Abstract
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 Rhododendron populations based on Tmax, an estimate of the maximum rate of freezing injury (ion leakage) in cold-acclimated leaves from juvenile progeny. Data from F2 and backcross populations derived from R. catawbiense and R. fortunei parents indicated significant annual increases in Tmax 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 F1 progenies from a R. catawbiense × R. dichroanthum cross. In contrast, CH of the mature parent plants (>10 years old) did not change significantly over the same evaluation period. In leaf samples from a natural population of R. maximum, CH evaluations over 2 years resulted in an average Tmax 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)—Tmax of 4-year-old ramets was significantly lower than the Tmax estimates for the 30- to 40-year-old source plants (ortets). In both the wild R. maximum 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.
Highlights
Woody perennials typically go through a juvenile period of growth in which they remain vegetative and do not respond to flowering stimuli
F2 POPULATION (Table 1) Comparison of 3- versus 4-year-old seedlings in this population revealed a significant 6.4◦C increase in mean Tmax in older plants, from −27.5◦C at age 3 to −33.9◦C at age 4. Frequency distributions of this population (Figure 1) show a normally distributed curve centered around mean Tmax values that shift toward greater cold hardiness (CH) after an additional year, still below the mature parental value of −43.0◦C The distribution of F2 phenotypes is consistent with a polygenic control of CH that has been reported for Rhododendron and other taxa (Guy, 1990; Lim et al, 1998b; Pellet, 1998)
Our study used Rhododendron populations segregating for CH to examine the relationship between juvenile and mature CH phenotypes
Summary
Woody perennials typically go through a juvenile period of growth in which they remain vegetative and do not respond to flowering stimuli This phase of development may last a few weeks in some plants to up to decades in certain shrub and tree species (Kramer and Kozlowski, 1979; Hartmann et al, 1997). Improvement in the CH of woody perennials for their suitability in northern climates has largely been attempted via classical plant breeding This includes recurrent phenotypic selection of juvenile progenies either based on their winter survival in the field or via laboratory-based CH tests which may include evaluation of cold acclimation (CA)—the timing, rate, and degree of CH increases during fall under inductive short-days and low temperatures. Anecdotal evidence indicates that field-grown juvenile seedlings of several landscape woody perennials are typically not as cold-hardy as their physiologically mature counterparts (Pellet, 1998; McNamara and Pellet, 2000), published literature on the relationship between juvenility, aging, and CH is scarce. Brown and Bixby (1976) observed that 1-month-old, coldacclimated black locust (Robinia pseudoacacia) seedlings were killed at 0◦C, whereas treated 3-month-old seedlings could survive −20◦C. McNamara and Pellet (2000) noted that stem tissues of 10- to 34-month-old Phellodendron sachalinense seedlings (half-sib populations) were hardy to ∼−5◦C in early www.frontiersin.org
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