Iron source affects iron reduction and re-greening of kiwifruit (Actinidia deliciosa) leaves

Among deciduous fruit plants, kiwifruit (Actinidia deliciosa) is one of the most susceptible to iron (Fe) chlorosis. To develop effective means for overcoming Fe chlorosis, it is of upmost importance to gain information about the reduction of Fe by leaf tissues, especially under conditions that lead to chlorosis. In the present study we have characterised the leaf Fe-chelate reductase (FCR) in Fe sufficient and Fe deficient kiwifruit leaves and for the first time tested the hypothesis that Fe-III-malate is a suitable source of Fe for FCR, in addition to Fe-III-citrate. Under field conditions, we have also tested the re-greening effects caused by the foliar application of different Fe sources, including Fe-III-malate, Fe-III-citrate, Fe-III-DTPA and an Fe-II source (FeSO4 + aminoacid-polypeptide mixture) on chlorotic leaves. The results demonstrated that, similarly to other species, mesophyll tissues of A. deliciosa leaves are able to perform an enzymatic Fe reduction prior to Fe uptake. Plasma membrane enriched material extracted from Fe sufficient leaves reduced Fem-malate and Fem-citrate. The pH optimum was 6.0-6.2 for Fe-III-malate and 6.5 for Fem-citrate. The substrate-dependence showed higher affinity for malate than for citrate. In contrast to the root level, the activity of the FCR of kiwifruit leaves was not enhanced under Fe deficiency. On the contrary, after two weeks of Fe depletion, the reduction of Fem-citrate was 4.5-fold lower in the Fe deficient plants than in the Fe sufficient ones, while the reduction of Fe-III-malate was not significantly affected. Under field conditions, the Fe solutions caused re-greening of chlorotic leaves, whose intensity and duration varied according to Fe source and Fe concentration. Among the treatments, the highest re-greening effect was caused by Fe-III-DTPA and especially by the Fen source. Fem-citrate and Fem-malate were less effective in stimulating chlorophyll formation. All treatments increased leaf Fe concentration and content. Although less Fe from malate than from citrate penetrated into the leaves, the re-greening effect from Fem malate was intermediate between that of Fem-DTPA and the one caused by Fe-III-citrate. The results suggest that if Fem-malate can reach the plasmamembrane it provides a good source of Fe for leaf Fe uptake.