An adequate general drought tolerance and the ability to acclimate to changing hydraulic conditions are important features for long-lived woody plants. In this study, we compared hydraulic safety (water potential at 50% loss of conductivity, (50)), hydraulic efficiency (specific conductivity, k(s)), xylem anatomy (mean tracheid diameter, d(mean), mean hydraulic diameter, d(h), conduit wall thickness, t, conduit wall reinforcement, (t/b)(h)(2)) and stomatal conductance, g(s), of forest plants as well as irrigated and drought-treated garden plants of Ligustrum vulgare L. and Viburnum lantana L. Forest plants of L. vulgare and V. lantana were significantly less resistant to drought-induced cavitation ((50)at 2.820.13MPa and 2.790.17MPa) than drought-treated garden plants (4.580.26MPa and 3.570.15MPa). When previously irrigated garden plants were subjected to drought, a significant decrease in d(mean) and d(h) and an increase in t and (t/b)(h)(2) were observed in L. vulgare. In contrast, in V. lantana conduit diameters increased significantly but no change in t and (t/b)(h)(2) was found. Stomatal closure occurred at similar water potentials ((sc)) in forest plants and drought-treated garden plants, leading to higher safety margins ((sc50)) of the latter (L. vulgare 1.63MPa and V. lantana 0.43MPa). These plants also showed higher g(s) at moderate , more abrupt stomatal closure and lower cuticular conductivity. Data indicate that the development of drought-tolerant xylem as well as stomatal regulation play an important role in drought acclimation, whereby structural and physiological responses to drought are species-specific and depend on the plants hydraulic strategy.
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