Green period characteristics and foliar cold tolerance in 12 iris species and cultivars in the Yangtze Delta, China

Abstract

Iris (Iris sp.) is a popular and widely planted herbaceous perennial. However, most iris species go dormant without any aesthetic quality for 5–6 months in the transition zone between the temperate and subtropical climates. To investigate the effects of species/cultivars, leaf shape, and air temperature conditions on the ability to stay green, 12 popular species and cultivars in the transition zone were evaluated. Iris tested included the following species: roof iris (I. tectorum), japanese iris (I. japonica), long leafed flag (I. halophila), yellow flag (I. pseudacorus), blood iris (I. sanguinea), japanese water iris (I. ensata), and small-flower iris (I. speculatrix) and the following cultivars: ‘Chinensis’ milky iris (I. lactea var. chinensis), ‘Bryce Leigh’ louisiana iris (I. hexagonae), ‘Black Swan’ german iris (I. germanica), ‘Careless Sally’ siberian iris (I. sibirica), and ‘Loyalty’ japanese water iris (I. ensata). We conducted a 2-year field study on mature iris populations and evaluated the percentage of green leaves during winter retention and spring recovery using a digital image analysis (DIA). Green period during this study was calculated using predicted sigmoid curves based on the percentage of green leaves. The present study revealed that iris species/cultivars and air temperatures had considerable influence on the duration of the green period. Both evergreen and deciduous iris phenotypes exist with three different leaf shapes, among which the average green period of fan-shaped leaf iris species and cultivars was the longest. Because there was no significant (P = 0.205) relationship between green period during this period and leaf lethal temperature (LT50), new cultivars with long green periods may be achieved without a simultaneous loss of cold tolerance in iris.