Effects of various irrigation regimes on water use efficiency and visual quality of some ornamental herbaceous plants in the field

Agricultural Water Management 212 (2019) 78–87

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Effects of various irrigation regimes on water use efficiency and visual quality of some ornamental herbaceous plants in the field

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Zahra Nazemi Rafi, Fatemeh Kazemi , Ali Tehranifar Department of Horticulture and Landscape, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: Herbaceous spices Drought Irrigation management Water use efficiency

Efficient use of water by selection of appropriate plant species is becoming increasingly important in semi-arid and arid regions to save scarce water resources. In this study, the effects of water deficit irrigation on physiological responses, water-use efficiency (WUE) and visual quality of four herbaceous ornamental species (Malva sylvestris, Althea rosea, Callistephus chinensis and Rudbeckia hirta) were investigated. The experiment was a splitplot treatment based on a randomized complete block design experiment with four irrigation levels (25%, 50%, 75%, and 100% reference evapotranspiration (ET0)) and four replications. Althaea rosea and Rudbeckia hirta showed dehydration avoidance by modifying their leaf shape, decreasing stomatal conductance and increasing Water Use Efficiency (WUEi). The lowest specific leaf area and highest root/shoot ratio were seen in Althaea rosea at 25% and 50% ET0, respectively. Malva sylvestris used a drought escape strategy by early flowering. In terms of landscape aesthetic values, Malva sylvestris performed well with a minimum of 75% ET0. Althaea rosea and Rudbeckia hirta were well-maintained with 25%–50 % ET0 irrigation levels. However, for an acceptable aesthetic landscape appearance, the minimum level of irrigation for Callistephus chinensis was 50% ET0. These results suggest that use of drought-tolerant spices such as Althaea rosea and Rudbeckia hirta can improve irrigation management and conserve aesthetic performance in urban landscapes.

1. Introduction Increased industrialization, rising population, and climate change have resulted in water conflicts especially in urban environments (LeaCox and Ross, 2001). In many urban communities, irrigation of urban landscapes is a considerable fraction of urban water consumption (between 40% and 70%) (Lockett et al., 2002). The absence of a perceptible association between landscape performance and resources such as water makes water conservation difficult. Water management is difficult when there are wide fields of expectations and end users (Kjelgren et al., 2000). Species diversity and their water need characteristics (Kjelgren et al., 2000), differences in regions, management practices, and landscape types make a determination of irrigation requirements more complex (Hilaire et al., 2008). To mitigate demand problems, water managers are looking to achieve significant long and short-term water saving strategies (Brown et al., 2004). Therefore, some conservation strategies are implemented such as using precision landscape irrigation (Kjelgren et al., 2000), using alternative water sources (Hilaire et al., 2008), using native and drought tolerant plant species (Lockett et al., 2002), applying deficit irrigation strategies (Hilaire et al., 2008; Mansour et al., 2017) and identifying the varying water

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needs in the developmental stages (Stabler and Martin, 2000). Studies to define the minimum irrigation need for maintenance of plants with acceptable aesthetics display are surprising possibilities for water conservation in landscaping. For example, many non-turf landscape plants can sustain acceptable aesthetics under some levels of water deficit protocols (Kjelgren et al., 2000). Investigations on irrigation of enera landscape groundcovers showed several species had acceptable performance at 20% or 50% of reference evapotranspiration (ET0) (Staats and Klett, 1995; Pittenger et al., 2001). A study showed 11 and 14 shrub species maintained acceptable aesthetic appearance at irrigation amounts equal to 0% and 18% of ET0, respectively (Shaw and Pittenger, 2004). For example, the species such as Workwood, Feathery cassia, Orchid spot rockrose, Pride of madeira, Bush snapdragon, Noell grevillea, Toyon, ‘Green Cloud’ texas ranger, Prostrate myoporum and ‘Santa Cruz’ firethorn, Goldmoss had acceptable appearance in no irrigation. Costello et al. (2005) reported no difference among irrigation at 0%, 25%, or 50% ET0 in the growth of 2- year oaks. Furthermore, the results from studies on water use of some landscape trees illustrated that increases of water use are associated with plant species type, plant size and/or soil moisture content (Devitt et al., 1994, 1995; Vrecenak and Herrington, 1984). Based on Stabler and Martin (2000, 2004),

Corresponding author. E-mail addresses: zahra_nazemi_rafi@yahoo.com (Z. Nazemi Rafi), [email protected] (F. Kazemi), [email protected] (A. Tehranifar).

https://doi.org/10.1016/j.agwat.2018.08.012 Received 24 January 2018; Received in revised form 11 June 2018; Accepted 9 August 2018 0378-3774/ © 2018 Elsevier B.V. All rights reserved.

