Responses of the Photosynthetic Apparatus to Plant Growth Regulators in two Sunflower Cultivars (Helianthus annuus L.)

GIANNAKOULA A. E & ILIAS I. F. 2011. Responses of the photosynthetic apparatus to plant growth regulators in two sunflower cultivars (Helianthus annuus L.). Phyton (Horn, Austria) 51(2): 245-259. The aim of the work was to examine the effect of growth regulators GA(3), GA(4) (gibberellic acid) and Prohexadione-Calcium on growth, net photosynthetic rate (P-N), and chlorophyll fluorescence of Helianthus annuus (L.) plants. The experiment was also conducted to investigate the combined effects of exogenous hormones on photosystem II (PSII) activity, irradiance response curves for electron transport rate (ETR), non-photochemical quenching (q(N)), photochemical quenching (q(P)) and real photochemical efficiency of PSII (Phi(PSII)) that were recorded under different photosynthetic active radiation. Exposure of sunflower plants to excess growth regulators (200 mu M) in hydroponical culture led to inhibition of growth and decreased maximum quantum yield of primary photochemistry (Fv/Fm) under Prohexadione-Ca while GA(3) and GA(4) induced stem elongation and enhanced photosynthesis. In addition Prohexadione-Ca treated plants exhibited decreased photosynthetic pigments and photosynthetic rates, whereas application of hormones (GA(3) and GA(4)) led to substantial preservation of chlorophylls and increases in chl a+b, net photosynthetic assimilation rate and stomatal conductance. Finally, plant biomass was reduced with Prohexadione-Ca. The alteration in chlorophyll a fluorescence characteristics observed under Prohexadione-Ca suggested a weakening of the photochemical processes near the PSII reaction centre.