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The aim of this study was to determine the effects of different irrigation regimens on five native aromatic and medicinal species including Ballota acetabulosa (Greek horehound), Helichrysum orientale (helichrysum), Melissa officinalis (lemon balm), Rosmarinus officinalis (rosemary), and Salvia fruticosa (Greek sage) when grown on adaptive green roof systems. The applied levels of irrigation were 100% (well-watered control), 75%, 50%, 25%, and 0% (no irrigation) of the daily pan evaporation (Epan). Measurements included the in situ determination of substrate moisture, stomatal resistance, and soil plant analysis development (SPAD) values. It was found that Greek horehound, helichrysum, and rosemary can sustainably grow at an irrigation of 25% Epan, whereas Greek sage and lemon balm require an irrigation of at least 50% Epan for sustainable growth in shallow adaptive green roof systems. The rapid development of contemporary cities has caused dramatic changes in urban landscape and climate. Increased population density, particularly in city centers, in conjunction with the sealing of major land portions through building and construction work, have resulted in the lack of urban open and green spaces (Ferguson, 1998). All the above have negatively affected the urban microclimate, including air and water quality, and have caused environmental deterioration, thus endangering public health, while also degrading life quality along with the comfort and wellbeing of the inhabitants. Green roofs provide contemporary technical solutions that could increase urban green spaces and contribute to the amelioration of environmental problems. Roofs and recessed penthouses cover a large area of the built urban spaces, especially in city areas which are characterized by dense building networks. Several researchers have reported, or forecasted through modeling, that green roofs could decrease ambient temperature (cooling) during summer, increase relative humidity, reduce infrared, and diffuse radiation (Kumar and Kaushik, 2005; Simmons et al., 2008). Further advantages resulting from green roof implementation include oxygen production by photosynthesis (Getter et al., 2009), building energy savings for summer cooling (Kotsiris et al., 2012a), reduction of air pollutants (Czemiel Berndtsson, 2010; Rowe, 2011), regulation of stormwater runoff and minimization of flooding events (Czemiel Berndtsson, 2010; Fioretti et al., 2010; Oberndorfer et al., 2007; Simmons et al., 2008; Van Woert et al., 2005), and amelioration of urban heat island effect (Akbari et al., 2001; Getter and Rowe, 2006). Thus, the general concept is the development of a green roof networking which can contribute to the improvement of the microclimate in multiple ways if green roofs are largely implemented in urban areas. Therefore, it is necessary to seek ways to construct them on top of existing buildings. Because of the minimum load-bearing capacity of most existing buildings, either extensive or adaptive green roofs (Nektarios et al., 2011, 2015; Ntoulas et al., 2012, 2013b) are appropriate for green roof implementation. Extensive green roofs are characterized by minimal substrate depths (2–15 cm) that result in loads between 20 to 120 kg·m. They are usually planted with succulent plants and require low or no maintenance (FLL, 2008). Adaptive green roof systems also use minimal substrate depth (5–15 cm), but are planted with various plant types such as aromatic and medicinal plants, turfgrasses, and groundcovers. In contrast with the extensive green roofs, the adaptive ones are accessible and require minimal irrigation inputs (Kotsiris et al., 2013; Ntoulas et al., 2012, 2013a, 2013b; Ntoulas and Nektarios, 2015). Both extensive and adaptive green roofs require the use of plant species that have adequate water-stress tolerance and are capable of growing in shallow substrate depths. Plant species for either extensive or adaptive green roof systems should be preferably native and adapted to local environmental conditions. Dimopoulos et al. (2013) reported that the prolific flora of Greece consists of 5752 kinds of plant species (1278 endemic), of which many of them are herbs, aromatics, and pharmaceuticals that could be considered appropriate for sustainable growth on Mediterranean green roof systems in conjunction with minimal water inputs (Benvenuti and Bacci, 2010; Kotsiris et al., 2012b, 2013; Nektarios et al., 2011, 2015; Papafotiou et al., 2013; Paraskevopoulou et al., 2015; Tassoula et al., 2015). The aim of the present study is to determine the irrigation threshold based on Epan for five medicinal and aromatic plant species. The findings of this research may be valuable in selecting the most appropriate plants for Mediterranean green roof systems and in predicting the minimal required irrigation inputs for sustainable growth. Materials and Methods Experimental setup The study was conducted on the roof of the campus restaurant at the Agricultural University of Athens (lat. 37 