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  • Author or Editor: Edward N. Ashworth x
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Abstract

Freezing injury is a major limiting factor in the production of horticultural crops. The effects of low temperatures vary and are noted as freezes to the Florida citrus crop; midwinter damage to deciduous fruit crops; frost damage to flowers, vegetables, and developing fruit; and limits to the range of perennial species. It is not the purpose of this paper to review the literature on freezing injury. Several recent books and review articles on the subject already exist (16, 17, 32–35, 50, 65). Instead, the purpose of this paper is to identify basic research areas and opportunities for horticulturists.

Open Access

The accumulation of total soluble sugars (TSS) and starch and their relationship to flower bud hardiness were studied in three Forsythia taxa: Forsythia ×intermedia `Spectabilis', Forsythia ×intermedia `Lynwood', and F. suspensa. Taxon hardiness was based on the mean temperature at which low temperature exotherms (LTEs) occurred during thermal analysis. Ethanol-extracted soluble sugars were quantified with anthrone, and starch was enzymatically digested and quantified with Trinder reagent. Qualitative changes in sugar content were determined with high-performance liquid chromatography and co-chromatography of authentic standards. Quantitative and qualitative changes in sugar content, similar for the three taxa, were observed in conjunction with fluctuations in flower bud hardiness, although neither TSS nor starch were correlated with mean LTE temperature. TSS was higher in acclimated than nonacclimated buds. However, after deacclimation began, sugars continued to increase with mean LTE temperature. Buds lacked starch except for a brief period during deacclimation. Galactose, stachyose, raffinose, and an unidentified carbohydrate were positively correlated with hardiness (P = 0.005, 0.001, 0.005, and 0.001, respectively).

Free access

Differential thermal analysis (DTA) was used to study the freezing behavior of `Berkeley' blueberry (Vaccinium corymbosum L.) flower buds at cooling rates of 10, 5, and 2C/hour. Experiments were conducted at various stages of hardiness on excised and attached (5 cm of stem) buds. The presence and number of low-temperature exotherms (LTEs) in hardy buds generally increased when analyses were conducted using faster cooling rates with excised buds. The number of LTEs detected in individual buds did not correlate (r 2 = 0.27) with the number of injured florets. The inability to detect LTEs in buds attached to stem segments and cooled at 2C/hour indicates that DTA cannot reliably estimate blueberry flower-bud hardiness in field plantings.

Free access

The location of ice crystals and their relationship to xylem vessels was studied in nonacclimated and acclimated `Berkeley' blueberry (Vaccinium corymbosum L.) flower buds. Light microscopy and low-temperature scanning electron microscopy (SEM) were used to detect ice crystals in the bud scales, floret scales, and bracts of dormant flower buds that had been frozen to -15C. No evidence of ice formation was observed in rachises, pedicels, and organs in florets when buds that had been fixed while frozen at -5C were examined with conventional SEM. This indicated that dormant buds underwent extraorgan freezing as a survival mechanism. Ice formation was not uniform in nonacclimated or deacclimated buds, although it was more prevalent in both than in acclimated buds. Large ice crystals were found in the ovaries of freeze-stressed nonacclimated buds. In deacclimated freeze-stressed buds, ice was found in the petals, rachises, pedicels, and ovaries. To determine whether this ice distribution pattern was correlated with the presence of mature xylem vessels, cleared flower buds were stained with basic fuchsin, which revealed the intact network of lignified elements. In nonacclimated buds (20 Sept.), mature xylem vessels extended through the rachises, connecting the bud scales with the floret scales and through the pedicels into the corollas of the florets. Although vascular development occurred in dormant buds, the greatest proliferation of vessels in the ovaries, petals, and sepals occurred coincident to the appearance of ice in these organs and the loss of hardiness.

