Growth enhancement of Brassica napus under both deficient and adequate iron supply by intercropping with Hordeum vulgare: a hydroponic study

Intercropping is a common agricultural practice improving acquisition of nutrients including iron (Fe). The mechanisms behind this effect, however, are still poorly understood particularly when intercropping cereals with non-legume dicots. To explore the role of root exudates in this positive interaction, oilseed rape (Brassica napus) and barley (Hordeum vulgare) were cultivated in hydroponics either as intercrop (IC) or monocrop (MC) under Fe-sufficient (+Fe) or Fe-deficient (–Fe) conditions. Intercropping improved chlorophyll (Chl) concentration, photosynthesis rates and the “active Fe” concentration in the young leaves of –Fe oilseed rape. Additionally, intercropping under Fe deficiency enhanced the root Fe-reductase activity and the release of organic acids and phenolic compounds by oilseed rape and the release of phytosiderophores by the barley plants. Resupply of sparingly soluble Fe(OH)3 to –Fe plants improved leaf Chl and increased the Fe concentration of barley plants under both MC and IC. Contrastingly, oilseed rape was able to use Fe(OH)₃ only under IC conditions. Moreover, oilseed rape-barley intercropping under + Fe conditions also improved Chl, photosynthesis and nitrogen uptake in oilseed rape. This positive effect of IC independent from Fe acquisition makes intercropping an attractive practice even under non-Fe stress conditions.