Species variability in boron requirement is correlated with cell wall pectin

Fourteen species of crop plants which differ in their reported tissue boron requirements were grown in B-replete or B-deflcient medium. Leaf samples were collected and analysed for B and cell wall components. There was a significant positive correlation among the species between B concentration in the leaf or the cell wall and uronic acid, rhamnose and galactose (indicative of pectin) in the cell wall. The concentration of cell wall pectin was also positively related with reported tissue-B requirements and observed sensitivity to B deficiency. Boron deficiency did not alter the amount of uronic acid present in cell walls, suggesting that there was no effect of B deficiency on pectin metabolism. Under B-deflcient conditions the amount of 'soluble' B (i.e. B not associated with the cell wall) declined dramatically while the proportion of cellular B that was 'insoluble' (i.e. B associated with the cell wall) increased. The positive relationship between pectin content, insoluble B and tissue-B requirement of diverse species suggests that the amount of cell wall pectin may be significant in determining the relative tissue-B requirements of the species. These results indicate that either (1) species with high cell wall pectin contents require greater amounts of B for the construction of the cell wall, or (2) pectin in cell walls forms an insoluble complex with B, thereby reducing its availability for other putative B-requiring metabolic functions. Thus, species with a high pectin content would have a higher tissue-B requirement.