Urban Forestry & Urban Greening 56 (2020) 126874
Available online 12 October 2020
1618-8667/© 2020 Published by Elsevier GmbH.
Assessment of physiological parameters to determine drought tolerance of plants for extensive green roof architecture in tropical areas
Metha Meetam
a,b, Naraporn Sripintusorn
c, Wisuwat Songnuan
b,c, Umaporn Siriwattanakul
c, Aussanee Pichakum
c,*
aDepartment of Biology, Faculty of Science, Mahidol University, 10400, Thailand
bCenter of Excellence on Environmental Health and Toxicology, CHE, Ministry of Education, Bangkok, 10400, Thailand
cDepartment of Plant Science, Faculty of Science, Mahidol University, 10400, Thailand
A R T I C L E I N F O Handling Editor: Richard Hauer Keywords:
Drought stress Extensive green roof Groundcover Plant physiology
A B S T R A C T
Extensive green roof architecture is an effective and low-maintenance solution to reduce energy cost of heating or cooling the buildings, and can be used to mitigate several urbanization problems such as heat island effect, lack of green space, and excessive stormwater runoff. However, designs and vegetation selections of the extensive green roofs have primarily been based on the studies conducted in the temperate areas. This study aimed to assess the suitability of ten groundcover plants in a tropical area using quantitative physiological parameters of plants, including relative water content (RWC), stomatal opening rate, maximum quantum yield of photosystem II (Fv/Fm), soil moisture, and leaf surface temperature. Upon severe drought stress, RWC could distinguish non- tolerant from tolerant species as early as 3 days after drought treatment (DAD). The drought-tolerant species had
>80 % RWC at 5 DAD. For non-tolerant species, Fv/Fm decreased to an undetectable level, whereas only 5.45 % reduction was found for the tolerant species at 5 DAD. Three plant species Sesuvium portulacastrum, Evolvulus nummularius and Callisia repens maintained >80 % stomatal opening at 5 DAD, but E. nummularius was not considered a tolerant species based on the RWC and Fv/Fm characteristics. Interestingly, under drought stress C. repens maintained >50 % soil moisture at 7 DAD and maintained canopy surface temperature comparable to the well-watered condition. Together the results suggest that these physiological parameters are useful for the assessment of drought tolerance ability of groundcover plants, and S. portulacastrum and C. repens should be considered for extensive green roof architecture in the tropical areas.
1. Introduction
As a consequence of growing urbanization, human activities such as extensive use of air conditioners, vehicles, and construction of heat- preserving buildings contribute to the “urban heat island” (UHI) phe- nomenon (Fan and Sailor, 2005; Ohashi et al., 2007). The heat from the city cannot be efficiently eliminated, leading to increased indoor tem- perature and higher use of air conditioning (Weng and Yang, 2004). This phenomenon creates a cycle of urban energy intensity (Liu, 2008). For example, the temperature in the urban area of Bangkok was as much as 5 higher than the rural area during December, 2004–2008 (Jongtanom et al., 2011). Other environmental problems such as pollution, urban flooding, and insufficient green space are also caused by urbanization, leading to detrimental physical and mental health of residents in cities
around the world.
Green roof architecture is a promising approach that has been used worldwide for expanding green urban areas and mitigate the heat island effect (Durhman et al., 2007; Lazzarin et al., 2005). Green roofs can shield solar radiation, decrease indoor temperature, lower air condi- tioner use, reduce air pollution, and slow rainwater runoff, while improving visually pleasing green scenery to the urban area (Getter and Rowe, 2006). The vegetation in the green roofs may also increase biodiversity of various animal inhabitants such as birds and various kinds of pollinators (Liu et al., 2012). However, the vegetation planted on the green roofs, especially extensive green roofs or green roofs with thin layers of substrates, regularly encounters extreme environments, and has traditionally been limited to a few species (Durhman et al., 2006). The following plant characteristics are recommended: 1) short
* Corresponding author at: Department of Plant Science, Faculty of Science, Mahidol University, 272 Rama 6 Rd., Phayathai, Ratchathewee, Bangkok, 10400, Thailand.
E-mail address: [email protected] (A. Pichakum).
