*3.1. Physical–Hydraulic Properties of Substrate*

Pressure heads of the experimental water retention curve of the substrate ranged between 0 and –100 cm providing important information concerning plant growth (Figure 2). In accordance to De Boodt and Verdonck [26], water retention in substrates with negative pressure heads greater than −100 cm decreases plant growth, while a negative pressure head that is less than −10 cm creates inadequate substrate aeration for plant growth. The main substrate hydraulic characteristics derived from the water retention curve are shown in Table 1.

**Table 1.** Hydraulic characteristics of the potted lavender plants' growth substrate Ps96:P<sup>4</sup> (Ps: pure sphagnum peat and P: perlite in a 96:4 volume ratio).


<sup>1</sup> water content at 0 cm pressure head (saturation); <sup>2</sup> air filled pores at <sup>−</sup>10 cm pressure head; <sup>3</sup> released amount of water between pressure heads of <sup>−</sup>10 and <sup>−</sup>50 cm; <sup>4</sup> released amount of water between pressure heads of −50 and −100 cm.

*Water* **2020**, *12*, x FOR PEER REVIEW 6 of 18

**Figure 2.** Water retention curve of the potted lavender plants' growth substrate Ps96:P4 (Ps: pure sphagnum peat and P: perlite in a 96:4 volume ratio). **Figure 2.** Water retention curve of the potted lavender plants' growth substrate Ps96:P<sup>4</sup> (Ps: pure sphagnum peat and P: perlite in a 96:4 volume ratio).

**Table 1.** Hydraulic characteristics of the potted lavender plants' growth substrate Ps96:P4 (Ps: pure sphagnum peat and P: perlite in a 96:4 volume ratio). **Total Porosity 1 Airspace 2 Water Content at Water Content Easily Available Water 3 Water Buffering Capacity 4**  With an aim to retain the substrate water content in the easily available water range, i.e., water content between 0.76 and 0.46 cm<sup>3</sup> cm−<sup>3</sup> (at −10 and −50 cm pressure head, respectively) (Table 1, Figure 2), when the TDR reading reached a volumetric water content of 46%, plants were irrigated with the corresponding NaCl treatments at amounts equal to the amount lost from evapotranspiration.

**(cm3 cm−3) (cm3 cm−3) −10 cm (cm3 cm−3) at −50 cm (cm3 cm−3) (cm3 cm−3) (cm3 cm−3)**  0.88 0.12 0.76 0.46 0.30 0.13 1 water content at 0 cm pressure head (saturation); 2 air filled pores at −10 cm pressure head; 3 released amount of water between pressure heads of −10 and −50 cm; 4 released amount of water between pressure heads of −50 and −100 cm. Particle size distribution affects the aeration and water retention properties of substrates [27,28]. Therefore, the above retention curve was defined in part by the substrate particle size distribution that was characterized by a relatively small percentage (≈11%) of particles >8 mm, a large percentage of particles (≈61%) in the range of 8-1 mm and a moderate percentage (≈28%) of particles < 1 mm (Table 2).


With an aim to retain the substrate water content in the easily available water range, i.e., water content between 0.76 and 0.46 cm3 cm−3 (at −10 and −50 cm pressure head, respectively) (Table 1, Fig. **Table 2.** Particle size distribution of the potted lavender plants' growth substrate Ps96:P<sup>4</sup> (Ps: pure sphagnum peat and P: perlite in a 96:4 volume ratio).

#### *3.2. Symptoms Induced by Salinity*

10–8 6.70 8–4 18.19 4–2 20.79 2–1 21.85 Indicator of the effect of salinity are the visible symptoms of the induced damage in plants (such as leaf chlorosis, browning and necrosis) that affects plant ornamental value [29]; an important consideration in floriculture [30]. Throughout the experiment, there was no plant mortality. At the high concentrations of 100 and 200 mM NaCl, all species demonstrated signs of salinity stress expressed initially with chlorosis of the leaves located at the middle and base of the plants followed by gradual necrosis of leaves and stems; at the end of the experiment (day 56), all lavender species irrigated with

20–16 1.18 16–10 2.09 200 mM NaCl had leaf necrosis >75% (Table 3). When irrigated with 100 mM NaCl, symptoms of leaf necrosis were more intense in both *Lavandula angustifolia* and *L. stoechas* than in both *L. dentata* var. *dentata* and *L. dentata* var. *candicans*. *L. stoechas* was the first species to demonstrate salinity stress (approximately 28 days since the start of the experiment), followed a week later by *L. angustifolia* and a further week later by both *L. dentata* var. *candicans* and *L. dentata* var. *dentata* (data not shown). On the other hand, all species irrigated with 50 mM NaCl demonstrated moderate signs of salinity stress at completion of the experiment expressed with chlorosis of the leaves (25%–50%) located at the middle and base of the plants; these symptoms reduce the commercial value of the plants. Furthermore, at 25 mM NaCl leaf chlorosis was inconspicuous in all species. Additionally, during the experiment it was noted that only the control of *L. dentata* var. *dentata* demonstrated mild signs of chlorosis suggesting nutrient deficiency; generally, *L. dentata* var. *dentata* developed greater growth (see Section 2.2) in comparison to the other species, therefore, is possible to have had a greater demand for nutrients.


**Table 3.** Assessment of visual symptoms of potted lavender plants induced by salinity based on a 6-point scale (0–5).

where, 0: plant mortality, 1: no leaf injury, 2: mild leaf chlorosis <25%, 3: moderate leaf chlorosis 25%–50% approximately, 4: leaf necrosis 50%-75% approximately, 5: leaf necrosis >75%.

Moderate symptoms of salinity at 50 mM suggest the need for further research to establish the effect of salinity in the range of 25–50 mM NaCl. The use of good quality water between saline water irrigations for strategic leaching of salts could contribute to reducing or delaying the effect of salinity [31]. To further reduce the use of good quality water natural resources for irrigation further research, studying the possibility of strategic leaching of salts with good quality water between saline water irrigations (>50 mM) is also suggested. Overall symptoms were more apparent in *L. dentata* var. *dentata* and *L. stoechas* due to the natural green color foliage of the particular species compared to the natural grey-green foliage of the other two species. Concerning *L. angustifolia* the above symptoms of plants irrigated with 100 and 200 mM NaCl (10.6 dSm−<sup>1</sup> and 20.7 dSm−<sup>1</sup> , respectively) were obtained after 56 days of irrigation leading to plant death if the experiment was prolonged further and were similar to the results obtained by Niu and Rodriguez [32] for *L. angustifolia* pot grown plants in peat-perlite based substrate and under greenhouse conditions after 11 weeks of irrigation with NaCl solutions; plants irrigated with 0.8 dS m−<sup>1</sup> showed good quality with acceptable reduction growth and little leaf injury, whereas plants irrigated with 3.2 dS m−<sup>1</sup> showed stunted growth and moderate leaf injury (25%–50%), while plants irrigated with either 6.4 dS m−<sup>1</sup> or 12.0 dS m−<sup>1</sup> died.
