Widening of xylem conduits and its effect on the diurnal course of water potential gradients along leaf venations

In plants, water flows from roots to leaves through the xylem vasculature following an axial gradient of water potential (��X). We hypothesized that xylem properties such as conduit widening govern the pattern of this gradient and determine the flow rate under the different ����X between leaves and soil occurring during any day. The steep gradient of ��X in leaf venations should strongly reflect the leaf physiology. Since stomatal conductance (gS) equilibrates the water losses through transpiration with the water flowing from soil to leaves, we expected gS to be synchronized with the maximum steepness of the gradient. We measured ��X and gS at the base of the apical leaflet and at the base of the petiole in leaves of Fraxinus excelsior from dawn to dusk. We found a slightly asynchronous diurnal course of ��X at the two different points along the leaf (��L���20 cm). ��X at the base of the apical leaflet decreased more rapidly in the morning and increased more slowly in the afternoon compared to ��X at the petiole base. This led to an original double-peaked pattern of ����X. A first peak (����X���-1 MPa) occurred two hours earlier than minimum ��X and was synchronized with the maximum gS. The second peak occurred after the gS drop in the afternoon, when the plant was likely under refilling. We demonstrated that a steep gradient of ��X develops in the main leaf vein and is strongly linked to maximum gS and potentially to other important aspects of leaf water relations. �� ISHS 2013.