Contents lists available at ScienceDirect
Urban Forestry & Urban Greening
journal homepage: www.elsevier.com/locate/ufug
https://doi.org/10.1016/j.ufug.2020.126874
Received 19 November 2019; Received in revised form 26 September 2020; Accepted 4 October 2020
period of establishment and fast reproduction, 2) low plant height with mat-forming habit, 3) shallow and spreading roots, and 4) succulent leaves with ability to store water (Li and Yeung, 2014; MacIvor and Lundholm, 2011). Many modern green roofs have been implemented in Europe and North America primarily using Sedum spp., succulent plants from the Crassulaceae family. (Thuring et al., 2010). However, these species are susceptible to high moisture and heavy rainfalls and thus many not be suitable for the tropical climate such as Thailand which experiences annual mean maximum temperature at ̴ 33 ◦C, mean annual rainfall at ̴ 2017.1 mm, mean annual rainy period at 130 days (Thai Meteorological Department, 2019). In addition, the use of local plant species may contribute to lower cost, higher stability, and improved biodiversity of green roofs.
Several physiological parameters have been used as quantitative indicators of plant health during abiotic stresses, including relative water content (RWC), stomatal opening rate, leaf surface temperature, and maximum quantum yield of photosystem II measured as Fv/Fm. These parameters change rapidly in plants that are not able to cope with the adverse environmental conditions, but less so in the tolerant plants.
Although some of these physiological parameters had been reported for a few studies of green roof plants (Farrell et al., 2013; Young et al., 2014), data from more studies are needed to evaluate the consistency and association between these attributes particularly in the selection of plants for tropical green roofs.
In this study, we evaluated the drought tolerance ability of ten groundcover plants that are commercially available for landscaping in Thailand. RWC, stomatal opening rate, maximum Fv/Fm, soil moisture, and leaf surface temperature were monitored under a seven-day drought condition to evaluate the plant performance. Plant candidates for the tropical extensive green roof architecture were determined based on these physiological parameters.
2. Materials and methods
2.1. Plant materials and growth condition
Ten species of groundcover plants were chosen for this experiment, including Sesuvium portulacastrum (L.) L., Phyla nodiflora (L.) Greene, Evolvulus nummularius (L.) L., Bacopa monnieri (L.) Wettst., Origanum vulgare L., Ficus radicans Desf., Callisia repens L., Erigeron karvinskianus DC., Mentha pulegium L., and Desmodium triflorum (L.) DC. Common names and botanical families of the selected species are listed in Table 1.
The experiment was performed using two-month old plants purchased from a local distributor. Plants were cultivated in 10 ×10 cm pots filled with 8 cm of planting substrate consisting of composted potting soil (Nong Mai Agriculture Fertilizer Factory, Pathum Thani, Thailand) mixed with 20 % rice husk and 20 % chopped coconut husk. The physical and chemical properties of the substrate mix (Table 2) were analyzed by Department of Soil Science, Faculty of Agriculture, Kaset- sart University, Kamphaeng Saen Campus, Thailand. The plants were acclimatized for a week in a greenhouse at Mahidol University, Salaya
sub-district, Nakhonpathom province, Thailand. During acclimatization and throughout the experiment, plants were irrigated using automated four-way sprinklers three times per day at 7 a.m., 12 a.m., and 5 p.m. for 30 min each, for the total of 2.7 mL/cm2/day.
The experiments were performed in three sets (22–29 May, 26 July-2 August, and 14–21 September in 2017). Each set contained 2–4 plant species with 20 pots per species. Drought condition was simulated by placing the pots under a clear plastic roof to shield them from the sprinklers (Fig. 1). Data were collected at 0, 1, 3, 5, and 7 days after the drought treatment (DAD).
2.2. Measurement of physiological parameters
For each plant species, five pots were reserved for non-destructive measurements and five for destructive sample collection. Non- destructive measurements, including Fv/Fm, soil moisture, and infrared photos were taken between 9 a.m. to 12 p.m.
RWC was determined by measuring fresh weight (FW), turgid weight (TW) and dry weight (DW) of each sample using Eq. (1) (Turner, 1981):
RWC(%) = FW− DW
TW− DW x 100 (1)
Stomatal aperture was measured by coating the mature leaf with nail polish, tracing with clear adhesive tape, and observing under a light microscope (Olympus BX53, Japan). Microscopic images were obtained from three leaves at three random fields per leaf. Stomatal aperture was determined using ImageJ software (Zelitch, 1961). Values presented are means of at least sixteen stomata.
Table 1
Plant species used in the study.
No. Common name Family Scientific name
1 Sea purslane Aizoaceae Sesuvium portulacastrum (L.) 2 Carpet grass Verbenaceae L. Phyla nodiflora (L.) Greene 3 Roundleaf bindweed Convolvulaceae Evolvulus nummularius (L.) L.
