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 withcontrasted resistances to root penetration (measured as penetrometerresistance, R_s. High R_s reduced the rates of leaf appearanceand expansion. Although the duration of expansion was increased,mature leaves were smaller. Underlying changes in leaf anatomywere investigated on cleared mature leaves, focusing on theepidermes. Three leaves were analysed: leaves 1 and 3 whichstarted their development in the embryo, and leaf 5 which wasinitiated on the seedling, after imposition of contrasted soilconditions. In all leaves, high R_s, caused a reduction in maturecell sizes, lengths and widths, and a shift in the relativeproportions of functionally different cell types, with a decreasein the relative proportions of stomata and associated cell types(interstomatal and sister cells) and an increase in the proportionsof unspecialized elongated epidermal cells and of trichomes.In leaves 3 and 5 the number of cellular files across the bladewas also reduced, while in leaf 1 it was similar at the twoR_s. These differences between leaves are attributed to differencesin 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 cells per file, suggestingthat this parameter is either largely insensitive to variationin root environment, or is programmed at the outset before stresswas perceived at the apex. Key words: Wheat, anatomy, mature epidermis, root impedance