Efficacy of Put and Spd sprayed on leaves from Brassica juncea plants against Cd2+-induced oxidative stress

The protective effect exerted by polyamines (Put and Spd) against cadmium (Cd) stress was investigated in Brassica juncea plants. Treatment with CdCl2 (75 micro-Mole) resulted in a rise of Cd accumulation, a decrease of fresh and dry weights in every plant organ, an increase of free polyamine content at limb and stem levels as well as a decrease at root level. On the other hand, the total conjugated polyamine levels in the stem tissues were unaffected by Cd. In the leaf tissues, this metal caused a reduction of chlorophyll a content, a rise of guaiacol peroxidase (GPOX) activity and an increase of malondialdehyde (MDA), soluble glucide, proline and amino acid contents. Exogenous application, by spraying, of putrescine (Put) and spermidine (Spd) to leaf tissues reduced CdCl2-induced stress. These polyamines proved to exert a partial, though significant, protection of the foliar fresh weight and to alleviate the oxidative stress generated by Cd through reductions of MDA amounts and GPOX (E.C.1.11.1.7) activity. The enhancement of chlorophyll a content in plants by Put and those of Chl a and Chl b by Spd both constitute evidences of their efficacy against the Cd2+-induced loss of pigments. Conversely to Put, Spd caused a decrease of Cd content in leave tissues and a rise in the stems and roots; these findings are in favour of a stimulation of Cd uptake by Spd. The proline stimulation observed with Cd was reduced further to the spraying of Put onto tissues, but the decrease induced by Spd was more limited. In the plants treated with Cd, the amino acid contents in the leaves were unaffected by Put and Spd spraying; on the other hand, Cd2+ disturbed polyamine levels (free and acido-soluble conjugated-forms); we notice the rise of total free PAs and the decrease of their conjugated-ones.

💡 Research Summary

The study investigated whether foliar application of the polyamines putrescine (Put) and spermidine (Spd) could mitigate cadmium‑induced oxidative stress in Brassica juncea. Plants were exposed to 75 µM CdCl₂, a concentration that reliably caused Cd accumulation in roots, stems and leaves, reduced fresh and dry biomass, lowered chlorophyll a content, and triggered classic oxidative stress markers: increased guaiacol peroxidase (GPOX, EC 1.11.1.7) activity, elevated malondialdehyde (MDA) levels, and higher concentrations of soluble sugars and proline. Free polyamine pools rose in stems and leaves but fell in roots, while total conjugated polyamines in stems remained unchanged.

After establishing the Cd stress model, the researchers sprayed 0.5 mM solutions of Put or Spd onto the foliage. Both polyamines provided statistically significant, though partial, protection. Put most effectively restored foliar fresh weight, reduced MDA accumulation, and suppressed GPOX activity, indicating a strong antioxidant role that likely involves direct scavenging of reactive oxygen species (ROS) and modulation of peroxidase expression. Put also fully recovered chlorophyll a levels, suggesting preservation of the photosynthetic apparatus. Moreover, Put lowered the Cd‑induced proline surge, further supporting its role in alleviating osmotic and oxidative disturbances.

Spd displayed a distinct pattern of action. While it also improved chlorophyll content (both a and b), it uniquely redistributed Cd within the plant: leaf Cd concentrations declined, whereas stem and root Cd levels increased. This suggests that Spd may facilitate translocation of Cd away from photosynthetically active tissue, possibly by altering cell‑wall binding sites or chelation dynamics. The antioxidant effect of Spd was evident but less pronounced than that of Put; MDA and GPOX were reduced, yet not to the same extent. Spd’s influence on proline was modest, and it did not significantly affect total leaf amino acid pools, which remained largely unchanged by either polyamine treatment.

Overall, the findings reveal that foliar polyamine spraying can counteract cadmium toxicity in B. juncea, but the mechanisms differ between Put and Spd. Put acts primarily as a direct antioxidant and chlorophyll protector, whereas Spd appears to modulate metal partitioning and supports pigment recovery. These complementary actions suggest that combined or sequential use of both polyamines could offer a more robust strategy for protecting crops from heavy‑metal stress. Future work should explore optimal dosing, long‑term field performance, and the applicability of this approach to other economically important species facing cadmium contamination.