The influence of water stress on nitrogen and phosphorus uptake and concentrations in Townsville stylo (Stylosanthes humilis)

Abstract

The phosphorus concentrations of Townsville stylo (TS) grown in the Northern Territory are lower than in Queensland, and fall below the requirements of grazing cattle for much of the year. An explanation was sought by measuring N and P in swards subjected to water stress at various times during growth. Stresses during early vegetative (EV), late vegetative (LV) and flowering (F) stages were compared with a well watered control. While nitrogen concentrations were affected little, water stress greatly reduced phosphorus concentrations, the relative effects being EV > LV > F. The EV stress depressed phosphorus concentration to 0.08% compared with 0.20% in the control. Phosphorus level recovered on the relief of stress to the same level as the control. Phosphorus concentration in stems was more sensitive to stress than that in leaves. These results show that if chemical analysis of plants is to be used to indicate the phosphorus status of the soil, the analyses will be unreliable unless the plants have grown without water stress for at least six weeks.