Green Period Characteristics and Foliar Cold Tolerance in 12 Iris
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leaves throughout the year, are un- Green Period Characteristics and Foliar Cold common among herbaceous perennials Tolerance in 12 Iris Species and Cultivars in the compared with deciduous ones which are leafless for some part of their an- Yangtze Delta, China nual cycle (Kikuzawa and Lechowicz, 2011). Because both evergreen and 1 deciduous iris phenotypes exist, better Danqing Li, Jiao Zhang, Jiaping Zhang, Kang Li, and Yiping Xia knowledge of the foliar color dynamics in winter and spring would be helpful ADDITIONAL INDEX WORDS. herbaceous perennial, digital image analysis, evergreen, to link with the factors that influence leaf lethal temperature, LT50 color retention and extended green period. Color changes during this pe- SUMMARY. Iris (Iris sp.) is a popular and widely planted herbaceous perennial. However, most iris species go dormant without any aesthetic quality for 5–6 riod in the transition zones have been months in the transition zone between the temperate and subtropical climates. To studied extensively in warm-season investigate the effects of species/cultivars, leaf shape, and air temperature condi- grasses. Pompeiano et al. (2014) found tions on the ability to stay green, 12 popular species and cultivars in the transition that zoysiagrass (Zoysia sp.) species/ zone were evaluated. Iris tested included the following species: roof iris (I. cultivars provided significantly longer tectorum), japanese iris (I. japonica), long leafed flag (I. halophila), yellow flag dormancy period than fine-leaved (I. pseudacorus), blood iris (I. sanguinea), japanese water iris (I. ensata), and small- species/cultivars. Also, color reten- flower iris (I. speculatrix) and the following cultivars: ‘Chinensis’ milky iris (I. lactea tion could be extended by increasing var. chinensis), ‘Bryce Leigh’ louisiana iris (I. hexagonae), ‘Black Swan’ german iris the photoperiod in months with (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 short daylengths (Esmaili and Salehi, evaluated the percentage of green leaves during winter retention and spring recovery 2012) and application of nitrogen using a digital image analysis (DIA). Green period during this study was calculated and trinexapac-ethyl in bermudagrass using predicted sigmoid curves based on the percentage of green leaves. The present [Cynodon dactylon (Richardson, 2002)]. study revealed that iris species/cultivars and air temperatures had considerable Longer color retention has been influence on the duration of the green period. Both evergreen and deciduous iris associated with poorer cold tolerance phenotypes exist with three different leaf shapes, among which the average green in a number of plants (Okeyo et al., period of fan-shaped leaf iris species and cultivars was the longest. Because there was 2011; Qian et al., 2001; Schwab no significant (P = 0.205) relationship between green period during this period and et al., 1996), which makes it hard to leaf lethal temperature (LT50), new cultivars with long green periods may be achieved without a simultaneous loss of cold tolerance in iris. selectively breed for cultivars with an extended green period and strong foliar cold tolerance. Previously, we ris, a genus of 300 species, is region, such as the local species roof preliminarily evaluated foliar cold tol- one of the most popular orna- iris, japanese iris, and small-flower iris erance and green period in six culti- I mental perennials in the North- (China Flora, 1985), long-leafed flag vars of german iris, among which ern Hemisphere (Austin and Waddick, and ‘Chinensis’ milky iris, which are ‘Bedtime Story’ stayed green the lon- 2005). However, a distinct drawback native to northeast or northwest China, gest (330 d/year), whereas ‘Caligula’ of this genus is that most species will and most horticulturally important was the shortest at 269 d and had be deciduous without any aesthetic species and cultivars (Han, 2008; Li much poorer cold tolerance than that quality as much as 5–6 months. Species/ et al., 2016; Tang et al., 2005). In this of ‘Bedtime Story’ in Hangzhou, cultivars with long green period (the regard, the existence of these irises in China (Wang et al., 2014). However, period when at least 50% of the leaves this region provides possibility for the relationship between green period are green) are preferred for landscape green period research from related and foliar cold tolerance is still not application. Obvious differences in the herbaceous species and cultivars that clear in iris. More iris species and green period of iris were found, and of differ obviously in foliar habits. cultivars need to be studied to have special interest is that several species Green period characteristics dur- a better understanding of the rela- and cultivars are evergreen in the ing winter green-down and spring tionship between these two traits. Yangtze Delta, China. The