
%0 Journal Article
%A Ranjith, SA 
%A Meinzer, FC 
%A Perry, MH 
%A Thom, M 
%T Partitioning of Carboxylase Activity in Nitrogen-Stressed Sugarcane and Its Relationship to Bundle Sheath Leakiness to CO<sub>2</sub>, Photosynthesis and Carbon Isotope Discrimination
%J Functional Plant Biology
%D 1995/12/01
%V 22
%N 6
%P 903-911
%K 
%R https://doi.org/10.1071/PP9950903
%U https://www.publish.csiro.au/paper/PP9950903 
%X We studied the effects of external nitrogen (N) supply on ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, phosphoenolpyruvate carboxylase (PEPC) activity, leaf gas exchange, carbon isotope discrimination (&#916;), and bundle sheath leakiness to CO2 (&#934;) in two cultivars of the C4 grass, sugarcane (Saccharum spp. hybrid). In addition to reducing overall levels of carboxylase activity and therefore photosynthetic rates, reduced N supply altered the partitioning of carboxylase activity. Under long-term N stress (4 months) Rubisco activity decreased more than PEPC activity causing significant reductions in the Rubisco/PEPC activity ratio, a measure of the ratio of C3 to C4 cycle activity. Concurrent determinations of &#916; for leaf dry matter and the prevailing ratio of intercellular to ambient partial pressure of CO2 (pI/pa) during leaf gas exchange suggested that the decreased partitioning of N to Rubisco activity under long-term N stress led to increased &#934; and reduced photosynthetic efficiency. The two cultivars studied maintained similar PEPC activities but differed genetically in regard to investment of N in Rubisco. Greater investment of N in Rubisco was associated with higher rates of photosynthesis and growth at similar or slightly lower leaf N content, indicating that greater relative investment of N in Rubisco activity also led to higher N-use efficiency. The results suggest that regulation of the ratio of C3 to C4 pathway activity and its consequences for &#934; may play a key role in the photosynthetic performance and growth of C4 grasses under both favourable and stressful conditions. 
