings, with northern populations generally being more cold hardy than southern populations (Sherwood et al. 2019).

Because winters are variable and unpredictable, controlled freezing experiments offer the most reliable information on cold tolerance at a given point during the winter (Pagter and Williams 2011; Hokanson and McNamara 2013; Pagter et al. 2014). These controlled freezing experiments can be used to identify populations or individuals that are more cold hardy or that deacclimate later than average (McNamara and Hokanson 2010). Identifying populations with increased cold tolerance or deacclimation resistance can provide an advantageous starting point for improvement. Although midwinter hardiness was not significantly different, the variation in deacclimation between ‘Alice’ and ‘Alison’ in controlled freezing tests (Dirr et al. 1993) indicates that genetic variation for winter survival traits may exist in H. quercifolia. A similar pattern was detected between H. macrophylla and H. paniculata where the latter, in addition to being far hardier in midwinter, retained its ability to withstand cold temperatures longer into an experimental warming period (Pagter et al. 2008a,b, 2011a,b). However, H. paniculata deacclimated faster than H. macrophylla, suggesting that midwinter hardiness and rate of deacclimation may need to be different breeding objectives.