Phylogenetic analyses in cornus substantiate ancestry of xylem supercooling freezing behavior and reveal lineage of desiccation related proteins.

The response of woody plant tissues to freezing temperature has evolved into two distinct behaviors: an avoidance strategy, in which intracellular water supercools, and a freeze-tolerance strategy, where cells tolerate the loss of water to extracellular ice. Although both strategies involve extracellular ice formation, supercooling cells are thought to resist freeze-induced dehydration. Dehydrin proteins, which accumulate during cold acclimation in numerous herbaceous and woody plants, have been speculated to provide, among other things, protection from desiccative extracellular ice formation. Here we use Cornus as a model system to provide the first phylogenetic characterization of xylem freezing behavior and dehydrin-like proteins. Our data suggest that both freezing behavior and the accumulation of dehydrin-like proteins in Cornus are lineage related; supercooling and nonaccumulation of dehydrin-like proteins are ancestral within the genus. The nonsupercooling strategy evolved within the blue- or white-fruited subgroup where representative species exhibit high levels of freeze tolerance. Within the blue- or white-fruited lineage, a single origin of dehydrin-like proteins was documented and displayed a trend for size increase in molecular mass. Phylogenetic analyses revealed that an early divergent group of red-fruited supercooling dogwoods lack a similar protein. Dehydrin-like proteins were limited to neither nonsupercooling species nor to those that possess extreme freeze tolerance.

Effects of formaldehyde-enriched mists on Pseudotsuga menziesii (Mirbel) Franco and Lobaria pulmonaria (L.) Hoffm.

The atmosphere in some areas is polluted with formaldehyde (HCHO); however, little is known about effects of HCHO on plants at concentrations resembling those in polluted areas. The effects of simulated fogwater enriched with HCHO on seedlings of Pseudotsuga menziesii (Mirbel) Franco (Douglas fir) and pendants of Lobaria pulmonaria (L.) Hoffm. were assessed. Plants were treated with HCHO-enriched fog (target concentrations of 100, 500, and 1000 microm) during five 4-night mist sessions. Growth and nitrogenase activity (acetylene reduction rate) for lichens and growth and timing of bud-break for Douglas fir were monitored. Nitrogenase activity was lowest in lichens treated at the highest HCHO concentration after all but the first mist session, and it declined significantly with increasing HCHO concentration after the final mist session (R(2) = 0.60, p = 0.02). However, differences in nitrogenase activity among treatments were generally not statistically significant (most p values from ANOVAs were >/= 0.20). Formaldehyde did not affect growth of the lichens. Budbreak of Douglas firs was slightly delayed and height growth was slightly depressed with increasing HCHO concentration, although effects were not statistically significant.

Effects of "near-lethal" stress on bud dormancy and stem cold hardiness in red-osier dogwood.

We studied the effects of "near-lethal" (NL, 47 degrees C for 1 h) heat stress, applied to intact shoots of red-osier dogwood (Cornus sericea L.) during early (October), deep (November) or late (December) dormancy, on bud dormancy release and development of stem tissue cold hardiness under natural conditions and at a constant temperature of 0 or 23 degrees C in the dark. The NL heat-stress treatment overcame bud dormancy when applied during the early and late stages of dormancy. During October and December, all plants in the 23 degrees C + dark post-stress environment broke bud within 35 and 12 days, respectively, whereas the corresponding values for days to bud break in the control plants were more than 150 and 110 days, respectively. Application of NL heat stress during deep dormancy caused only slightly earlier bud break compared to the control plants. In the 0 degrees C + dark post-stress environment, all NL heat-treated plants died within 9 weeks. Under natural post-stress conditions, bud break in plants receiving NL heat stress during early and deep dormancy occurred at the same time as in control plants, whereas bud break of plants receiving NL heat stress during late dormancy occurred 55 days earlier than in control plants. Under both natural and 23 degrees C + dark post-stress conditions, cold hardiness of plants receiving NL heat stress during early dormancy was similar to that of controls. Application of NL heat stress during deep dormancy hastened the rate of deacclimation under the 23 degrees C + dark post-stress conditions but had no effect on deacclimation under natural post-stress conditions. Application of NL heat stress during late dormancy enhanced deacclimation of plants in both the 23 degrees C + dark and natural post-stress environments.

Recovery of plants from "Near-Lethal" stress.

This study reports on the dieback and recovery of red-osier dogwood, Cornus sericea L. plants from "near-lethal" (NL, sublethal) stress after varying lengths of post-stress environment (PSE). Intact dormant stems were subjected to 47° C for one hour during either October, November or December, and then placed into either constant 0° C or 23° C (dark condition) or kept under natural conditions at Corvallis, OR. Plants exposed to NL-heat stress in October died prior to 9 weeks of 0° C PSE, while none of the plants from other PSE treatments showed signs of injury. For plants exposed to NL-heat stress during November and December, stemdieback occurred at 0° C after 12 and 15 weeks, respectively. None of the plants from the other PSE treatments were injured. Post-stress temperatures of 0° or 5° C following NL-heat in October were lethal while temperatures above 10° C allowed recovery. Post-stress exposure to 0° C injured excised stems within 48 h, whereas irreversible damage to whole plants occurred by two weeks. Dormant plants exposed in October to other stresses, e.g., freezing temperature and hydrogen cyanamide, at NL dosages showed that these stresses also caused plant dieback at 0° C and little or no dieback at 23° C PSE.