Lithium distinguishes between growth and circumnutation and augments glutamate-induced excitation of Helianthus annuus seedlings

An effect of lithium (Li+) on growth, circumnutation, and glutamate-induced excitation in sunflower (Helianthus annuus L.) seedlings was investigated using time-lapse photography and extracellular electrical potential measurements. The seedlings were treated with a micro and millimolar concentration of lithium chloride (LiCl) both persistently in a hydroponic medium and acutely through Li+ injection. The length of hypocotyls, fresh weight of seedlings, intensity and period of circumnutation, and the number of action potentials (APs) after glutamate (Glu) injection were determined. It was found that the circumnutation intensity and period did not depend on hypocotyl length and fresh weight of seedlings. Under persistent Li+ treatment, the circumnutation intensity was constant at a concentration between 0.2 and 20 mM although the hypocotyls were significantly shorter in relation to the control, whereas at a concentration of 40 mM circumnutation intensity decreased without any changes in the hypocotyl length. Under persistent treatment with 0.5 and 20 mM Li+, the period of circumnutation was significantly prolonged. The number of APs in a Glu-induced series significantly increased in the seedlings exhibiting an Li+-induced decrease in circumnutation intensity (in 40 and 60 mM Lit). Additionally Li+ injection before Glu injection also augmented the series of APs in seedlings growing without Li+ in hydroponic medium. These Li+ - sensitive responses demonstrated that circumnutation and growth are partly independent processes and reveal a relationship between circumnutation intensity and excitability in Helianthus annuus seedlings.