4 Water hyssop Plantaginaceae Bacopa monnieri (L.) Wettst.
5 Oregano Lamiaceae Origanum vulgare L.
6 Rooting fig Moraceae Ficus radicans Desf.
7 Miniturtle plant Commelinaceae Callisia repens L.
8 Seaside daisy Asteraceae Erigeron karvinskianus DC.
9 Pennyroyal Lamiaceae Mentha pulegium L.
10 Three flowered
beggarweed Fabaceae Desmodium triflorum (L.) DC.
Table 2
Physical and chemical properties of the planting substrate.
Soil characteristics: very fine
Soil taxonomy: isohyperthermic Vertic Endoaquepts Composition
Sand (%) 0.8
Slit (%) 33.8
Clay (%) 65.4
Moisture content (%) 49
Density (g/cm3) 0.323
Hydraulic conductivity (m/day) 149
pH 6.7
Electrical conductivity (dS/m) 0.133
Cation exchange capacity (cmol/kg) 29.3
Organic matter (%) 19.1
Available phosphate (mg/kg) 37.4
Exchangeable potassium (mg/kg) 195
Fig. 1.Plant treatment conditions. Well-watered plants were grown in inside a greenhouse with automated irrigation. Drought-treated plants were shielded from the irrigation using a plastic cover.
Fv/Fm was measured using a field portable modulated chlorophyll fluorometer FMS2 (Hansatech, UK). Mature leaves were dark-adapted using leaf clips for at least 30 min prior to measurement. After dark adaptation, each clip was opened and measurements were taken ac- cording to the manufacturer’s instructions.
Infrared photos were captured to collect leaf surface temperature according to Jones (2004) using a thermal imaging camera FLIR ONE (FLIR systems, USA). The camera was connected to an Android-based mobile phone. Temperature was measured using the mobile applica- tion provided by the manufacturer. The camera distance was approxi- mately 30− 40 cm away from the plants with 0.95 emissivity setting.
Three measurements were made for each plant at each time point.
To assess the growth condition, soil moisture and ambient temper- ature were measured throughout the experimental period. Soil moisture was determined around 10 a.m. by inserting a soil moisture meter (Plant Soil Mate Meter ETP306, QINGDAO TLEAD International, China) into the soil approximately 5 cm below the surface. An automatic data logger (HOBO Pendant, USA) was used to continuously monitor the ambient temperature in the greenhouse according to Parton and Logan (1981).
2.3. Statistical analysis
Statistical significance between data obtained from the drought and control conditions was analyzed using Microsoft Excel 2019 (USA). The data were verified for normal distribution using the Shapiro-Wilk test prior to the analysis.
3. Results
3.1. Environmental conditions during the experiment
The physiological responses of plants were highly dependent on the environmental factors. In this study, the experiments were performed in three sets from May and September, which were during the rainy season of Bangkok. The plants were grown inside the greenhouse and were subjected to higher air temperature than normally experienced in Thailand due to the greenhouse effect. The average daytime air tem- perature inside the greenhouse ranged from 31.9 to 35.1 ◦C, with the maximum temperature of 46.1 ◦C for the control and 53.8 ◦C for the drought condition (Table 3). In general, the air temperature was 2− 3 ◦C higher in the drought condition than in the control condition partly because of the additional layer of roof to shield water from the auto- mated irrigation. The average nighttime temperature ranged from 26.2 to 27.8 ◦C with the minimum of 24.3 ◦C. The average daytime and
nighttime temperature was not significantly different between the three sets of the experiment.
3.2. Overall morphology after drought treatment
Ten locally available groundcover plant species selected for this study were subjected to drought stress under greenhouse conditions.
Two out of ten species, namely S. portulacastrum and C. repens, could tolerate the drought based on their overall morphology. At 7 DAD these two species under the drought condition still appeared as succulent and green as the irrigated plants under the control condition (Fig. 2). The other eight species exhibited severe symptoms of drought stress at the end of drought treatment. The earliest signs of stress were wilting and yellowing of the leaves, which could be observed as early as 3 DAD. The first species to display the symptoms was P. nodiflora, which appeared dead after 3 DAD, whereas the majority of species in this study wilted and died between 5–7 DAD. Interestingly, F. radicans started wilting at 7 DAD, but could recover after re-watering.
