Eleven peach scion-rootstock combinations were evaluated for cold hardiness by controlled freezing after acclimation in controlled environments at temperatures of 20/14 or 10/4°C day/night and photoperiods of 9, 12 or 15 hours. Temperature and scion cultivar had much greater effects on cold hardiness of buds and bark than did photoperiod or rootstock. Trees held at 10/4° were more cold hardy than those held at 20/14°. ‘Siberian C’ scions were more cold hardy than ‘Harrow Blood’ or ‘Elberta’ scions. Short days increased cold acclimation of fruit buds at 20/14° but not at 10/4°. Rootstock influence on scion acclimation was small but some cultivar differences were apparent.
Low tissue-water content and increased osmotic concentration of cell sap are associated with frost resistance. Changes in total osmotic concentration of cell sap are due mainly to changes in concentration of sugars. Generally, sugar content increases with hardening and decreases with dehardening. This study examined the effect of elevated sucrose levels (3% to 15%) in the medium on the cold hardiness of `Festival' red raspberry (Rubus idaeus L.) shoots in vitro. To determine whether expected hardening is caused by elevated sucrose levels or by osmotic stress, different levels of mannitol in the media have been tested. After growing raspberry shoots on media with different levels of sucrose and mannitol for 2 weeks, shoot moisture content (percent) was determined. Cold hardiness of the shoots was determined by using differential thermal analysis or artificially freezing the shoots and assessing the survival by regrowth test and visual rating.
Effects of fall fertilization programs on cold hardiness of young Cornus alba `Argenteo-marginata' and Weigela florida `Rumba' plants were examined. At the end of Summer 1992, four fertilization programs were applied to 1-year-old woody plants that were propagated in 1991 by cuttings. Fertilization treatments were as follows: 1) discontinuation of liquid fertilizer treatments on 30 Aug., 2) decreasing N concentration (100 to 0 mg·liter-1 of 20N–20P–20K) from 15 Aug. to 30 Sept., 3) constant N concentration (100 mg·liter-1 of 20N–20P–20K) from 15 Aug. to 30 Sept., and 4) high K concentration (110 mg·liter-1 of 7N–11P–27K) from 15 Aug. to 30 Sept. Whole plants were then removed from pots and roots were cleaned. Plants were then placed under freezing temperatures from 0 to –20C at 2C intervals, with plant samplings done three times during fall—at the end of September, October, and November. After the freezing test, plants were stored at –2C and repotted in May 1993 for winter injury evaluation. Preliminary results indicated that the four fertilization programs did not induce a significant effect on cold hardiness of the two species. However, it was clear that the degree of cold hardiness was different for each species: Weigela was ≈10 degrees less hardy compared to Cornus in September and October. In November, species demonstrated hardiness at temperatures less than –20C. Cornus also showed cold hardiness at less than –20C in October.
Late spring frosts are a major concern to blueberry growers in the southeastern United States. Cold hardiness of flower buds (stages 4 to 6) was evaluated in three southern highbush blueberry cultivars (`Cooper', `O'Neal', and `Gulfcoast'). Differential thermal analysis (DTA) and tissue browning tests revealed that the critical temperature and ovary damage occurred at –11C in `Cooper', –12C in `O'Neal', and –13C in `Gulfcoast'.
Near-lethal abiotic stresses, e.g., low or high temperatures, chemicals, etc., can break endodormancy prematurely and reduce cold hardiness in woody plants. It is not well-ducumented whether biotic stresses can cause the same effect. Botryosphaeria dothidea causes canker in redbud (Cercis canadensis) and many other woody plants and is one of the most limiting factors growing redbud in the landscape. Two-year-old seedlings were planted in a nursery in May 1998 at The Morton Arboretum. Trees were inoculated (n = 10/treatment) with the fungus in Sept. 1998 using the stem slit method (a slit was cut about 5 cm above the base of the trunk and the wound was covered with parafilm after treatment). The treatments were T1 = control (PDA, Potato Dextrose Agar),T2 = 1-mm mycelium plug, T3 = low spore suspension (25 μL), T4 = high spore suspension (25 μL). Stem cold hardiness was evaluated by artificial freezing tests in Nov. 1998. The mean LT50 (the temperature at which 50% of the tissues is killed) from ion leakage were T1 (Control) = -29.3 °C, T2 (mycelium): -24.05 °C, T3 (low spore) = -18.75 °C, and T4 (high) = -16.4 °C. T3 and T4, the low- and high-spore inoculation, significantly reduced cold hardiness in redbud stem tissues. The LST (lowest survival temperature) based on visual observation of the samples after 7 days indicated all Botryosphaeria dothidea-treated plants had lower cold hardiness compared to control. Endodormancy was broken in B. dothidea-treated plants after placing plants under 16 h of light and 23 /18 °C day/night temperature for 1 month after the treatment. The highest percent budbrealk was for T4 (high spore), followed by T3 (Low Spore) and T2 (Mycelium).
An increase in mechanical pruning of Concord grapevines in Washington has led to a marked increase in yield. In 1993 the average yield for the 20,000 plus acres was slightly greater than 12 ton/acre. As part of a long term study, initiated in 1987, to evaluate the effects of mechanical pruning on Concord yield and fruit quality, we have also followed bud cold hardiness and winter injury over several years. Cold hardiness was monitored using low temperature exotherm analysis of excised buds. Winter injury was evaluated by visual examination of bud and cane tissues collected from vines with different croploads. In 1990 the average yield for mechanically pruned vines was 13T/ac and for balance pruned vines about 8T/ac. Winter injury during December 1990 showed significantly less injury to mechanically pruned vines whether primary, secondary or tertiary buds were examined. During the winter of 1991-92 and 1993-94 bud cold hardiness of individual vines showed no relationship to cropload per vine.
