Phosphorus responses of C₃ and C₄ species

An hypothesis was formulated that phosphorus (P) partitioning in tissues of C₄ leaves would permit C₄ plants to resist P deficiency better than C₃ plants. To test this hypothesis, 12 C₃, C₄, and C₃-C₄ intermediate species were grown at adequate, deficient, and severely deficient P supply in a solid-phase-buffered sand culture system to characterize photosynthetic and growth responses. Species differed considerably in response to P stress. The growth of C₃ species was more sensitive to P supply than C₄ species, but C₃ and C₄ species had similar photosynthetic P use efficiency, and C₄ species did not have low leaf P content, contrary to our hypothesis. In fact, leaf photosynthetic rates were not correlated with growth responses. Moncots had lower leaf P content and better maintenance of leaf production under P stress than dicots, because of greater inhibition of branching (dicots) than of tillering (monocots). The most P efficient species in this survey was Brachiaria, a C₄ monocot that increased root biomass allocation under stress while maintaining P allocation to the shoot. It is concluded that C₄ species are not inherently more P efficient than C₃ species, but that monocots are more P efficient than dicots, because of contrasting P and biomass allocation under stress.