3.3. Effects of drought treatment on physiological parameters
Overall, RWC of the plants remained stable in the control condition and decreased over time after the drought treatment. Fig. 3A showed RWC at 5 DAD of the plants under drought in comparison to the well- watered plants. Six species, namely P. nodiflora, E. nummularius, O. vulgare, E. karvinskianus, M. pulegium, D. triflorum had less than 50 % RWC and were not be able to recover. Two species: B. monnieri and F. radicans had RWC at 58 % and 61 %, respectively, and could be considered moderately drought tolerant. The two drought tolerant species: S. portulacastrum and C. repens had over 80 % RWC at 5 DAD.
Furthermore, the differences between the drought tolerant and non- tolerant plants could be observed in the onset and rate of RWC reduc- tion, as demonstrated in Fig. 3 B–E. RWC of P. nodiflora declined sharply between 1 and 3 DAD and became significantly different from the con- trol condition as early as 3 DAD (t(2) = -13.1, p = 0.003). For M. pulegium, its RWC slowly declined during 1–3 DAD, and then showed significantly declined (t(4) =-12.5, p =0.000) during 3–5 DAD to 10 % RWC. In contrast, RWC of the drought-tolerant S. portulacastrum decreased only slightly from 1 to 5 DAD, and 63 % RWC could still be retained at 7 DAD. Remarkably, RWC of C. repens stayed above 97 % throughout the experiment.
Stomatal opening typically reflects the plant’s water potential. While a large stomatal aperture allows an increased rate of photosynthesis, it is associated with a high rate of transpiration and its closure is triggered in response to dehydration. To test whether the stomatal opening rate could be used to indicate the dehydration stress, average stomatal aperture was therefore measured. After 5 DAD, seven plant species either closed their stomata completely or their leaves were too damaged to measure the stomatal opening. In contrast, three plant species still had opened stomata after 5 DAD, namely S. portulacastrum (98.32 %), E. nummularius (93.23 %), and C. repens (83.36 %) (Fig. 4 A). It should be noted that E. nummularius was not considered a drought-tolerant species based on the previous RWC data, suggesting that the plant species may not be able to efficiently regulate its stomatal closure in response to the dehydration.
Fv/Fm is used to described the maximum quantum yield of photo- system II and can indicate the level of cellular damages sustained by plant leaves under drought or high light stress. In this study, control plants had healthy Fv/Fm values between 0.72− 0.89. Fv/Fm could not be determined for two species, namely B. monnieri and C. repens, due to their small leave sizes. At 5 DAD, Fv/Fm reached the undetectable level for four plant species: P. nodiflora, E. karvinskianus, M. pulegium, D. triflorum (Fig. 4B). Fv/Fm decreased only slightly in the drought- tolerant species S. portulacastrum from 0.844 to 0.798, which is equiv- alent to 5.45 % reduction. Three other plant species exhibited moderate Fv/Fm reduction, namely E. nummularius from 0.778 to 0.302 (61.18 %), Table 3
Air temperature at daytime and nighttime during the three sets of experiments.
Date Condition
Air temperature in daytime
(◦C) Air temperature in
nighttime (◦C)
Max Min Average Max Min Average
22/5/
2017 to 29/
5/ 2017
control 46.1
±4.5 24.9
±1.1 31.9 ±
2.0 29.6
±1.1 24.9
±1.0 27.2 ± 1.0
drought 45.7
±3.6 25.1
±3.6 34.1 ±
2.7 30.8
±1.4 25.2
±1.3 27.8 ± 1.2 26/7/
2017 to 2/
8/ 2017
control 43.8
±2.7 24.8
±0.4 32.2 ±
1.5 29.4
±1.1 24.9
±0.6 26.7 ± 0.8 drought 44.7
±1.7 24.6
±0.6 35.1 ±
2.1 30.5
±1.2 24.9
±0.6 27.5 ± 0.9 14/9/
2017 to 21/
9/ 2017
control 46.5
±2.3 25.3
±0.5 34.1 ±
2.0 30.0
±1.1 25.1
±0.5 26.7 ± 0.4
drought 53.8
±5.6 24.9
±0.5 35.1 ±
3.5 29.4
±1.1 24.3
±0.5 26.2 ± 0.5
Fig. 2.Representative images of plants under the control or drought-treated condition at 7 DAD. S. portulacastrum (A), P. nodiflora (B), E. nummularius (C), B. monnieri (D), O. vulgare (E), F. radicans (F), C. repens (G), E. karvinskianus (H), M. pulegium (I), D. triflorum (J).
