Phosphorus responses of C-3 and C-4 species

An hypothesis was formulated that phosphorus (P) partitioning in tissues of C-4 leaves would permit C-4 plants to resist P deficiency better than C, plants. To test this hypothesis, 12 C-3, C-4, and C-3-C-4 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 C-3 species was more sensitive to P supply than C-4 species, but C-3 and C-4 species had similar photosynthetic P use efficiency, and C-4 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 C-4 monocot that increased root biomass allocation under stress while maintaining P allocation to the shoot. It is concluded that C-4 species are not inherently more P efficient than C-3 species, but that monocots are more P efficient than dicots, because of contrasting P and biomass allocation under stress.