The purpose of the present study was to apply the Richards function to fit electrolyte and phenolic leakage data for several taxa of woody plants subjected to freezing stress and to determine how the curve inflection point relates to the lethal temperature range. The lowest survival temperature of Fraxinus americana, Lagerstroemia cv. Natchez, Magnolia grandiflora, Rhododendron cv. Red Ruffle and Zelkova serrata was determined based on visual evaluation of oxidative browning facilitated by a modified regrowth test and differential thermal analysis. Lethal injury occurred in all cases within a range of 3°C below lowest survival temperature. Using the Richards function inflection point as an estimate of lethal temperature led to an overestimation of freezing tolerance in most taxa. This overestimation was greater for stems than leaves, and was greater in winter than in summer. The lethal temperature range generally coincided with the initial increase in leakage caused by freezing. The lethal temperature range also was determined by using a point of interception of the lower asymptote of a curve with a line tangential to the inflection point. In most taxa tested estimated lethal temperature based on the point of interception provided an improvement over the estimate based on the point of inflection.

The time-domain reflectometry (TDR) method of measuring water content has been applied to mineral soils but not to organic growing media. We investigated the applicability of TDR for measuring the water content of organic media in containers. TDR calibration was conducted for sand, peat, composted pine bark, sand and peat mix, sand and bark mix, and a commercial growing medium (Metro Mix 300). Regression analysis of volumetric water content was conducted with the ratio of apparent: physical length of the probe (La: L) as an independent variable. The calibration curve for Metro Mix 300 was compared to curves generated for a range of soils by other investigators. Additionally, water-content and La: L changes were monitored in Metro Mix 300 for 10 months and were compared to predicted values from the calibration curve. Organic media had a higher water content than sand for the same La: L value. Equations developed by previous authors generally underestimated water content when compared with the calibration curve for Metro Mix 300. We attribute this difference to a large fraction of highly decomposed organic matter or vermiculite and, thus, to the presence of more bound water. Specific calibration of TDR may be required to determine the absolute water content of organic growing media.

We determined whether increase in cold hardiness of Rhododendron cv. Catawbiense Boursault induced by water stress was correlated with changes in tissue water relations. Water content of the growing medium was either maintained near field capacity for the duration of the study or plants were subjected to drought episodes at different times between 15 July and 19 February. Watering during a drought episode was delayed until soil water content decreased below 0.4 m3 m−3 then watering was resumed at a level to maintain soil water content between 0.3 and 0.4 m3 m−3. Cold hardiness was evaluated in the laboratory with freeze tolerance tests on detached leaves. Water relations parameters were determined using pressure‐volume analysis. Exposure to drought episodes increased cold hardiness during the cold acclimation stage in late summer and fall but not during the winter. When water‐stressed plants were re‐watered to field capacity, the previous gain in cold hardiness gradually disappeared. Water relations parameters correlating with seasonal changes of cold hardiness included dry matter content (r =−0.67). apoplastic water content (r =−0.60), and water potential at the turgor loss point (r = 0.40). Changes of cold hardiness in water‐stressed plants in reference to well‐watered plants were correlated with changes of all water relations parameters, except for osmotic potential at full turgor (r = 0.13). It is proposed that water stress reduced the hydration of cell walls, thereby increasing their rigidity. Increased rigidity of cell walls could result in a development of greater negative turgor pressures at subfreezing temperatures and therefore increased resistance to freeze dehydration.

The primary cause of losses in evergreen azaleas injured by early freeze is bark split on lower stems. Delayed acclimation in the fall is thought to permit this injury. We examined whether reduced water supply affects acclimation of Rhododendron L. `Coral Bell', `Hinodegiri', and `Red Ruffle'. Containerized plants were grown under four watering regimes and placed outdoors or in the greenhouse. The water content of the growing medium was maintained at either 0.3 to 0.4 or 0.5 to 0.6 m3·m-3 from 16 June to 30 Aug. 1993, when half of the plants under each of these regimes was switched to the other watering regime. Freeze tests were conducted on 30 Aug. and 9 (let. Injury to leaves, and lower, middle, and upper stems was evaluated visually. Acclimation of leaves and upper stems before the August test, in most cases, was not stimulated by reduced water content, while the response of lower and middle stems was cultivar- and location-specific. The lower water content treatment after 30 Aug. generally increased freeze tolerance of all plant parts regardless of the previous watering regime. The higher water content treatment after 30 Aug. either prevented or delayed acclimation. This study demonstrated that the reduced water supply provided a feasible means of promoting acclimation of evergreen azaleas in late summer.

The objective of the present study was to develop an empirical cold hardiness model applicable to several taxa of deciduous trees. Cold hardiness expressed as lowest survival temperature of Acer rubrum, Betula nigra, Liquidambar styracifiua, Fraxinus pennsylvanica, Prunus serotina and Quercus alba was evaluated at approximately weekly intervals during the winters of three consecutive years. Plant samples and meteorological data were collected from Georgia Experiment Station, Griffin, Georgia. Maximum, minimum and average temperatures, hourly chill and heat accumulation. day length and time of year were used as input variables for model development. The statistical method of stepwise procedure of regression analysis was employed to select variables for the model. Based on the assumption that model components should be the same for all taxa included in this study and all three winters, the following independent model variables were selected as valid inputs: day length, number of accumulated hours with temperature above 20°C and number of accumulated hours with temperature below 10°C. Equation coefficients of species‐specific models were determined for each species. Cold hardiness predictions were compared to actual observations for each species. The model components were interpreted as representing two processes: (1) internally regulated and independent of actual temperature, and (2) externally regulated and dependent on the amount of accumulated chill or heat. The model allowed for comparisons of cold hardening and dehardening between the studied taxa and between years.