Bearing trees of ‘Loring’, ‘Redhaven’ and ‘Babygold 5’ peach trees (Prunus persica (L.) Batsch) on Siberian C seedlings defoliated earlier than those on the other seedling rootstocks. Early cold acclimation of scions in the fall and scion cold hardiness in mid-winter were enhanced more by Siberian C seedlings than by those of the other seedling rootstocks. Bud survival and fruit set of ‘Redhaven’ and ‘Bablygold 5’ scions were greater on Siberian C seedlings than on any of the other seedling rootstocks following an outdoor cold stress at -23.4°C in January. The cold hardiness of phloem, cambium, and xylem stem tissues were closely correlated with each other in the fall, but were not correlated with cold hardiness of flower buds on the same shoots. Seedlings of Siberian C appeared to enhance early scion dormancy and they increased scion bud hardiness by as much as 4.7° in the fall, and 1.3° in mid-February, compared with those of other rootstocks tested.
Greenhouse-cultured, container-grown seedlings of interior Douglas fir [Pseudotsuga menziesii var. glauca (Beissn.) France], Engelmann spruce [Picea engelmannii (Parry) Engelm.], and ponderosa pine (Pinus ponderosa var. scopulorum Engelm.) were acclimated and deacclimated to cold in growth chambers over 19 weeks. Heat tolerance and cold hardiness of needles, and bud dormancy, were measured weekly. Heat tolerance of Douglas fir and Engelmann spruce needles increased with development through the first complete annual cycle: new needles on actively growing plants; mature needles, not cold-hardy, on dormant plants; cold-hardy needles on dormant and quiescent plants; and mature, needles, not cold-hardy, on actively growing plants. Heat tolerance of ponderosa pine needles differed in two respects. New needles had an intermediate tolerance level to heat, and fully cold-hardy needles were the least tolerant. Thus, the physiological changes that conferred cold hardiness were not associated with greater heat tolerance in all the conifers tested. In none of these species did the timing of changes in heat tolerance coincide consistently with changes in cold hardiness or bud dormancy.
Canes and flower buds of selected red raspberry cultivars (Rubus idaeus L. `Maurin Makea', `Muskoka', and `Ottawa') were sampled from a field (latitude, 61 °20'N; longitude, 24 °13'E) at 1-month intervals during Winter 1996-97 to study the interaction of dormancy and cold hardiness, hardiness retention, and rehardening capacity. One set of canes was subjected to dehardening (3 days) and two sets to dehardening + rehardening (3 and 7 days) treatments before cold hardiness determination. Maximum midwinter hardiness occurred in January, after breaking of endodormancy. Cold hardiness of canes and buds reached -28.6 to -37.2 °C and -24.2 to -31.6 °C, respectively. Throughout the winter, raspberry canes were hardier than buds. Endodormancy had a greater influence on dehardening and rehardening in buds than in canes, and cultivars differed in their response. Dehardening of `Maurin Makea' canes and buds, and `Muskoka' buds was slightly enhanced by breaking of dormancy, whereas dehardening in `Ottawa' was not affected by dormancy. Raspberry canes and buds could reharden even after dormancy release. Rehardening capacity was affected by the state of dormancy only in `Maurin Makea' buds. Changes in dormancy status failed to explain cultivar differences regarding dehardening and the capacity to reharden suggesting other factors may be involved.
Carolina buckthorn [Rhamnus caroliniana Walt. or Frangula caroliniana (Walt.) Gray] is an attractive and water-stress-resistant shrub or small tree distributed extensively in the southeastern United States that merits use in managed landscapes. Due to substantial climatic differences within its distribution (30-year normal midwinter minima range from 13 to -8 °C), selection among provenances based on differences in cold hardiness is warranted. Before selections are marketed, the potential of carolina buckthorn to be invasive also merits investigation. Ecological problems resulting from the introduction of Rhamnus L. species in the United States, most notably the dominance of R. cathartica L. (common buckthorn) over neighboring taxa, are due in part to early budbreak. Consequently, we investigated depth of cold hardiness and vernal budbreak of carolina buckthorn and common buckthorn. Stem samples of carolina buckthorn and common buckthorn collected in midwinter survived temperatures as low as -21 and -24 °C, respectively. Although the cold hardiness of carolina buckthorns from Missouri was greater than that of carolina buckthorns from Ohio and Texas on 2 Apr. 2003, there were no differences in cold hardiness of stems from Missouri and Texas on all three assessment dates in the second experiment. All plants survived at both field locations except for the carolina buckthorns from southern Texas planted in Iowa, which showed 0% and 17% survival in 2003 and 2004, respectively. Budbreak of both species with and without mulch in Ames, Iowa, was recorded from 9 Apr. to 10 May 2002. Mean budbreak of common buckthorn was 5.7 days earlier than budbreak of carolina buckthorn, and buds of mulched carolina buckthorns broke 4.2 days earlier than did buds of unmulched carolina buckthorns. We conclude that the cold hardiness of carolina buckthorn is sufficient to permit the species to be planted outside of its natural distribution. Populations of carolina buckthorn in Ohio and Missouri should be the focus of efforts to select genotypes for use in regions with harsh winters. Phenology of its budbreak suggests carolina buckthorn will not be as invasive as common buckthorn, but evaluation of additional determinants of invasiveness is warranted.