CASSAVA VARIETAL RESPONSE TO PHOSPHORUS FERTILIZATION .2. PHOSPHORUS UPTAKE AND USE EFFICIENCY

In Latin America, where cassava is commonly grown, yield is partly limited by low-P soil. Previous trials conducted at Centro Internacional de Agricultura Tropical (CIAT) indicated a wide range of response for adaptation to low-P soils in cassava germplasm. The objectives of this study were to identify plant characteristics and possible mechanisms involved in varietal response to P and to assess the relative importance of P uptake and internal use. Two-year trials with four varieties were conducted in a field low in P with three levels (0, 50 and 100 kg ha-1) of P application. Five sequential harvests (with determination of P uptake) were carried out during each of two growth cycles, and VAM infection rates of fine roots and root length density in the surface soil layer were measured. In all varieties P uptake increased with P fertilization, and cultivar variation in P uptake was related to differences in fine root length density more than to VAM infection rates. Uptake efficiency of roots (as uptake rate per unit root length) did not differ among varieties. Phosphorus-use efficiency (as root yield per accumulated P in the whole plant) and patterns of dry matter allocation to roots and shoots were significantly different among varieties. It was concluded that cultivar differences in response to applied P were mainly due to contrasting patterns of dry matter partitioning and P-use efficiency. Adaptation to low-P soils could be enhanced by selection of varieties with high fine-root length density, moderate top growth and stable harvest index.