Comparative anatomical, morphological, and physiological parameters controlling photosynthesis in two Populusxeuramericana clones during short-term osmotic treatment

The relationships between drought response and anatomical/physiological pro perties were assessed in two poplar clones belonging to the Aigeros section : Populus x euramericana clone Dorskamp (drought-tolerant) and clone Luisa Avanzo (drought-sensitive). Cuttings of both clones were exposed for 12 h t o 0 mM (control), 50 mM (osmotic potential -0.112 MPa), and 150 mM (-0.336 MPa) mannitol. Ln control, Dorskamp had smaller stomata than Luisa Avanzo, one or two layers of palisade cells, a spongy mesophyll, and high concentra tions of antioxidative compounds (ascorbate, glutathione). After exposure t o 50 or 150 mM mannitol, both clones closed their stomata: leaf conductance and opening of stomata decreased. When exposed to 50 mM mannitol, net phot osynthetic rate (P-N) and chlorophyll (Chl) and total solute contents remai ned stable; ribulose-1,5-bisphosphate carboxylase/oxygenase activity, Chi s ynthesis and turn-over, ascorbate peroxidase and glutathione reductase acti vities were less affected in Dorskamp than in Luisa Avanzo. Following an ex posure to 150 mM mannitol, Dorskamp exhibited higher P-N and higher content s of antioxidants (ascorbate, glutathione) and antioxidative enzymes (ascor bate peroxidase, glutathione reductase) than Luisa Avanzo. Hence the drough t-tolerant poplar was able to better avoid and tolerate osmotic stress.