The influences of drying down of soil on theapparent photosynthesis, transpiration, amountof carbohydrate reserves and growth were studied with 1-yea-old seedlings of Akamatu (
Pinus densiflora SIEU. et ZUCC.) under the fieldconditions. The test seedlings received the treatment of gradaallp drying of soil for once (A) or twice (B) in the summer of 1952, meanwhile the soil moisture in the pots of control seedlings (C) were held nearly at the field capacity. Details of the treatment are given in Fig. 2. and its explanation.
It is evident from Fig. 2. that deficient amounts of soil moisture resulted in subnormal rates of apparent photosynthesis and transpira- tion, but the degree of depression of the rates depended not only upon the soil moisture butalso the air humidity. When water was applied to the soil in which test seedlings were grown, the apparent photosynthesis and transpiration of tests recovered to their original relationship with the controls in next day after the watering. Fig. 3. shows that a slight increase in appafent photosynthesis was associated with a little dec- rease in soil moisture below the field capacity. The percentage of carbohydrate reserves was slightly higher-in test seedlings (A, B) in comparison with coirtrols (C) just after the treatment, but these differences diminished in early spring of 1953. (Table 1.) The rate of diameter growth of tests was markedly depres- sed during the period of drying down of soil, on the contrary, after the treatment the rate was higher in test seedlings. (Fig. 4.)
Although a marked reduction in the production of carbohydrate occured under the drought con- ditions, the percentage of it was higher in tests, and it is supposed that this higher percentage is due to decreased consumption of carbohydrate reserves for growth, because the rate of growth was depressed remarkably during the period of drying of soil.
The effect on the growth in next year 1953 was not obvious.
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