Circumnutation is a helical organ movement widespread among plants. It is variable due to a different magnitude of trajectory (amplitude) outlined by the organ tip, duration of one cycle (period), ...

Increasing temperature leads to intensive water evaporation, contributing to global warming and consequently leading to drought stress. These events are likely to trigger modifications in plant physiology and microbial functioning due to the altered availability of nutrients. Plants exposed to drought have developed different strategies to cope with stress by morphological, physiological, anatomical, and biochemical responses. First, visible changes influence plant biomass and consequently limit the yield of crops. The presented review was undertaken to discuss the impact of climate change with respect to drought stress and its impact on the performance of plants inoculated with plant growth-promoting microorganisms (PGPM). The main challenge for optimal performance of horticultural plants is the application of selected, beneficial microorganisms which actively support plants during drought stress. The most frequently described biochemical mechanisms for plant protection against drought by microorganisms are the production of phytohormones, antioxidants and xeroprotectants, and the induction of plant resistance. Rhizospheric or plant surface-colonizing (rhizoplane) and interior (endophytic) bacteria and fungi appear to be a suitable alternative for drought-stress management. Application of various biopreparations containing PGPM seems to provide hope for a relatively cheap, easy to apply and efficient way of alleviating drought stress in plants, with implications in productivity and food condition.

An endogenous, helical plant organ movement named circumnutation is ubiquitous in the plant kingdom. Plant shoots, stems, tendrils, leaves, and roots commonly circumnutate but their appearance is still poorly described. To support such investigations, novel software Circumnutation Tracker (CT) for spatial-temporal analysis of circumnutation has been developed.

Action potentials (APs), i.e. long-distance electrical signals, and circumnutations (CN), i.e. endogenous plant organ movements, are shaped by ion fluxes and content in excitable and motor tissues. The appearance of APs and CN as well as growth parameters in seedlings and three-week old plants of Helianthus annuus treated with osmotic and salt stress (0-500 mOsm) were studied. Time-lapse photography and extracellular measurements of electrical potential changes were performed. The hypocotyl length was strongly reduced by the osmotic and salt stress. CN intensity declined due to the osmotic but not salt stress. The period of CN in mild salt stress was similar to the control (approx. 164 min) and increased to more than 200 min in osmotic stress. In sunflower seedlings growing in a hydroponic medium, spontaneous APs (SAPs) propagating basipetally and acropetally with a velocity of 12-20 cm min-1 were observed. The number of SAPs increased 2-3 times (7-10 SAPs 24h-1plant-1) in the mild salt stress (160 mOsm NaCl and KCl), compared to the control and strong salt stress (3-4 SAPs 24h-1plant-1 in the control and 300 mOsm KCl and NaCl). Glutamate-induced series of APs were inhibited in the strong salt stress-treated seedlings but not at the mild salt stress and osmotic stress. Additionally, in three-week old plants, the injection of the hypo- or hyperosmotic solution at the base of the sunflower stem evoked series of APs (3-24 APs) transmitted along the stem. It has been shown that osmotic and salt stresses modulate differently hypocotyl growth and CN and have an effect on spontaneous and evoked action potentials in sunflower seedlings. We suggested that potassium, sodium, and chloride ions at stress concentrations in the nutrient medium modulate sunflower excitability and CN.

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 Li⁺). 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.