Abstract
The relation between cell division in the root tips and soil temperature was investigated by comparing Japanese persimmon trees cultured in a plastic house with those grown and outdoors. The air temperature in a plastic house was kept at or over 19 °C from 19 March to 2 June. At every sampling date the 40 root tips from five to ten 23-year-old trees were collected at a distance of 1 m around the trunk and from 10 to 40 cm depth. The tips were dipped in 0.002 M 8-hydroxyquinolin (5°C) for three hours and then fixed in acetic acid-alcohol for microscopy.
1. The root tips from trees growing in a plastic house or outdoors exhibited cell division on 22 March. The mitotic index of the roots cultivated in a plastic house increased from 22 March to 2 June when it reached maximum; the high level was sustained until 18 July. It decreased gradually after August; no cell division figure was observed in root tips sampled in 2 October. The mitotic index of roots cultivated outdoors increased gradually until 2 August and then decreased; no mitotic figure was observed in samples collected on 17 September.
2. A significant positive correlation existed between the mitotic index of the roots and soil temperatures between March and July. Extrapolating from the relationship, the temperature which induces cell division was 11.7°C.
3. The cell division-temperature relationship obtained from August to October indicated that the soil temperature, when the mitotic index decreased, was 21°C.
4. Whether the root tips were sampled from trees cultivated indoors or outdoors, they could be white or black. About 40 percent of the root tips sampled between 2 May and August from indoor trees was white; the same frequency of white roots on outdoor trees was observed between 2 June and August. No white root tips were observed after September.
5. The mean mitotic index of black root tips was about 0.1%, whereas that of white one was about 0.4%.
6. The proportion of white root tips increased as the soil temperature exceeded 19°C.
7. The appearance of white root tips had relation with the end of shoot growth or anthesis.
