Effect of Salt Stress and Foliar Application of Salicylic Acid on Morphological, Biochemical, Anatomical, and Productivity Characteristics of Cowpea (Vigna unguiculata L.) Plants

Abstract

The present study aimed to investigate the impact of salinity on vegetative growth, chemical constituents, and yields of cowpeas (Vigna unguiculata) and the possible benefits of salicylic acid (SA) on these plants after damage from salinity. To achieve these objectives, two pot experiments were carried out at the Faculty of Agriculture, Al-Azhar University, Egypt, during the two growing seasons of 2019 and 2020. The results revealed that salinity significantly decreased, and SA treatment substantially increased the plant height, number of compound leaves, number of internodes per plant, fresh weights of leaves and stems, productivity, photosynthetic pigments content, and concentrations of nitrogen (N), phosphorus (P), and potassium (K) of the cowpea plants compared with the control. The anatomical structure of stems and leaves of the plants were also investigated, and it was found that positive variations in the anatomical structure of the median portion of the main stems and blades of mature foliage leaves were detected in the stressed and SA-treated plants. In conclusion, SA treatment increased the salt stress tolerance of cowpea plants by improving the morphological and physiological attributes of the plants.

Keywords: SA; Vigna unguiculata L.; anatomy; growth; photosynthetic pigments; productivity; salinity stress.

Figure 1

Habit of cowpea plant at blooming stage (the age of 90 days) as affected by salinity stress. (A) Untreated plant (control), (B) plant treated with 2000 ppm salinity, (C) plant treated with 4000 ppm salinity, (D) plant treated with 6000 ppm salinity, and (E) plant treated with 8000 ppm salinity.

Figure 2

Vegetative growth characters of cowpea as affected by different levels of saline water and foliar application with salicylic acid (SA) at concentration of 100 ppm (average of the two seasons 2019 and 2020 combined). (A) Plant height, (B) no. of internodes, (C) no. of compound leaves, and (D) fresh weight of leaves and stem. Data are means of five replicates (n = 5), and for each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s HSD range test at p ≤ 0.05.

Figure 3

Habit of mature plants, at blooming stage, of cowpea plants as affected by SA and salinity stress. (A) Untreated plant (control), (B) plant grown under 6000 ppm of salinity stress, and (C) plant grown under 6000 ppm salinity stress and sprayed with 100 ppm of SA.

Figure 4

Impact of foliar application with SA on photosynthetic pigments and concentrations of free Pro in cowpea leaves, at full blooming stage, grown under various levels of salinity (average of the two growing seasons, 2019 and 2020 combined). (A) Ch_a, (B) Ch_b, (C) Car, and (D) Proline. Data are means of five replicates (n = 5), and for each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s HSD range test at p ≤ 0.05.

Figure 5

Mean values of N, P, and K in dry leaves of cowpea at full blooming stage, grown under different levels of salinity stress and foliar application with SA (average of the two seasons, 2019 and 2020 combined). (A) N content, (B) P content, and (C) K content. Data are means of five replicates (n = 5), and for each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s HSD range test at p ≤ 0.05.

Figure 6

Yield characters of cowpea as affected by different levels of saline water and foliar application with SA at a concentration of 100 ppm (average of the two seasons 2019 and 2020 combined). (A) No. of seeds/pod, (B) no. of pods/plant, (C) weight of pods (g)/plant, and (D) weight of 100 seeds. Data are means of five replicates (n = 5), and for each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s HSD range test at p ≤ 0.05.

Figure 7

Cross sections through median portion of the main stem of cowpea plant, aged 75 days, as affected by foliar spray with SA. (A) Untreated plant (control), (B) plant grown under salinity stress of 6000 ppm, and (C) plant grown under 6000 ppm salinity stress and sprayed with 100 ppm of SA. Abbreviations: Co Z = cambium zone; Co. = cortex; Ep. = epidermis; Fi St. = fiber strands; Ph. = phloem; P = pith; V = vessel; and Xy = xylem.

Figure 8

Microphotographs of cross sections through the blade of the terminal leaflet of the compound leaf developed on the median portion of the main stem of cowpea plant, aged 75 days as affected by SA and salinity stress. (A) Untreated plant (control), (B) plant grown under salinity stress of 6000 ppm, and (C) from plant grown under 6000 ppm of salinity stress and sprayed with 100 ppm of SA. Abbreviations: Uep = upper epidermis; L ep., = lower epidermis; Pal. = palisade tissue; Spo. = spongy tissue; M.b = midvein bundle; M.r. = midvein region; and T = trichome.