No abstract

The low temperature exotherms (LTE) of 1-year-old twigs of Haralson apple (Malus pumila Mill.), shagbark hickory (Carya ovata IMill.1 K. Koch), green ash (Fraxinus pennsylvanica Marsh), honey locust (Gleditsia triacanthos L.), American chestnut (Castanea dentata IMarshi Borkh.), and red oak (Querecs rubra L.) were determined by differential thermal analysis (DTA). In one type of experiment freezing during a DTA experiment was halted for up to 2.5 hours after part of the supercooled water had frozen at temperatures between -25 and -42 C. Upon resumption of cooling the freezing started within 2 C of the stopping temperature. In a second type of experiment living and dead cells were microscopically observed in the same ray after partial freezing in the DTA apparatus. In another experiment, the LTE persisted even after tangential and radial sectioning of the twig to 0.13 millimeters. In a final experiment the LTE of a single multiseriate ray of red oak had the same shape as the LTE of wood with many uniseriate rays.These experiments confirm that the deep supercooled water in woody xylem or pith freezes in numerous independent events over a span of as much as 20 C. The units which freeze in an event are single cells or small groups of cells. Ice grows very slowly if at all from these units, and water moves very slowly from unfrozen cells to frozen ones. Deep supercooling of ray parenchyma does not require an intact ray.

Mahi pwnila Mill. twigs were collected from September through De-cember and stored at 5°C until the low temperature exotherms of the xylem were determined by differential thermal analysis. During the differential thermal analysis, cooning was interrupted, and temperatures of 5 to -18°C were held for 0.4 to 10 hours before cooling to -500C was resumed. Control twigs were cooled to -50°C without interruption. Holding the twigs at 1.3 to -5°C shifted the start of the low temperature exotherm from about -20 to -30°C. Slightly higher (2.6°C) and lower (-10°C) temperatures were occasionally effective. The shift began within 20 to 30 minutes and increased progressively to 150 minutes. The acclmation was reversibly inhibited by N2 atmosphere.

Abies koreana Wilson is an endemic tree species that is facing critical population declines in Korea. To identify factors affecting the natural regeneration of A. koreana, we examined the role of seed pathogens in the overwintering survival of seeds in natural seedbeds on Mount Halla, Korea. In September 2003, seeds of A. koreana were placed on seedbeds in the following three types of sites: Sasa dominated, shaded by rocks, or forest floor; seeds were then recovered from beneath the snow cover in April 2004 and were analyzed for the occurrence of harmful microfungi. Racodium therryanum Thuem. was the fungus most often isolated from retrieved seeds and was also the most detrimental of the eight fungal species tested in a pathogenicity trial. In vitro, R. therryanum caused a total loss of germination ability in A. koreana seeds at 0 °C after 100 days. The infection rate of R. therryanum was negatively correlated with the seed germination rate. The infection rate of R. therryanum was highest on the forest floor and increased with the duration of snow cover. The occurrence of R. therryanum was temporally restricted to the period of snow cover and spatially to the thick A0 soil layer on the forest floor. This study suggests that R. therryanum may be a significant factor inhibiting the natural regeneration of A. koreana at the seed stage. Abies koreana Wilson est une espèce d’arbre indigène dont la population décline de façon inquiétante en Corée. Dans le but d’identifier les facteurs qui affectent la régénération naturelle d’A. koreana, nous avons étudié le rôle des pathogènes qui affectent les graines dans la survie des semences après qu’elles aient passé l’hiver dans des lits de germination naturels sur le mont Halla, en Corée. En septembre 2003, des graines d’A. koreana ont été placées sur des lits de germination dans les trois types suivants de sites : dominé par Sasa, à l’ombre de rochers ou sur le parterre forestier. Les graines ont ensuite été récupérées sous le couvert de neige en avril 2004 et examinées pour détecter la présence de champignons microscopiques nocifs. Racodium therryanum Thuem. est le champignon qui a été le plus souvent isolé des graines récupérées et ce champignon était également le plus nuisible des huit champignons testés dans un test de pathogénicité. In vitro, R. therryanum a causé la perte totale du pouvoir de germination des graines d’A. koreana après 100 jours à 0 °C. Le taux d’infection par R. therryanum était négativement corrélé au taux de germination des graines. Le taux d’infection par R. therryanum était le plus élevé sur le parterre forestier et augmentait avec la durée du couvert de neige. L’occurrence de R. therryanum était restreinte dans le temps à la période durant laquelle il y avait un couvert de neige et dans l’espace à l’épais horizon A0 du sol sur le parterre forestier. Cette étude indique que R. therryanum peut être un facteur significatif en inhibant la régénération d’A. koreana dès le stade de la semence