**4. Discussion and conclusion**

55%, *Acer pseudoplatanus* with 52.2%, Pyracantha *coccinea* with 52%, and *Ailanthus altissima* with

Not a big difference occurred in these results under -6 Bar water stress level. The species had similar ranks to the ranks under -4 Bar water stress level. Once again, the highest values in the comparison of germination percentages to the control group percentages were observed in *Pinus nigra* (76.1%), *Cupressus sempervirens* (58.6%), *Pinus brutia* (54.2%), and *Thuja orientalis* (50.1%). The rate was below 50% in all of the other species. Under -6 Bar water stress level, the rate was 39.4% for *Pinus sylvestris* and 22.3% for *Cedrus libani,* while other species went below 20%. *Pyracantha coccinea* had a rate of 19.9%; *Acer pseudoplatanus* had a rate of 13.2%; and *Sophora japonica* had a rate of 12.8%. The highest proportional fall under -6 Bar water stress level was observed in *Ailanthus altissima*, and germination percentage could only reach 2% compared

Under -8 Bar water stress, which is the highest water stress level, *Ailanthus altissima* and *Acer pseudoplatanus* did not germinate. Under these conditions, the changes in germination percen‐ tages compared with the control group are as follows 2.2% for *Sophora japonica*, 7.4% for *Cedrus libani*, 10.9% for *Pyracantha coccinea*, 16.9% for *Pinus sylvestris*, 22.2% for *Thuja orientalis*, and 31.5% for *Cupressus sempervirens*. Under the highest water stress level, which is -8 Bar, the highest germination percentages were observed in *Pinus nigra* (64.8%) and *Pinus brutia* (46.5%). The graph showing the reduction rates in the germination percentages of species due to

**Figure 2.** The reduction rates in the germination percentages of the species due to increasing water stress.

20.3%.

50 Water Stress in Plants

with the control group.

increasing water stress is given in **Figure 2**.

In today's modern life, it has been accepted that the presence of plants in cities is an indicator of their quality and inhabitability [29]. Plants reduce the air pollution and noise in their surrounding areas [30–35]. They also increase aesthetic value [36], have a good influence on psychology [37, 38], save energy [39, 40], prevent erosion [41], and decrease the speed of winds. Since they penetrate into soil with their roots, they prevent transportation of soil by precipi‐ tation and streams. They also protect wild life and hunting resources. Green and open areas surrounded by plants are important activity areas for both adults and children [42, 43]. In addition, indoor plants increase the productivity of people working in these places [44]. They relieve people psychologically and reduce stress and negative feelings [45–47].

Due to these functions of plants, a lot of issues such as plants' spread areas, [48–52], protection [53–59], production [60–64], tolerance against stress factors [65, 66], use in various areas [67– 69], genetic variability [70–72], relationships with environment and other living beings [73– 79], and raising awareness about them, as well as their legal aspects [80–82] have become main study areas. Therefore, a lot of studies have been conducted on these subjects.

In addition to these functions of plants, their contributions to the aesthetic aspect of the places they are in should be dwelt on under a separate title. Landscape practices for which various species and varieties are used have gained a distinct importance in the modern world. The desire to use diverse species has led to an intense use of plants outside their natural spread areas. The species which are not part of the natural flora of the region draw more attention when they are used for landscape practices and increase landscape quality. However, these practices also cause such plants to deal with ecological and climatic conditions which they are not used to. Therefore, maintenance and watering costs of such species are higher. However, global warming makes it necessary to have a reasonable and thrifty attitude in the use of water.

It is inevitable that climate change manifests its effects all around the world due to global warming. It is expected that the increase in temperature and changing precipitation will increase water problems. It is expected that there will be changes in the frequency and severity of droughts and floods throughout Europe, which may result in loss of lives and property [83]. Therefore, there is a need to determine species which are tolerant to water stress and use such species in both landscaping and forestation practices.

This study is an attempt to reveal the tolerances of certain plant species, which are commonly used for landscape practices, against drought stress. The results of the study indicate that increasing water stress reduced the germination percentage in all the species examined. Many previous studies have reported similar results for many other species so far. Sevik and Cetin [84] conducted a study to determine species' tolerances towards water stress and reported that the species which were most affected by water stress were *Sophora japonica*, *Ailanthus altissi‐ ma*, and *Cupressus arizonica,* whereas the species which were most tolerant against water stress were *Pinus nigra*, *Cupressus sempervirens*, and *Pinus brutia*.

