*3.3. Plant Growth*

With the exception of measurements taken near completion of the experiment (42 and 49 days) in all four species, two-way ANOVA for data concerning the various plant growth variables measured throughout the duration of the experiment showed no significant interactions of the main experimental factors that are between species and irrigated NaCl solutions. In most measured plant variables (plant height, shoot canopy diameter, growth index, ΦPSIIo, inflorescence number) there was a significant effect of the main factors from the beginning of the experiment until day 42. Significant interactions were shown on day 49 and 56 for plant height, day 35 and 56 for ΦPSIIo and at the end of the experiment (day 56) for leaf thickness, inflorescence number and length, peduncle length and root dry weight.

Overall results confirm that salinity stress is initially expressed in the above ground growth parameter of plants [33]. More specifically, plant height was significantly reduced at 200 mM NaCl from day 21 onwards and until day 42 (Table 4). Overall *L. angustifolia* showed the smallest height while *L.* dentata var. *dentata* showed the greatest plant height (*p* < 0.05) between day 7 and 42. Both *L. stoechas* and *L. dentata* var. *candicans* showed similar development in plant height between them that was intermediate of the other two species (*p* < 0.05). Following, near completion of the experiment (days 49 and 56) *L. angustifolia* continued to show the least plant height compared to the other species. On both day 49 and day 56, plant height at 200 mM NaCl was significantly reduced compared to the control in *L. stoechas* and *L. dentata* var. *candicans.* Furthermore, on day 56, plant height at both 100 and 200 mM NaCl was significantly reduced compared to the control in *L. dentata* var. *dentata.* Our results agree with previous research findings that showed salinity induced the decrease in plant height of pot grown *Arbutus unedo* in peat, sand and clay-loam based substrate and irrigated for 16 weeks with 52 mM and 105 mM [34].

Similar results to plant height were obtained for shoot canopy diameter. Shoot canopy diameter was reduced significantly at both 100 and 200 mM NaCl compared to the control from day 49 of the experiment onwards (Table 5). Overall *L. angustifolia* showed the smallest shoot canopy diameter while *L. dentata var. dentata* the greatest one throughout the duration of the experiment (*p* < 0.05). *L. dentata* var. *candicans* showed a similar development in shoot canopy diameter with *L. dentata var. dentata* until day 35; afterwards, it showed a similar shoot canopy development with *L. stoechas* that was between that of the other two species (*p* < 0.05). Results were also consistent for the plant growth index. Plant growth index was reduced significantly at 200 mM NaCl compared to the control from day 35 of the experiment onwards (Table 6). Overall *L. angustifolia* showed the smallest growth index while *L. dentata var. dentata* the greatest one throughout the duration of the experiment (*p* < 0.05). *L. dentata* var. *candicans* showed a similar growth index with *L. dentata var. dentata* on day 7. Afterwards, it showed a similar growth index with *L. stoechas* that was between that of the other two species (*p* < 0.05). Similarly, in our results, the canopy and biomass of both *Cistus albidus* and *C. monspeliensis* grown in pots containing peat-pelite based substrate were reduced after 4 months irrigation with 70 and 140 mM NaCl [35]. Also, the canopy and biomass of potted *Asteriscus maritimus* in peat and sand based substrate were reduced after 150 days of irrigation with 70 and 140 mM NaCl [33].

