| Ultraviolet-B radiation (UVB, 280–320nm) induces many plant defense compounds, including phenols. This study tested the hypothesis that UVB may precondition seedlings against temperature extremes. The effects of a prior treatment with low, ambient or twice-ambient levels of UVB on high and low temperature tolerance were investigated in cucumber (Cucumis sativus L.) and two conifer species: jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana Mill., BSP).; Exposure to UVB increased the accumulation of UV-absorbing phenols in all species. Low and ambient levels of UVB enhanced heat tolerance in cucumber. This was demonstrated by the decline in Membrane Injury Index (MII), improved survival and faster recovery of stressed seedlings. In contrast, twice-ambient levels of UVB increased lipid peroxidation and the plants became more heat-sensitive.; UVB pretreatment reduced needle browning in black spruce and MII in both conifer seedlings when they were challenged by a later heat and freezing stress. A logistic model was developed to describe MII response to a range of stress temperatures. MII varied depending on the UVB dose, exposure duration and type of stress. A longer exposure to low and ambient levels of UVB was more effective in reducing MII, compared to a higher dose or a shorter exposure. Such a reduction in MII was more pronounced during freezing than under heat stress.; Phenylalanine-ammonia-lyase (PAL) inhibitor, 2-amainoindan-2-phosphonic acid (AIP), suppressed the accumulation of phenolic compounds in UVB irradiated jack pine seedling. This, in turn, diminished the ability of UVB pretreatment to reduce MII under freezing temperatures. A functioning phenol biosynthetic pathway was, therefore, essential for UVB to induce frost hardening. These results suggest that at lower doses, UVB increases stress tolerance by promoting phenol production in plants.; UVB pretreatment also improved survival and biomass accumulation in black spruce, although not in jack pine, when the seedlings were transplanted to a boreal clear-cut in northern Ontario. Given the deficiency of ambient UVB in most greenhouses, supplemental UVB may be used to harden seedlings before they are transplanted into the field. |
