The root electrical capacitance (CR) method is suitable for assessing root growth and activity, but soil water content (SWC) strongly influences the measurement results. This study aimed to adapt the method for field monitoring by evaluating the effect of SWC on root capacitance to ensure the comparability of CR detected at different SWC. First a pot experiment was conducted with maize and soybean to establish CR–SWC functions for the field soil. Ontogenetic changes in root activity were monitored under field conditions by simultaneously measuring CR and SWC around the roots. The CR values were normalized using SWC data and experimental CR–SWC functions to obtain CR*, the comparable indicator of root activity. The effect of arbuscular mycorrhizal fungi (AMF) inoculation on the CR* and biomass of field-grown soybean was investigated. The pot trial showed an exponential increase in CR with SWC. CR–SWC functions proved to be species-specific. CR showed strong correlation with root dry mass (R2 = 0.83–0.87). The root activity (CR*) of field-grown crops increased until flowering, then decreased during maturity. This was consistent with data obtained with other methods. AMF inoculation of soybean resulted in significantly higher CR* during the late vegetative and early flowering stages, when destructive sampling concurrently showed higher shoot biomass. The results demonstrated that the root capacitance method could be useful for time course studies on root activity under field conditions, and for comparing single-time capacitance data collected in areas with heterogeneous soil water status.

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Appropriate selection and well-timed measurement of plant developmental, morphological and physiological parameters are essential to maximize efficacy and minimize time consumption of experiments. To select for the most sensitive indicators of drought or salt stress, three independent pot experiments with diverse setups were analysed with 20-20 measured parameters. Parameters of plant growth, phenology and symbiotic interactions, visual stress symptoms, photosynthetic activity, nutrient composition and vitality were studied and the result matrices were evaluated with principal component analysis (PCA). Stress effects manifested in PC1 of two experiments and in PC2 of the third one. Traits assumed to be adequate for stress indication were characterized by high PC1 or PC2 loading values. Beside parameters of biomass production, growth and visible stress symptoms, less evident traits e.g. root electrical capacitance, membrane stability index in roots and leaves, relative water content of leaves and SPAD units were identified.