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water use efficiency (WUE) for established Cercidium floridum, Nerium oleander, and Leucophyllum frutescens could be optimized by deficit irrigation regimens. Water conservation can also be achieved using such strategies as Water Sensitive Urban Design (Kazemi et al., 2009, 2011) while, keeping landscape aesthetic values at the same time is still a challenge (Hurd et al., 2006). Several research works have been conducted in this area which focuses on how to create low maintenance and attractive meadows in urban landscapes (Hitchmough et al., 2004; Dunnett and Hitchmough, 2007). Due to the beauty of flowering herbaceous species, it is hard to convince people to eliminate them from urban landscaping (Rutgers Master Gardeners, 2016). These plants are the first group which shows symptoms of water deficiency. Therefore, to optimize landscape aesthetic performances of water-wise landscaping (Sun et al., 2012), studies have been focused on drought effects on herbaceous flowering plants (Moosavi et al., 2014). Due to morphological and physiological adaptations, using native plants is as a way to maintain acceptable landscape appearance (Starman and Lombardini, 2006) and support local wildlife (Ober and Knox, 2017). However, the complexities of urban microclimates have resulted in not adapting of some native plants to a harsh new habitat (Wade et al., 2010). To obtain an exceptional landscape performance, a mix of adapted exotics and native plant species should be applied in municipal landscaping. The wholeplant characters (Blum, 2005) and atmospheric demand for moisture have effects on plant water needs (Kjelgren et al., 2000). Plants adjust their water demand by dehydration avoidance characters such as creating more trichomes (Ehleringer et al., 1976), reducing lower specific leaf area (Fonseca et al., 2000), increase their rooting depth (Kjelgren et al., 2009) and changing their stomatal size and density (Masle et al., 2005). Native plants obtain accommodation mainly by dehydration escape and dehydration avoidance rather than dehydration tolerance (Blum, 2005). However, having drought tolerance potential, plants experience some degree of wilting, leaf burn, reduction in growth, flower size, number of flowers and quality (Zollinger et al., 2006). Therefore, understanding the minimum water needs for desirable plant quality (Henson et al., 2006) and the varying degree of adaptation and water use efficiency may assist in keeping landscape visual quality to an acceptable level (Cameron et al., 2006; Andrew et al., 2013). In the current study, the effect of water deficit irrigation on physiomorphological and landscape performance of two native herbaceous species, Althaea rosea, Malva sylvestris, and two adapted exotic plant species to Iran, Rudbeckia hirta and Callistephus chinensis were examined. The main aim was to provide knowledge on the lowest watering level for acceptable maintenance, which may help to design future water management framework for a mix of native and non-native urban landscaping.

The experimental design was a split-plot treatment based on a randomized complete block design with four replications. The irrigation treatments were in four levels including 25%, 50%, 75% and 100% of the reference evapotranspiration (ET0) (required to optimize growth of Poa pratensis (Kentucky bluegrass). The hourly weather data (air temperature, relative humidity, solar radiation, wind speed and rainfall) were achieved from local weather station records. Local reference evapotranspiration (ET0) was obtained from weather data using Penman–Montieth equation (Allen et al., 2000). The study examined four herbaceous flowering plant species including Rudbeckia hirta, Callistephus chinensis, Althaea rosea, Malva sylvestris. The size of each of the 64 plots was 1 m × 1.5 m and they were separated by a 1-meter distance. The plots of Althaea rosea, Malva sylvestris, Rudbeckia hirta and Callistephus chinensis species we consisted of 6–12 plants, respectively. Althaea rosea, Malva sylvestris and rudbeckia hirta, Callistephus chinensis species were spaced a minimum of 0.5, 0.3 m apart, (4 and 8 plants per square meter) respectively. In April 2016, the seeds were sown in plastic trays and uniform seedlings (6 to 8-leaf stage) were transplanted approximately one month after planting the seeds. The plants were allowed to establish one month prior to the treatment initiation. The weeds were removed using a hand removal. Then the plants were treated with four irrigation levels until the end of the season. Rainfall per day was subtracted from the amount of each treatment. The total Available moisture content in the soil was 14.9% (Table 1). Pure Irrigation Depth (PID) which is dependent on the root depth of the experimental species, was calculated 4.47 cm or 44 mm. The maximum irrigation interval (4 days) was calculated as the ratio of PID to maximum water requirement at the maximum growth stage which was 10 mm/day. In this research, irrigation intervals were considered every second days. The total amount of water taken up by the plants in each plot was calculated in all the treatments during the period of the experiment to measure water use efficiency. 2.2. Measurements At the end of the study period, landscape impact, floral impact and appearance impact of the studied species were assessed applying a modified method of Rozum (2014). Appearance impact rating was evaluated base on a 1–5 Likert- type scale. Complete death of plants and leaves was scored 1= dead, 2= visible witling, firing and green leaf < 50%, 3= slow growth, green leaves = 50%, 4= good growth, green leaves = 75% and 5=exceptional growth. Floral impact rating was based on how much plant’s appearance is improved by flowering. It was rated on a scale of 1–5 where 1= no flowering; 2 = 25% impact; 3 = 50% impact; 4 = 75% impact and 5 = 95% impact, very showy inflorescence. Landscape impact rating was assessed based on how the general appearance of the plants, their flowering and their insect and disease symptoms impacted on the landscape quality. The gradations of the scales were as follows: 1= having a weak growth in the landscape, sparse flowering or without ornamental value; 5=having a pleasurable landscape appearance. Althea rosea displayed a degree of leaf rolling, which was rated on a scale of 1–5: 1=no rolling; 5=full leaf rolling. After conducting a visual rating, the plants were harvested, their roots were washed, and then the fresh weight of the roots and shoots were measured. To measure the dry mass (DM) and root/shoot ratio, the plants were placed