Free access

Abstract

The capacity of woody tissues to deep supercool has been associated with the presence of a marked dehydrative resistance (i.e., the capacity of a tissue to retain moisture against a vapor pressure gradient). Naturally occurring seasonal changes in the extent of deep supercooling and dehydrative resistance in xylem tissues of Cornus florida L., Prunus persica (L.) Batsch, and Salix babylonica L. were monitored to determine if such a relationship could be established. Results indicated that the relationship between these 2 parameters is quite complex and may be more qualitative than quantitative. Greatest seasonal fluctuation in dehydrative resistance occurred in xylem tissue of peach and dogwood equilibrated at 90% RH. Seasonal changes in willow were relatively small. The capacity of peach and dogwood to retain water at 90% RH generally increased as supercooling increased. At 86% and 81% RH the same general trend was present, but the degree of seasonal fluctuation was much less than at 90% RH. The predicted relationship was not present at 95%, 78%, and 58% RH. The capacity to withstand a desiccation stress equivalent to 90% RH (− 140 bars) may somehow be integrally related to the capacity to maintain a stable supercooled system.

Open Access

Examination of both frozen specimens and -5C freeze-fixed buds showed that ice crystals were not uniformly distributed in blueberry flower buds. Localized freezing was also evidenced by detection of multiple freezing events using differential thermal analysis (DTA). Upon cooling, an initial exotherm occurred just below 0C and coincided with ice formation in adjacent woody tissue. Multiple low temperature exotherms (LTE), which have been reported to correspond with the freezing of individual blueberry florets (Bierman, et al. 1979. ASHS, 104(4):444-449), occurred between -7C and -28C. The presence and temperature of LTEs was influenced by cooling rates and whether buds were excised. LTE temperatures did not correlate with hardiness of buds frozen under field-like conditions. Results suggested that DTA of excised buds was not an appropriate method for determining hardiness.

Free access

Abstract

The freezing behavior of peach [Prunus persica (L.) Batsch] flower buds was influenced by the temperature at which ice formation was initiated. Buds seeded with ice just below 0°C were more likely to exhibit deep supercooling, and water in the primordia would supercool to lower temperatures than in unseeded excised flower buds. This effect was not always expressed and varied with the stage of acclimation. Researchers using differential thermal analysis to estimate bud hardiness will need to evaluate this effect. Seeding specimens with ice may be warranted to obtain results comparable with field conditions.

Open Access

Thermal analysis of Forsythia × intermedia `Spectabilis' flower buds had previously detected the occurrence of low temperature exotherms (LTE) during freezing. The LTE apparently resulted from the freezing of supercooled water and corresponded to the death of the florets. The genus Forsythia encompasses a wide array of species and interspecific crosses ranging in flower bud hardiness and floret size. The ability of buds to supercool, the relationship between the LTE and flower bud hardiness, and the extent to which floret size affects both were studied in flower buds of the following Forsythia species: F. × intermedia `Spectabilis', F. × intermedia `Lynwood', F. `Meadowlark', F. suspensa var. fortunei, F. `Arnold Dwarf, F. europaea, F. giraldiana, F. × intermedia `Arnold Giant', F. japonica var. saxatilis, F. mandshurica, F. ovata, and F. viridissima. Flower buds used for thermal analysis were also used in subsequent size determinations. Hardiness evaluations were conducted using controlled freezing tests, and the sampling interval defined using the temperature range of the LTEs. Initial evaluation indicated a high degree of correlation (α>.50) between mean LTEs and mean killing temperatures. The Forsythia genus, with its broad range of bud hardiness and size provides an excellent system in which to study the mechanisms of supercooling. Thermal analysis of cultivars which exhibit LTEs can accurately assess bud hardiness with minimal plant material.

Free access

Abstract

An attempt was made to alter the late-season growth habit of eastern thornless blackberry plants (Rubus sp.) with foliar sprays of growth regulators and plant nutrients. While fall applications of ethephon hastened leaf abscission, they did not affect shoot dieback or yield. Cultivar effects were very strong. ‘Dirksen’ plants had the least cane dieback and highest yield, ‘Hull’ plants were intermediate, and ‘Smoothstem’ plants were the least hardy and had the lowest yield. A significant negative correlation was found between the number of leaves retained throughout November and the subsequent yield of these 3 cultivars. Artificial freezing tests throughout the winter demonstrated that buds were less hardy than bark tissue. Bark and pith tissue from plants defoliated 30 Sept, were injured more than those tissues from plants defoliated later in the fall. No apparent relationship between fall defoliation and bud hardiness was observed. ‘Dirksen’ buds were 0.2° to 5.9°C hardier than were ‘Smoothstem’ buds. Differential thermal analysis showed that a freezing event occurred in both stems and buds at about −5°. A 2nd exotherm occurred in buds at about the LT50.

Open Access