Caprio, J. M. and Quamme, H. A. 1999. Weather conditions associated with apple production in the Okanagan Valley of British Columbia. Can. J. Plant Sci. 79: 129-137. An analytical method that generates χ 2 indices of association was used to determine the "critical" levels of daily weather occurrences associated with annual variations in apple production (Malus domestica Borkh.) in the Okanagan Valley of British Columbia over a 72-yr period, 1920-1991. The weather was recorded in the centre of the region (the Pacific Agri-Food Research Centre, Summerland). High temperatures during flower bud initiation in June (≥ 30°C) and during flower bud development in August (≥26°C) were associated with poor production the following year. Low temperature during November, December and February (critical value range, ≤-7°C to ≤-29°C) was the main climatic factor limiting apple production. These time periods correspond to the occurence of historical winter injury events. Daytime temperatures ≥5°C at times during January also affects apple production adversely, probably because warm weather leads to tree de-acclimation. Daily precipitation in March (≥1.0 mm) was associated with good production years which may indicate a requirement for early irrigation. Production was favoured by mild temperatures during bloom (≥21°C daytime, ≥11°C nighttime) and adversely affected by both very low temperatures (≤10°C nighttime) and unseasonably high temperatures (≥28°C daytime) coinciding with the temperature requirements of pollination and pollen tube growth. Good production years were also associated with a lack of low nighttime temperature (≤5°C) in spring, probably associated with frost in low lying areas. Hot, dry weather during August of the harvest year (≥33°C daytime, ≤2 mm) had a negative impact on apple production possibly because of loss in net photosynthesis, lower fruit size or apple sunburn. Warm weather during harvest (≥17°C daytime) favoured production, probably because of improved conditions for harvest operations and low fruit losses from autumn frosts.Key words: Malus domestica Borkh., tree fruit, climate, heat stress, spring frost, winter injury Caprio, J. M. et Quamme, H. A. 1999. Conditions météorologiques associées à la production des pommes dans la Vallée de l'Okanagan en Colombie-Britannique. Can. J. Plant Sci. 79: 129-137. Nous avons utilisé une méthode analytique produisant des indices d'association χ 2 pour déterminer les niveaux critiques des phénomènes météorologiques quotidiens reliés aux variations annuelles qui ont affecté la production des pommes (Malus domestica Borkh.) dans la vallée de l'Okanagan en ColombieBritannique pendant 72 ans, soit durant la période 1920-1991. La météo était enregistrée au centre de la région, plus précisément au Centre de recherches agroalimentaires du Pacifique à Summerland. Les températures élevées survenant durant l'initiation des boutons floraux en juin (≥30°C) et durant leur phase de développement en août (≥26°C) étaient associées à une faible production de fruits l'année suivan...
In acclimated winter twigs of Haralson apple (Pyrus Malus L.), a lag in temperature during cooling at a constant rate was observed at about -41 C by differential thermal analysis. The temperature at which this low temperature exotherm occurred was essentially unaffected by the cooling rate. During thawing there was no lag in temperature (endotherm) near the temperature at which the low temperature exotherm occurred, but upon subsequent refreezing the exotherm reappeared at a somewhat higher temperature when twigs were rewarmed to at least -5 C before refreezing. These observations indicate that a small fraction of water may remain unfrozen to as low as -42 C after freezing of the bulk water in stems. The low temperature exotherm was not present in twigs freeze-dried to a water content below 8.5% (per unit fresh weight), but it reappeared when twigs were rehydrated to 20% water. When freeze-dried twigs were ground to a fine powder prior to rehydration, no exotherm was observed. Previous work has shown that the low temperature exotherm arises from xylem and pith tissues, and that injury to living cells in these tissues invariably occurs only when twigs are cooled below, but not above the temperature of the low temperature exotherm. This study revealed that the low temperature exotherm resulted from the freezing of a water fraction, that the freezing of this water was independent of the freezing of the bulk water, that the exotherm was associated with some gross structural feature but not the viability of the tissue, and that injury to living cells in the xylem and pith was closely and perhaps causally related to the initial freezing of this water.Time-temperature profiles have been studied during the freezing of a number of plants (6-8), but in only rare instances has it been possible to demonstrate that sudden exothermic changes are associated with freezing injury to hardy plants. An exception is the flower primordia of dormant winter buds of hardy deciduous azaleas in which Graham (2) has found a distinct temperature rebound (exotherm) associated with injury at temperatures as low as -43 C. Also, the second supercooling point hypothesis proposed by (12) is based on calorimetric and microscopic studies of stems during freezing which reveal a calorimetric lag in the freezing process at temperatures slightly above the killing point and a discrete release of heat and loss of fluorescence of cells at the moment of injury.Recently, exotherms in apple twigs have been studied by Quamme (10) using differential thermal analysis. This technique involves measuring temperature differences between a test twig and a reference (dried twig) when both are cooled at the same constant rate. The freezing of water or any other phase change in the sample results in a temperature lag that is detected as a temperature rebound or exotherm on the differential thermal profile. Differential thermal analysis is more sensitive for detecting exothermic changes than conventional time-temperature plots of sample temperature during freez...
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