Effects of soil resistance to root penetration on leaf expansion in wheat (Triticum aestivum L): Kinematic analysis of leaf elongation

Wheat leaves (Triticum aestivum L.) elongated 50% more slowly when plants were grown in soils with high mechanical resistance to penetration (R(s)). The profiles of epidermal cell lengths along the growth zone of expanding leaves and the locations of newly formed walls were recorded in order to compare the kinetics of elongation and partitioning of both meristematic and non-meristematic cells. In leaf 5, which completely developed under stress, high R(s) did not affect the flux of mature cells through the elongation zone; leaf elongation was reduced only because these cells were shorter. This reduced size reflected a reduction in cell length at partitioning, associated with shorter cycling time. The relative rates of cell elongation before and after partitioning were unchanged. Cell fluxes were similar because the population of meristematic cells was reduced, offsetting their increased partitioning rate. In contrast, in leaf 1, high R(s) had no effect on the number of dividing cells; elongation rate was reduced because of slower relative cell expansion rate and slower cell partitioning rate. These differences could reflect differences in the stage at which successive leaves perceived root stress and also time-dependent changes in the responsiveness of leaf development to stress-induced root signals or in the nature of these signals. The data reveal that cell cycling time may in fact be decreased by unfavourable growth conditions and is not directly related to cell expansion rates; they also show that the elongation rate of meristematic cells is partly independently controlled from that of nonmeristematic cells.