Hormone-induced protection of sunflower photosynthetic apparatus against copper toxicity

The effects of excess Cu as affected by the application of exogenous hormones (gibberellic acid - GA(3) and indole-3-acetic acid - IAA) with respect to sunflower (Helianthus annuus L.) growth, physiology, and metabolism were studied. Application of 100 mu M IAA lessened the toxic effects of 80 W Cu in roots indicating greater root length and root hair formation, while addition of 100 mu M GA(3) ameliorated the toxic effect mainly to the shoot. The content of photosynthetic pigments significantly declined under Cu stress, whereas application of hormones led to a substantial preservation of chlorophylls and carotenoids. Under Cu stress, the rate constant of energy trapping by photosystem 2 (PS2) reaction centres (RCs) was reduced as a result of physical dissociation of the light-harvesting complex (LHC) from PS2 core, while application of IAA and especially GA(3) resulted in stability of the LHC of PS2 RCs. The drop in net photosynthetic (P-N) and transpiration (E) rates with preserved or slightly reduced variable to maximum fluorescence ratio (F-v/F-m) in the presence of 80 mu M Cu could be explained by a possible inhibition of the enzymatic processes in the Calvin cycle. Application of 100 mu M IAA and 100 mu M GA(3) lessened Cu effects mainly on P-N. Water use efficiency was also improved under hormone exposure.