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 analyse d for gas exchange and chlorophyll fluorescence after 24 days from the transplanting. Under N-stress the decline in photosynthesis and the r ise in stomatal conductance determined at light saturation level were accompanied by an increase of the intercellular CO2 concentration (abo ut +60%). This indicates that the decline in A(max) (photosynthetic ac tivity at light saturation level) was due to limitation related to a r educed mesophyllic activity, rather than to stomatal limitation. At lo w 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 t he carbon partitioning trend had shifted towards sucrose. This result is in accordance with the lower shoot/root ratio observed in nitrogen- stressed plants.