Mixed cropping has the potential to enhance flood tolerance of drought-adapted grain crops

Recently, the occurrences of extreme flooding and drought, often in the same areas, have increased due to climate change. Wetland plant species are known to oxygenate their rhizospheres by releasing oxygen (O₂) from their roots. We tested the hypothesis that wetland species could help upland species under flood conditions; that is, O₂ released from the wetland crop roots would ameliorate rhizosphere O₂-deficient stress and hence facilitate upland crop root function. Flooding tolerance of upland-adapted staple crops-pearl millet (Pennisetum glaucum) and sorghum (Sorghum bicolor) mix-cropped with rice (Oryza spp.) was investigated in glasshouse and laboratory. We found a phenomenon that strengthens the flood tolerance of upland crops when two species-one wetland and one drought tolerant-were grown using the mixed cropping technique that results in close tangling of their root systems. This technique improved the photosynthetic and transpiration rates of upland crops subjected to flood stress (O₂-deficient nutrient culture). Shoot relative growth rates during the flooding period (24 days) tended to be higher under mixed cropping compared with single cropping. Radial oxygen loss from the wetland crop roots might be contributed to the phenomenon observed. Mixed cropping of wet and dryland crops is a new concept that has the potential to overcome flood stress under variable environmental conditions.