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- Author or Editor: Norman E. Pellett x
Nitrogen fertility levels during the summer and fall had little effect on cold acclimation of root or stem tissues of container-grown plants of Forsythia intermedia Zab. cv. Lynwood by mid-November 1969. Tissue N levels of roots decreased from August 1 to September 3, but had increased again on October 30. Nitrogen fertilization rates affected tissue levels of P and K in Forsythia roots. Nitrogen and P levels that resulted in acceptable plants had limited effect on cold acclimation of Cornus alba L. var. sibirica Loud and Forsythia roots and stems in late October or early November 1970. Increased N and P fertilization affected tissue levels of N, P, K, Ca, and Mg.
Forsythia is a popular plant for the landscape in many regions of the northern hemisphere. Inadequate cold hardiness of flower buds often results in few spring flowers on most cultivars in northern regions. Forsythia mandschurica Uyeki has flowered every year since 1970 at the University of Vermont Horticultural Research Center (3). This species is described in Chinese language with a plate of herbarium specimens and a plant distribution map (2).
Flower buds of eight ecotypes representing three native North American azalea species being grown in Burlington, Vermont were compared for cold hardiness by laboratory freezing during the cold acclimation period for three years. Species were Rhododendron calendulaceum, R. prinophyllum, and R. viscosum. There was a high variation in the number of florets killed within an inflorescens in response to freezing temperatures. There was little difference in the cold hardiness of florets of R. Pinophyllum and R. calendulaceum florets, but R. viscosum florets were hardier. Some differences were noted in cold hardiness of florets of ecotypes, but these were not necessarily related to latitude of origin. Cold hardiness showed a relationship with the daily mean temperature of the three days preceding freezing tests.
Most of 36 crabapple and 19 other woody plant taxa demonstrated the ability, when dormant, to grow a continuous row of callus along the cambial region on split-stem pieces within 5 to 7 days of incubation at 25 °C. The ability to grow callus after freezing tests was compared with discoloration and electrical conductivity for determining laboratory freeze injury to selected taxa. Hardiness levels were determined using the procedures of callus growth, discoloration, and electrical conductivity after freezing stem pieces of Jack crabapple [Malus baccata (L.) Borkh. `Jacki'], pink bud Sargent crabapple [M. sargentii Rehd. `Rosea'], Mary Potter crabapple [Malus sp. `Mary Potter'], and snowberry mountainash [Sorbus discolor (Maxim.) Maxim.]. Sampling dates for laboratory freezing tests were chosen to represent midwinter cold hardiness and partial hardiness of either late fall or early spring. There was a high correlation between discoloration and callus ratings for most plants; however, the two methods usually did not identify the same critical temperature (T50) for injury. The critical temperatures identified by callus growth was often 3 to 6 °C lower than for discoloration. For many taxa, callus growth was easier to see than discoloration of cambium and phloem, providing a less subjective evaluation of injury. TTC (2,3,5-triphenyl tetrazolium chloride) treatment was sometimes useful to identify callus growth that died after forming. The critical temperature (Tc), the highest temperature at which relative electrical conductivity differed significantly from the control temperature, was higher in most cases, indicating less cold hardiness than the T50 for callus and discoloration. The callus procedure may have value for evaluating injury to the cambial zone from freezing and other plant stresses because it determines the ability of the plant to continue growth.
Leaves and stems from 7 field-grown cultivars of Rhododendron were frozen in laboratory tests from September to December or January for 2 successive winters to determine the temperature causing injury to selected organs. In all cultivars, the leaf midrib, petiole, and stem extraxylary (cambium, phloem, and cortex) organs were the least cold-hardy, while the stem xylem and leaf intervein were the most cold-hardy. Organs of ‘Lee’s Dark Purple’ and ‘Cataw-biense Boursalt’ rhododendrons were the most cold-hardy by December while ‘Caractacus’, ‘America’, and ‘Nova Zembla’ were the least cold-hardy. Field temperatures similar to those causing injury in the laboratory resulted in leaf and stem damage in early December.
Five evergreen rhododendron cultivars were compared for flower bud cold hardiness in laboratory freezing studies on 6 dates. ‘Roseum Elegans’, ‘Catawbiense Boursalt’ and ‘Boule de Neige’ were more cold hardy on most sampling dates than ‘America’ and ‘Lee’s Dark Purple’. The relative cold hardiness of these 5 cultivars was consistent on several winter dates over several years. Injured florets were black and readily separated from uninjured white florets upon dissection of the inflorescence bud after freezing.
Florets of eight provenances representing three native North American azalea species [Rhododendron calendulaceum (Michx.) Torr., R. prinophyllum (Small) Millais, and R. viscosum (L.) Torr.] being grown in Burlington, Vt., were compared during three seasons for cold hardiness by laboratory freezing during cold acclimation. There was a large variability in the number of florets killed within an inflorescence in response to freezing temperatures. Cold hardiness of florets of the three species ranked, from most to least hardy, were R. viscosum, R. prinophyllum, and R. calendulaceum. Some differences were noted in cold hardiness of florets of provenances, but these were not necessarily related to latitude or elevation of origin. Cold hardiness of most provenances showed a significant linear relationship with the daily mean temperature of the 3 days preceding freezing tests. Ambient temperatures just before subfreezing test temperatures may affect winter injury more than provenance differences for these species.
Maintenance of selected moisture and N levels in the soil throughout the fall did not significantly affect the rate of cold acclimation of container grown Juniperus chinensis cv. ‘Hetzi’ roots or tops. Two levels of soil N resulted in N of .79% ppm and 1.65% in the tops, and root N of .70% and 1.29% on December 2, 1963.
Plants receiving no N fertilization after September 2 decreased in total root N from 1.27% on September 11 to 0.70% on December 2. Tops decreased in total N from 1.62% to 0.84% during the same time period. Different soil moisture levels did not differentially affect the tissue moisture percentages, or cold acclimation.
Roots of container grown ‘Hetzi’ juniper developed cold hardiness to -10°C on December 2, 1963 in St. Paul, Minnesota as determined by controlled freezing tests. The temperature of container soil, under natural conditions, did not fall below 0° until after December 2. Once frozen, the soil temperature responded rapidly to falling ambient temperatures. Container soil temperatures of -10° occurred several times after December 2 resulting in root injury.
Tops developed cold hardiness from -15°C on September 11 to greater than -39° on December 2, 1963. No top injury occurred at any stage during the study. Winter injury common to container grown Hetzi Juniper in Minnesota is apparently root injury.
Changes in total sugars, reducing sugars, total N and protein N showed little relationship to changes in cold acclimation. Root and top moisture percentages decreased during the fall, the rates closely paralleling the increase in cold acclimation. It is postulated that a decrease in the cellular moisture, resulted in increased concentration of cellular solutes and closer spatial arrangement of water binding substances. Cold acclimation may have resulted from the higher bound water/free water ratio.