developed in several stages ( Howell and Weiser, 1970 ; Tromp, 2005 ). Studies on the frost tolerance of the overwintering organs of woody plants demonstrated that the hardening processes taking place during the first half of dormancy could be divided into
Abstract
Dormancy (absence of budbreak) in Amelanchier alnifolia Nutt., was greatest from 1 September to 13 October 1981 in ‘Smoky’ and from 1 September to 27 October 1981 in ‘Pembina’. About 576 hours for ‘Smoky’ and 696 hours for ‘Pembina’ at 0° to 7°C, were required to break bud dormancy. One week of additional chilling (3°) had little effect during deep rest, but had a significant effect when the rest requirement was nearly satisfied. Flower bud hardening began in early September and attained –44° LT50 for florets on 30 October. Vascular connective tissue at the base of the bud was less hardy than floral primordia, throughout the dormant season. Flower buds dehardened from –60° to –30° after exposure to 20° for 7 days, to –14° after exposure to 20° for 14 days. No low-temperature exotherms were detected.
Abstract
Seedlings of 18 citrus types were exposed to artificial hardening conditions. ‘Nagami’ kumquat, false hybrid satsuma and ‘Cleopatra’ mandarins were the most cold hardy and ‘Lisbon’ lemon, ‘Mexican’ lime, Rangpur’ lime, and ‘Calamondin’ hybrid kumquat the least cold hardy. Three mandarins, 1 tangelo, 3 oranges, and 4 grapefruit types were intermediate in hardiness. Generally, the most hardy types hardened some at 70° day and 50°F night temperatures, and 60°/40° as well as at lower temperatures while the least hardy types hardened primarily at 50°/30° and 45°/26°. Sugar accumulation was associated with hardening.
Abstract
2,3,5-Triphenyltetrazolium chloride (TTC) reduction measures glucose equivalents of substances diffusing from plant tissues. Amounts of diffusing substances are greater from cold-hardened than unhardened citrus. These differences are colorimetrically distinguishable and identify citrus plants exposed to low temp in controlled environment studies.
Abstract
Tests of citrus seedlings exposed to a series of hardening temperatures showed that kumquat, Fortunella hindsii (Champ.) Swing., acquired more hardiness at 21°/10°C than did ‘Redblush’ grapefruit, Citrus paradisi Macf., or citron, C. medica L. After 8 weeks’ hardening kumquat was the most cold hardy; citron, the least. Leaf photosynthetic CO2 uptake decreased, and leaf diffusion resistance (sec/cm) increased with hardening in all cultivars, but did not reflect the degree of hardening attained. Stomatal closure during hardening was not caused by moisture stress. Ethylene evolution from leaves did not change during hardening of kumquat, mandarin, C. reticulata Blanco, or grapefruit, but did increase from hardened citron leaves.
Cold hardening is an effective method for conditioning meristems for cryopreservation. ABA plays a role in hardening and produces increased hardiness in suspension cultured cells. This study was designed to determine if growth, in vitro, on ABA (5×10-5 M) for one week, would substitute for one week of cold hardening, and if ABA would provide additional conditioning when added in combination with cold hardening treatments. In vitro plantlets of Rubus spp. were grown for one week with or without cold hardening and with or without ABA. Meristems from these plants were frozen at 0.8C* min-1 to -35 C, then plunged into LN2, thawed, and plated on recovery medium. One month after thawing, cold-hardened plants with and without ABA treatment had recovery rates of up to 83%. Survival of plants grown at room temperature ranged from zero to 8% and zero to 28% for plants grown on ABA at room temperature. At the rates tested, ABA is less effective than cold hardening in conditioning apical meristems of in vitro Rubus plants for cryopreservation and provides no additional protection to cold-hardened meristems.
Postharvest shelf life of fresh sweet basil (Ocimum basilicum L.) at 5°C is only 3 to 4 d due to development of chilling injury symptoms. Plants chill-hardened at 10°C for 4 h daily (2 h at end of the light period and 2 h at the beginning of the dark period) for 2 d prior to harvest had 3 d extended shelf life at 5°C. Increasing the duration of preharvest chill-hardening did not further improve the shelf life. Plants were chill-hardened at 10°C for 4 h daily for one week at different periods during the day. Four-, 5- and 6-week-old basil were used in each of three consecutive runs. With the 4- and 5-week-old basil, chill-hardening at the beginning of the day extended average shelf life by 1 and 1.5 d at 5°C, respectively. Shelf life was either decreased or not affected by the other periods of preharvest chilling. Postharvest chill-hardening of packaged sweet basil for 1 d at 10°C before transfer to 5°C increased shelf life by 5 d. Postharvest chill-hardening has potential for reducing chilling injury of packaged sweet basil.
Abstract
In vitro shoot cultures of MM 106 apple (Malus domestica Borkh.) and ‘Smoky’ saskatoon (Amelanchier alnifolia Nutt.) that were subjected to a 10-week, short-photoperiod, low-temperature hardening treatment, including a −3°C exposure followed by 5-7 days at 2°, were 4-8° hardier than untreated shoot cultures. Apple shoot cultures grown on media with elevated sucrose concentrations (3-14%), but not subjected to acclimating conditions, had reduced shoot moisture content and increased up to 6° in hardiness. Apple and saskatoon shoot cultures given a short-photoperiod, low-temperature hardening treatment and apple cultures grown on medium containing a high sucrose level developed red and purple leaf coloration.
Abstract
Roots of ‘Mailing (M) 26’ and ‘Malus robusta (MR) 5’ were less hardy than stems in winter; hardened more slowly in fall; and dehardened later in spring. In 1967 roots were hardier than in 1968, despite slightly higher soil temperatures. This difference in hardiness was associated with much less rainfall in 1967 leading to a lower level of root hydration. While overall soil temperature-hardiness relationships were unclear, short-term changes in root hardiness were correlated with soil temperature during the preceding week. Hardening in apple roots appeared to be influenced by soil temperature and level of root hydration.
Plantlets of Solarium tuberosum L. `Russet Burbank', `Sangre', and `Centennial Russet' were grown in vitro from nodal cuttings. A medium overlay was used to reduce the humidity of the in vitro environment. This treatment was tested for its effect on plant growth and on the rate of water loss from detached leaves. The latter was assayed as indicative of hardening and consequent survival of plantlets once removed from in vitro culture. The paraffin medium overlay reduced the rate of water loss from detached leaves of cultured plantlets, but also reduced root growth.