Description:
<p>Nitrogen deficiency reduces the photosynthetic capacity of both C3 and C4 plants. The regulation of photosynthetic gas exchange in eight clones of the C4 grass, sugarcane (Saccharum spp.), grown at three levels of N availability was studied to determine whether N stress diminishes the efficiency of the C4 CO2 concentrating system in addition to reducing overall rates of photosynthesis. The quantum yield for CO2 uptake decreased linearly with decreasing leaf N content. Genetic variation in quantum yield at a given level of N supply was also observed. Leaf tissue carbon isotope discrimination (Δ) increased linearly with decreasing quantum yield. Concurrent determinations of the prevailing ratio of intercellular to ambient partial pressure of CO2 (pi/pa) during leaf gas exchange suggested that the observed variation in Δ was almost entirely attributable to variation in bundle sheath leakiness to CO2 (Φ) rather than pi/pa. Taken together, these results point to substantial environmental and genetic variation in the efficiency of the CO2 concentrating system in sugarcane. Reduced partitioning of carboxylase activity to ribulose—1,5-bisphosphate carboxylase relative to phosphoenolpyruvate carboxylase in N-deficient plants suggested that the associated increase in Φ and decline in quantum yield may have been attributable largely to a decline in C3 cycle activity in the bundle sheath relative to C4 cycle activity in the mesophyll. Quantum yield and intrinsic water use efficiency (WUE) were negatively correlated. In contrast with the trade-off between intrinsic light- and water use efficiency, photosynthetic nitrogen-use efficiency and intrinsic WUE were positively correlated.</p>