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Abstract
Intact, mature apple (Malus pumila, Mill. cv. Richared Delicious) fruit exhibited 2 exotherms, at −2.1 ± 0.1°C and at −4.7 ± 0.3° during freezing. Freezing only through Exotherm 1 caused fruit softening. Freezing to inception of Exotherm 2 caused increased softening, and also slightly increased ion leakage. Freezing to termination of Exotherm 2 produced a 3rd increment of softening, greatly increased ion leakage, and reduced respiration. Only with freezing through termination of Exotherm 2 did visual injury symptoms occur; these symptoms were evident several hours after thawing, and apparently reflected lethal injury to the fruit. Injury was no greater after 3 months than after 1 month of storage at 0° following freezing.
Abstract
A simple technique for determining ion leakage from hardy woody tissues was applied to apple (Malus pumila Mill.) fruit tissue. Parameters for this application are presented. Leakage was unaffected by concentrations of 0.4 to 0.7 M sucrose in the incubating medium or by incubating time of 12 to 36 hours at 23°C but varied between samples taken from different areas of the same fruit. Preparatory procedure to obtain the smallest sample without excessive injury was determined. The technique reproducibly identified increases in ion leakage of 5% or more over the controls.
Abstract
Richared Delicious’ apple (Malus pumila, Mill.) fruit tissues exhibited 2 distinct freezing points (exotherms) during freezing. Exotherm 1 occurred at −1.5° to −2.2°C and Exotherm 2 occurred between −4.2° and −7.7°C. The inception temperature of Exotherm 2 varied considerably both within a fruit and among different fruits. During fruit maturation from August to October, no significant change in freezing pattern occurred, although soluble solids increased 55% during the period tested. Inception of Exotherm 1 caused no distinct change in physical properties of cortex tissue. Inception of Exotherm 2 was associated with significant softening and increased ion leakage from the tissue. Just beyond termination of Exotherm 2, the tissue lost its capacity to exhibit multiple exotherms on thawing and refreezing, and underwent massive changes in firmness, ion leakage, and respiration; it was concluded that freezing to this point was lethal to the tissue. The concept that tissue injury is related to specific points on the freezing curve of apples is presented.
Twelve shrub rose cultivars were evaluated for pest resistance in the southern Blue Ridge mountains under high humidity and rainfall (1.34 inches per week average during the 1994 and 1995 growing seasons). `Albo plena', `Blanc Double de Coubert', `Fru Dagmar Hastrup', `Roseraie de l'Hay', R. rugosa `Alba', `Sarah van Fleet', and `Topaz Jewel' were highly resistant to black spot and Cercospora sp. leaf spot. `Alba Meidiland', `Linda Campbell', `Pink Meidiland', and `Scarlet Meidiland' were susceptible, while `Bonica' displayed intermediate resistance to both diseases. `Sarah van Fleet' foliage and the flowers of `Albo plena', `Blanc Double de Coubert', and R. rugosa `Alba' were damaged by Japanese beetle feeding. No other cultivars were damaged by Japanese beetles.
In order to evaluate and compare adaptability to dry sites, plant water relations and leaf gas exchange were compared in response to water stress among six birch species: monarch birch (Betula maximowicziana), river birch (B. nigra), paper birch (B. papyrifera), European birch (B. pendula), `Whitespire' Japanese birch (B. platyphylla var. japonica `Whitespire'), and gray birch (B. pendula). After 28 days without irrigation, Japanese birch maintained significantly higher stomatal conductance (gs) and net photosynthesis (Pn) than did any of the other species, despite having one of the lowest mid-day water potentials. Evaluation of tissue water relations, using pressure-volume methodology, showed no evidence of osmotic adjustment for any of these species in response to water stress. However, there was substantial variation among species in the water potential at the turgor loss point; varying from a high of -1.34 MPa for river birch to a low of -1.78 MPa for Japanese birch. Rates of Pn and gs under mild stress (mean predawn leaf water potential of -0.61 MPa) were negatively correlated with leaf osmotic potential at full turgor and the leaf water potential at the turgor loss point.