Zn-localization and anatomical changes in leaf tissues of green beans (Phaseolus vulgaris L.) following foliar application of Zn-lignosulfonate and ZnEDTA

Understanding the effective penetration of Zn fertilizers, following foliar application, requires powerful techniques to trace over a period of time the pathway of Zn within the leaf. In this paper, atomic absorption spectroscopy, Cryo-scanning electron microscopy (Cryo-SEM) and energy-dispersive X-ray microanalysis (CEDX) was used to study Zn uptake and localization at leaf cellular level, at different times after the foliar-application of a Zn-lignosulfonate (Zn-LS) complex in comparison with a ZnEDTA chelate. Green bean plants (Phaseolus vulgaris L., cv. Linera) were grown in pots and Zn-LS and ZnEDTA fertilizers were compared in foliar application on the oldest leaves. Zinc concentration in Treated Non-treated leaves and “washing away” water was determined by atomic absorption spectrophotometry at 6 h, 4 and 30 days. Significant differences in Zn penetration inside the leaves were observed, the most efficient being the Zn LS complex which showed fastest absorption after 6 h. At this time, also significant differences in Zn localization inside the leaf tissues were detected by CEDX, the mesophyll showing the highest absorption of Zn LS. Zn were detected in the mesophyll at the highest concentration in leaves of plants treated with Zn-LS also at the 4th and 30th days, whereas in those treated with ZnEDTA it was in the lower epidermis. The analyses of structural changes on frozen-hydrated leaf samples by Cryo-SEM showed that Zn-LS complex treatment causes an increase of the total thickness of the leaf and in particular of the spongy mesophyll, the innermost and physiologically active tissue layer.