Growth strategies and utilization of phosphorus in Cajanus cajan L Millsp and Desmodium tortuosum (Sw) DC under phosphorus deficiency

Leguminous plants have developed several growth strategies to withstand nutrient limitations imposed by the soil and different species depend on different mechanisms plant roots can use under a given soil condition for the acquisition of inorganic and organic phosphorus (P). The objective of this study was to provide appropiate physiological indicators to describe plant performance under P deficiency early in the vegetative cycle of the annual non-cultivated Desmodium tortuosum (Sw) D.C. and the perennial Cajanus cajan (L.) Mllsp grown inside a highly ventilated greenhouse in 950-mL pots with sand irrigated with either 1.0 (T1) or 0.01 (T2) nM P in the Hoagland solution for a period of 35 or 45 days when growth pressure made differences between P treatments apparent. Results indicated that total leaf area, relative growth rate (RGR), but not relative leaf area expansion rate (RLAER), were drastically reduced in n plants in both species and that root length was reduced by 50% in Desmodium severely stressed T2 plants. The activity of acid phosphatase secreted by the roots in vivo increased under P deficiency in both species. The kinetic constants (Km and Vmax) of the secreted enzyme calculated from Lineweaver-Burk linear plots were different for the enzyme from T1 or T2 plants in both species. The numerical value of the Km for the substrate p-nitrophenyl-P provided a means of comparing the enzyme from high- or low-P plants. and it is suggested that Km, enzyme activity (Vmax), total leaf area and RGR (during exponential growth), may be used as physiological indicators to differentiate plants grown under P deficiency or sufficiency