2. Material and methods 2.1. Plant materials and experimental design This field study was conducted in Mashhad Botanic Garden (Longitude: E59° 36', Latitude: N36° 15' and 985 m above sea level, average rainfall per year: 233.8 mm) located in south part of the city of Mashhad, Iran during 2016. Physical and chemical characteristics of the soil of the experimental site are listed in Table 1. Table 1 Characteristic of the soil in the experimental field. pH

Organic C %

N

P ppm

K

Sand %

Silt

Clay

Soil texture

BD g/cm3

FC v/v%

PWP

7.80

0.5

0.04

14.3

210

38.8

36

c

loam

1.45

25.2

10.2

Field capacity (FC)- Permanent Wilting Point (PWP)- Bulk Density (BD). 79

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chinensis, respectively. Also, no significant difference was found between Althaea rosea and Malva sylvestris. In the case of 25% ET0, Malva sylvestris produced the highest biomass, followed by Althaea rosea. The highest (70%) and the lowest (58%) reductions in biomasses were found in Althaea rosea and Malva sylvestris compared to the control irrigation treatment, respectively (Table 3). Drought is one of the most important environmental stresses to limit plant establishment and growth (Cregg, 2002). While the prime effect of water stress on agricultural crops is yield reduction (Passioura, 1996); the most important drought stress effect in ornamental plants is negative effects on their visual quality. In ornamental plants, the intensity of response to water stress (declining the daily ET) is dependent on the species/variety (Lenzi et al., 2009). Species responses to drought stress have been discussed by many researchers. For example, biomass accumulation in stressed plants is almost limited to C. monspeliensis, Cistus albidus, oleander, rose and Geranium sp. (Sánchez-Blanco et al., 2002; Niu et al., 2008; Niu and Rodriguez, 2009; Álvarez et al., 2013). Andrew et al. (2013) observed that the seedlings of E. excelsa and E. grandis in well-watered situations (100% field capacity) produced greater total biomass (54.6 g and 27.7 g, respectively) compared to the drought treatments (12.5, 25 and 50 field capacity). Moreover, González Rodríguez et al. (2010) showed that the plants in 100% field capacity had large foliage compared to them in 40% field capacity. This means that the higher drought stress levels induced the lower leaf emergence rate, lower photosynthesis rate and thus lower biomass assimilation (Andrew et al., 2013). Our results on Rudbeckia hirta were in contrast to the results in Henson’s report (2005). He found that irrigation at 0%, 25%, 50%, 75% or 100% had no effect on biomass of Rudbeckia hirta. Also, irrigation at 50%–100% ET0 had no significant effect on the growth of Rudbeckia hirta. In the current study, poor visual quality and less biomass reduction in Malva sylvestris, suggest that having high biomass or keeping it during the drought does not necessarily guaranty high aesthetic values.

in an oven at 65 °C until a constant weight was obtained (Kazemi et al., 2011). In this research work, two concepts were used to describe water use efficiency. Water use efficiency based on growth index (WUEi) and WUE based on biomass production (WUEb). The first was calculated by dividing the growth index (GI) difference (subtracting the final from the initial GI) by the total water applied (Fernandez et al., 2009). Growth index was measured by multiplying the plant height to canopy width on a north-south axis and canopy width on an east-west axis. WUE based on biomass production (WUEb), was measured by dividing the difference of initial and final total biomass by the amount of water applied to the plants during the experiment (Medrano et al., 2015). Leaf area (cm2) was calculated using a leaf area meter (LI-3000C; LI-COR Ltd., Lincoln, Nebraska USA). Specific Leaf Area (SLA) was calculated by dividing the leaf area by leaf dry weight. The ash content was obtained by burning 2.0 g sample on a muffle furnace at 550 °C for 4 h and then weighing the residue (Latimer, 2012). Leaf stomatal conductance was measured on the young, fully expanded leaves by a SC-1 porometer (SC-1, Decagon, Pullman, USA) between 12:30 and 1:30 pm. Mean of the two readings per plant were taken every two weeks. The canopy temperature was measured by a Handheld Infra-Red Thermometer (Sixth Sense LT 300 Infrared Thermometer, USA) at a height of 45.5 cm above each plant height at noon every four weeks and before irrigation was applied. Mean of the two readings was recorded. 2.3. Data analysis The data were subjected to the analysis of variance (ANOVA) using the mixed procedure in SPSS software package (version 25). The irrigation treatments and plant species were considered as fixed factors. The significance of between-treatment means was tested at p ≤ 0.05 levels. The experiment was analyzed as a randomized complete block design with a split-plot arrangement. The relationships between the parameters were measured using Pearson's simple correlation tests.

