Spatial Patterns of Carbon Isotope Discrimination and Allocation of Photosynthetic Activity in Sugarcane Leaves

Longitudinal variation in carbon isotope discrimination (Δ), photosynthetic gas exchange, phosphoenolpyruvate carboxylase (PEPC) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities, chlorophyll and N content were characterised in the uppermost fully expanded leaf of sugarcane (Saccharum spp. hybrid) plants. Δ values of lamina tissue and the water-soluble fraction increased from the base to near the tip of the leaf then remained essentially constant, or declined along the rest of the leaf. Both the CO2 assimilation rate (A) and stomatal conductance (g) also increased from the base to the tip. The relative increase in A was much greater than that of g, causing the ratio of intercellular to ambient partial pressure of CO2 (pi/pa) to decrease substantially from leaf base to tip. Activities of both PEPC and Rubisco increased from leaf base to tip. Based on current models of factors determining Δ in C4 plants, the increase in Δ from the base to tip of sugarcane leaves appeared to be entirely attributable to decreasing pi/pa. Bundle sheath leakiness to CO2, the other major determinant of Δ in C4 plants, was nearly constant from the leaf base to tip, consistent with the nearly constant ratio of Rubisco and PEPC activity. Spatial allocations of carboxylase activity, chlorophyll, and N were highly correlated with prevailing levels of incident light at different points along the leaf. Partitioning of N and photosynthetic activity within individual leaves was consistent with allocation of N and photosynthetic capacity along vertical gradients of incident light reported for entire canopies.