Falusi et al. [85] analyzed the influence of increasing water stress on the germination percen‐ tages of four origins of *Pinus halepensis* and revealed that there were great differences between germination percentages of the origins. The origin, which was least affected by water stress, had a germination percentage of 94.10% in the control group, while this rate was 63.64% under -8 Bar water stress level. The origin, which was most affected by water stress, had a germination percentage of 90.1% in the control group, while this rate reduced to 11.8% under -8 Bar water stress level.

Tilki and Dirik [86] performed experiments on various origins of *Pinus brutia*. They reported that Silifke‐origin seeds had a germination percentage of 78.7% in the control group. This rate reduced to 44.8% under -0.4 MPa level. As for the Cehennemdere‐origin seeds, they showed a germination percentage of 33.2% in the control group while this rate reduced to 4% under -0.4 MPa level. In this study, germination percentage of *Pinus brutia* was 45.2% in the control group while this rate was 40% under -2 Bar, 36.3% under -4 Bar, and 24.5% under -6 Bar water stress levels.

Boydak et al. [87] conducted a study with *Pinus brutia* seeds of 6 different origins. The germination percentage was observed to be 84.3% in the control group. This rate changed to be 88.7% under -2 Bar. It dramatically reduced after -4 Bar water stress level. Under -4 Bar water stress level, the percentage was 80.6%, whereas it was 55.5% under -6 Bar water stress level. The germination percentage was 25.2% under -5 Bar water stress level and was 29.8% under -8 Bar water stress level, according to what they report.

Ahmadloo et al. [88] conducted a study on *Cupressus sempervirens* and *Cupressus arizonica* and created -2, -4, -6 and -8 Bar water stress levels. They analyzed germination percentages. It was seen that *Cupressus arizonica* yielded 18.75% germination percentage in the control group. This rate reduced to 14.5% under -2 Bar water stress level. It was found to be 10.5% under -4 Bar, 9.25% under -6 Bar, and 7% under -8 Bar water stress levels. Germination percentage of *Cupressus sempervirens* in the control group was 27.75%. Under -2 Bar water stress level, this rate reduced to 18.5%, 18% under -4 Bar, 11.75% under -6 Bar, and 7.5% under -8 Bar water stress levels. These results generally support the results of this study. In this study, germination percentage of *Cupressus sempervirens* L. was 62.6% in the control group. This rate reduced to 54.3% under -2 Bar, 45.8% under -4 Bar, 36.7% under -6 Bar, and 19.7% under -8 Bar, which is the highest water stress level. Therefore, it was seen that germination percentage reduced to 31.5% of the control group under -8 Bar water stress level. Ahmadloo et al. [88] reported this number as 27%.


germination percentages of the origins. The origin, which was least affected by water stress, had a germination percentage of 94.10% in the control group, while this rate was 63.64% under -8 Bar water stress level. The origin, which was most affected by water stress, had a germination percentage of 90.1% in the control group, while this rate reduced to 11.8% under -8 Bar water

Tilki and Dirik [86] performed experiments on various origins of *Pinus brutia*. They reported that Silifke‐origin seeds had a germination percentage of 78.7% in the control group. This rate reduced to 44.8% under -0.4 MPa level. As for the Cehennemdere‐origin seeds, they showed a germination percentage of 33.2% in the control group while this rate reduced to 4% under -0.4 MPa level. In this study, germination percentage of *Pinus brutia* was 45.2% in the control group while this rate was 40% under -2 Bar, 36.3% under -4 Bar, and 24.5% under -6 Bar water

Boydak et al. [87] conducted a study with *Pinus brutia* seeds of 6 different origins. The germination percentage was observed to be 84.3% in the control group. This rate changed to be 88.7% under -2 Bar. It dramatically reduced after -4 Bar water stress level. Under -4 Bar water stress level, the percentage was 80.6%, whereas it was 55.5% under -6 Bar water stress level. The germination percentage was 25.2% under -5 Bar water stress level and was 29.8%

Ahmadloo et al. [88] conducted a study on *Cupressus sempervirens* and *Cupressus arizonica* and created -2, -4, -6 and -8 Bar water stress levels. They analyzed germination percentages. It was seen that *Cupressus arizonica* yielded 18.75% germination percentage in the control group. This rate reduced to 14.5% under -2 Bar water stress level. It was found to be 10.5% under -4 Bar, 9.25% under -6 Bar, and 7% under -8 Bar water stress levels. Germination percentage of *Cupressus sempervirens* in the control group was 27.75%. Under -2 Bar water stress level, this rate reduced to 18.5%, 18% under -4 Bar, 11.75% under -6 Bar, and 7.5% under -8 Bar water stress levels. These results generally support the results of this study. In this study, germination percentage of *Cupressus sempervirens* L. was 62.6% in the control group. This rate reduced to 54.3% under -2 Bar, 45.8% under -4 Bar, 36.7% under -6 Bar, and 19.7% under -8 Bar, which is the highest water stress level. Therefore, it was seen that germination percentage reduced to 31.5% of the control group under -8 Bar water stress level. Ahmadloo et al. [88] reported this


under -8 Bar water stress level, according to what they report.

stress level.