O. vulgare from 0.812 to 0.474 (41.62 %), and F. radicans from 0.816 to
0.709 (13.11 %). 3.4. Soil moisture and canopy surface temperature
Reduction of soil moisture is the major cause of drought stress in plants. Conversely, some drought tolerant plants can help the soil retain moisture, which is also beneficial for temperature insulation and Fig. 3. Relative water content (RWC) after drought treatment compared to the well-watered condition of plants at 5 DAD (A). RWC of representative drought- tolerant (B, C) and non-tolerant (D, E) plant species from 0 DAD to 7 DAD. Error bars represent SE (n =5). *, **, *** indicate significant difference (p ≤0.05, 0.01, 0.001; one-tailed t-test).
rainwater retention in the roof gardens. Under the well-watered condi- tion, the soil moisture at 5 DAD was similar to that of 0 DAD (Fig. 5).
Time-course monitoring of the soil moisture showed that the values rapidly decreased during 3–5 DAD, and the values were below 20 % at 5 DAD for most of the plant species. For drought tolerant plants S. portulacastrum and C. repens, however, the soil moisture gradually decreased. At 5 DAD, the soil moisture for C. repens remained higher than the others at 78.5 %, whereas the soil moisture of S. portulacastrum remained at 32 %. The soil moisture contents of C. repens and S. portulacastrum were above 20 % at the end of the experiment.
Plant canopy surface temperature was measured using infrared im- aging. The well-watered plants had lower surface temperature than the plants under drought stress (Fig. 6). The average temperature difference between the control and drought-treated plants at 5 DAD was 3.76 ◦C.
The maximum difference of 8 ◦C was observed in B. monnieri, whereas C. repens showed a minimal difference in the temperature.
4. Discussion
Vegetation selection is important in the design and maintenance of the extensive roof architecture. It is also important that the selected plants are readily and widely available. We aimed to explore ground- cover plant species that may be suitable for extensive roof gardens in the tropical regions. Thus, we evaluated physiological traits and overall morphology under drought stress of ten candidate species that are commercially available for landscaping in Thailand. The severe drought condition in this study was simulated by complete water withdrawal for one week. In addition, the plants were maintained inside the greenhouse and experienced higher temperature, with the average maximum tem- perature as high as 53.76 ◦C during the daytime. Although not originally intended, this may ensure that plants selected were able to withstand extreme heat from full radiation of the rooftop structure. In comparison, a previous study of thin-layer green roofs in Taiwan showed that the daytime ambient temperature of 33.5 ◦C could result in the maximum
roof ground temperature as high as 55.0 ◦C (Liu et al., 2012).
Such extreme conditions rapidly and severely affect morphological and physiological characteristics of plants. In this study, most plants withered within the first few days and only those adapted to tolerating drought stress survived. However, to further differentiate the plant ca- pabilities to withstand the condition, we also monitored changes in their physiological parameters, which are more quantitative and perhaps could be detected before the morphological symptoms. The three physiological parameters investigated in this study were RWC, stomatal opening, and Fv/Fm, which are commonly used to distinguish plants with high and low tolerance to drought stress. RWC directly measures the dehydration status of the plant. However, this method requires destructive harvesting of the whole plant, which can practically be applied only at the end of a study. Another potential pitfall of this method is that some plant species may also be able to survive extreme dehydration, i.e. low RWC, for an extended period. Thus, in this study, we examined two additional parameters. The measurement of stomatal aperture is relatively simple, although tedious, i.e. only a light- microscopic skill is required. The stomatal opening directly affects water status, and is in turn regulated by the water status of the plants.
Therefore, stomatal opening is highly related to RWC. Plants must control stomatal opening to balance the benefit of drawing in carbon dioxide for photosynthesis and the detriment of dehydration. However, certain plants with Crassulacean acid metabolism (CAM) can increase water use efficiency by opening the stomata to obtain CO2 at night to reduce water loss due to evaporation. One drought-tolerant species in this study – S. portulacastrum, was shown to have CAM metabolism. In addition, as previously discussed, some plant species may be deficient in their ability to induce stomatal closure in response to the drought stress.
Fv/Fm measures chlorophyll fluorescence that is emitted back from the light energy that exceeds the photochemistry capacity of photosystem II.
In plants sustaining cellular damages from drought stress, the ratio be- tween closed to open photosystem II increases, leading to a decrease in Fv and hence Fv/Fm (Maxwell and Johnson, 2000). Fv/Fm is one of the most convenient methods to obtain real-time measurement of plant physiological status. However, a rather expensive equipment is required. Another caveat is that the plant leaf must be large enough for the instrument’s measuring device.
