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

Abstract

Wheat seedlings (Triticum aestivum L.) were grown on soils with contrasted resistances to root penetration (measured as penetrometer resistance, R2). High R3 reduced the rates of leaf appearance and expansion. Although the duration of expansion was increased, mature leaves were smaller. Underlying changes in leaf anatomy were investigated on cleared mature leaves, focusing on the epidermes. Three leaves were analysed: leaves 1 and 3 which started their development in the embryo, and leaf 5 which was initiated on the seedling, after imposition of contrasted soil conditions. In all leaves, high Rs caused a reduction in mature cell sizes, lengths and widths, and a shift in the relative proportions of functionally different cell types, with a decrease in the relative proportions of stomata and associated cell types (interstomatal and sister cells) and an increase in the proportions of unspecialized elongated epidermal cells and of trichomes. In leaves 3 and 5 the number of cellular files across the blade was also reduced, while in leaf 1 it was similar at the two Rs. These differences between leaves are attributed to differences in their developmental stage when root stress was first perceived. Remarkably, Rs had no effect (leaf 1) or relatively small effects (leaves 3 and 5) on the total number of cells per file, suggesting that this parameter is either largely insensitive to variation in root environment, or is programmed at the outset before stress was perceived at the apex.