Hydroponics in physiological studies of trace element tolerance and accumulation in plants focussing on metallophytes and hyperaccumulator plants

Background: When studying metallophytes and hyperaccumulator plants, it is often desired to assess the level of tolerance of a specific trace metal/metalloid in a putative tolerant species, to determine root and shoot accumulation of the trace metal/metalloid of interest, or to establish whether a trace metal/metalloid has an essential function. The use of hydroponics has proven to be a powerful tool in answering such questions in relation to the physiological regulation of metal/metalloids in plants. Carefully designing experiments requires considering nutrient solution formulation, dose rate regime, and environmental conditions, but this is often overlooked. Aims: This review aims to bring together key information for hydroponics studies in physiological, evolutionary, and genetics/molecular biological research of trace metal/metalloid tolerance and accumulation in plants, focussing on metallophytes and hyperaccumulator plants. Conclusions: It is not possible to define a ‘universal’ nutrient solution that is both sufficient and non-toxic for all plants, although it is often possible, dependent on plant species under study and the research question to be addressed, to ‘adapt’ commonly used ‘standard formulations’. Well-designed and executed hydroponics experiments can yield powerful insights in the regulation of essential and toxic metal/metalloid trace elements, and this extends far beyond hyperaccumulator plants.