A decrease in shoot dry weight is also an initial effect of reduced growth due to salinity [31]. Shoot dry weight was significantly reduced compared to the control at both 100 and 200 mM NaCl (Table 7). In a similar experiment, the total plant dry weight of *L. multifida* decreased when irrigated with 100 mM and 200 mM NaCl [20,24]. Both *L. dentata* var. *dentata* and *L. dentata* var. *candicans* developed similar shoot dry weights that were significantly greater than the other two species. Furthermore *L. angustifolia* developed the smallest shoot dry weight amongst the studied species (*p* < 0.05). It is possible that there is the presence of some interspecies variation, as the salinity shoot dry weight decrease of the corresponding control at 200 mM was the smallest for *L. angustifolia* (3%), followed in ascending order by *L. dentata* var. *candicans* (18%), *L. dentata* var. *dentata* (27%) and *L. stoechas* (32%). Although *L. angustifolia* (3%) showed the least decrease in shoot dry weight from the corresponding control, the appearance of the plant at the end of the experiment in comparison to the other species exhibited a large amount of leaf and stem necrosis at both 100 mM and 200 mM NaCl (see Section 2.1). The above results agree with the decrease in shoot dry weight due to irrigation with NaCl solutions in other pot grown ornamental species under greenhouse conditions such as *Nerium oleander* [36], *Achillea millefolium*, *Agastache cana*, *Gaillardia aristata* [37], and *Rosmarinus o*ffi*cinalis* [38]. On the other hand, although root dry weight in all species increased at low NaCl concentrations followed by a decrease at 100 and 200 mM NaCl, root dry weights compared to the control were not significantly different. As there were non-significant differences between the control and various NaCl solutions, it is possible that *L. dentata* var. *candicans* developed the greatest root dry weight, while the other species developed similar root dry weights (*p* < 0.05) due to interspecies variation.




and mean values are not shown.



**Table 5.** The effect of irrigation with different NaCl solutions on shoot canopy diameter (cm) of *Lavandula* species (n = 6, *p* < 0.05). Differences between means ± S.E.

ns: non-significant; \* denotes significant differences between means at *p* < 0.05, shown with different letters within columns.





**Table 7.** The effect of irrigation with different NaCl solutions on shoot and root dry weights (g) of *Lavandula* species (n = 6, *p* < 0.05). Differences between means ± S.E. shown with different letters (Tukey HSD, *p* < 0.05).

ns: non-significant; \* denotes significant differences between means at *p* < 0.05, shown with different letters within columns. †When interactions are significant, factors are not considered, and mean values are not shown.

Leaf thickening is a common response to salinity [31,39], whereby salt content in the leaves is diluted by increased succulence [40]. Leaf thickness increased significantly compared to the control only in *L. angustifolia* at 50 mL NaCl (Figure 3); however, non-significant leaf thickness increase compared to the control was generally observed at concentrations >50 mM NaCl in the remaining lavender species. It is possible over the 56 days, NaCl levels, especially in the higher concentrations (100 and 200 mM NaCl), exceeded the threshold of long-term acclimation mechanisms related to leaf thickness. Further research with smaller NaCl levels is necessary to study the effect of salinity on leaf thickness.

are not shown.

*L. dentata* var. *candicans* x 0 mM (control) ns 13.887±0.501ab

*L. dentata* var. *dentata* x 0 mM (control) ns 5.315±0.501cd

ns: non-significant; \* denotes significant differences between means at P < 0.05, shown with different letters within columns. †When interactions are significant, factors are not considered, and mean values

Leaf thickening is a common response to salinity [31,39], whereby salt content in the leaves is diluted by increased succulence [40]. Leaf thickness increased significantly compared to the control only in *L. angustifolia* at 50 ml NaCl (Figure 3); however, non-significant leaf thickness increase compared to the control was generally observed at concentrations >50 mM NaCl in the remaining lavender species. It is possible over the 56 days, NaCl levels, especially in the higher concentrations (100 and 200 mM NaCl), exceeded the threshold of long-term acclimation mechanisms related to leaf

 25 mM ns 14.847±0.501ab 50 mM ns 15.180±0.501a 100 mM ns 15.422±0.501a 200 mM ns 12.277±0.501b

25 mM ns 4.800±0.501cd 50 mM ns 5.533±0.501cd 100 mM ns 4.667±0.501cd 200 mM ns 4.703±0.501cd **Fspecies/sig.** 156.577/ 0.000\* 415.648/ 0.000 **FΝaCl/sig.** 10.444/ 0.000\* 6.470/ 0.000 **Finteraction/sig.** 1.312/ 0.223 2.396/ 0.009\*

**Figure 3.** The combined effect of the interaction between lavender species and NaCl solution irrigation treatments (0, 25, 50, 100, 200 mM) on leaf thickness of plants (n= 6, *P* < 0.05). Differences between means ± S.E. shown with different letters (Tukey HSD, *P* < 0.05). Note c: control. **Figure 3.** The combined effect of the interaction between lavender species and NaCl solution irrigation treatments (0, 25, 50, 100, 200 mM) on leaf thickness of plants (n = 6, *p* < 0.05). Differences between means ± S.E. shown with different letters (Tukey HSD, *p* < 0.05). Note c: control.

