Response of Common Ice Plant (Mesembryanthemum crystallinum L.) to Sodium Chloride Concentration in Hydroponic Nutrient Solution (2024)

Plant material.

Seeds of ice plant (Baker Creek Heirloom Seeds, Mansfield, MO) were started in 2.5-cm rockwool cubes that were pre-soaked with 21N–2.2P–16.5K Jack’s All Purpose Fertilizer (JR Peters, Allentown, PA) at a concentration of 150 mg·L⁻¹ N and placed in trays (Fig. 1). For crop cycles when seeds were germinated in February and March, the plug trays were placed on a heat mat maintaining a 20 °C rockwool temperature. All seed trays were fertilized daily with the aforementioned fertilizer solution and raised in a controlled greenhouse environment with 19.8 ± 0.5 °C (mean ± sd) day temperature and 19.0 ± 0.6 °C (mean ± sd) night temperature. A photoperiod of 20 h·d⁻¹ and supplemental lighting were supplied by high-pressure sodium fixtures set to provide 55 μmol·m⁻²·s⁻¹ at the plant canopy. Germination generally occurred 2–3 d after sowing, and seedlings were thinned to one per rockwool cube 3 weeks after sowing. The entire germination stage took 4 weeks. The seedlings were then selected for uniformity (Fig. 2) and transplanted to the experimental setting.

Experimental setting.

Mini-pond hydroponic systems were created using 4-L containers filled with nutrient solution (Fig. 3). A 2.5-cm hole was drilled on the cover of each container to fit the rockwool cube. Each single seedling was transplanted into one container with the bottom of the rockwool cube and plant roots touching the nutrient solution. Air pumps supplied air to the nutrient solution through tubing connected to air stones placed into the container to maintain saturated dissolved oxygen. All plants were grown in controlled greenhouse environment with 22.5 ± 0.7 °C (mean ± sd) day temperature, 21.2 ± 0.8 °C (mean ± sd) night temperature, and a 14-h photoperiod. High-pressure sodium lights provided 88.9 ± 9.8 μmol·m⁻²·s⁻¹ (mean ± sd) as measured at plant canopy. Supplemental light was on from 0600 to 2000 hr daily, which provided a supplemental daily light integral of 4.48 mol·m⁻²·d⁻¹.

The hydroponic nutrient solution was made by combining equal parts (0.75 g·L⁻¹ each) of 5N–5.2P–21.6K Jack’s Professional Water-Soluble Fertilizer (J. R. Peter’s) and 15.5N–0P–0K YaraLiva Calcinit (Yara International, Oslo, Norway) following the lettuce recipe by Mattson and Peters (2014). This nutrient recipe provided 150 mg·L⁻¹ nitrogen (N), 39 mg·L⁻¹ phosphorus (P), 162 mg·L⁻¹ potassium (K), 139 mg·L⁻¹ calcium (Ca), 47 mg·L⁻¹ magnesium (Mg), 62 mg·L⁻¹ sulfur (S), 2.3 mg·L⁻¹ iron (Fe), 0.38 mg·L⁻¹ manganese (Mn), 0.11 mg·L⁻¹ zinc (Zn), 0.38 mg·L⁻¹ boron (B), 0.113 mg·L⁻¹ copper (Cu), and 0.075 mg·L⁻¹ molybdenum (Mo). The pH of the nutrient solution was adjusted every 3 d to the range of 5.6–6.0, using 1 M potassium hydroxide (KOH) and 1 M sulfuric acid (H₂SO₄). After 1 week for plant establishment (7 d after transplanting into the hydroponic containers), the base nutrient solution was replaced, and five different concentrations of NaCl were added to the base nutrient solution (0, 0.05, 0.10, 0.20, and 0.40 M NaCl). Nine plants were randomly assigned to each NaCl treatment, and therefore a total of 45 plants were used for each crop cycle, with a total of three replicate crop cycles in the experiment. The nutrient solution plus corresponding NaCl was replaced every week (at day 14 and 21 after transplanting) to maintain electrical conductivity (EC). The EC was measured every 3 d and after each replacement of the nutrient solution and averaged 1.8 ± 0.1, 7.5 ± 0.1, 13.1 ± 0.3, 23.4 ± 0.3, and 42.3 ± 0.2 dS·m⁻¹ (mean ± sd) for the 0, 0.05, 0.10, 0.20, and 0.40 M NaCl treatments, respectively.

Measurements.

Three plants from each treatment (a total of 15 plants at each sample date) were harvested at day 7, 14, and 21 after NaCl treatment. At each harvest, the following measurements were taken: fresh weight (FW) and DW (following 72 h in an oven at 70 °C) of shoot (stem and leaf), leaf, root, and whole plant; number of leaves on main stem; and leaf surface area with a leaf surface area meter (LI-3100; LI-COR, Lincoln, NE). Shoots from the three plants per treatment per harvest date were pooled together for mineral nutrient tissue analysis at the Cornell Nutrient Analysis Laboratory (Ithaca, NY). EBCs were observed under ×16 magnification (field of view or field size is equal to 1.44 mm). A random plant was selected from each treatment. Three leaves were removed from each plant at separate locations (bottom, middle, and upper). These representative aliquots were sampled for qualitative and visual data. The experiment was replicated over time for a total of three times.

Experimental design and statistical analysis.

