Investigating Exogenous Tyrosine Supplements on the Responses of the Kale Plant to Salinity Stress

Abstract

The study investigated the role of exogenous tyrosine (TYR) supplements in extending kale tolerance to NaCl stress at various concentrations (50, 100, and 200 mM). The salt stress was induced by irrigating the soil with salt water, and TYR was supplied through foliar spraying. The impact of TYR supplementation under NaCl stress was assessed by evaluating growth parameters, enzymatic and non-enzymatic defense, oxidative stress markers, and mineral composition. The results revealed that TYR significantly increased the levels of β-carotene, lycopene, anthocyanins, and polyphenols as well as the activities of polyphenol oxidase (PPO), catalase (CAT), and superoxide dismutase (SOD). TYR enhanced the activities of ascorbate peroxidase, CAT, and SOD under 200-NaCl, increased PPO activity at all NaCl concentrations, and reduced the MDA content only at 200-NaCl. The Mg, P, K, Ca, Na, and the ratio K/Na increased under 200-NaCl, while Ca, Na, and Cl declined with lower NaCl. TYR raised Ca and Na levels at 100-NaCl but decreased Na, Cl, and the Na/K ratio at 200-NaCl. In conclusion, high NaCl levels suppressed Chl-a, β-carotene, lycopene, sucrose accumulation, and the activities of PPO, APX, CAT, and SOD, which led to reduced leaf, shoot, and root growth; however, these negative impacts were alleviated by TYR supplementation. The study suggests that to promote agricultural sustainability, it may be advisable to extend tolerance thresholds for moderately tolerant crops, enhance the tolerance of salt-sensitive vegetables in saline regions, and incorporate exogenous TYR.

Keywords: antioxidants; nutrition; sustainability; tolerance.

FIGURE 1

Defining the promotive dose of TYR in kale seedling germination. *Mean values (n = 3) in the same column for each trait in each group with the same lower‐case letter are not significantly different by Tukey's multiple range test at p ≤ 0.05. TYR, tyrosine.

FIGURE 2

Variation of (a) chlorophyll a (Chl‐a), chlorophyll b (Chl‐b), (b) total chlorophyll (Chl), lutein, (c) carotenoid, β‐carotene (β‐car), and lycopene (Lyc) in the kale leaves. *Mean values (n = 3) in the same column for each trait in each group with the same lower‐case letter are not significantly different by Tukey's multiple range test at p ≤ 0.05. TYR, tyrosine.

FIGURE 3

Variation of (a) anthocyanin, (b) total polyphenol (TPC), and (c) polyphenol oxidase (PPO) activity in the kale leaves. *Mean values (n = 3) in the same column for each trait in each group with the same lower‐case letter are not significantly different by Tukey's multiple range test at p ≤ 0.05. TYR, tyrosine.

FIGURE 4

PCA biplot analysis for the amount of (a) Chl‐a, Chl‐b, total Chl. (b) carotenoid, lycopene, β‐carotene. (c) lutein, anthocyanin, polyphenol, and the activity of polyphenol oxidase (PPO).

FIGURE 5

Variation of (a) proline, (b) glucose (GLU), and sucrose (SUC) content of kale leaves. *Mean values (n = 3) in the same column for each trait in each group with the same lower‐case letter are not significantly different by Tukey's multiple range test at p ≤ 0.05. TYR, tyrosine.

FIGURE 6

Variation of (a) malondialdehyde (MDA), (b) hydrogen peroxide (H₂O₂) concentration, and activity of (c) APX, CAT, POD, and (d) SOD in the kale leaves.*Mean values (n = 3) in the same column for each trait in each group with the same lower‐case letter are not significantly different by Tukey's multiple range test at p ≤ 0.05. TYR, tyrosine.

FIGURE 7

PCA biplot analysis for proline, glucose, sucrose, MDA, and H₂O₂, concentrations, and the activity of APX, CAT, POD, and SOD. C, control; TYR, tyrosine.