Bundle sheath lignification mediates the linkage of leaf hydraulics and venation

The lignification of the leaf vein bundle sheath (BS) has been observed in many species and would reduce conductance from xylem to mesophyll. We hypothesized that lignification of the BS in lower-order veins would provide benefits for water delivery through the vein hierarchy but that the lignification of higher-order veins would limit transport capacity from xylem to mesophyll and leaf hydraulic conductance (K-leaf). We further hypothesized that BS lignification would mediate the relationship of K-leaf to vein length per area. We analysed the dependence of K-leaf, and its light response, on the lignification of the BS across vein orders for 11 angiosperm tree species. Eight of 11 species had lignin deposits in the BS of the midrib, and two species additionally only in their secondary veins, and for six species up to their minor veins. Species with lignification of minor veins had a lower hydraulic conductance of xylem and outside-xylem pathways and lower K-leaf. K-leaf could be strongly predicted by vein length per area and highest lignified vein order (R-2=.69). The light-response of K-leaf was statistically independent of BS lignification. The lignification of the BS is an important determinant of species variation in leaf and thus whole plant water transport. The lignification in cell wall, which blocks apoplastic water flow, has been found at bundle sheath cells of leaf veins in many plant species, but few studies have addressed the effect of the lignified bundle sheath on leaf water transport. This study detected the lignified bundle sheath cells at the minor veins in six of 11 temperate woody species and indicated that the six species with lignified bundle sheath had lower leaf hydraulic conductance (K-leaf). In addition, measured K-leaf was strongly predicted from a multiple regression against vein length per area and the highest lignified vein order with R-2 values of 69%, although there was no significant relationship between K-leaf and vein length per area across our species set. These results provide clear evidence of an importance on the lignification for leaf water-transport capacity.