In this study S. portulacastrum and C. repens were categorized as drought-tolerant plant species based on their ability to maintain 80 % RWC at 5 DAD. This was supported by the ability of both species to maintain stomatal opening above 80 % at 5 DAD. Fv/Fm of S. portulacastrum only decreased by 5.45 % at this stage, although Fv/Fm
of C. repens could not be recorded due its small leaf size. In comparison, Sedum sp., a popular plant for green roofs, was shown to have RWC about 70–90 % under drought conditions (Vahdati et al., 2017). It can survive and maintain the Fv/Fm values about 0.5− 0.8 even in drought condition for four months (Durhman et al., 2006). S. portulacastrum is a succulent species that had been previously reported to have high drought tolerance and is a good candidate for green roof architecture in the tropics (Tan and Sia, 2009) C. repens has also been recommended for extensive green roof systems (Liu et al., 2012; Feitosa and Wilkinson, 2018; Chen, 2013). Thus, the two plant species identified in this study have a high potential for the tropical extensive green roof architecture. A longer period of study may be required to evaluate their performance.
In addition to the drought tolerance ability, C. repens stood out in their ability to regulate its surface temperature, displaying minimum temperature difference between the plants under the control and drought stress. In comparison, S. portulacastrum showed approximately 2 ◦C difference between the two conditions. Furthermore, C. repens maintained the soil moisture twice more than that of the other plant species tested including S. portulacastrum, the next best candidate.
Together these factors may help a C. repens roof garden conserve cooling energy of a building. However, the effectiveness of a plant on temper- ature reduction may be variable depending on plant density and height (Liu et al., 2012). Thus, these attributes should be investigated in more Fig. 4. Stomatal opening (A) and Fv/Fm (B) of plants at 5 DAD. Stomatal
opening was averaged from at least 16 stomata at three positions on each leaf, three leaves per plant species. Fv/Fm was averaged from three leaves per plant, five plants per species. Fv/Fm of B. monnieri and C. repens were not measured due to small leaf size. Error bars represent SE.
detail.
In this study, other moderately drought-tolerant plant species were also identified. These include B. monnieri and F. radicans. The latter is an interesting candidate. Although F. radicans could maintain a moderate amount of RWC and wilted at 7 DAD, but it could recover after re- watering. This is supported by its ability to maintain high Fv/Fm after the drought treatment, suggesting that it could avert the drought-
induced damages. F. radicans was also the second-best in the canopy surface temperature at 5 DAD. Thus, this plant species may be a good candidate for the roof gardens under periodic drought stress.
In conclusion, our study has identified potential candidates for the tropical extensive green roof architecture based on commercially available plant species for landscaping. We have also demonstrated the usefulness and pitfalls of some physiological parameters that may be Fig. 5.Soil moisture at 5 DAD of ten plant species relative to 0 DAD (A). Soil mositure of representative drought-tolerant (B, C) and non-tolerant (D, E) plant species from 0 DAD to 7 DAD. Error bars represent SE (n =10). *, **, *** indicate significant difference from value at 0 DAD (p ≤0.05, 0.01, 0.001; one-tailed t-test).
Fig. 6. Leaf surface temperature after drought treatment. Representative infared images of well-watered and drought-treated plants at 5 DAD of drought non-tolerant plant species: B. monnieri (A), O. vulgare (B), F. radicans (C); and drought tolerant species: C. repens (D). Average differences in temperature between drought and control treatments at 5 DAD (E). Error bar represents SE (n =3).
applied in the screening of the candidate plants. Further long-term evaluation and testing on a model roof are needed.
CRediT authorship contribution statement
Metha Meetam: Conceptualization, Visualization, Writing - original draft. Naraporn Sripintusorn: Investigation, Visualization, Writing - original draft. Wisuwat Songnuan: Conceptualization, Visualization, Writing - original draft. Umaporn Siriwattanakul: Investigation.
Aussanee Pichakum: Conceptualization, Visualization, Writing - re- view & editing.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
This research was supported by the DPST research grant [grant no.
022/2557] to MM from the Institute for the Promotion of Teaching Science and Technology (IPST), Thailand, and the research grant enti- tled “Plant material for low-maintenance green roof to conserve energy and carbon footprint” (2017-2018) from National Research Council and Mahidol University, Thailand.
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