Apoplastic and symplastic zinc concentration of intact leaves of field onion (Allisum cepa) as affected by foliar application of ZnSO4 and Zn-amino chelates

Abstract

A better understanding of the physiological mechanisms by which zinc (Zn) is absorbed by plant leaves may provide additional basic information to help in the improvement of Zn foliar application programs. In this two-year field experiment, apoplastic and symplastic distribution of Zn in the leaves and re-translocation from the leaves to the bulb of onion (Allisum cepa L. cv. Behbahan) as affected by foliar application of different Zn fertilizer sources was investigated. Four Zn fertilizer sources, i.e., ZnSO4 and Zn-lysine (Zn-Lys), Zn-methionine (Zn-Met), and Zn-threonine (Zn-Thr) were applied as foliar spray at a concentration of 0.5% Zn. Free amino acids (i.e., Lys, Met, and Thr) were used to separate the effect of Zn and accompanying amino acids. A free of Zn and amino acid treatment was also considered as control. The Zn-form revealed significant influence on apoplastic Zn concentration. Foliar application of Zn-amino chelates particularly Zn-Lys increased contribution of symplastic to apoplastic Zn in onion leaves by decreasing the Zn retention on the apoplastic spaces. Higher leaf symplastic Zn in the plants supplied with the Zn-Lys in comparison with those supplied with the other Zn fertilizer sources was accompanied with higher bulb Zn accumulation. This was probably due to larger re-translocation of Zn from the leaves to the bulb at the Zn-Lys treatment. Spraying Zn and amino acids either free or complexed with Zn produced an increase in Zn concentration provoking a decrease of nitrate concentration in the bulbs and the highest effect was observed at the Zn-Lys treatment. According to the results, Zn-Lys was more effective than ZnSO4 in enhancing the symplastic to apoplastic Zn ratio in the leaves and resulted higher accumulation of Zn in the bulbs.