The sensitivity of photosynthesis to phosphorus deficiency differs between C-3 and C-4 tropical grasses

Phosphorus (P) is an important determinant of plant productivity, particularly in the tropical grasslands of Australia, which contain both C-3 and C-4 species. Few studies have compared the responses of such species to P deficiency. Previous work led us to hypothesise that C-3 photosynthesis and the three subtypes of C-4 photosynthesis have different sensitivities to P deficiency. To examine their dynamic response to P deficiency in more detail, four taxonomically related tropical grasses (Panicum laxum (C-3) and Panicum coloratum, Cenchrus ciliaris and Panicum maximum belonging to the C-4 subtypes NAD- ME, NADP-ME and PCK, respectively) were grown under contrasting P supplies, including P withdrawal from the growing medium. Changes in photosynthesis and growth were compared with leaf carbohydrate contents and metabolic fingerprints obtained using high-resolution proton nuclear magnetic resonance (H-1-NMR). The response of CO2 assimilation rates to leaf contents of inorganic phosphate ([Pi]) was linear in the C-3 grass, but asymptotic for the three C-4 grasses. Relative growth rate was affected most by low P in the C3 species and was correlated with the leaf content of glucose 6-phosphate more than with carbohydrates. Principal component analysis of the H-1-NMR spectra revealed distinctive profiles of carbohydrates and amino acids for the four species. Overall, the data showed that photosynthesis of the three C-4 subtypes behaved similarly. Compared with the C-3 counterpart, photosynthesis of the three C-4 grasses had a higher P use efficiency and lower Pi requirement, and responded to a narrower range of [Pi]. Although each of the four grass species showed distinctive H-1-NMR fingerprints, there were no differences in response that could be attributed to the C-4 subtypes.