Abstract
To make clear the water consumption at the different levels of the soybean canopy, the water fluxes in the stem were measured at three different parts of the stem (4th internode, z1; 8th internode z2; 13th internode, z3) using a newly developed apparatus (Fig. 1) at flowering stage in the field. Two plant spacings (wide spacing, 30×30cm; close spacing, 20×20 cm) were prepared. The differences of water fluxes between z1 and z2, between z2 and z3, and the water flux at z3 provided the water consumption of the lower-, the middle- and the upper-foliage, respectively (Fig. 2). 1. The diurnal courses of the water flux in gH2O per 30 min at the three different parts of the stem were in phase with one another (Fig. 4a). 2. In those three foliages, the water consumption in gH2O per dm2 leaf per 30 min was greatest at the upper foliage which was distributed in the upper layer of the canopy directly exposed to the sun. The water consumption of lower foliage of the plant with wide spacing, the branches of which elongated to the upper layer of the canopy, was almost similar to that of the upper foliage. The water consumptions of the middle foliage of the plants at both spacings and of lower foliage at close spacing were 1/3-1/2 of the upper foliages (Fig. 4b). 3. The water consumption ratio (the ratio of the water consumption of a foliage to water flux of the basal part of the stem) of the upper foliage gradually decreased during daytime. This may be attributed to increase of water stress of the upper foliage with increase of solar radiation (Fig. 4c). 4. The vertical profile of water consumption of the canopy at the wide spacing showed a peak at the layer just below the uppermost layer, while at the close spacing canopy, the peak was at the uppermost layer (Fig. 5). 5. The water amount consumed by the soybean plants per unit ground area was greater at close spacing than at wide spacing.
