Root Morphology, Proton Release, and Carboxylate Exudation in Lupin in Response to Phosphorus Deficiency

Narrow-leafed lupin (Lupinus angustifolius L.) is widely planted in infertile acidic soils where phosphorus (P) deficiency is one of the major limiting factors for plant growth. A hydroponic experiment was conducted to examine the morphological and physiological responses of roots of narrow-leafed lupin in response to altered P supply at 0, 1, 10, 25 or 75 mu M P as monopotassium phosphate (KH2PO4). Low P (P-0 and P-1) significantly decreased the plant biomass, but the supply of 10 mu M P was sufficient to produce similar plant biomass as the maximal P supply (P-75), indicating an efficient P acquisition by narrow-leafed lupin. Phosphorus deficiency did not enhance rates of carboxylate exudation and proton release by plant roots, indicating that carboxylate exudation and proton release are not the mechanisms for efficient P acquisition. In contrast, low P supply evidently modified the root morphology by increasing the primary root elongation, and developing a large number of cluster-like first-order lateral roots with dense root hairs, thus allowing efficient P acquisition by narrow-leafed lupin under low P supply.