Abstract:
The leaching of water soluble phenolic compounds from
cold stressed Rhododendron leaves was found to be a reliable
indicator of tissue damage. In sublethally stressed leaf
tissues, more and novel phenolic compounds appeared and were
hypothesized to be precursors of cell wall components such
as lignin and suberin. Subsequent radiolabeling of
sublethally cold stressed tissues showed a preferential
incorporation of the phenolic precursor phenylalanine into
cell walls. The appearance of more and new phenolic
compounds in intact leaves was found in four varieties of
artificially cold acclimated Rhododendron.
Phenolic polymers including suberin and lignin were
also found in cold hardened Azalea (Rhododendron sp.) floral
buds. Hardy Azalea floral buds appear to possess an ice
nucleation barrier within bud tissues, which seems to
prevent ice present in the stem and scales from nucleating
water in the primordia. Non-hardy buds do not have such
barriers. This physical barrier can be removed by a
sublethal heat treatment of the buds; treated buds showed
decreased hardiness as measured by differential thermal
analysis (DTA), evidenced by a decrease in the number of low
temperature exotherms (LTE's). Histochemical and
photomicrographical studies suggest a lignified, suberized
barrier occurs at the base of bud scales and pedicels, and
beneath the bud axis.
