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 - GA3 and indole-3-acetic acid - IAA) with respect to sunflower (Helianthus annuus L.) growth, physiology, and metabolism were studied. Application of 100 μM IAA lessened the toxic effects of 80 μM Cu in roots indicating greater root length and root hair formation, while addition of 100 μM GA3 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 GA3 resulted in stability of the LHC of PS2 RCs. The drop in net photosynthetic (PN) and transpiration (E) rates with preserved or slightly reduced variable to maximum fluorescence ratio (Fv/Fm) in the presence of 80 μM Cu could be explained by a possible inhibition of the enzymatic processes in the Calvin cycle. Application of 100 μM IAA and 100 μM GA3 lessened Cu effects mainly on P N. Water use efficiency was also improved under hormone exposure.