3.2. Root/shoot ratio (R/S)

3. Results and discussion

Comparisons among the plant species showed significant differences in their root/shoot ratio. Althea rosea had the greatest root/shoot ratio (0.419), followed by Malva sylvestris, Rudbeckia hirta and Callistephus chinensis. Table 3 showed that the plants treated with 50% ET0 ranked the highest in their root/shoot ratio (0.278); however, there were no significant differences on root/shoot ratio of the plants treated by 75% ET0 and 25% ET0. In Callistephus chinensis, no significant differences were detected between 75% ET0 and 25% ET0. There was no significant difference between Rudbeckia hirta under 25% ET0 treatment and Callistephus chinensis under 75% ET0 treatement. In Rudbeckia hirta and Althea rosea, the plants treated with 50% ET0 significantly had an increase in their R/S ratio as compared to the control conditions (2.02 and 1.59, respectively). Due to fast osmotic adjustment in roots (Sharp, 2004), drought stress had less effect on root growth than on shoot growth (Franco et al., 2011). Cai et al. (2012) reported that increased root/shoot ratio is one

The results of the analysis of variance showed that the effects of drought stress and plant species types on the measured factors were significant at P ≤ 0.01. Also, interactions between the drought levels and the plant species were significant for all the measured traits except for stomatal conductance (P ≤ 0.01) (Table 2). 3.1. Plant biomass The treatment effects on the biomass of the four plant species were highly significant. In all the plant species, the plants treated with 100% ET0 had the greatest biomass while the lowest biomasses were associated with 25% ET0. Comparisons between the biomass of the plant species showed that the highest (1071.20 g) and the lowest biomass values (199.53 g) were obtained in Althea rosea and Callistephus

Table 2 Analysis of variance of the effect of drought (irrigation) levels and plant species types on some morpho-physiological traits. Source of Variation

Replication Irrigation Error a Species Irrigation × Species Error b

d.f.

3 3 9 3 9 36

Mean Square WUEi (cm/l)

WUEb (g/l)

Biomass (g)

SLA (cm2 g−1)

Root/Shoot Ratio (g/g)

Ash content (%)

Δ Canopy Temperature (°C)

Stomatal Conductance (mm s−1)

0.47NS 25.17** 0.14NS 116.32** 7.98** 0.26

0.03NS 0.26** 0.06NS 29.65** 0.19** 0.05

2880NS 2,079,922** 4621NS 2,641,446** 187,599** 2311

950** 15,170** 18NS 109,825** 833** 166

0.002NS 0.036** 0.001NS 0.298** 0.031** 0.001

0.1NS 75.9** 0.2NS 115.7** 2.5** 0.2

1.4NS 161.2** 0.3NS 135.7** 8.2** 0.8

1.1NS 356.8** 1.5NS 30.4** 3.1NS 1.9

80

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Table 3 Effect of irrigation treatment and species on Biomass, SLA and Root/Shoot Ratio. Irrigation treatment (ET0) Parameter Biomass (g)

SLA

100%

75%

50%

25%

Species

Callistephus chinensis

290.2j

233.1j

161.3k

113.4k

199.53C

Rudbeckia hirta Althea rosea Malva sylvestris Irrigation

1313c 1757.5a 1531.7b 1223.11A

981.2e 1362.6c 1212.3d 947.30B

720.8f 637.8g 775.9f 573.95C

432.3i 526.9h 639.5g 428.03D

861.83B 1071.20A 1039.83A 793.10

Callistephus chinensis

163.79d

148.57de

116.07ghi

90.59jk

129.76B

Rudbeckia hirta Althea rosea Malva sylvestris Irrigation

137.67ef 140.22ef 334.70a 194.09A

125.71fgh 132.79efg 321.65a 182.18B

112.33hi 104.86ij 260.03b 148.32C

100.97ijk 85.47k 231.08c 127.02D

119.17C 115.84C 286.86A 162.90

Callistephus chinensis

0.274de

0.066j

0.059j

0.053j

0.113D

Rudbeckia hirta Althea rosea Malva sylvestris Irrigation

0.118i 0.350c 0.265e 0.252B

0.172gh 0.309cd 0.200fg 0.187C

0.239ef 0.556a 0.257e 0.278A

0.070j 0.459b 0.150hi 0.183C

0.150C 0.419A 0.218B 0.225

(cm2 g−1)

Root/Shoot Ratio (g/g)

Means with the same letter are not significantly different.

prediction of drought tolerance, but also for competitive effects on landscape plantings. The lowest specific leaf area was found in Althaea rosea due to a significant rolling (Blum, 1996) and higher tissue density under drought stress conditions (Castro-Díez et al., 2000). Leaf rolling is one of the adapting mechanisms that reduces leaf area (Blum, 1996), transpiration through abaxial stomatal closure (Begg, 1980), water usage and preserves photosystem II (PSII) performance (Nar et al., 2009).

of the characteristics of drought tolerant species. This phenomenon affects faster establishment and maintenance of the seedlings in such conditions (Andrew et al., 2013). With an exception of 25% ET0 treatment, the root/shoot ratio increased in Rudbeckia hirta in the other irrigation treatments. The higher root/shoot ratio in Althaea rosea and Rudbeckia hirta enhances source/sink ratio for nutrients and water in semi-arid environments. Nevertheless, this ratio is not always changed by drought stress. Sánchez-Blanco et al. (2014) reported that the root to shoot ratio in Pistacia lentiscus and Phillyrea angustifolia was not affected under drought condition. With 75% ET0 exception, the same response was observed in Callistephus chinensis. The opposite effect of severe stress compared to moderate stress on root/shoot ratio in Malva sylvestris was in agreement with the findings of Navarro et al. (2009). They mentioned that differences in water deficit levels led to differences in the root/shoot ratio of Myrtus communis. After 5 months, a moderate water stress had no significant effect on root development while a severe water deficit significantly reduced root and shoot ratio in Myrtus plants.

