Trends in wood density and structure are linked to prevention of xylem implosion by negative pressure.

Wood density (D t), an excellent predictor of mechanical properties, is typically viewed in relation to support against gravity, wind, snow, and other environmental forces. In contrast, we show the surprising extent to which variation in D t and wood structure is linked to support against implosion by negative pressure in the xylem pipeline. The more drought-tolerant the plant, the more negative the xylem pressure can become without cavitation, and the greater the internal load on the xylem conduit walls. Accordingly, D t was correlated with cavitation resistance. This trend was consistent with the maintenance of a safety factor from implosion by negative pressure: conduit wall span (b) and thickness (t) scaled so that (t/b)2 was proportional to cavitation resistance as required to avoid wall collapse. Unexpectedly, trends in D t may be as much or more related to support of the xylem pipeline as to support of the plant.