INFLUENCE OF PHOSPHORUS LIMITATIONS ON THE GROWTH, NUTRIENT PARTITIONING AND PHYSIOLOGY OF BALSA (OCHROMA PYRAMIDALE) SEEDLINGS

Effects of P fertilisation rates (0, 0.56, 5.6 and 56 mg L⁻¹) on the growth and physiology of balsa seedlings in soilless medium were examined. Phosphorus application rate significantly affected most biometric and physiologic variables, and the greatest response occurred with the increase from 5.6 to 56 mg L⁻¹. All growth response parameters were greatest at 56 mg L⁻¹. Foliar P concentration was within sufficiency range only at 56 mg L⁻¹; therefore a minimum sufficiency level was between 5.6 and 56 mg L⁻¹ applied P. All foliar macro-and micronutrient concentrations were influenced by P supply, except Mn. Foliar P, Ca and Mg concentration increased with increasing P, and the concentration of most other nutrient decreased. Phosphorus, N and C content in each tissue increased with increasing P application rate due to dry mass increase. The highest concentrations of malic, succinic and lactic acids in xylem fluid occurred at the highest P rate. Physiological adaptations of balsa to P limitations included the preferential allocation of dry mass, P and N to plant roots, reduced specific leaf area, and increase in P-utilisation efficiency, root phosphatase activity and foliar boron concentration.