Drought effects on hydraulic conductivity and xylem vulnerability to embolism in diverse species and provenances of Mediterranean cedars

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Tree Physiology, 25:1109–1117

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Drought effects on hydraulic conductivity and xylem vulnerability to embolism in diverse species and provenances of Mediterranean cedars

Mehdi Ladjal (1), Roland Huc (1, 2) and Michel Ducrey (1)

1. INRA, Unité de Recherches Forestières Méditerranéennes, Institut National de la Recherche Agronomique, Avenue A. Vivaldi, F-84000, Avignon, France / 2. Corresponding author (huc@avignon.inra.fr) / Received September 1, 2004; accepted December 17, 2004; published online July 4, 2005

Summary

We studied hydraulic traits of young plants of the Mediterranean cedar species Cedrus atlantica (Endl.) G. Manetti ex Carrière (Luberon, France), C. brevifolia (Hook. f.) Henry (Cyprus), C. libani A. Rich (Hadeth El Jebbe, Lebanon) and C. libani (Armut Alani, Turkey). With an optimum water supply, no major differences were observed among species or provenances in either stem hydraulic conductivity (K_s) or leaf specific conductivity (K_l) measured on the main shoot. A moderate soil drought applied for 10 weeks induced marked acclimation through a reduction in K_s, particularly in the Lebanese provenance of C. libani, and a decrease in tracheid lumen size in all species. Cedrus atlantica, which had the smallest tracheids, was the species most vulnerable to embolism: a 50% loss in hydraulic conductivity (Ψ_PLC50) occurred at a water potential of –4.4 MPa in the well-watered treatment, and at –6.0 MPa in the moderate drought treatment. In the other species, Ψ_PLC50 was unaffected by moderate soil drought, and only declined sharply at water potentials between –6.4 and –7.5 MPa in both irrigation treatments. During severe drought, K_s of twigs and stomatal conductance (g_s) were measured simultaneously as leaf water potential declined. For all species, lower vulnerability to embolism based on loss of K_s was recorded on current-year twigs. The threshold for stomatal closure (10% of maximum g_s) was reached at a predawn water potential (Ψ_pd) of –2.5 MPa in C. atlantica (Luberon) and at –3.1 MPa in C. libani (Lebanon), whereas the other provenance and species had intermediate Ψ_pd values. Cedrus brevifolia, with a Ψ_pd (–3.0 MPa) close to that of C. libani (Lebanon), had the highest stomatal conductance of the study species. The importance of a margin of safety between water potential causing stomatal closure and that causing xylem embolism induction is discussed.

Keywords: Cedrus atlantica, Cedrus brevifolia, Cedrus libani, drought acclimation, stomatal conductance.