Photosynthesis and chlorophyll fluorescence in sunflower (Helianthus annuus L.) leaves as affected by phosphorus nutrition

Abstract

The effects of phosphate concentration on plant growth and photosynthetic processes in primary leaves of young sunflower (Helianthus annuus L.) plants were examined. Plants were grown for 3 weeks on half-strength Hoagland's solution containing 0, 0.1, 0.5, 1.0, and 3.0 mol m−3 orthophosphate (Pi). It was shown that optimal photosynthesis and the highest light utilization capacity were achieved at 0.5 mol m−3 Pi in the growth medium, which was in good agreement with the maximum content of organic phosphorus in the leaves. Low phosphate in the medium inhibited plant growth rate. Phosphate deficiency appreciably decreased photosynthetic oxygen evolution by leaves, the efficiency of photosystem two (PSII) photochemistry and quantum efficiency of PSII electron transport. High oxidation state of PSII primary electron acceptor QA, at 0.1 mol m−3 Pi, however, indicates that photosynthetic electron transport through PSII did not limit photosynthesis in Pi-deficient leaves. The results indicate that diminished photosynthesis under sub- and supra-optimal Pi was caused mainly by a reduced efficiency of ribulose 1, 5-bisphosphate (RuBP) regeneration at high light intensities. These results suggest that, under non-limiting C02 and irradiance, photosynthesis of the first pair of leaves could be diminished by both sub- and supra-optimal phosphorus nutrition of sunflower plants.