Research PaperKinnow mandarin plants grafted on tetraploid rootstocks are more tolerant to Cr-toxicity than those grafted on its diploids one

•Cr-toxicity inhibited the growth and physio-biochemcial attributes in citrus.•Ploidy level of rootstock had significant impact on Cr-tolerance in Kinnow mandarin.•High Cr-tolerance of 4x plants to Cr-toxicity is highly associated with Cr-accumulation in leaves.•Plants with tetraploid rootstocks had more Cr concentration in their roots than leaves.

This study compared the effects of chromium toxicity on the biomass, photosynthesis, antioxidants, reactive oxygen species, and nutrient concentration in Kinnow mandarin (Citrus nobilis Lour x Citrus deliciosa Ten) plants grafted on diploids (2x) and tetraploids (4x) of Poncirus trifoliata [L.], Citrus reshni, and Citrus limonia Osbeck. Plants were grown under controlled conditions and irrigated with complete nutrient solution supplemented with chromium (0.75 mM). After 120-d of growth under chromium stress, plant biomass, total chlorophyll concentrations, photosynthetic activity, stomatal conductance, transpiration rate, water use efficiency, H2O2 concentrations, rate of O2− generation, lipid peroxidation, antioxidant activities (superoxide dismutase, peroxidase, catalase) and mineral nutrient concentration (Ca, Mg, K, P and Cr) were determined. Chromium inhibited plant growth, and decreased chlorophyll concentration, photosynthetic activity, stomatal conductance, transpiration rate, water use efficiency, and activities of antioxidant enzymes, but the rate of lipid peroxidation and formation of reactive oxygen species were increased. Chromium stress also caused alterations in nutrients concentration in roots and leaves. Nevertheless, it was observed that Kinnow mandarin plants grafted on the tetraploid rootstocks showed high tolerance to chromium toxicity as reported by maintaining greater biomass accumulation and less reduction in the attributes commented before. In addition, plants with 4x rootstocks had higher Ca, Mg, K and P concentration in their roots and leaves in comparison to 2x rootstocks. Plants grafted on 4x rootstock presented higher quantity of chromium in roots than leaves, suggesting that the chromium tolerance of plants with tetraploid rootstocks may be attributed to chromium sequestration to roots with lower transfer to leaves; consequently avoiding oxidative damage to green pigments and the photosynthetic apparatus.