Abstract
In the present paper, the author deals with the experiments to determine the causal agents of “yuzuhada” (hard end or black end like physiological disease) development in Nijisseiki pear fruits. The results obtained are summarized as follows.
1. As to the seasonal changes of osmotic pressure, in the mature leaves, it indicated higher values (ca. 16_??_17 atms.) in May and July and lower values in June (ca. 14 atms.). During May-June the osmotic pressure in the young leaves was approximately two atms. lower than one in the mature l _??_ ves. On the other hand, the osmotic pressure in the fruits was almost constant (10_??_11 atms.) from May to middle July, and after that time gradually increased until it came up to 13. 5 atms. at the harvest period. The moisture content in the leaves decreased by approximately 10 percent from May to September, whereas it increased by about 10 percent in the fruits.
2. The difference of osmotic pressure between leaf and fruit showed a maximum value in July (ca. 6 atms.), and then decreased in August as a consequence of the osmotic pressure of fruit.
3. As to the daily changes of osmotic pressure in hot and dry summer days, it showed a minimum value at 5 a. m., then rose reaching a maximum value at 2 p. m., and then lowered towards evening. The fluctuation of osmotic pressure in the daytime came up to ca. 4_??_7 atms., specially under the dry condition.
4. As to the daily changes of osmotic pressure in the “yuzuhada” tree, though there was little difference in them between the “yuzuhada” tree and the healthy one in the daytime, the gradual declining degree of osmotic pressure towards night in the “yuzuhada” tree was less than that in the healthy one. Consequently, in the former the remarkable difference of osmotic pressure between leaves and fruits was maintained for a longer period.
5. As soon as the soil moisture decreased in the neighbourhood of field capacity, the deficiency of moisture content and the increase of osmotic pressure in leaves was revealed. And under the more decreased soil moisture condition, that is, the so called first permanent wilting point, the moisture content in leaves decreased by approximately 10 percent and the osmotic pressure increased by 5_??_6 atms. The decrease of the moisture content and the increase of the osmotic pressure in fruit was revealed, when the soil moisture decreased till half-way between the field capacity and the first permanent wilting point. Under the first permanent wilting point, the moisture content in fruit decreased by 2_??_2.5 percent, and the osmotic pressure in it increased by 2_??_2.5 atms.
6. When the difference of osmotic pressure between leaves and fruits reached about 5 atms., the decrease of moisture content and the increase of osmotic pressure in fruits was recognized. It has been supposed that Yuzuhada develops when fruits are deprived of their moisture by leaves under water deficient condition. Accordingly, from the above mentioned observations, it was understood that is must be in July and hot and dry summer that “yuzuhada” remarkably develops.
7. Water deficiency in the first part of July produced extremely high “yuzuhada” percentage, and the disorder developed in 20_??_25 days after the treatment (the middle or latter part of August)., Field observation revealed that most of “yuzuhada” fruits developed during August.
8. Pyres betulaefolia stock is known to produce little “yuzuhada” on the Nijisseiki scion grafted on it. The osmotic pressure of leaves of P. betulaefolia was always higher than those of P. serotina and Nijisseiki pear (P. serotina var. culta).
9. Between osmotic pressure of leaves of Nijisseiki grafted on P. betulaefolia and that one grafted on P. serotina
