BORON AS LIMITING FACTOR IN PHOTOSYNTHESIS AND GROWTH OF SUNFLOWER PLANTS IN RELATION TO PHOSPHATE SUPPLY

The effect of varied boron supply (0, 0.5, 1, 25 and 100 mu M) in the nutrient solution containing different phosphate concentrations on nut rient utilization, plant growth and photosynthesis was studied in youn g sunflower (Helianthus annuus L.) plants, Boron deficiency decreased the dry matter yield of plants, leaf area and chlorophyll content in t he leaves. The decrease was significantly higher in the presence of su b-(0.1 mM) and supra-optimal phosphate concentrations (3 mM) in the nu trient solution. Boron deficiency appreciably decreased photosynthetic oxygen evolution per unit leaf area and proton gradient across the th ylakoid membrane, but it did not influence photochemical and electron transport efficiency of photosystem two. The diminished rate of photos ynthesis in boron-deficient sunflower leaves could be correlated to th e reduction in chlorophyll content, electron transport to CO2 and phot ophosphorylation. In P-deficient plants nonphotochemical chlorophyll f luorescence quenching increased with baron concentration, implying an increase in the proton gradient, and confirming the role for boron in maintaining the integrity of thylakoid membranes.