The development of pectin structural features during the differentiation of cambial derivatives was investigated in aspen (Populus tremula L. x P. tremuloides Michx.) using biochemical and immunocytochemical methods. Comparisons were also made between active and resting tissues. Active tissues, in particular cambial cells and phloem derivatives, were characterized by a high pectin content. Use of antibodies raised against arabinan side chains of rhamnogalacturonan 1 (LM6), as well as biochemical analysis, revealed an obvious decrease from the cortex to the differentiating xylem. Galactan side chains, detected with LM5 antibodies, were present mainly in the cambial zone and enlarging xylem cells. In contrast, they were totally absent from sieve-tube cell walls. Image analysis of LM5 immunogold labelling in the cambial zone showed a clustered distribution of galactan epitopes in the radial walls, a distribution which might result from the association of two different periodic processes, namely the exocytosis of galactan and wall expansion. Cessation of cambial activity was characterized by cell wall thickening accompanied by a sharp decrease in the relative amount of pectin and a lowering of the degree of methylesterification. The data provide evidence that the walls of phloem and xylem cells differ in their pectin composition even at a very early stage of commitment. These differences offer useful tools for identifying the initial cells among their immediate neighbours.

To characterize the structural events associated with incompatibility of graft development, we conducted a histological study of compatible and incompatible pear/pear and pear/quince grafts that had been grown for five months in a greenhouse. Multivariate analysis of histological data describing the structure of the graft union allowed us to discriminate between compatible and incompatible combinations before either macroscopic examination or qualitative microscopic examination differences between graft combinations became evident. The histological variables responsible for the discrimination between incompatible and compatible unions were related to three typical symptoms of graft incompatibility: bark discontinuity, which was the main feature; cambial dysfunction; and accumulation of starch in the scion. Little cell necrosis was observed at the interface of incompatible grafts at the 5-month stage of graft development. Multivariate analysis of histological data provides a new tool for studying early structural events resulting from the graft incompatibility response and for diagnosing early graft incompatibility.