Iris, a genus of 300 species, is one of the most popular ornamental perennials in the Northern Hemisphere (Austin and Waddick, 2005). However, a distinct drawback of this genus is that most species will be deciduous without any aesthetic quality as much as 5–6 months. Species/cultivars with long green period (the period when at least 50% of the leaves are green) are preferred for landscape application. Obvious differences in the green period of iris were found, and of special interest is that several species and cultivars are evergreen in the Yangtze Delta, China. The delta is located in the transition zone between subtropical and temperate climates (Zhang et al., 2005), with asynchrony in growth among species and higher species diversity (Loreau and Hector, 2001). Irises from different origins have adapted to the climate in this region, such as the local species roof iris, japanese iris, and small-flower iris (China Flora, 1985), long-leafed flag and ‘Chinensis’ milky iris, which are native to northeast or northwest China, and most horticulturally important species and cultivars (Han, 2008; Li et al., 2016; Tang et al., 2005). In this regard, the existence of these irises in this region provides possibility for green period research from related herbaceous species and cultivars that differ obviously in foliar habits.
Green period characteristics during winter green-down and spring green-up deserve more attention for most herbaceous ornamental perennials because they contribute to the plants’ ornamental value. Evergreen plants, which can retain their functional leaves throughout the year, are uncommon among herbaceous perennials compared with deciduous ones which are leafless for some part of their annual cycle (Kikuzawa and Lechowicz, 2011). Because both evergreen and deciduous iris phenotypes exist, better knowledge of the foliar color dynamics in winter and spring would be helpful to link with the factors that influence color retention and extended green period. Color changes during this period in the transition zones have been studied extensively in warm-season grasses. Pompeiano et al. (2014) found that zoysiagrass (Zoysia sp.) species/cultivars provided significantly longer dormancy period than fine-leaved species/cultivars. Also, color retention could be extended by increasing the photoperiod in months with short daylengths (Esmaili and Salehi, 2012) and application of nitrogen and trinexapac-ethyl in bermudagrass [Cynodon dactylon (Richardson, 2002)].
Longer color retention has been associated with poorer cold tolerance in a number of plants (Okeyo et al., 2011; Qian et al., 2001; Schwab et al., 1996), which makes it hard to selectively breed for cultivars with an extended green period and strong foliar cold tolerance. Previously, we preliminarily evaluated foliar cold tolerance and green period in six cultivars of german iris, among which ‘Bedtime Story’ stayed green the longest (330 d/year), whereas ‘Caligula’ was the shortest at 269 d and had much poorer cold tolerance than that of ‘Bedtime Story’ in Hangzhou, China (Wang et al., 2014). However, the relationship between green period and foliar cold tolerance is still not clear in iris. More iris species and cultivars need to be studied to have a better understanding of the relationship between these two traits.
This study quantified the winter color retention (discoloration) and spring recovery response of 12 iris species/cultivars in a field environment in the Yangtze Delta, China. Moreover, the relationship between green period, calculated using predicted sigmoid curves, and foliar cold tolerance, measured using LT50, was studied to provide a theoretical basis for molecular marker-assisted breeding of new cultivars that combine the characteristics of a long green period and improved foliar cold tolerance.
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