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We assessed the effects of near-lethal heat stress on bud break, heat-shock proteins (HSPs) and ubiquitin in hybrid poplar (Populus nigra (L.) Charkowiensis x P. nigra (L.) incrassata). Shoots, with 10-15 buds each, were collected from September to March and exposed to temperatures between 20 and 60 degrees C for 2 h. Shoots were then placed in a greenhouse at 18-22 degrees C with supplemental light and cumulative bud break was recorded over a 4-week period. Samples of bud tissues were collected during and up to 96 h after heat treatment for protein analysis. De novo synthesis of proteins was monitored by exposing excised buds to [(35)S]-methionine for 3 h before, during, or after heat treatment. Heat treatments of 40-45 degrees C resulted in both a release from endodormancy and a decrease in thermal units needed for bud break during ecodormancy. The response to near-lethal heat stress was complex and was affected by intrinsic thermal sensitivity. Heat treatments were least effective during August and became progressively more effective as endodormancy progressed. In the later stages of ecodormancy, a heat treatment of 45 degrees C either inhibited bud break or killed the buds. Although temperatures of 42.5 to 45 degrees C inhibited incorporation of [(35)S]-methionine into proteins for at least 48 h, several HSPs were synthesized in response to temperatures of 40-45 degrees C. Immunoblots indicated that one of the heat-induced proteins was immunologically related to HSP70. Increases in free and conjugated forms of ubiquitin were also observed in response to heat treatment. Production of HSPs and ubiquitin, however, was not consistently associated with the heat treatments that induced the highest percentage of bud break. The roles of heat-induced protein degradation, HSPs, and ubiquitin in overcoming dormancy by near-lethal heat stress are discussed.

Poplar branches were ringed in late spring to determine whether the interruption of the phloem flow could induce the accumulation of vegetative storage proteins (VSPs) in the bark of adult trees. Eight days after ringing, an increased deposition of starch as well as a premature rise in the soluble-protein level occurred in the bark tissues located 1 cm above the ring. Changes in the SDS-PAGE pattern of bark proteins were characterized by the accumulation of three polypeptides (32, 36 and 38 kDa), which exhibited the same molecular weight as VSPs described in poplar bark during winter, cross-reacted to antibodies raised against a poplar VSP, and bound to several lectins in the same way as poplar bark VSPs. These results indicate that during the vegetative period, ringing induces the accumulation of VSPs in the bark of poplar.

Summary Overwintering bark of poplar ( Populus x euramericana cv. ›Robusta‹) contains three major storage polypeptides with 32, 36 and 38 kDa. They exhibit very similar physicochemical and immunological properties, and could be related to a storage polypeptide with 32 kDa isolated from the wood of dormant poplars. The present study describes the structural and immunological homologies between vegetative storage proteins in bark and wood of poplar.

Sublethal heat stress has been shown to decrease or eliminate deep supercooling of flower buds in woody plants and to release plants from endodormancy. Experiments were conducted to characterize the effect of heat stress on endodormancy and ecodormancy in peach (cv Loring) and two hybrid poplars. Protein synthesis (de novo) and patterns of protein expression were also monitored. In order to determine optimum treatment temperatures, shoots, collected September-March, were exposed to a range of temperatures (35-60 C) under wet or dry conditions for 1-6 h. Shoots were then placed in the greenhouse and cumulative budbreak was monitored over 4 weeks. Samples of bud and bark tissues were collected during and up to 72 h after heat treatment for SDS-PAGE analysis. Data indicate: 1) twigs must be immersed in water for the heat treatments to be effective; 2) heat treatments resulted in a release from endodormancy and a decrease in thermal units needed for budbreak during ecodormancy; 3) 40 C for 2-4 h was optimum in fall and late winter whereas 45 C was the optimum temperature to induce budbreak in midwinter; 4) optimum temperature for peach floral buds (37.5 C/2h) was lower than for vegetative buds (40 C/4h), and 5) heat treatments also decreased cold hardiness. Protein synthesis decreased significantly following heat treatment but was significantly greater than controls (room temp) 24-48 h after heat treatment.