Studies on Leaf Water Stress in Fruit Trees

Abstract

Relationships between soil factors and leaf water potential were studied during summer for satsuma mandarin (Citrus unshiu MARC.) and maltese blood (Citrus sinensis OSBECK) trees.1. Leaf water potential measured before sunrise (ψψmax) was curvilinearly related to the soil matric potential in the main root zone. When soil matric potential dropped below pF 3.5 to 4.0, the slope of the curve began to increase rapidly.Even under the same atmospheric and soil moisture conditions, leaf water potentials during the day (ψψmin) were always higher for the trees grown in the granite soil than in the rhyolite soil. The difference described above may be due to the differences in soil properties such as hydraulic conductivity and porosity between the two soil types.2. Both soil matric potential and ψψmax for trees at the site with shallow soil profile continued to fall rapidly as days went on, whereas at the site with deeper soil profile the rate of decrease of the both values began to retard after ψψmax reached around -4.5 bars. Since no difference was found in the extent of root depth between the two sites, the difference should be attributed to the supplemental water flow from the subsoil.3. Root system of a tree was divided into two parts; one was held in moist soil (irrigated daily), the other in dry soil (nonirrigated). The moisture of nonirrigated soil decreased as days went on, the values of ψψmax, however, were remained constant according to the proportion of rootlets in moist soil.4. The submerging treatment was carried out by using trees grown in pots. Initially, ψψmin in this treatment was lower than that of control plants irrigated daily. After 5 to 7 days, however, ψψmin and leaf diffusion resistance in the submerged plants became abnormally higher than those of controls.