A secondary effect of reduced growth induced by salinity includes the potential reduction in photosynthesis of plant leaves [16]. Concerning ΦPSIIo, *L. dentata* var. *dentata* showed the least ΦPSIIo compared to the other species (*P* < 0.05) on day 14 (Table 8), suggesting the presence of interspecies variation in plant nutrient demand, as there were non-significant differences between the control and the different NaCl solutions applied and the fact that *L. dentata* var. *dentata* ΦPSIIo values in the control (0.77) were relatively less than optimum (circa 0.83) [41]. With regards to the latter, *L. dentata* var. *dentata* also showed mild signs of chlorosis in the control, suggesting a greater demand for nutrients in comparison to the other species (see Section 2.1). Following this, ΦPSIIo significantly decreased in plants irrigated with 200 mM NaCl compared to the control on day 35 only in *L. angustifolia*, and on A secondary effect of reduced growth induced by salinity includes the potential reduction in photosynthesis of plant leaves [16]. Concerning ΦPSIIo, *L. dentata* var. *dentata* showed the least ΦPSIIo compared to the other species (*p* < 0.05) on day 14 (Table 8), suggesting the presence of interspecies variation in plant nutrient demand, as there were non-significant differences between the control and the different NaCl solutions applied and the fact that *L. dentata* var. *dentata* ΦPSIIo values in the control (0.77) were relatively less than optimum (circa 0.83) [41]. With regards to the latter, *L. dentata* var. *dentata* also showed mild signs of chlorosis in the control, suggesting a greater demand for nutrients in comparison to the other species (see Section 2.1). Following this, ΦPSIIo significantly decreased in plants irrigated with 200 mM NaCl compared to the control on day 35 only in *L. angustifolia*, and on day 56 in both *L. angustifolia* and *L. dentata* var. *candicans*. The ΦPSIIo values for all species irrigated with 200 NaCl was lower than the optimum ΦPSIIo value for most plant species (circa 0.83), indicating the exposure of plants to stress, in this study salt stress [41]. Although no significant differences were shown, the remaining ΦPSIIo values of plants irrigated with either 100 mM or 200 mM were also less than the optimum ΦPSIIo in all species. The decrease in ΦPSIIo suggests the presence of salt stress that led to chlorosis and premature senescence of mature leaves [42]. These results agree with the symptoms or leaf chlorosis and necrosis observed on the plants irrigated with 100 and 200 mM NaCl and also with Munns' [43] findings that high potassium concentrations cause premature senescence, chlorosis, and necrosis in leaves due to the disrupt of plant protein synthesis.

Plants subjected to salt stress could reduce the inflorescence number, influence flowering time (speed or delay) and duration, reduce inflorescence and reduce peduncle length. Throughout the duration of the experiment, only two lavender species flowered (*L. stoechas* and *L. dentata* var. *dentata*). The experiment took place in late winter–early spring (Feb-March) and as the control of both non-flowered lavenders (*L. angustifolia* and *Lavandula dentata* var. *candicans*) did not produce inflorescences, it is possible that the season in which the experiment took place was too early for these species to flower. It is reported that *L. dentata* var. *dentata* flower all year round in mild-winter areas such as Athens [44] and *L. stoechas* flowers in early spring [45], whereas *L. dentata* var. *candicans* from early spring to late fall and *L. angustifolia* flowers from early to midsummer [44]. Anthesis in both *L. stoechas* and *L. dentata* var. *dentata* started earlier at 100 and 200 mM NaCl compared to the control (Figure 4). At the end of the experiment (day 56), the inflorescence number for the two lavender species that flowered was reduced significantly at 100 and 200 mM in comparison to the control only for *L. stoechas*, suggesting a tendency for earlier inflorescence death under salt stress. Similarly, the inflorescence length was reduced significantly at 100 and 200 mM only for *L. stoechas*. On the other hand, the inflorescence peduncle was reduced significantly compared to the control in

