EFFECTS OF ROOTSTOCKS AND INTERSTOCK ON CELL DIMENSIONS IN SCION STEMS OF APPLE (MALUS PUMILA MILL.)

Summary Fibre and vessel element dimensions were studied in Cox's Orange Pippin scion stems grafted directly on to the dwarfing M.8 and M.9 rootstocks, or on to the vigorous MM. 104 rootstock with interstocks of MM.104 or the very dwarfing M.20 varieties. In the xylem formed in trees aged between 5 and 7 years, the total cross‐sectional areas of fibres and the lumen of vessels were very closely related to the width of the growth rings. Variations in the ratio (total cell area):(lumen area) and in fibre wall thickness were mostly attributable to intrinsic seasonal effects, as reflected by the position of sampling within the growth ring. Differences in cell length and mean wall thickness of fibres between the rootstocks M.8, M.9 and MM.104/MM.104 were relatively small, whereas the M.20 interstock was associated with significantly smaller values for these attributes. The effects of the rootstocks studied appeared to be exerted through changes in the rate of stem growth as a whole, whilst the very dwarfing interstock altered the relationships between the lateral expansion of cells and the elongation and wall thickening of fibres.