2018 ASHS Annual Conference

Occurrences of extreme temperature fluctuations during late winter/early spring can subject overwintering perennials to unseasonal de-acclimation. Consequently, these dehardened plants become vulnerable to returning freezing temperatures. Freeze-damage from such scenario to economically important horticultural species has been noted in recent years with increasing frequency. But little is known about the physiology of deacclimation response (timing and speed). To that end, mid-winter hardiness and seasonal de-acclimation were investigated during January to March 2014 in the leaves of 10 evergreen azalea cultivars (Rhododendron section Tsutsusi) (‘Changchunerhao’, ‘Dazhusha’, ‘Elsie Lee’, ‘Hongshanhu’, ‘Nuccio’s Pink Bobble’, ‘Shiyandujuan’, ‘Yudaizhirong’, ‘Zhuangyuanhong’, ‘Zihudie’ and ‘Zi’e’) grown under natural conditions in eastern China. Leaf samples were collected bi-weekly and leaf freezing tolerance (LFT) was evaluated using a temperature-controlled freeze-thaw protocol followed by ion-leakage-based injury assessment. Based on the mid-winter / cold-acclimated LFT, these cultivars were grouped as ‘more-hardy’ (‘Changchunerhao’, ‘Elsie Lee’, ‘Nuccio’s Pink Bobble’, ‘Shiyandujuan’ and ‘Yudaizhirong’) versus ‘less-hardy’. ‘Elsie Lee’ was the hardiest and ‘Dazhusha’ was the least-hardy in midwinter. Eight of the 10 cultivars first showed de-acclimation when daily mean temperature over two-week period preceding the LFT measurement was ~9.5 °C. De-acclimation for other two cultivars (‘Nuccio’s Pink Bobble’ and ‘Shiyandujuan’) was somewhat delayed and might have involved ‘de-acclimation-reacclimation’ cycling before eventual de-acclimation. Our data indicate that the ‘more-hardy’ group de-acclimated slower than the ‘less-hardy’ ones over the first half of the de-acclimation period. This trend reversed during second half of the de-acclimation period. Accordingly, ‘more-hardy’ and ‘less-hardy’ cultivars depicted a ‘curvilinear’ and ‘reverse curvilinear/linear’ de-acclimation kinetics, respectively. Though somewhat preliminary, these results may indicate suitability/ vulnerability of these cultivars to various scenarios of changing climate; for example: ‘more-hardy’ group may survive hard frosts after unseasonal warm spells during midwinter, whereas ‘less-hardy’ group could survive sudden frost in the early spring. ‘More-hardy’ cultivars generally had higher total soluble sugars (TSS) than ‘less hardy’ ones at acclimated state. TSS declined during de-acclimation in all cultivars and the loss was positively correlated with the loss in LFT. Leaf starch content generally followed opposite trend to that of TSS, i.e. it was at lowest during acclimated state and increased during de-acclimation.