Boron-mediated Amelioration of Copper-toxicity in Sweet Orange [Citrus Sinensis (L.) Osbeck Cv. Xuegan] Seedlings Involved Reduced Damage to Roots and Improved Nutrition and Water Status

'Xuegan' (Citrus sinensis) seedlings were fertilized 6 times weekly for 24 weeks with 0.5 or 350 μM CuCl and 2.5, 10 or 25 μM HBO. Cu-toxicity increased Cu uptake per plant (UPP) and Cu concentrations in leaves, stems and roots, decreased water uptake and phosphorus, nitrogen, calcium, magnesium, potassium, sulfur, boron and iron UPP, and increased the ratios of magnesium, potassium, calcium and sulfur UPP to phosphorus UPP and the ratios of leaf magnesium, potassium and calcium concentrations to leaf phosphorus concentration. Many decaying and dead fibrous roots occurred in Cu-toxic seedlings. Cu-toxicity-induced alterations of these parameters and root damage decreased with the increase of boron supply. These results demonstrated that B supplementation lowered Cu uptake and its concentrations in leaves, stems and roots and subsequently alleviated Cu-toxicity-induced damage to root growth and function, thus improving plant nutrient (decreased Cu uptake and efficient maintenance of the other nutrient homeostasis and balance) and water status. Further analysis indicated that the improved nutrition and water status contributed to the boron-mediated amelioration of Cu-toxicity-induced inhibition of seedlings, decline of leaf pigments, large reduction of leaf CO assimilation and impairment of leaf photosynthetic electron transport chain revealed by greatly altered chlorophyll a fluorescence (OJIP) transients, reduced maximum quantum yield of primary photochemistry (F/F), quantum yield for electron transport (ET/ABS) and total performance index (PI), and elevated dissipated energy per reaction center (DI/RC). To conclude, our findings corroborate the hypothesis that B-mediated amelioration of Cu-toxicity involved reduced damage to roots and improved nutrient and water status. Principal component analysis showed that Cu-toxicity-induced changes of above physiological parameters generally decreased with the increase of B supply and that B supply-induced alterations of above physiological parameters was greater in 350 μM Cu-treated than in 0.5 μM Cu-treated seedlings. B and Cu had a significant interactive influence on C. sinensis seedlings.