Reduction in Leaf Water Potential and Hydraulic Conductance of Young Rice Plants (Oryza Sativa L.) Grown in Wet Compacted Soils

Abstract

Lower leaf water potential (Ψ₁) was observed in young rice plants grown in wet and highly compacted soil. Our objectives were to establish why the reductions of Ψ₁ occurred and to find the effect of plant hydraulic conductance on Ψ₁. One seedling of a lowland cultivar Nipponbare and an upland cultivar Senshou were grown in plastic pipes with a diameter of 5.4 cm and a height of 45 cm filled with a soil mixture. Soil bulk density (SBD) was set at four levels and wet soil conditions were maintained. In a growth chamber Ψ₁, transpiration rate per unit leaf area (T_L), leaf area and root length were measured at 35 days after sowing. In both cultivars, plant hydraulic conductance (C_P=-T_L/Ψ₁) decreased as SBD increased and there was a positive correlation between C_P and root length per unit leaf area. The greater decreases in root length than in leaf area, in the soil with a high bulk density, were suggested to reduce C_P, thus resulting in lower Ψ₁. In both cultivars, root hydraulic conductance per unit root length (C_R), estimated using a pressure-flux method, increased with increase in SBD. The increases in C_R were accompanied by the increases in T_L per unit root length at a high SBD. We suggest that the suppression of root length in rice by highly compacted soil causes lower C_P which in turn reduces Ψ₁, even if the soil is wet. C_R, water absorption rate per unit root length and the diameter of the primary roots, clearly increased under highly compacted soil conditions, but this might not be able to compensate for the greater reduction in root length than in leaf area, and so may not permit C_P and Ψ₁ to be maintained.