3.4. Canopy temperature It was clear from Table 4, that the variation in ΔTc (canopy temperature – air temperature) between the plant species was considerable. Malva sylvestris obtained the greatest ΔTc (3.52 °C), followed by Callistephus chinesis, Althea rosea and Rudbeckia hirta in decreasing order. It was evident that increasing the stress level resulted in a significant increase in ΔTc. There was no significant difference in ΔTc between Malva sylvestris and Callistephus chinesis at 25% ET0. Similar results was obtained at 50% of ET0. Rudbeckia hirta and Althea rosea with 25% ET0 also had no significant differences with the control plants of Malva sylvestris in terms of ΔTc. ΔTc in Rudbeckia hirta was negative in 100% to 50% ET0. Among the plant species, Malva sylvestris was the only plant species that showed positive ΔTc (1.44 °C) even in the control irrigation treatment. Callistephus chinesis had the highest change in ΔTc (10.48 °C) from 100% ET0 to 25% ET0 as compared to the other plant species. In stressed irrigation levels (75% ET0, 50% ET0 and 25% ET0), the lowest ΔTc values were observed in Rudbeckia hirta, followed by Althaea rosea. Leaf shape and size variations can affect canopy temperature in similar conditions (Basra, 1997). The difference between canopy temperature (Tc) and air temperature (Ta) has been used to assess stress responses (Howell et al., 1986), water use (Pinter et al., 1990) and tolerance to drought in plants (Blum et al., 1989). Therefore, this trait may have more utility for indirect estimating of gs in breeding schedules (Roohi et al., 2015). The lower ΔTc in Rudbeckia hirta, and Althaea rosea could be explained by changes in leaf angle (Zheng et al., 2008) and the degree of pubescence of the stems and leaves (Gersony et al., 2016). In fact, low ΔTc led to drought tolerance by maintenance of physiological function and turgor (Olivares-Villegas et al., 2007). The canopy temperatures of Malva sylvestris at all levels of irrigation were greater than air temperatures. Henson (2005) reported similar responses in Impatiens x walleriana and Begonia carrieri hort. ‘Vodka’. In his

3.3. Specific leaf area The average effects of water regimes on all the plant species showed that with increasing the drought stress, SLA significantly decreased (Table 3). Species comparisons indicated that Malva sylvestris was superior in terms of this factor compared to the rest of the species (286.86 cm2 g−1). There was no significant difference between Rudbeckia hirta and Althea rosea in terms of SLA. Under the control irrigation treatment Malva sylvestris had the highest SLA (334.70 cm2 g−1) and this value was followed by the equivalent value for Callistephus chinensis (163.79 cm2 g−1). However, no significant difference was detected among Callistephus chinensis, Rudbeckia hirta and Althea rosea under 25% ET0 irrigation treatment. Comparisons also revealed that SLA had no significant difference between 50% ET0 and 75% ET0 in Rudbeckia hirta. Specific leaf area has been reported to be declined under drought stress (Marcelis et al., 1998). Low specific leaf area is an index of drought resistance due to having more photosynthetic capacity (Painawadee et al., 2009), which can be used to select herbaceous species for adverse environmental conditions (Maroco et al., 2000). Specific leaf area varied strongly across the plant species within the control treatment in the current study, so that the papery leaves (higher SLA), in Malva sylvestris, can be used as a species indicator not only for 81

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Table 4 Effect of irrigation treatment and species on Ash content, Δ Canopy Temperature and Stomatal Conductance. Irrigation treatment (ET0) Parameter

100%

75%

50%

25%

Species

Callistephus chinensis

14.63gh

14.39h

12.41j

11.42k

13.21D

Rudbeckia hirta Althea rosea Malva sylvestris Irrigation

22.98a 17.83cd 18.30c 18.43A

21.95b 16.45e 18.05cd 17.71B

17.73d 15.05fg 15.25f 15.11C

16.25e 13.33i 14.18h 13.79D

19.73A 15.66C 16.44B 16.26

Δ Canopy Temperature (°C)

Callistephus chinensis Rudbeckia hirta Althea rosea Malva sylvestris Irrigation

−4.20i −6.53j −3.45hi 1.44def −3.18D

2.24de −6.13j −2.20h 2.33de −0.94C

3.85bc −0.75g 0.80f 4.25b 2.04B

6.28a 1.06ef 2.60cd 6.05a 4.00A

2.04B −3.08D −0.56C 3.52A 0.48

Stomatal Conductance (mm s−1)

Callistephus chinensis

35.95cde

35.41de

30.80f

28.08g

32.56C

Rudbeckia hirta Althea rosea Malva sylvestris Irrigation

38.40b 41.46a 37.79bc 38.40A

37.09bcd 38.91b 36.22cd 36.91B

31.42f 33.99e 31.01f 31.80C

27.65g 28.68g 28.12g 28.13D

33.64B 35.77A 33.29BC 33.81

Ash content (%)

Means with the same letter are not significantly different.

