Shoot and root vulnerability to xylem cavitation in four populations of Douglas-fir seedlings

The objectives of this study were to assess the range of genotypic variatio n in the vulnerability of the shoot and root xylem of Douglas-fir (Pseudots uga menziesii (Mirb.) France) seedlings to water-stress-induced cavitation, and to assess the trade-off between vulnerability to cavitation and conduc tivity per unit of stem cross-sectional area (k(s)), both within a species and within an individual tree. Douglas-fir occupies a broad range of enviro nments and exhibits considerable genetic variation for growth, morphology, and drought hardiness. We chose two populations from each of two varieties (the coastal var, menziesii and the interior var. glauca) to represent envi ronmental extremes of the species. Vulnerability curves were constructed fo r shoots and roots by plotting the percentage loss in conductivity versus w ater potential. Vulnerability in shoot and root xylem varied genetically wi th source climate. Stem xylem differed in vulnerability to cavitation betwe en populations; the most mesic population, coastal wet (CW), was the most s usceptible of the four populations. In the roots, the most vulnerable popul ation was again CW; the interior wet (TW) population was moderately suscept ible compared with the two dry populations, coastal dry (CD) and interior d ry (ID). Root xylem was more susceptible to cavitation than stein xylem and had significantly greater k(s). The trade-off between vulnerability to cav itation and k(s), however, was not evident across populations. The most vul nerable population (CW) had a shoot k(s) of 0.534 +/- 0.067 mu mol m(-2) s( -1) MPa-1, compared with 0.734 +/- 0.067 mu mol m(-2) s(-1) MPa-1 for the l ess vulnerable CD stems. In the roots, TW was more vulnerable than ID, but had the same k(s).