Freeze-thaw injury and recovery were studied in unhardened seedlings of Scots pine (Pinus sylvestris L.) at the end of the second growing season. Visual damage scoring, microscopy, gas exchange, and plasma membrane H + -ATPase activity were used to determine the degree of damage to the needles. The measurements were performed immediately after the frost treatments and after a 21-day recovery period under favorable conditions. The first signs of injuries were found by light microscopy immediately after the frost treatments in the mesophyll cells of needles exposed to -2°C. Low H + -ATPase activity indicated that injuries occurred at -3.5°C and gas exchange at -5°C. The seedlings exposed to -6.5°C or below lost their needles and died. The seedlings exposed to -5°C showed incomplete recovery and irreversible damage after 21 days when assessed structurally and by gas exchange and visual scoring. At the microscopic level, recovery was complete in the needles exposed to -3.5 and -2°C. The needles subjected to -3.5°C showed high H + -ATPase activity, indicating ongoing repair. Accordingly, the temperature range for cellular damage to unhardened needles was between -2 and -5°C, depending on the method used, but the critical temperature for irreversible damage was between -3.5 and -5°C.Résumé : Les dommages dus à l'alternance du gel et du dégel et la récupération ont été étudiés chez des semis non endurcis de pin sylvestre (Pinus sylvestris L.) à la fin de la deuxième saison de croissance. L'évaluation visuelle des dommages, la microscopie, les échanges gazeux et l'activité de l'H + -ATPase dans la membrane plasmique on été utilisés pour déterminer le degré de dommages aux aiguilles. Les mesures ont été effectuées immédiatement après les traitements de gel et après une période de récupératon de 21 jours dans des conditions favorables. Les premiers signes de dommages ont été observés au microscope optique immédiatement après les traitements de gel dans les cellules de mésophylle des aiguilles exposées à -2°C. La baisse d'activité de l'H + -ATPase indiquait que des dommages survenaient à -3°C et les échanges gazeux, à -5°C. Les semis exposés à -6,5°C ou à une température plus froide ont perdu leurs aiguilles et sont morts. Les semis exposés à -5°C n'avaient pas complètement récupéré et avaient subi des dommages irréversibles après 21 jours lorsqu'ils furent évalués structurellement, par échanges gazeux et visuellement. Au niveau microscopique, la récupération était complète chez les aiguilles exposées à -3,5 et 2°C. L'activité de l'H + -ATPase était élevée chez les aiguilles soumises à -3,5°C, ce qui indiquait que le processus de réparaton était en cours. Par conséquent, l'écart de température qui endommageait les cellules des aiguilles non endurcies se situait entre -2 et -5°C selon la méthode utilisée, mais la température critique qui causait des dommages irréversibles se situait entre -3,5 et -5°C.[Traduit par la Rédaction]
Light microscopic structure of needles of Scots pine (Pinus sylvestris L.) in relation to air pollution and needle element concentrations in S.E. Finland and the Karelian Isthmus, N.W. RussiaAbstractmThe mesophyll structure of current and second year needles of mature Scots pines (Pinus sylvestris L.), growing on a survey line from the Karelian Isthmus (N.W. Leningrad region, Russia) to S.E. Finland, was studied at the light microscopic level in 1992 and 1994. The area is affected by sulphur dioxide and nitrogen oxide emissions from Russia (200 000±285 000 ton/year SO2 and about 100 000 ton/year NOX), Estonia (140 000±320 000 ton/ year SO2 and 30 000±50 000 ton/year NOX) and Finland (6000±9000 ton/year SO2 and 23 000±25 000 ton/year NOX) and by unknown amounts of neutralizing compounds from Estonia. Mean concentrations of SO2 were between 1.0 and 1.5 mg m ±3 and those of ozone between 52 and 61 mg m ±3 during growing seasons of 1991±1994. The concentrations tended to be lower in 1993/94 than in 1991/92. The results showed cellular changes related to exposure to sulphur dioxide, ozone and acid wet deposition. The percentage of cells with injuries related to acid deposition was higher in 1994 than in 1992, possibly due to higher reduction of neutralizing than of acidifying compounds in the main emission areas, while the percentage of cells with injuries related to sulphur dioxide correlated positively with needle sulphur concentrations and showed a declining trend between Karelian Isthmus and S.E. Finland in the needles sampled in 1992. These changes and those related to ozone were considerably less numerous in 1994 than in 1992, possibly on account of the lower concentrations of SO2 and ozone in 1993/94 than in 1991/92, especially in early summer, the time of the most sensitive phase of needle growth. In addition, periods of lower wet deposition in summer 1992, and considerably lower temperatures in autumn/winter 1992 than in 1994 may have enhanced the injuries in the needles collected in October/December 1992, relative to those collected in 1994. The results of the microscopic work agreed well with the simultaneous chemical measurements, pointing to the usefulness of cellular analysis for diagnostic work in the field.
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