The sensitivity of photosynthesis to phosphorus deficiency differs between C₃ and C₄ tropical grasses

Phosphorus (P) is an important determinant of plant productivity, particularly in the tropical grasslands of Australia, which contain both C₃ and C₄ species. Few studies have compared the responses of such species to P deficiency. Previous work led us to hypothesise that C₃ photosynthesis and the three subtypes of C₄ 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 (C₃) and Panicum coloratum, Cenchrus ciliaris and Panicum maximum belonging to the C₄ 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 (¹H-NMR). The response of CO₂ assimilation rates to leaf contents of inorganic phosphate ([P_i]) was linear in the C₃ grass, but asymptotic for the three C₄ 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 ¹H-NMR spectra revealed distinctive profiles of carbohydrates and amino acids for the four species. Overall, the data showed that photosynthesis of the three C₄ subtypes behaved similarly. Compared with the C₃ counterpart, photosynthesis of the three C₄ grasses had a higher P use efficiency and lower P_i requirement, and responded to a narrower range of [P_i]. Although each of the four grass species showed distinctive ¹H-NMR fingerprints, there were no differences in response that could be attributed to the C₄ subtypes.