EFFECTS OF SOIL RESISTANCE TO ROOT PENETRATION ON LEAF EXPANSION IN WHEAT (TRITICUM-AESTIVUM L) - COMPOSITION, NUMBER AND SIZE OF EPIDERMAL-CELLS IN MATURE BLADES

Wheat seedlings (Triticum aestivum L.) were grown on soils with contra sted resistances to root penetration (measured as penetrometer resista nce, R(s)). High R(s) reduced the rates of leaf appearance and expansi on. Although the duration of expansion was increased, mature leaves we re smaller. Underlying changes in leaf anatomy were investigated on cl eared mature leaves, focusing on the epidermes. Three leaves were anal ysed: leaves 1 and 3 which started their development in the embryo, an d leaf 5 which was initiated on the seedling, after imposition of cont rasted soil conditions. In all leaves, high R(s) caused a reduction in mature cell sizes, lengths and widths, and a shift in the relative pr oportions of functionally different cell types, with a decrease in the relative proportions of stomata and associated cell types (interstoma tal and sister cells) and an increase in the proportions of unspeciali zed elongated epidermal cells and of trichomes. In leaves 3 and 5 the number of cellular files across the blade was also reduced, while in l eaf 1 it was similar at the two R(s). These differences between leaves are attributed to differences in their developmental stage when root stress was first perceived. Remarkably, R(s) had no effect (leaf 1) or relatively small effects (leaves 3 and 5) on the total number of cell s per file, suggesting that this parameter is either largely insensiti ve to variation in root environment, or is programmed at the outset be fore stress was perceived at the apex.