all NaCl concentrations for both flowered *L. stoechas* and *L. dentata* var. *dentata*. The above results agree with the findings of several authors that have studied the effect of salinity on various ornamental plant species. García-Caparrós and Lao [46] state that salinity could bring forward and shorten the duration of anthesis. Salinity has been reported to decrease the inflorescence number in gerbera [47,48], different cultivars of *Rosa* x *hybrida* L. [48,49], *Matricaria chamomilla* [50], inflorescence length in *Eustoma grandiflorum* [18] as well as the peduncle length / stem length (measured from the basis of the plant to the first flower) in *Dianthus caryophyllus*, *Gerbera jamesonii* L [47], *Eustoma grandiflorum* [18], *Rosa hybrida* 'Kardinal' [51], and *Matricaria chamomilla* [50]. Results also showed that *Lavandula stoechas* developed significantly more inflorescences than *L. dentata* var. *dentata* on both day 42 and 49, which is possibly due to interspecies differences, as there were no differences between the control and applied NaCl solutions.


**Table 8.** The effect of irrigation with different NaCl solutions on leaf ΦPSIIo of *Lavandula* species (n = 6, *p* < 0.05). Differences between means ± S.E. shown with different letters (Tukey HSD, *p* < 0.05).

ns: non-significant; \* denotes significant differences between means at *p* < 0.05, shown with different letters within columns. † When interactions are significant, factors are not considered, and mean values are not shown.

and applied NaCl solutions.

Ld: *L. dentata* var. *dentata*. Bars represent S.E.

Plants subjected to salt stress could reduce the inflorescence number, influence flowering time (speed or delay) and duration, reduce inflorescence and reduce peduncle length. Throughout the duration of the experiment, only two lavender species flowered (*L. stoechas* and *L. dentata* var. *dentata*). The experiment took place in late winter–early spring (Feb-March) and as the control of both nonflowered lavenders (*L. angustifolia* and *Lavandula dentata* var. *candicans*) did not produce inflorescences, it is possible that the season in which the experiment took place was too early for these species to flower. It is reported that *L. dentata* var. *dentata* flower all year round in mild-winter areas such as Athens [44] and *L. stoechas* flowers in early spring [45], whereas *L. dentata* var. *candicans* from early spring to late fall and *L. angustifolia* flowers from early to midsummer [44]. Anthesis in both *L. stoechas* and *L. dentata* var. *dentata* started earlier at 100 and 200 mM NaCl compared to the control (Figure 4). At the end of the experiment (day 56), the inflorescence number for the two lavender species that flowered was reduced significantly at 100 and 200 mM in comparison to the control only for *L. stoechas*, suggesting a tendency for earlier inflorescence death under salt stress. Similarly, the inflorescence length was reduced significantly at 100 and 200 mM only for *L. stoechas*. On the other hand, the inflorescence peduncle was reduced significantly compared to the control in all NaCl concentrations for both flowered *L. stoechas* and *L. dentata* var. *dentata*. The above results agree with the findings of several authors that have studied the effect of salinity on various ornamental plant species. García-Caparrós and Lao [46] state that salinity could bring forward and shorten the duration of anthesis. Salinity has been reported to decrease the inflorescence number in gerbera [47,48], different cultivars of *Rosa* x *hybrida* L. [48,49], *Matricaria chamomilla* [50], inflorescence length in *Eustoma grandiflorum* [18] as well as the peduncle length / stem length (measured from the basis of the plant to the first flower) in *Dianthus caryophyllus*, *Gerbera jamesonii* L [47], *Eustoma grandiflorum* [18], *Rosa hybrida* 'Kardinal' [51], and *Matricaria chamomilla* [50]. Results also showed that *Lavandula* 

