Evaluation of Perennial Herbaceous Species for their Potential Use in a Green Roof under Mediterranean Climate Conditions

In Mediterranean climates, the establishment and survival of vegetation on extensive green roofs can be limited by drought stress. The aim of this research was to test nine succulent and fifteen non-succulent species for their tolerance to drought. For both succulent and non-succulent species, a randomized-complete block design with three replicates was used to test the factorial combination of genotypes and irrigation scheduling (W-1 = watering when substrate water content was reduced by 10% of the container capacity; W-2 = watering when substrate water content was reduced by 65% of the container capacity). The tolerance to drought stress was similar among the non-succulent species except for Hypericum calycinum and Satureja illirica that showed a lowest growth on drought condition. Delosperma cooperi was more tolerant to drought stress compare to the other succulent species. Hypericum calycinum and Phlox douglasii showed the lowest transpiration values among non-succulent species while in succulent species the lowest value was recorded in Sedum reflexum. The physiological parameters confirmed the effects of drought stress on plants with an higher leaf temperature in plants exposed to drought stress compared to the values recorded on non stressed plants. Leaf chlorophyll and carotenoids content were generally reduced in non succulents species by drought stress while an opposite behavior was observed in succulent species.