delta is green-up deserve more attention for This study quantified the winter located in the transition zone between most herbaceous ornamental peren- color retention (discoloration) and subtropical and temperate climates nials because they contribute to the spring recovery response of 12 iris (Zhang et al., 2005), with asynchrony plants’ ornamental value. Evergreen species/cultivars in a field environment in growth among species and higher plants, which can retain their functional in the Yangtze Delta, China. Moreover, species diversity (Loreau and Hector, 2001). Irises from different origins have adapted to the climate in this Units To convert U.S. to SI, To convert SI to U.S., Department of Horticulture, College of Agriculture multiply by U.S. unit SI unit multiply by and Biotechnology, Zhejiang University, 866 0.3048 ft m 3.2808 Yuhangtang Road, Hangzhou, Zhejiang, 310058, 2 2 China 0.0929 ft m 10.7639 2.54 inch(es) cm 0.3937 1 Corresponding author. E-mail: [email protected]. 25.4 inch(es) mm 0.0394 doi: 10.21273/HORTTECH03692-17 (°F – 32) O 1.8 °F °C(°C · 1.8) + 32 • June 2017 27(3) 399 RESEARCH REPORTS the relationship between green period, intelligent temperature and humidity plot (with 100% being full retention calculated using predicted sigmoid recorder (ZDR-F20; Zeda Instru- and 0% being completely withered). curves, and foliar cold tolerance, mea- ments, Hangzhou, China) moni- Digital images were taken with a digital sured using LT50,wasstudiedtopro- tored daily air temperature (Fig. 1). camera (EOS 60D; Canon, Tokyo, vide a theoretical basis for molecular In Table 1, relative chlorophyll Japan) and analyzed individually by marker-assisted breeding of new culti- content of functional leaves (the third MATLAB (version R2016a maci64; vars that combine the characteristics of and fourth actively growing leaves MathWorks, Natick, MA). Because a long green period and improved foliar from central part on both sides of a the leaves of several iris species/ cold tolerance. plant) was determined by a chlorophyll cultivars are long and perpendicular meter (SPAD-502 PLUS; Konica to the ground, the top-down image Materials and methods Minolta Sensing, Osaka, Japan) to used in turfgrass (Richardson et al., PLANT MATERIALS AND ESTABLISH- compare the leaf color differences 2001) was not applicable for deter- MENT. The experiment was conducted among 12 irises. Green period, namely mining the percentage of green leaves from Sept. 2014 to May 2016 on the number of days per year when the in iris. A tripod 1.2 m in height was mature iris populations at the percentage of green leaves in a popu- used to fix the camera, and the lens Resources Nursery for Flower Bulbs lation exceeding 50% (Guo et al., was mounted down at a 45° angle and Herbaceous Perennials, Zhejiang 2006), was obtained by visual rating from a horizontal axis. This allowed University, Hangzhou, China (lat. during 2012–14. Leaf width was the camera to be positioned in such 29°11#Nto30°33#N, long. 118°21#E measured at the maximum width of a way that the images taken would be to 120°30#E).On10Sept.2013, functional leaves. Leaf shape of iris similar to a person observing plants uniformly sized bare rhizomes of each species/cultivar was classified based in the plot. A script file was developed species/cultivar, with an average max- on their foliar shape and characteris- in the MATLAB programming lan- imum width measured by an electronic tics. Fan-shaped iris leaves are usually guage using color values in the hue, digital caliper (Syntek, Hangzhou, hypertrophied and arrayed into a fan saturation, and brightness (HSB) sys- China) and shown in Table 1, were shape. Sword-shaped leaf irises have tem. To selectively identify iris green obtained from clonally propagated erect and relatively hard leaves, whereas leaves, a hue ranging from 60° to 200°, plants. Experimental plots of each bar shaped-leaf ones have thin and a saturation ranging from 10% to 100%, species/cultivar planted at 20 plants/m2 soft blades. and a brightness ranging from 10% in a silt loam soil were 1.5 · 1.5 m with DIGITAL IMAGE ANALYSIS OF THE to 100% were adopted after preliminary three replications and arranged in PERCENTAGE OF GREEN LEAVES AND work on the pictures, and the number a complete randomized block design. THE CALCULATION OF GREEN PERIOD. of selected green pixels (selectPixCnt- Conventional management and fertil- Iris winter color retention and spring green) was obtained (Fig. 2). Simi- ization procedures for irises in East recovery were evaluated on 10-d inter- larly, the number of selected brown China were adopted as Hu and Xiao vals, using DIA techniques to quantify pixels for withered leaves was re- (2012) recommended. An on-site the percentage of green leaves for each corded as selectPixCnt-brown, which Table 1. Introduction, classification, rhizome size, and foliar characteristics of the 12 iris species/cultivars used to analysis green period characteristics and foliar cold tolerance.