**Figure 4.** The combined effect of the interaction between lavender species and NaCl solution irrigation treatments (0, 25, 50, 100, 200 mM) on plant inflorescence number (n= 6, *P* < 0.05). Note: Ls: *L. stoechas*, **Figure 4.** The combined effect of the interaction between lavender species and NaCl solution irrigation treatments (0, 25, 50, 100, 200 mM) on plant inflorescence number (n = 6, *p* < 0.05). Note: Ls: *L. stoechas*, Ld: *L. dentata* var. *dentata*. Bars represent S.E.

Considering all of the above, the overall performance of the studied lavender species under the effect of salinity was satisfactory at levels <25 mM NaCl in all species. Further research is necessary to establish the effect of salinity between 25–50 mM NaCl or possibly >50 mM in combination with the interchanged use of irrigation with good quality water with the aim to contribute towards the conservation of good quality water natural resources. The adverse effects induced by high levels of NaCl (>100 mM) amongst lavender species in ascending order was *Lavandula dentata* var. *dentata*, *L. dentata* var. *candicans*, *L. stoechas* and *L. angustifolia*. Therefore, in areas with saline irrigation water, Considering all of the above, the overall performance of the studied lavender species under the effect of salinity was satisfactory at levels <25 mM NaCl in all species. Further research is necessary to establish the effect of salinity between 25–50 mM NaCl or possibly >50 mM in combination with the interchanged use of irrigation with good quality water with the aim to contribute towards the conservation of good quality water natural resources. The adverse effects induced by high levels of NaCl (>100 mM) amongst lavender species in ascending order was *Lavandula dentata* var. *dentata*, *L. dentata* var. *candicans*, *L. stoechas* and *L. angustifolia*. Therefore, in areas with saline irrigation water, the use of the better performed in the current study under saline irrigation conditions *Lavandula dentata* var. *dentata* and *L. dentata* var. *candicans* is proposed.

#### **4. Conclusions**

The study of the effect of saline irrigation in floriculture is important to consider for producing nursery crops without signs of salinity injury. Amongst other factors, the level of salinity stress induced on plants is dependent on plant species and varieties. The effect of salinity through irrigation on the growth of four lavender species was determined. The applied irrigation method allowed plants to receive the adequate amount of water for plant growth (easily available water), ensuring the effect of irrigation was induced by water quality i.e., applied NaCl solutions.

The effect of salinity was initially expressed in most of the above ground growth variables of the plants studied. Generally, growth was satisfactory in all species irrigated with <25 mM NaCl. Symptoms of salinity injury were moderate at 50 mM NaCl, affecting the commercial value of the ornamental species. At high NaCl levels (100 and 200 mM), plants showed severe symptoms of salt stress that included leaf and stem necrosis. Only two lavender species flowered, possibly due to season variation between species. Anthesis time was quicker and anthesis duration was reduced for plants irrigated with high saline concentrations.

Overall results of the effect of salinity were consistent, allowing to rank species in descending order of plant development as follows: *Lavandula dentata* var. *dentata*, *L. dentata* var. *candicans*, *L. stoechas* and *L. angustifolia*. Throughout the duration of the experiment, both *Lavandula dentata* var. *dentata* and *L. dentata* var. *candicans* showed better growth, and hence are suggested for areas with poor water quality using saline water for irrigation.

**Author Contributions:** Conceptualization, A.T.P.; methodology, A.T.P., P.A.L. and G.L.; formal analysis, A.T.P. A.K.K. and K.B.; investigation, A.K.K. and A.T.P.; writing—original draft preparation, A.T.P.; writing—review and

editing, A.T.P., P.A.L., G.L. and K.B.; supervision, A.T.P., P.A.L. and G.L.; project administration, A.T.P. A.K.K. and K.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** We would like to thank Kalantzis Plants for the free supply of lavender plants.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


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