Vol. 72 No: 4

Title:
Air Temperature, Humidity and Rooting Volume Affecting Freezing Injury to Rhododendron and Other Perennials

Authors:
R.W.F. CAMERON and G.R. DIXON

pp: 553-562

Abstract:
Intensive husbandry of woody plants grown in containers may increase the opportunities for cold induced damage especially where rooting volumes are restricted. Research reported here considered the ranges of air temperature, rates of temperature change and the impact of humidity on cold induced damage to leaves and the inter-relation with rooting volume using species of Acer, Ceanothus, Euphorbia, Magnolia and Rhododendron. Damage was expressed as visual assessments and as increasing ion leakage from the tissues. Exposing Rhododendron leaves to 2°C for 24 h prior to freezing at -6°C for 5 h was less damaging than the same treatment after exposure to 25 or 16°C. Damage resulting from treatments with -8 and -10°C was similar for leaves previously exposed to 25, 16 or 2°C. The extent of injury was also influenced by the environment during freezing. During short periods of freezing temperatures below -4°C, tissues submerged in water, and then frozen, generally showed less injury than those surrounded by air. At lower subzero temperatures and longer periods of exposure, however, the greatest injury formed in tissues placed in water. When exposed to -6°C, submerged leaves were damaged after 5 h whereas leaves placed in air withstood freezing for up to 7 h. Similar results were obtained with studies of the moisture contents of leaves and the ambient humidity during freezing. At -4°C leaves saturated with moisture suffered significantly less damage than those placed in dry atmospheres. Extending the drying period from lh to 24 h prior to exposure to freezing, however, reduced tissue injury especially at the lowest temperatures. Results obtained by freezing plants grown in containers of several sizes for Acer, Ceanothus, Euphorbia and Magnolia were pooled. This experiment confirmed that survival increased significantly (P<0.05) in larger containers as air temperature was reduced over time, with the possible exception of Acer.

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