Theoretical considerations of optimal conduit length for water transport in vascular plants

Vascular plants have shown a strong evolutionary trend towards increasing l ength in xylem conduits. Increasing conduit length affects water transport in two opposing ways, creating a compromise that should ultimately define a n optimal conduit length. The most obvious effect of increased length is to decrease the sequential number of separate conduits needed to traverse the entire pathway, and thereby to reduce the number of wall-crossings and the hydraulic resistance to flow within the xplem. This is an essential evolut ionary pressure towards the development of the vessel, a conduit of multice llular origin whose length is not restricted by developmental constraints. The vessel has been an essential component in all plant lineages, achieving transport tissues with very high specific conductivity. A countering effec t, however, arises from the partitioning of the cavitation response, a proc ess whereby individual xylem conduits drain of water and lose conducting ca pacity. Flow in the xylem is down a gradient of negative pressure, which is necessarily most negative in the distal regions (i.e. near the foliage). C avitation can be caused directly by negative pressures, and results in a to tal loss of the hydraulic conductance of the individual conduits within whi ch it occurs. If cavitation is triggered by ion; pressure experienced only at the very distal end of a long conduit, the conduit nevertheless loses it s conducting capacity along its entire length. Pathways composed of long co nduits will therefore suffer greater total conductance loss for equivalent pressure gradients, because the effects of cavitation are not effectively r estricted to the tissue regions within which the cavitation events are gene rated. By contrast, short conduits can restrict cavitation to distal region s, leaving trunk and root tissues less seriously affected. The increased to tal conductance loss of a system made entirely of very long conduits transl ates into a lower maximum rate of water transport in the xylem. The loss in hydraulic capacity associated with failure to partition the flow pathway f ully, and locally contain the effects of cavitation, theoretically reaches a maximum of 50% for the extreme case in which a single set of conduits tra verses the entire pathway. Shorter conduits confine individual cavitation e vents to smaller regions and permit the pathway as a whole to have a more g radual conductance Loss in conjunction with the pressure gradient. A compro mise exists between (1) minimizing total conductance loss from cavitation v ia fine partitioning of the pathway with many tiers of short conduits, and (2) reducing total wall resistance via coarse partitioning with a few tiers of long conduits. An analysis is presented of the optimal number of end wa lls (i.e. mean conduit length relative to total pathway length) to maximize transport capacity. The principle of optimal containment of cavitation als o predicts that conduits should not be of equal length in all portions of t he pathway. The frequency of end walls should rather be proportional to the magnitude of the water-potential gradient at each point, and conduits shou ld be longest in the basal portion (roots) and progressively shortened as t hey move up the stems to the foliage. These concepts have implications for our understanding of the contrasting xylem anatomies of roots and shoots, a s well as the limits to evolution for increased hydraulic conductance per x ylem cross-sectional area. They also indicate that to model the hydraulic b ehaviour of plants accurately it is necessary to know the conduit length di stribution in the water flux pathway associated with species-specific xylem anatomy.