Root oxygen use as a measure for ion uptake from slightly different nutrient solutions

The current system of nutrient supply in horticulture may be improved by ion specific measuring in combination with real-time nutrient adjustments. Frequent measurements of the solution around the roots and related adjustments of the solutions supplied, will result in plant roots experiencing smaller fluctuations in nutrient concentrations and ratios than in the current system of analyzing and adjusting the nutrients once in 1-2 weeks. To find out how much effect on growth more frequent measurements and adjustments could have, we measured the oxygen uptake of roots growing in nutrient solutions of different ionic compositions. The assumptions underlying this approach are that nutrient uptake requires energy and the energy is generated by the oxidation of assimilates. Thus root oxygen use is a measure of the energy use for ion uptake and via assimilate use also related to above-ground production. To measure root oxygen use in optimal and slightly sub-optimal nutrient solutions, four air-tight vessels to grow plants in nutrient solutions were constructed. Each vessel was connected to an airbag and a peristaltic pump to circulate the air. Consequently tomato, cucumber and sweet pepper young plants were grown on optimal and slightly sub-optimal nutrient solutions. Oxygen sensors recorded the decrease in oxygen concentration. So far, a 5-18% higher oxygen consumption was found for a NO₃ solution as compared to a NO₃/Cl solution. In conclusion root oxygen use for a whole root system during growth can be accurately measured and as a first result the uptake of nitrate requires more energy than the uptake of chloride. Consequences for the application of ion specific measurements as well as for the definition of optimised target solutions for the root environment are discussed.