Using a biochemical C4 photosynthesis model and combined gas exchange and chlorophyll fluorescence measurements to estimate bundle-sheath conductance of maize leaves differing in age and nitrogen content

Bundle-sheath conductance (gbs) affects CO2 leakiness, and, therefore, the efficiency of the CO2-concentrating mechanism (CCM) in C4 photosynthesis. Whether and how gbs varies with leaf age and nitrogen status is virtually unknown. We used a C4-photosynthesis model to estimate gbs, based on combined measurements of gas exchange and chlorophyll fluorescence on fully expanded leaves of three different ages of maize (Zea mays L.) plants grown under two contrasting nitrogen levels. Nitrogen was replenished weekly to maintain leaf nitrogen content (LNC) at a similar level across the three leaf ages. The estimated gbs values on leaf-area basis ranged from 1.4 to 10.3 mmol m-2 s-1 and were affected more by LNC than by leaf age, although gbs tended to decrease as leaves became older. When converted to resistance (rbs = 1/gbs), rbs decreased monotonically with LNC. The correlation was presumably associated with nitrogen effects on leaf anatomy such as on wall thickness of bundle-sheath cells. Despite higher gbs, meaning less efficient CCM, the calculated loss due to photorespiration was still low for high-nitrogen leaves. Under the condition of ambient CO2 and saturating irradiance, photorespiratory loss accounted for 3–5% of fixed carbon for the high-nitrogen, versus 1–2% for the low-nitrogen, leaves.