The experiments were conducted to evaluate the effects of irrigation, temperature and shading on the acidity and the potassium content of tomato fruits (var. Fukuju No. 2) from plants grown in pots containing 13 litres of volcanic ash soil. Plants were placed under the treatments during the period between the flowering stage and the incipient colour stage of fruits on the second truss. All fruits were harvested at the incipient colour stage.
1. On the beginning of experiment in 1966, each pot was left over night after sufficient watering and weighed. Deficit from this weight of pot was determined daily thereafter. Irrigation was made in three ways to compensate the deficit daily and when it reached 1.5 and 2.5 kg per pot, respectively.
The titratable, total and combined acidities of fruits, expressed on fresh weight basis, from the plants irrigated daily were the lowest in three treatments and the highest were those of fruits from the plants which were not irrigated until 2.5kg of moisture was lost. The same trend was found for the potassium content. The titratable acidity represented as a percentage of the total acidity was not affected by these treatments. Although neither timing nor amount of irrigation was so strictly regulated on the experiment in 1965, the changes of the acidities caused by irrigation treatments were found almost the same with those of 1966 experiment.
In order to make certain of the possibility of dilution effect of irrigation on the acidities and the potassium content, the above determinations were recalculated on dry weight basis. In this case, effects of irrigation could not be observed. This suggested that the changes of the acidities and the potassium content on fresh weight basis were mainly dependent on the differences of moisture content of fruits, that is, the explanation by dilution effect.
The more frequent the irrigation was, the lower the refractometer reading and the greater the fruit weight. pH showed no apparent changes. Gelatinous pulp weight expressed as a percentage of whole fruit was not affected by treatments.
2. In the experiment of temperature, the plants in pots were held all day in 1964, and for eight hours in the daytime daily in 1965, respectively, in the rooms controlled at 20°C or at 30°C. In both experiments, the titratable, total and combined acidities and the potassium content of fruits at 30°C were higher than those of fruits at 20°C. The titratable acidity percent was also higher at 30°C than at 20°C.
pH of fruit juice was higher and fruit weight was lower at 30°C than at 20°C. Differences of temperature brought no significant change of the gelatinous pulp percent.
3. In the experiment of shading effect, light intensity was lowered to 50 or 25 percent of that of unshaded control by the screen of lawn.
Shading effects were small, if any, on the acidities and the ptassium content of tomato fruits.
The stronger the extent of shading, the lower the fruit weight and the greater the gelationus pulp percent.
Changes of the titratable, total and combined acidities of tomato fruits enveloped in aluminium foil bags during their development had almost the same pattern with that found on the fruits developed under natural light condition.