25% ET0 showed the most decreases in this trait as compared to their control plants (%31 and 28%, respectively).

report, Rudbeckia hirta had the greatest canopy temperatures at 0% ET0 and 25% ET0 (5.169 and 2.62 °C, respectively) and there was no significant difference through 50% ET0 to 100% ET0 and through 25% ET0 to 75% ET0.Greater ΔTcs and higher leaf desiccation values in Malva sylvestris and Callistephus chinesis in 25% ET0 were in agreement with the results obtained by Blum et al. (1999) on rice. The data also indicated that there were significant negative relationships between ΔTc and biomass for plant species under drought stress conditions (Table 6). Landscape plant species are assessed for their aesthetic appearance and not for their yield. Therefore, the conception of optimum yield and growth is not applicable to urban landscape plants (Shaw and Pittenger, 2004). As previously reported, cooler canopy led to more grain yield in wheat, triticale and barley (Ayeneh et al., 2002; Roohi et al., 2015). Our results indicated a significant negative correlation between ΔTc and plants appearance, floral and landscape impact rating under drought conditions. A positive correlation was found between ΔTc and SLA in Althaea rosea under drought stress condition (r = .996, P < 0.01), while there was no significant correlation for these factors among other species. Similar to these results, a significantly positive correlation between SLA and canopy temperature was previously reported in three cereal species under drought conditions (Roohi et al., 2015). The ΔTc was not correlated with WUEi and WUEb in our current study. Contrary to these results, Alderfasi and Morgan (1998) reported that there was negative associations between canopy temperature and crop water use efficiency of biomass. In addition, a negative correlation was observed between ΔTc and stomatal conductance under drought conditions (Table 6). Roohi et al. (2015) had similar report for relationship of canopy temperature and gs on cereal species.

3.6. Ash content A comparison between the leaf ash means of the four species illustrated that Rudbeckia hirta had the highest value, followed by Malva sylvestris, Althaea rosea and Callistephus chinesis, respectively (Table 4). As expected, there was a significant decreasing trend in ash content of the plants as drought stress treatment become severe. Leaf ash varied from 14.63 to 22.98% under the control irrigation conditions, with the minimum and maximum belonging to Callistephus chinesis and Rudbeckia hirta, respectively. In Althaea rosea and Rudbeckia hirta, this trait was significantly decreased as the irrigation level was decreased. Rudbeckia hirta and Althaea rosea under 25% ET0 indicated a greater decrease in this trait as compared to their corresponding control plants (%29 and 26% respectively). Leaf ash content was decreased with increasing the level of drought stress as previously observed in barley and wheat (Voltas et al., 1998; Araus et al., 1998). Drought stress affects the species by inducing stomata closure (Cai et al., 2012). The stomatal closure as a response to drought stress levels were similar among Rudbeckia hirta, Callistephus chinensis and Malva sylvestris species in the current study. Callistephus chinensis and Malva sylvestris had less sensitive stomatas to drought levels compared with Althaea rosea. It has been documented that osmotic adjustment plays an important role in maintenance of higher stomatal conductance and drought resistance. Therefore, as an additional drought resistance strategy, higher visual quality in Rudbeckia hirta, like other Rudbeckia species, could be related to osmotic adjustment (Prevete et al., 2000). Observational comparisons of the results showed that the pattern of stomatal conductance (gs) between the plant species was similar to that for ash contents among the species (Table 4). Furthermore, significant positive correlations were observed between the leaf ash content and the stomatal conductance (gs) across the plant species when treatments were plotted together (See Fig. 5). There is a close correlation between water status of the plants (gs) and mineral accumulation in leaves which is discussed by Cabrera-Bosquet et al. (2009) on maize.

3.5. Stomatal conductance Regardless of the irrigation regime, there was no significant difference in stomatal conductance of Malva sylvestris when compared with Rudbeckia hirta and Callistephus chinesis. Plants in the control treatment (100% ET0) had 4%, 17% and 27% higher stomatal conductance than the plants treated with 75% ET0, 50% ET0 and 25% ET0, respectively. No significant difference was observed between all the plant species at 25% ET0. Malva sylvestris, Rudbeckia hirta and Callistephus chinesis had similar pattern at 50% ET0 and 75% ET0. Comparison of the different irrigation treatments within Althaea rosea showed a significant decreases in stomatal conductance. Althaea rosea and Rudbeckia hirta with

3.7. Water use efficiency (WUE) In ranking the plant species, Rudbeckia hirta and Althaea rosea were 82

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Callistephus chinensis has been reported in Pelargonium zonale (Andersson, 2000). In case of Althaea rosea and Rudbeckia hirta, higher thickness, better light interception (Chaves et al., 2002), presence of trichomes (Dunkić et al., 2001) and decreased leaf area (Lazaridou and Koutroubas, 2004; Huang et al., 2005) may lead to a better water saving and improvement in WUE. Therefore, WUE could be considered as an index of the ability of the plant species to save water under dry conditions (Fang et al., 2010).

Table 5 Effect of irrigation treatment and plant species on water use efficiency index (WUEi) and water use efficiency biomass (WUEb). Irrigation treatment (ET0) Parameter WUEi (cm/l)

WUEb (g/l)

