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

Abstract

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.