Semester

Fall

Date of Graduation

1999

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Agricultural & Extension Education

Committee Chair

Rajeev Arora.

Abstract

An in vitro laboratory protocol for assessing cold hardiness (CH) in rhododendrons based on electrolyte-leakage was developed. The raw leakage data were transformed into percent-adjusted injury, which showed high correlation with injury data based on visual observations. The percent-adjusted injury data---fitted to the Gompertz function coupled with the Jackknife method---allowed for reliable and statistically sound estimations of Rhododendron leaf freezing-tolerance (LFT). Utilizing this method, Tmax values for Rhododendron cultivars, species, and populations were determined and ranked relatively. Our observation of a continuous distribution of LFT in F2 and backcross (BC) populations derived from a R. catawbiense x R. fortunei cross suggested that as few as three genes with strong additive effects were controlling variation in CH. Differences in mid-winter freezing tolerance (FT) were primarily due to differences in acclimating ability among the progenies and were independent of non-acclimated FT. A 25-kDa dehydrin correlated with LFT differences among cold-acclimated F2 segregants. Moreover, its expression increased as LFT increased with both chronological age and developmental phase-change of wild and cultivated plants. The presence or absence of the 25-kDa dehydrin may serve as a genetic marker for 'super' CH in Rhododendron. Rhododendron seedlings (in F1, F2 and BC populations segregating for T max) increased their yearly average LFT by about 5--6°C during the period of juvenile growth. This change in CH is likely the reflection of an increased ability to cold acclimate, however, the degree of CH increment in individual progenies varied yearly. Bulked segregant analysis (a PCR-RAPD-based technique) was used to identify molecular markers in Rhododendron progenies segregating for CH. No markers were found to be associated with super cold-hardy genes, however a putative marker linked with low CH bulk was detected. The data from a nutritional study showed no correlation between nutrient status and seasonal CH of Rhododendron.

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