Physiological And Biochemical Responses Of Citrus Sinensis Seedling Roots,Upper And Lower Leaves To Magnesium-deficiency

Magnesium(Mg)-deficiency is a widespread problem adversely affecting quality and yield of many important agricultural crops,including citrus.Despite of its important roles for the normal growth and development of higher plants,Mg has been overlooked by botanists and agronomists relative to the other macronutrients such as nitrogen(N),phosphorus(P),calcium(Ca)and is referred to as "the forgotten element".To our best knowledge,so far,few researchers comparatively investigated the physiological and biochemical responses of plant roots,upper and lower mature leaves to Mg-deficiency.In China,Mg-deficiency is a common problem responsible for the poor citrus fruit quality and the loss of productivity.Based on these results,we used the 'Xuegan'(Citrus sinensis)seedlings as the materials and took the lead in investigating the effects of Mg-deficiency on plant growth,leaf mass per area(LMA),contents of pigments and activity of Rubisco in upper and lower leaves,and gas exchange,contents of nonstructural carbohydrates,organic acids,total soluble proteins,amino acids,total phenolics and anthocyanins,and activities of key enzymes related to organic acid,amino acid and phenolic metabolisms in roots,upper(three quarter height)and lower(quarter height)leaves.The objectives of this work was to determine the differences in the Mg-deficiency-induced alterations of gas exchange,contents of major metabolites and activities of key enzymes among roots,upper and lower leaves and to provide theoretical basis for high yield and high quality cultivation of citrus.Our results clearly showed that the effects of Mg-deficiency on gas exchange(CO2 assimilation and dark respiration),the contents of the major metabolites(organic acids,amino acids and phenolics)and the activities of the related key enzymes were more pronounced in the lower leaves than in the upper leaves except for the nonstructural carbohydrates,but the change trends were similar between the two.Despite increased accumulation of nonstructural carbohydrates,the lower CO2 assimilation in the Mg-deficient leaves was not caused by the feedback inhibition mechanism via sugar accumulation.Both dark respiration and organic acid metabolism were induced in the Mg-deficient lower leaves to 'consume' the excessive carbohydrates,and inhibited in the Mg-deficient roots with less accumulation of nonstructural carbohydrates to keep the net carbon balance.The contents of total phenolics and anthoyanins were increased and decreased in the Mg-deficient lower leaves,respectively,whereas the contents of both anthoyanins and total phenolics were reduced in the Mg-deficient roots.Interestingly,the biosynthesis of amino acids was repressed in the Mg-deficient roots and lower leaves,thus lowering the level of total free amino acids in these roots and leaves.To conclude,great differences existed in the Mg-deficiency-induced alterations of gas exchange,major metabolites and key enzymes among roots,upper and lower leaves.