Drought resistance of two Douglas fir species (Pseudo- tsuga menziesii (Mirb.) Franco and Pseudotsuga macro- carpa (Torr.) Mayr.): relative importance of water use efficiency and root growth potential

G. Aussenac, P. Grieu J.M. Guehl

Laboratoire de Bioclimatologie et Ecophysiologie Forestière, Station de Sylviculture et de Produc- tion, INRA Centre de Nancy, Champenoux, 54280 Seichamps, France!

Introduction maintained at 4.6 Pa!KPa-!, and 2) in response to increasing d4V in well-watered plants. The measurements were performed with an assimi- In order to optimize the choice of species lation chamber in which air temperature, for reforestation in regions subjected to ambient C02 concentration, photosynthetic flux and water summer water it is to photon density vapor pressure shortage, important were controlled. Analyzing the data trough A determine which physiological compo- vs Intracellular C02 molar fraction (Ci) graphs, nents may be involved in their drought we could determine stomatal and mesophyll adaptation. Therefore, we have carried out components of changes in A (Prioul et al., this aimed at the 1984; Jones, l!185). The gas exchange re- study comparing ecophy- to zH!Vwas also examined with behavior of 2 fir sponse respect siological Douglas spe- to the optimization theory of Cowan and cies: Pseudofsuga menziesii (Mirb.), origi- Farquhar (1977). from a wet coastal nating relatively In a second experiment (Grieu and Ausse- zone (Ashford, WA, U.S.A., 300 m eleva- nac, 1988), non-destructive measurements of tion) and Pseudotsuga macrocarpa (Torr.) the number of growing roots and root elonga- Mayr., growing under drier conditions (CA, tion were made on 3 mo old seedlings of the 2 species grown in root boxes. U.S.A., 1315 m elevation).

Materials and Methods Results

Carbon dioxide assimilation rate (A), stomatal Data of Fig. 1 a show that the peak rates of conductance (g! and transpiration rate (E) A were higher in P. menziesii than in were measured on 2 old yr seedlings: 1) during P. macrocarpa. After an initial increase, a soil water depletion cycle (the water status of remained un- the plants was determined by predawn water mesophyll photosynthesis potential measurements), while the leaf-to-air changed over a wide predawn water water vapor pressure difference (4 W) was potential range in both species, while gs

was reduced. At higher soil drought, a dra- matic decline of mesophyll photosynthesis was noted starting from -1.5 MPa in P. macrocarpa and -1.9 MPa in P. menzie- sii. In both species, stomatal conductance and especially mesophyll photosynthesis were dramatically decreased, as d W was increased (Fig. 1 b). P. macrocarpa had the highest and the most unstable - and thus non-optimal - values of aEf7A in response to in- creasing AW (Fig. 2a). P. macrocarpaalso had the highest c7Elc?A at high water poten- tial (Fig. 2b). Growth of the pre-existent roots was more important in P. macrocarpa than in P. menziesii and growth of the new roots was similar in both species (Fig. 3). Furthermore, P. macrocarpa explored the deep soil layers more quickly tha P. men- ziesii (Fig. 4.).

Conclusions gas exchange rates and exhibited the Unexpectedly, in the present comparative least conservative water economy. study the results show that the drought- The high drought adaptation of P. resistant P. macrocarpa had the lowest macrocarpa seems to be due mainly to

80 f-n I efficient root growth and soil exploration menziesii, Pseudotsuga macrocarpa et Cedrus abilities, whereas, surprisingly, no adapta- atlantica. Ann. Sci. For. 45, 117-124 Grieu Guehl J.M. & Aussenac G. The tion features seem to have developed at P., (1988) effects of soil and on the leaf level. atmospheric drought pho- tosynthesis and stomatal control of gas ex- change in three coniferous species. Physiol. Plant. 73, 97-104 References Jones H.G. (1985) Partitioning stomatal and non-stomatal limitations to photosynthesis. Plant Cell Environ. 8, 95-104 Cowan LR. & G.D. Stomatal Farquhar (1977) Prioul J.L., Comic G. & Jones H.G. Dis- function in relation to leaf metabolism and envi- (1984) cussion of stomatal and non-stomatal ronment. Soc. BioL 471-505 compo- Symp. Exp. 31, nents in leaf photosynthesis decline under Grieu P. & Aussenac G. (1988) Croissance et stress conditions. In: Advances in Photosyn- d6veloppement du systbme racinaire de semis thesis Research (Sybesma C., ed.), vol. IV, Mar- de trois esp6ces de conif6res: Pseudotsuga tinus Nijhoff/W. Junk, The Hague, pp. 375-378