Measurement of the Frost Hardiness in Black Tea Varieties Using the Freezing Test of Matured Leaves (Part 2)

Abstract

For the purpose of standardizing the method of measuring the frost hardiness using the matured leaves, responses of the varieties showing the various hardiness to the frost injury were investigated under four levels of freezing temperature (-3°C, -5°C, -7°C, and -9°C) and eight levels of freezing time (30 min., lhr., 2hr., 3hr., 5hr., 7hr., 9hr., and 15hr., ) in this study.As a result of this experiments the standardized method of measuring the frost hardiness of Assam hybrid was determined as follows : The shoots with matured leaves were placed into a freezer and kept at 0°C for 30 min., and then kept at -9°C for 2hr. in the freezer. After the treatment the shoots were water-cultured at room temperature for 2days, and the degree of the frost injury was graded from one to ten by visual observation. For this test, five check varieties were chosen as follows:Variety Yabukita Chinshin-OolongGlade 1 3Hatsumomiji F1 552 Ai505 7 9Clear varietal differences of the critical temperature occurring the frost injury of matured leaves were observed among the following varieties in this experimental condition.F1 552, F1 635 (Assam hybrid) -3--5°C.Hatsumomiji, Benikaori (Assam hybrid)-5--7°C.Tadanishiki, Chinshin-Oolong (Assam hybrid, China) -7--9°C.The critical temperature of each variety was approximately invariable regardless of the lengthened freezing time. The frost injury became more severe as temperature was dropped under the critical point, and increased slightly with the lapse of freezing time in the range of above-mentioned temperatures.It was confirmed that rapid freezing and rapid thawing also promoted the degree of the frost injury, but the relative frost hardiness of different varieties was practically unchanged.The frost hardiness of Hatsumomiji in the various cultural conditions was found to fluctuate from 4 to 8 grade, but this variety was usually graded 5 or 6. On account of the fluctuation of the frost hardiness varying with the field conditions, hardiness test must be carried out comparing with the check variety in the same field.