59#, long. 23 42#, 36 a.s.l.). The study period was between 23 July and 9 Sept. 2013 and it was conducted on 90 rectangular plastic containers with internal dimensions of 24 cm · 24 cm · 11 cm.Within each container, a green roof system was simulated containing three layers having a total height of 2.7 cm. The layers were placed in the following order initiating from the bottom of the containers toward the substrate: a) protection and moisture retention fabric (VLS 300; Diadem, Landco Ltd., Athens, Greece); b) undulated drainage (Diadrain 25H; Diadem, Landco Ltd.); and c) geotextile (VLF 150; Diadem, Landco Ltd.). As a growing substrate, a mix of pumice (75% v/v), clinoptilolite zeolite (10% v/v), peat (8% v/v), and compost from garden waste and dairy manure (7% v/v) was placed on top of the geotextile at a depth of 8 cm. The physical and chemical properties of the substrate are listed in Table 1 whereas the mechanical analysis and the water-retention capacity are listed in Table 2. Plant material The study included five species of native aromatic and medicinal plants of the Greek flora: B. acetabulosa (Greek horehound, 15 plants),H. orientale (helichrysum, 20 plants), M. officinalis (lemon balm, 15 plants), R. officinalis (rosemary, 20 plants), and S. fruticosa Received for publication 18 Nov. 2015. Accepted for publication 23 Mar. 2016. This article is part of anMSc thesis submitted to the Department of Crop Science, Agricultural University of Athens, by Iro Kokkinou. The project (Urban BioRoof) with the code number 12CHN136 was funded by the Hellenic General Secretariat of Research and Technology under the Operational Programme ‘‘Competitiveness and Entrepreneurship’’ (EPAN II) and by the Regional Operational Programmes of the five Regions of transitional support, under the Action ‘‘Bilateral Research and Technological Cooperation between Greece and China 2012–2014.’’ Themention of a trademark, proprietary product or vendor does not imply endorsement by the authors nor does it imply approval to the exclusion of other products that may also be suitable. Corresponding author. E-mail: pan@aua.gr. 608 HORTSCIENCE VOL. 51(5) MAY 2016 (Greek sage, 20 plants). Differences in the number of plants used were due to plant losses during their propagation from the wild. The plants were collected from Euboea (lemon balm), Attiki (rosemary), the islands of Kythnos (Greek horehound), and Crete (helichrysum and Greek sage). They were collected as cuttings from their natural habitat in May 2012 except for lemon balm that was collected in June 2012. Plant propagation was performed under mist for 5–8 d depending on the species at the Laboratory of Floriculture and Landscape Architecture of the Agricultural University of Athens. After their propagation, the plants were acclimatized for 2 weeks and then transferred to the experimental roof in 10-cm diameter pots where they grew for 1 year. On 16 June 2013, a single plant was transplanted into each experimental container that was equipped with green roof layers and substrate. At the initiation of the study, the heights of the plants were similar between replications of each species.More specifically, the Greek horehound had a height of 19.2 cm (±1.2 SE), helichrysum 14.2 cm (±0.5 SE), lemon balm 21.2 cm (±0.9 SE), rosemary 17.8 cm (±1.1 SE), and Greek sage 25.5 cm (±1.8 SE). After transplanting, all containers were placed on the building roof under mild shade for 20 d and irrigation was provided as needed. On 5 July 2013, the plants were transferred to full sun. On 23 July 2013, irrigationwas applied to saturation to produce uniform wet conditions in between treatments at the initiation of water-stress period. The water-stress period lasted from 24 July to 9 Sept. 2013. During the water-stress period, five different irrigation regimens were implemented based on daily evaporation (Epan), which was determined by a Class A evaporation pan. For each plant species, irrigation was applied daily either at 100% Epan (well-watered control) or at 75%, 50%, 25% (as deficit irrigation treatments), or 0% (drought) of Epan. All treatments were watered by hand at 0800 HR using a watering can equipped with perforated nozzle to ensure uniformity. Experimental design, treatments, and statistical analysis The plot arrangement followed a completely randomized design with helichrysum, rosemary, and Greek sage having four replications and Greek horehound and lemon balm having three replications per treatment. To determine the irrigation threshold for sustainable growth of each of the five plant species, a comparison between the five different irrigation regimens (100%, 75%, 50%, 25%, and 0% of daily Epan) was performed for each plant species independently. Oneway analysis of variance was performed on the collected data employing the statistical analysis software Statgraphics Centurion, version 15.2.11 (Statpoint Technologies Inc., Warrenton, VA). Our main research interest focus

Response of Native Aromatic and Medicinal Plant Species to Water Stress on Adaptive Green Roof Systems