Growth and stomatal responses of wheat seedlings to spatial and temporal variations in soil strength of bi-layered soils

Leaf growth and stomatal behaviour are sensitive to variations in soil mechanical resistance to penetration (R(s)). That resistance is strongly influenced by soil water content, density and texture. As such it is therefore an inherently variable and changing characteristic of the roots natural environments. Leaf responses to spatial and temporal variations in R(s) were analysed in wheat using two kinds of simplified model systems: (a) bi-layered soils made of either a low R(s) layer on top of a high R(s) layer, or the converse, (b) soils where, after enduring high R(s), the whole root system was suddenly exposed to lower R(s) by raising soil water content. Both leaf expansion rate and stomatal conductance responded to some roots meeting a new Soil layer and also to a step change in impedance to the bulk of roots. These responses could not be ascribed to variations in water or nutrient status per se and strengthen the case for the involvement of some kind of chemical signalling of Rs to leaf cells. Moreover, a striking and novel feature of these responses is that they were always detected with a significant time-lag after the change in Rs had first been experienced. It is concluded that leaf biological age is a paramount factor in explaining such a lag. These data reveal that leaf sensitivity to R(s) is mostly confined to early developmental stages preceding blade emergence. However, they also point to the contribution of additional factors, raising the questions of the role of root parts behind the tip and of threshold-type leaf responses to stress induced root signals.