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We studied the water relations of 6 shrub and 3 tree species typical of the mediterranean climate region of central Spain to identify differential responses to water stress between and within species, and to determine if free proline concentration in leaves could be used as a water stress indicator. Predawn and midday water potentials (Ψw) on a seasonal basis, relative water content (RWC), leaf mass per area, foliar nitrogen and free proline concentrations were measured. The lowest water potentials were observed at the end of the summer, with recovery to higher water potentials in the fall and winter seasons. Species differed regarding the annual Ψw fluctuation. Thymus zygis, Halimium viscosum, Genista hirsuta and Juniperus oxycedrus exhibited the most negative midday and predawn Ψw (both less than -6 MPa) with a large magnitude of response to changing conditions in soil moisture of the upper horizon of the soil. Lavandula pedunculata and Cistus ladanifer showed a moderate response. Quercus rotundifolia, Quercus faginea and Retama sphaerocarpa showed a modest response. The Ψw of different size individuals of Quercus rotundifolia and Cistus ladanifer were compared. The annual Ψw fluctuation was greater in small individuals as compared to large individuals. In every species, there was an increase in proline concentration of bulk leaf tissues when predawn Ψw dropped below -5 MPa. Small plants of Cistus ladanifer reached lower water potentials and also higher concentration of proline than bigger plants. Proline could possibly be used as a drought stress indicator in every species except Q. rotundifolia. It is suggested that in addition to water stress avoidance due to deep root systems, some mechanisms of water stress tolerance may operate among shrub and tree species of central Spain.

Summary Some substituted urea herbicides (chlortoluron, diuron and methabenzthiazuron) were tested for possible use in the cultivation of Euphorbia lathyris L. Diuron showed high phytotoxicity for this species, which was tolerant to chlortoluron and methabenzthiazuron. In preliminary experiments two triazine herbicide (atrazine and simazine) were also used, but both were so toxic to E. lathyris that their use was not continued beyond this stage. In glasshouse experiments, chlortoluron showed slight phytotoxicity to E. lathyris but this did not appear in field experiments. Both chlortoluron and methabenzthiazuron were very efficient against weeds which were present and significantly increased yield of E. lathyris in comparison with unweeded plots. The poor competitive ability of E. lathyris against weeds was confirmed. Rasuma Emploi d'herbicides a base d'urees substituees pour le desherbage de Euphorbia lathyris L., source potentielle de combustible vegetal Certains herbicides a base d'urees substituees (le chlortoluron le diuron et le meth a benzthiazuron) ont ete testes pour le desherbage eventuel de cultures de Euphorbia lathyris L. Le diuron a fait preuve d'une forte phytotoxicite envers cette espece, qui a tolere le chlortoluron et le methabenzthiazuron. Deux triazines (l'atrazine et la simazine) ont egalement ete utilisees dans les premiers essais mais elles se sont revelees tellement toxiques envers E. lathyris qu'elles ont ete abandonnees. Dans des essais en serre, le chlortoluron a fait preuve envers E. lathyris d'une faible phytotoxicite qui ne s'est pas fait voir en plein champ. Le chlortoluron et le meethabenzthiazuron se sont montres tres efficaces, tous les deux, contre les mauvaises herbes presentes dans ces essais et ont amene des augmentations significatives dans les rendements d'E. lathyris par rapport aux parcelles non-sarclees. Ces travaux ont confirme la faible capacite de concurrence avec les adventices deja signalee chez E. lathyris. Zusammenfassung Die Verwendung von Harnstoffherbiziden zur Unkrautbekampfung in Euphorbia lathyris L., einer moglicherweise zur Brennstoffproduktion geeigneten Pflanze Einige substituierte Harnstoffherbizide (Chlortoluron, Diuron, Methabenzthiazuron) wurden auf ihre mogliche Verwendbarkeit in kultivierter Euphorbia lathyris L. untersucht. Diuron wirkte stark toxisch auf diese Spezies, welche sich aber gegenuber Chlortoluron und Methabenzthiazuron als tolerant erwies. In Vorversuchen wurden auch zwei Triazinherbizide (Atrazin und Simazin) gepruft; beide wirkten aber derart phytotoxisch, dass sie nicht mehr weitergetestet wurden. In Gewachshausversuchen wirkle Chlortoluron leicht phytotoxisch an E. lathyris; in Feldversuchen wurden jedoeh keine negativen Symptome festgestellt. Chlortoluron und Methabenzthiazuron waren sehr wirksam gegen die vorhandene Unkrautflora und erhohten das Erntegewicht von E. lathyris signifikant im Vergleich zu dem von ungejateten Parzellen abgeernteten Ertrag. Die Versuche bestatigten die geringe Konkurrenzfahigkeit von E. lathyris gegenuber der Unkrautflora.

Juvenile plants of Thymus mastichina, Thymus zygis, Lavandula pedunculata, Genista hirsuta, and Cistus ladanifer were transplanted from the field to the greenhouse; the soil used was the same in which they had grown at their natural site. The plants were grown to maturity to determine comparative growth, water status, and mycorrhizal colonization under greenhouse conditions and to determine the influence of drought on the symbiosis. After 24 months, L. pedunculata showed the most developed root system; G. hirsuta showed the lowest root/shoot ratio and the lowest percentage of mycorrhizal colonization. This percentage ranged from 32.0% to 82.4% for G. hirsuta and L. pedunculata, respectively. There was a positive relationship between root biomass and root/shoot ratio with mycorrhization percentage, and a positive response of water potential to that percentage also occurred. After a moderate drought treatment over 4 weeks, root and shoot biomass and root/shoot ratio as well as mycorrhizal colonization decre...