The effects of foliar-applied manganese on major cell wall polysaccharide fractions and soluble sugar composition of cucumber leaves

Increasing evidence shows that micronutrients can induce changes in cell wall (CW) properties in plants. A detailed knowledge of these changes will contribute to understand the beneficial and detrimental impacts of trace elements on plant development and defence. In the present work, the influence of foliar spray of manganese (Mn) on the composition of CW polysaccharides and soluble sugars was investigated in cucumber leaves using spectrophotometric and chromatographic techniques. Cucumber seedlings were hydroponically cultured under controlled situations for 21 days. At the two-leaf stage, the above facade of cucumber leaves was fertilized with MnSO₄ solutions at various dosages (1.8, 4.5 and 7.2 mM). Results showed that the foliar Mn spray decreased the galacturonic acid (GalA) content, while it increased the degree-of-methyl-esterification (DM) of pectin. The amounts of fucose, arabinose and galactose diminished upon Mn application. The contents of rhamnose and xylose decreased or did not change by Mn treatment. The glucose level enhanced at low concentration of Mn (1.8 mM), but it decreased at medium and high concentrations of Mn (4.5 and 7.2 mM). The increase of mannose content was significant only at medium concentration of Mn. Overall, the amount of total hemicellulose increased at 1.8 mM concentration of Mn, while a decrease was observed at 4.5 and 7.2 mM concentrations of Mn. The cellulose content decreased at low and high concentrations of Mn, although it remained unchanged at medium concentration of Mn. Generally, the total contents of CW polysaccharides increased at low concentration of Mn, but they decreased at the higher concentrations of Mn. Such changes can be important in plant resistance to environmental stresses via reduction of permeability against fungal pathogens and decrease of water loss from tissues under stresses such as drought and salinity.