The effect of nitrogen deficiency on leaf gas exchange and chlorophyll fluorescence parameters in sunflower

Fully expanded leaves of sunflower (Helianthus annuus L.) growing with either complete or nitrogen-deficient nutrition solution were analysed for gas exchange and chlorophyll fluorescence after 24 days from the transplanting. Under N-stress the decline in photosynthesis and the rise in stomatal conductance determined at light saturation level were accompanied by an increase of the intercellular CO2 concentration (about +60%). This indicates that the decline in A(max) (photosynthetic activity at light saturation level) was due to limitation related to a reduced mesophyllic activity, rather than to stomatal limitation. At low light intensity photosynthetic efficiency of PSII was not affected. In N-stressed plants starch content decreased significantly in leaves. The sucrose/starch ratio rose in N-stressed plants, indicating that the carbon partitioning trend had shifted towards sucrose. This result is in accordance with the lower shoot/root ratio observed in nitrogen-stressed plants.