AUTHOR=Lim Chon-Chong , Krebs Stephen L. , Arora Rajeev 

TITLE=Cold hardiness increases with age in juvenile Rhododendron populations

JOURNAL=Frontiers in Plant Science

VOLUME=5

YEAR=2014

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2014.00542

DOI=10.3389/fpls.2014.00542

ISSN=1664-462X

ABSTRACT=<p>Winter survival in woody plants is controlled by environmental and genetic factors that affect the plant’s ability to cold acclimate. Because woody perennials are long-lived and often have a prolonged juvenile (pre-flowering) phase, it is conceivable that both chronological and physiological age factors influence adaptive traits such as stress tolerance. This study investigated annual cold hardiness (CH) changes in several hybrid <italic>Rhododendron</italic> populations based on <italic>T</italic><sub>max</sub>, an estimate of the maximum rate of freezing injury (ion leakage) in cold-acclimated leaves from juvenile progeny. Data from F<sub>2</sub> and backcross populations derived from <italic>R. catawbiense</italic> and <italic>R. fortunei</italic> parents indicated significant annual increases in <italic>T</italic><sub>max</sub> ranging from 3.7 to 6.4°C as the seedlings aged from 3 to 5 years old. A similar yearly increase (6.7°C) was observed in comparisons of 1- and 2-year-old F<sub>1</sub> progenies from a <italic>R. catawbiense</italic> × <italic>R. dichroanthum</italic> cross. In contrast, CH of the mature parent plants (&gt;10 years old) did not change significantly over the same evaluation period. In leaf samples from a natural population of <italic>R. maximum</italic>, CH evaluations over 2 years resulted in an average <italic>T</italic><sub>max</sub> value for juvenile 2- to 3-year-old plants that was 9.2°C lower than the average for mature (~30 years old) plants. A reduction in CH was also observed in three hybrid rhododendron cultivars clonally propagated by rooted cuttings (ramets)—<italic>T</italic><sub>max</sub> of 4-year-old ramets was significantly lower than the <italic>T</italic><sub>max</sub> estimates for the 30- to 40-year-old source plants (ortets). In both the wild <italic>R. maximum</italic> population and the hybrid cultivar group, higher accumulation of a cold-acclimation responsive 25 kDa leaf dehydrin was associated with older plants and higher CH. The feasibility of identifying hardy phenotypes at juvenile period and research implications of age-dependent changes in CH are discussed.</p>