100%

75%

50%

25%

Species

Callistephus chinensis

0.47i

0.51i

0.47i

0.53i

0.49D

Rudbeckia hirta Althea rosea Malva sylvestris Irrigation

3.95ef 3.63fg 2.38h 2.61D

5.54d 3.74fg 2.69h 3.12C

6.49c 4.56e 2.79h 3.58B

11.04a 7.31b 3.04gh 5.48A

6.76A 4.81B 2.72C 3.70

Callistephus chinensis

0.49g

0.47g

0.44g

0.42g

0.45D

Rudbeckia hirta Althea rosea Malva sylvestris Irrigation

2.50e 3.34c 2.53e 2.21B

2.43ef 3.62bc 2.48ef 2.25B

2.46ef 3.90ab 2.18f 2.25B

3.02d 4.06a 2.45ef 2.49A

2.60B 3.73A 2.41C 2.30

3.8. Relationship between WUE and stress-related parameters There were negative relationships between WUEb and stomatal conductance (R2 = 0.71 and R2 = 0.70) and a highly negative relationship between WUEi and stomatal conductance (R2 = 0.86 and R2 = 0.89) for Rudbeckia hirta and Althaea rosea, respectively. It was shown that there was negative relationship between WUEi and stomatal conductance for Malva sylvestris (R2 = 0.61). Also, Ash content had negative relationships with WUEi (R2 = 0.85, R2 = 0.88) and WUEb (R2 = 0.59, R2 = 0.74) in Rudbeckia hirta and Althaea rosea, respectively. Such negative relationships between SLA and WUEi (R2 = 0.87, R2 = 0.87) and SLA and WUEb (R2 = 0.59, R2 = 0.66) was also observed in Rudbeckia hirta and Althaea rosea. As mentioned above, there was no relationship between WUE and ΔTc among the plant species. Some researchers reported a negative correlation between WUE and SLA, assuming an adaptive value of SLA by water loss reduction in thicker leaves (Warren et al., 2005). Stomatal conductance had a critical role for water balance and WUE (Tuberosa, 2012). Our result was consistent with McKay et al. (2003), who reported that decreasing stomatal conductance and increasing WUE could led to more drought resistance. Also, the relationship between ash content and WUE indicated that using ash as selection criteria for species with higher WUE would present a reasonable cost and simple procedure (Frank et al., 1997).

Means with the same letter are not significantly different.

seen as superior, in term of WUEi (6.76 cm/l) and WUEb, (3.73 g/l) compared to the rest of the plant species, respectively. This WUEi value was 93, 60 and 29% greater than the WUEi values of Callistephus chinensis, Malva sylvestris and Althaea rosea, respectively. The amount of WUEb was 30, 35 and 88% higher than WUEb amounts of Rudbeckia hirta, Malva sylvestris and Callistephus chinensis, respectively. Treatment comparisons (average for all species) indicated that differences in WUEi were significantly increased as the degree of drought stress increased. In case of WUEb, none of the irrigation treatments except 25% ET0 was found to be significantly different. Irrigation treatment regimens had no significant effects on WUEb and WUEi of Callistephus chinensis. In case of WUEi, similar response was found in Malva sylvestris. Compared with the control irrigated plants, WUEi of Rudbeckia hirta, Althaea rosea and Malva sylvestris was increased by 179, 102 and 28%, respectively, under 25% ET0 (Table 5). There is no general agreement on how water deficiency affects water-use efficiency. Tanner and Sinclair (1983) discussed that reduction of dry matter is almost as much as the decline in transpiration is, thus WUE is not affected. Higher WUE is mostly influenced by a consequence of decreased water use rather than assimilation or pure enhancement in plant production (Blum, 2005). Unlike agriculture, in urban landscape, improving WUE does not necessarily mean improving the overall yield or growth. The ultimate goal is rather to achieve more efficient use of water for suitable aesthetic appearance and plant fitness (Stabler and Martin, 2000). An increase in WUE under drought stress has been observed by several researchers in ornamental plant species such as woody ornamentals (Cameron et al., 2006), Catharanthus roseus (Jaleel et al., 2008), Dianthus (Álvarez et al., 2009) and Callistemon citrinus (Álvarez and Sánchez-Blanco, 2013). Our estimation of higher WUE under lower irrigation agreed with those of Tyler et al. (1996) who observed Cotoneaster dammeri ‘Skogholm’ under a leaching fraction (LF) (0.0–0.2) had 29% greater water use efficiency than plants under high LF (0.4–0.6). Similar response of water stress on WUE which we achieved in

3.9. Irrigation level thresholds The experiment determined the following irrigation level thresholds for each species to attain satisfying growth and visual quality. In case of Malva sylvestris, critical level of irrigation was between 75% and 100% ET0; for Althaea rosea it lied between 25% and 50% ET0; for Callistephus chinensis, it was between 50% and 75% ET0 and; for Rudbeckia hirta, it was between 25% and 50% ET0. The critical level of irrigation of Malva sylvestris and Callistephus chinensis were similar to the critical irrigation level in Impatiens x walleriana and Begonia carrieri hort. ‘Vodka’ Henson (2005). The lowest irrigation level for maintaining acceptable quality in Rudbeckia hirta was opposed to Henson (2005) results. In his report, the critical level of irrigation for acceptable visual scores were between 0% and 25% ET0. While, the essential level of irrigation for final percent cover was between 25% and 50% ET0. 3.10. Visual ratings Landscape impact, appearance impact and floral impact were meaningfully different at the treatment, species and species/treatment

Table 6 Pearson’s correlation coefficients (r) among morphological and some physiological traits of trail species under drought condition (75% ET0 to 25 ET0).

