Role of bundle sheath conductance in sustaining photosynthesis competence in sugarcane plants under nitrogen deficiency

The role of bundle sheath conductance ( g bs ) in sustaining sugarcane photosynthesis under nitrogen deficiency was investigated. Sugarcane was grown under different levels of nitrogen supply and g bs was estimated using simultaneous measurements of leaf gas exchange and chlorophyll fluorescence at 21% or 2% [O 2 ] and varying air [CO 2 ] and light intensity. Maximum rates of PEPC carboxylation, Rubisco carboxylation, and ATP production increased with an increase in leaf nitrogen concentration (LNC) from 1 to 3 g m −2 . Low nitrogen supply reduced Rubisco and PEPC abundancies, the quantum efficiency of CO 2 assimilation and g bs . Because of reduced g bs , low photosynthetic rates were not associated with increased leakiness under nitrogen deficiency. In fact, low nitrogen supply increased bundle sheath cell wall thickness, probably accounting for low g bs and increased estimates of [CO 2 ] at Rubisco sites. Effects of nitrogen on expression of Sh PIP2;1 and Sh PIP1;2 aquaporins did not explain changes in g bs . Our data revealed that reduced Rubisco carboxylation was the main factor causing low sugarcane photosynthesis at low nitrogen supply, in contrast to the previous report on the importance of an impaired CO 2 concentration mechanism under N deficiency. Our findings suggest higher investment of nitrogen into Rubisco protein would favour photosynthesis and plant performance under low nitrogen availability.