Studies on the cold injury of citrus trees. III

On the development of cold injury to leaves as affected by some freezing or melting treatments

Abstract

1. In early February, young orange trees of Unshu (Citrus unshiu) and Natsumikan (C. natsu-daidai) were frozen by degrees from 0°C to -7°C in the refrigerator at night (9:30 p.m, to 9:00 a.m.). On the following day, they were taken out and each one third of them was treated in different ways. The first group was placed in the open air of 13.5°C under the direct ray of the sun, the second group was kept in the warm room of 13.5 °C, and the third group was warmed slowly up to 6-10°C by opening the door of the refrigerator. No apparent differences were found in the beginning among the treated and the non-treated plants. However, in early July the treated plants were very inferior in their growth to the non-treated plants in order of the first, second and third groups.
2. In early March, plants were treated with-5°C minimum temperature at night for 5 days successively. The longer the time of treatment, the higher was the freezing temperature, and the severer was the damage. Particularly, plants frozen at night for 4 or 5 days long sheded their leaves remarkably after melting. The tendency was more marked by Natsumikan than by Unshu oranges.
3. Plants frozen at night transpired more water in the daytime than plants not frozen, resulting in a decreased water content of leaves. On the fifth day of the freezing treatment, treated plants lost 1.3_??_1.4 times as much water as the non-treated plants, the water content of their leaves being 65% of the non-treated plants on fresh weight basis. On the other hand, plants suffered less cold injuries when watered before the treatment.