Callistephus chinensis Rudbeckia hirta Althea rosea Malva sylvestris

Plant biomass

Appearance impact

Floral impact

Landscape impact

Stomatal conductance

−.731 −.928** . −.857** −.894**

−.911 −.864** −.694* −.895**

−.884 −.916** −.830** −.735**

−.873 −.947** −.854** −.901**

−.892 −.947** −.887** −.900**

**

ΔTc

**

**

ΔTc = The difference between canopy temperature and air temperature; n.s = not significant. * P ≤ 0.05. ** P ≤ 0.01. 83

**

**

SLA ns

.40 .535 ns .996** .321ns

Growth index −.154 ns −.815** −.051 ns −.359 ns

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Fig. 1. Landscape impact rating of Callistephus chinensis, Rudbeckia hirta, Althaea rosea, Malva sylvestris at 25%, 50%, 75%, 100% ET0 irrigation treatment levels in September 2016. Different letters show significant differences at P ≤ 0.05.

Fig. 3. Floral impact rating of Callistephus chinensis, Rudbeckia hirta, Althaea rosea, Malva sylvestris at treatment levels (25%, 50%, 75%, 100% ET0) in September 2016. Treatments with different letters were significantly different at P ≤ 0.05.

scores or more (out of 5), for the lowest watering level of 25% ET0. The plant appearance findings indicated that 75% ET0 was a minimum irrigation level to maintain plant appearance in Callistephus chinensis and Malva sylvestris, whereas, this level was 50% ET0 for Rudbeckia hirta (See Fig. 4). Regarding the conducted investigations in Malva sylvestris and Althaea rosea, there are differences on trichome types and also on the length of the trichomes (Kim and Lee, 2005; Romitelli and Martins, 2013). Therefore, longer four types of trichomes of Althaea rosea had more important role in protection of the plant against drought stress compared to the shorter one type of the trichomes in Malva sylvestris. In spite of being native to Iran, Malva sylvestris had less ability to maintain landscape performance and adapt to conditions of limited available water than an exotic species as Rudbeckia hirta. Therefore, a native plant is not certainly the most drought resistant (Wade et al., 2010). Combining well adapted exotic with native species is as a way to create

Fig. 2. Leaf rolling index of Althaea rosea for irrigation treatments (25%, 50%, 75%, 100% ET0) averaged across three measurements in 2016. Treatments with different letters are significantly different at P ≤ 0.05.

levels. Landscape impact rating indicated the overall influence of plants on the landscape appearance from floral and foliage appearance to the appearance that pest or disease problems may bring. Through all the drought stress levels, Rudbeckia hirta and Althaea rosea had the greatest landscape impacts (See Fig. 1). Althaea rosea showed a significantly different leaf rolling under 50% and 25% irrigation levels (See Fig. 2). In presence of direct sunlight and once every two days irrigation frequency, Malva sylvestris’s leaf edges were burned, even under 100% ET0 irrigation treatment. Therefore, daily irrigation and growing in partly shade conditions would be important factors for increasing visual quality of Malva sylvestris. In our study, Althaea rosea and Callistephus chinensis took the highest and lowest floral impact, in 25% ET0 irrigation treatment respectively. Drought had detrimental effects on foliage and plant appearance of Malva sylvestris; however, there was no significant effect on its floral appearance across irrigation treatment levels from 75% to 25% ET0 (See Fig. 3). Only Althaea rosea maintained a plant appearance rating of three

Fig. 4. Appearance impact rating of species Callistephus chinensis, Rudbeckia hirta, Althaea rosea, Malva sylvestris and treatment levels (25%, 50%, 75%, 100% ET0) in September 2016. Treatments with different letters are significantly different at P ≤ 0.05. 84

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Fig. 5. Relationships between ash content and stomatal conductance under drought stress for the studied plant species.

a beautiful landscape with conserving water. Several factors contribute to drought tolerance of plants. For example, Rudbeckia hirta and Althaea rosea reduced their water loss by modifying their leaf shape or orientation (Lampinen et al., 2004), decreasing their leaf and shoot growth (Cameron et al., 1999) and closing their stomata (Panković et al., 1999).

Acknowledgements

4. Conclusion

References

Our results showed that drought tolerance of plants is associated with fundamental plant traits and there is a large variation on drought tolerance mechanism among the plant species. The higher thickness, decreased leaf area and trichomes are suitable indices for prospecting drought tolerance in species. Althaea rosea and Rudbeckia hirta use stomatal closure to fit as drought-tolerant plants. Althaea rosea and Rudbeckia hirta, compared to other plant species had higher WUEi under drought. It appears, Callistephus chinensis and especially Malva sylvestris are not proper species under minimal landscape irrigation due to witling and leaf firing. Responses of Rudbeckia hirta and Althaea rosea to drought stress can be attributed to their desiccation avoidance strategies. These strategies are achieved via decreasing water loss by trichomes, modifying shape and leaf orientation, reducing stomatal aperture and leaf area. These responses are more aesthetically acceptable than desiccation tolerance mechanisms such as wilting in landscape planting. All in all, considering propensity responses for drought tolerance in Rudbeckia hirta and Althaea rosea, these plant species appears to be appropriate plant candidates for water-wise landscaping. The results of this study showed that even though Malva sylvestris was a native species, it did not pass the test for urban landscaping with low water use. Rudbeckia hirta was not native but showed a great positive response to drought stress. Therefore, having a strategy to use both native and exotic drought resistant plants may better assist in achieving more sustainable low water use landscaping.

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