The experiment was designed as a randomized complete block design. The above methods were replicated three times, and these crop cycles were treated as different blocks. Within each block, the experimental unit was one plant in a 4-L hydroponic container. Within each crop cycle, containers were randomly placed on the benches in the greenhouse. There were nine experimental units for each of the five NaCl treatments. Three experimental units from each NaCl treatment were randomly selected for destructive harvest at three time points (day 7, 14, and 21 after treatment). The data were analyzed using JMP 14.0 software (SAS Institute, Cary, NC). Analysis of variance and Tukey’s honestly significance difference test were used to determine differences among NaCl treatments for each harvest date. Block effect was considered in the analysis. Data represent block centered means (± se) which removed the block effect and more accurately reflected the difference among treatments.

Fresh weight.

In the first week of NaCl treatment, plants treated with lower NaCl concentrations (0.05 and 0.10 M) did not differ from the control, but plants treated with higher NaCl concentrations (0.20 and 0.40 M) started to show stunting effects of NaCl in terms of total FW, shoot FW, and root FW (Table 1). After 2 weeks of NaCl treatment, plants treated with 0.05 M NaCl exhibited the greatest shoot FW and therefore the greatest total FW. The control (0 M), 0.10, and 0.2 M NaCl groups had shoot FW smaller than the 0.05 M group but greater than the 0.40 M NaCl group. A similar pattern was observed at week 2 for total FW. Root FW for all treatments exhibited the pattern whereby increasing NaCl level in the nutrient solution decreased the plant root FW. After 3 weeks of NaCl treatment, the 0.05 M NaCl group exhibited greater shoot and total FW than the other groups, although it had only half of the root FW of the control. The 0.10 M NaCl group was ranked second and better than the control in terms of shoot and total FW. Similarly, it had much less root FW than the control but higher shoot FW, which resulted in higher total FW. In general, plants treated with lower NaCl concentrations (0.05 and 0.10 M NaCl) sacrificed some root weight but accumulated even more shoot weight. The 0.20 M NaCl group exhibited similar shoot and total FW as the control but lower root FW. The 0.40 M NaCl group exhibited the poorest result, showing root FW only one-tenth that of the control. From the timeline perspective, all groups showed highly significant (P < 0.001) linear growth in total FW, shoot FW, and root FW (Table 1).

Dry weight.

DW generally showed a similar trend as FW with a few notable differences. DW was less affected than FW by NaCl at lower concentrations (0–0.1 M). In other words, plants treated with 0.05 and 0.10 M NaCl accumulated water at a higher rate than accumulating dry mass. This was partially evidenced by the leaf water content result (Table 3). Second, the stimulating effect in terms of DW for the 0.05 M treatment did not appear until the last week. In other words, the outperformance of the 0.05 M NaCl group in terms of FW in the second week was mostly due to the accumulation of extra water. Third, the stunting effect of 0.40 M NaCl on DW was not as large as that on FW. This was also evidenced by the lowest leaf water content of this group (Table 3). Last, as NaCl level increased, the degree of decreased root DW was not as large as the degree of decreased root FW. In other words, when treated with higher NaCl levels, the reduction in water content was more dramatic than the reduction in dry matter content. From the timeline perspective, all plants showed highly significant (P < 0.001) linear growth in total, shoot, and root DW (Table 2).

Shoot/root ratio.

Shoot/root ratio was higher with increased NaCl level. This trend gradually appeared from week 1 to week 3 (Table 2). However, the 0.40 M NaCl led to an increase in shoot/root ratio even by the first harvest due to poorer root performance than shoot performance. After the first week, both the control group and the 0.40 M NaCl group had relatively constant shoot/root ratio over time. In other words, they both gained shoot and root weights at the same rate, but this rate for the 0.40 M NaCl group was much lower than that for the control group. The other groups started with shoot/root ratios close to that of the control and over time had increased shoot/root ratios that were eventually close to that of the 0.40 M group. In other words, for these groups, the rate of accumulating shoot weight is higher than the rate of accumulating root weight.

Leaf number on the main stem, leaf fresh weight, leaf dry weight, and specific leaf area.

Plants treated with higher NaCl concentrations developed fewer but thicker leaves (i.e., lower specific leaf area) (Tables 3 and 4). Visually, plants treated with higher NaCl concentrations had brighter green color (Fig. 4). Leaf FW showed a similar trend as shoot and total FW. The 0.05 M NaCl group started to stand out after 2 weeks of NaCl treatment and largely outperformed the control after 3 weeks of NaCl treatment. The 0.10 M NaCl group did not differ from the control group in the first 2 weeks of NaCl treatment but had greater leaf FW than control in the last. In terms of leaf FW and DW, the 0.40 M NaCl group was stunted in the first week of NaCl treatment, and this stunting effect became more obvious in the following weeks.

Nutrient analysis: Epidermal bladder cells.

Plants treated with NaCl accumulated much greater Na and Cl in the leaf tissue than control plants. There was also a pattern whereby total Na and Cl concentration increased even further as NaCl in nutrient solution increased from 0.05 to 0.40 M, although Na and Cl in 0.10 M and higher treatments were not statistically different from 0.05 M (Table 5). Plants containing more Na and Cl had a visually greater number of EBCs (Fig. 5). Although EBCs per unit area were not quantified, visually, within the same treatment, upper leaves that represented new growth had greater EBC density than middle and bottom leaves that represented older growth. The bottom leaves showed similar amounts of EBCs, indicating similar growing status before NaCl application.

The leaf concentration of macronutrients N and P was decreased by increased NaCl in hydroponic nutrient solution, whereas the leaf concentration of K remained relatively stable (Table 6). The intake of secondary macronutrients Ca, Ma, and S was inhibited by increased NaCl in hydroponic nutrient solution (Table 7).