Hardy buds of Azalea spp. were subjected to a sublethal heat stress of 45°C for 2 hours. Two hours after treatment the low temperature exotherms (LTE's) of the primordia were determined by differential thermal analysis (DTA). The LTE's of buds exposed to heat stress were either fewer in number and/or occurred at higher temperatures than in controls. Visual examination of the flower buds following DTA confirmed that treated buds were killed at higher temperatures than control buds. Cold hardy buds which have been pretreated as described also have lower resultant hardiness compared to controls as evidenced by controlled freezing experiments. These studies suggest the presence of an ice barrier within the buds of the plants which avoid freezing by supercooling, and that the barrier is partially overcome by sublethal heat stress.
Biodynamics is a form of organic agriculture first described in the 1920s by Rudolph Steiner, and practitioners can become certified biodynamic farmers by following specified practices. What distinguishes biodynamic from organic certification is the required use of nine preparations thought to improve soils and increase crop yields. This literature review focuses on the published, peer-reviewed science behind the use of biodynamic preparations, with the goal of providing objective information to extension educators, including Master Gardeners.
Leaf disks of Rhododendron cv. `English Roseum' were vacuum infiltrated with buffered solutions of L- or D-phenylalanine. Controls were infiltrated with buffer. Following a 24 hour period under lights (23°C, 16 hr light: 8 hr dark), the disks were cold shocked at 0°C for one hour, then held under the same light regime for three days. Disks were then subjected to a series of cold treatments, thawed, and analyzed for damage visually and by phenolic leakage. Disks infiltrated with D-phenylalanine consistently showed greater damage at warmer temperatures than did disks infiltrated with the L isomer. We believe the inhibitory action of D-phenylalanine upon phenylalanine ammonia-lyase is responsible for this decrease in cold hardiness.
It has been previously shown that dormant, cold-hardy floral buds of Azalea possess layers of highly lignified and suberized cells below the bud axis and beneath each bud scale. Two species of deciduous Azalea were analyzed bi-weekly using differental thermal analysis (DTA) throughout their dormant season to determine the development of cold hardiness as denoted by low temperature exotherms (LTEs). Other buds collected at the same time were observed using fluorescence microscopy to document the relationship between the development of the barrier and the onset of cold hardiness. Preliminary analysis showed when buds were maximally cold hardy the barrier was most intact, and as buds began to lose hardiness, the layer started to degrade. These results suggest that in fact this layer of cells does act as the long-proposed bud barrier. In a comparison between the species, the hardier species (R. japonicum) was found to have a denser layer of phenolic-rich cells compared to buds of the less hardy species (R. occidentale). This finding further supports the relationship between the layer of cells and the existence of cold hardiness in bud tissues.
The change in the cold hardiness of Rhododendron (cv. `English Roseum' following chronic exposure to ultraviolet-B radiation (280-320 nm) was studied. Leaf disks removed from ultraviolet-B exposed plants exhibited a greater tolerance to freezing temperatures than plants which received no ultraviolet-B exposure. Visual browning and percent phenolic leakage indicated that UVB-exposed leaf disks were killed at -11 °C, while control disks were killed at -8°C. The increase in phenolics seen in UV-B exposed plants most likely contributed to their increased resistance to cold temperature through the synthesis of cell-wall associated components such as lignin and suberin.