Salicylic acid improved lead and zinc stress tolerance in maize by enhancement of growth and physiological characters

Heavy metal contamination of arable land is an increasing problem for crop production and food safety. The effect of salicylic acid (SA) on crop performance depends not only on the plant species, but also on the concentration of SA and the method of application. The effects of exogenous application of SA at concentrations of 0, 750 and 1500 µM by foliar spraying and seed priming methods were investigated on maize plants under lead and zinc stress. Heavy metal stress significantly reduced photosynthetic parameters and pigment levels, with the greatest reduction observed under the combined application of these metals (Jan and Parray Jan, S., Parray, J.A. (2016) Heavy metal uptake in plants. In Approaches to Heavy Metal Tolerance in Plants; Springer: Singapore, 1–18. 10.1007/978-981-10-1693-6_1). Comparison of photosynthetic rates during early growth and tasseling showed that under unstressed conditions, photosynthesis increased markedly as the plants developed. On the other hand, under stressed conditions, photosynthesis decreased over time. This decrease was less pronounced when metals were applied individually, but significantly more pronounced when they were applied in combination. The application of SA mitigated the effects of heavy metal stress. Comparison of application methods showed that seed priming was more effective than foliar spraying in the early growth stage. However, there were no significant differences between these methods at the tassel stage. Hydropriming and distilled water spraying showed no advantage over the control treatment. Heavy metal stress reduced leaf area, stomatal size, vascular bundle and mesophyll cell diameter and plant biomass, while increasing stomatal frequency and leaf thickness. Although the relative water content (RWC) of plants under stress conditions decreased, SA application alleviated this damage. The decrease in RWC was accompanied by an increase in stomatal resistance and leaf temperature. This reduction in RWC could be due to a decrease in vascular diameter and water flow to the leaves. The reduction in stomatal closure and transpiration rate under stress conditions can be considered as a water saving mechanism. Salicylic acid, especially at the highest concentration, improved plant performance and increased maize biomass. This improvement was attributed to the increase in photosynthetic pigment content, photosynthetic rate and leaf area expansion. The increase in leaf area and decrease in leaf thickness with SA application indicate further development of leaf cell size and leaf expansion.