Effects of soil moisture supplied at various growth stages on the growth and fruiting of young citrus trees

Abstract

Experiments were carried out during three years period, from 1966 through 1968, to clarify the effects of soil moisture supplied at various growth stages on the growth, yield, and fruit qualities of young Satsuma orange trees.
Six to eight years old trees (Oiwa No.5) grown on sandy loam in earthware pipes (60cm in diameter ×60cm in depth) were used as the materials.
Water-regimes were designed as follows:
Plot Periods of water regimes
sign May-June July-Aug. Sept.-Oct.
A High Medium Medium(Low)
B Low Medium Medium(High)
C Medium(Low) High Medium
D Medium(High) Low Medium
E Medium Medium(Low) High
F Medium Medium(High) Low
G Medium Medium Medium
Soil moisture tension and moisture content on a percentage basis of water-holding capacity were pF=1 and 66% in wet-, pF=2 and 44% in medium-, pF=3 and 29% in dry-regime, respectively, on an average during the decade of 2 month. Sign in parenthesis indicates the water regime in 1968.
Results obtained were summarized as follows:
1. The growth of plants in new shoot, leaf number, trunk diameter and tree crown was good when supplied with wet- and medium-moisture in soil. The effect of drought period on the growth was decisive as the extreme during from July to August, followed by plot F regimed during from September to October in 1966. The number of flowers in the next spring was the largest in wetregimed during from July to August, and the next largest during from May to June, while the smallest in dry-regimes during the periods mentioned above, especially from July to August. There was a negative correlation between the number of shoot sprouting in the fall and the flower bearing in the next spring.
2. Thickening growth of fruits was inhibited by the dry-regimes during each period, especially at the earlier stages of growth. Young fruits werealso dropped by this treatment. Yield and average weight of fruits was higher in wet- than in dryregimes. Color development in fruits was the best in dry-regime during the period from September to October, while the least in the other two dryregimes. Part of fruit rind was enlarged in dryregimes towards the later stages of growth. The content of soluble matter and citric acid in fruit juice was the highest in dry-regime during the period from September to October, while the lowest in wetregime during the same period. Fruit juice in the other two dry-regimes presented no good taste.
3. The degree of water saturation deficit (W. S. D.) and diffusion pressure deficit (D. P. D.) in leaf was higher in dry- than in wet-regimes. There was a negative correlation between W. S. D. or D. P. D. and the rate of apparent photosynthesis and transpiration. Nitrogen content in leaf was higher in dry- than in wet-regimes, while phosphorus content in it was just the contrary.
The pore space in dry-regimed soil was occupied by a higher proportion of soil air with less water. Soil pH was high in wet-regimes. Soluble salts and NO₃-N concentrations in the soil solution were increased by lowering the soil moisture. In conclusion, the medium- and wet-regimes at various growth stages were highly desirable for the growth and yield of young citrus trees. Considering fruit qualities, the sequence of soil moisture supply seemed to be the best in wet- or medium- during the period from May to June, in medium- during from July to August, and in dry-regime during from September to October.