52 Water Stress in Plants

stress levels.

number as 27%.

Boydak et al. [87] conducted a study with 6 different origins of *Pinus brutia* seeds. The average germination percentage of the seeds was 84.3% in the control group. The rate reduced to 25.2% under -8 Bar water stress level. That study also revealed that origins differed in their tolerances towards water stress. For instance, germination percentage in the 2nd origin was 91% in the control group, and this rate reduced to 40.5% under -8 Bar. In addition, the 5th origin had a germination percentage of 94.5% in the control group, and this rate reduced to 15% under -8 Bar water stress. Proportional values make the results clearer. The 2nd origin's proportional germination percentage was found to be 44.5% under -8 Bar water stress level, while this rate was found to be 15.8% for the 5th origin under the same conditions.

Kaufmann and Eckard [89] stated that water stress at a level of -8 Bar may reduce the germi‐ nation percentages of Pinus *contorta* and *Picea engelmannii* seeds at a rate of 50%. Djavanshir and Reid [90] evaluated germination percentages in *Pinus ponderosa* and *Pinus elderica*. They concluded that increasing water stress affected the germination percentage; and germination percentage reduced nearly to zero in *Pinus ponderosa* under -8 Bar and in *Pinus elderica* under -12 Bar water stress levels.

Semerci et al. [5] studied the influence of water stress on various *Pinus nigra* origins. They created -2, -4, and -6 Bar water stress levels. They analyzed the seeds' germination percentages based on the origins. They concluded that germination percentage varied between the origins, and increasing water stress led to a significant reduction in germination percentages. The study included seeds originating in Ballıköy in Tavsanlı. The germination percentage of the seeds in the control group was 98%. This rate reduced to 76% under -2 Bar, 52% under -4 Bar, and 16% under -6 Bar water stress levels. However, the seeds originating in Göksun B. Çamurlu had a germination percentage of 62% in the control group, while this rate was 58% for Andırın Akifiye‐origin seeds. The germination was at a rate of 1% under -2 Bar water stress level for these seeds, while no germination took place under -6 Bar water stress level.

The same study also reported that proportional germination percentages largely vary between the origins. For instance, proportional germination percentage in seeds originating in Tavsanlı Ballıköy was 77% under -2 Bar, 53% under -4 Bar, and 17% under -6 Bar water stress levels. Similarly, proportional germination percentage in seeds originating in Mengen Daren was 75% under -2 Bar, 63% under -4 Bar, and 16% under -6 Bar water stress levels. However, the same study revealed that the proportional germination percentage of seeds originating in Göksun B. Çamurlu and Andırın Akifiye was only 2% under -2 Bar water stress level. Topacoglu et al. [91] reported that *Pinus nigra* originating in Ankara Uluhan yielded 95.08% cumulative germination percentage under -8.0 Bar water stress level, while the seed originating in Isparta Tota yielded a cumulative germination percentage of 85.41%.

Buyurukçu [27] compared the tolerance of Anatolian pinus nigra (*Pinus nigra* Arnold. subsp. *pallasiana* (Lamb.) Holmboe) clones from clonal seed garden in Hanönü Günlüburun against the drought. In order to compare the clones against drought during the phase of germination, the seeds were subjected to -2, -4, -6, and -8 Bar water stress levels using PEG 6000 solution. All in all, it was seen at the end of ANOVA and Duncan's test results that clones had different responses of tolerance against water stress. Germination percentage, which was 48% on average in the control group, reduced to 16% under -2 Bar, 15% under -4 Bar, 2% under -6 Bar, and 0.4% under -8 Bar. However, what is remarkable in this study is the great differences between clones. For instance, the 17th clone had a germination percentage of 29% in the control group, while this rate reduced to 2% under -2 Bar water stress level. The seed did not germinate under the higher water stress levels. Similarly, the 9th clone had a germination percentage of 78% which reduced to 0% under -2 Bar water stress level. However, the 14th clone had a germination percentage of 49%, which reduced to 8% under -6 Bar water stress conditions and 5% under -8 water stress conditions. Taking into account the fact that these clones belong to the same stand of origin and are grown under the same conditions, it is possible to say that genetic structure may be prominent in determining the tolerance towards water stress.
