Several factors are involved in controlling iron (Fe) availability in the apoplast of leaf mesophyll tissues, including pH, organic acid:Fe ratio, reducing power and Fe source. While Fe-II is able to cross directly the plasmamembrane (PM), Fe-III has to undergo a reduction by a ferric chelate reductase (FCR) before entering the cells. In this study we have tested the effects of foliar sprays of Fe-III-sulphate, malic (M) and citric (C) acids and sorbitol (S) and their combinations on the re-greening of one-year old micropropagated chlorotic kiwifruit plants grown in pots on a calcareous soil. Treatments also included Fe-III-EDTA and ferrous sulphate (Fe-II) sprays. With the exception of Fe-II-sulphate and Fe-III-sulphate+sorbitol solutions, all sprayed treatments had pH adjusted to 6.0. Sprays were applied three times at weekly intervals and leaf chlorophyll was estimated non destructively by SPAD 502 meter. Iron concentration in control leaves ranged from 134 to 282 mug g(-1) DW. The influence of these treatments varied with the severity of chlorosis. In case of severely chlorotic leaves, the application of the mixture of malic acid, citric acid and sorbitol in combination with Fe-III-sulphate resulted in a rapid and persistent re-greening, which is presumably due to the chelating ability of citrate along with the supply of reducing power to mesophyll cells by malic and sorbitol. Iron (Fe-II)-sulphate and Fe-EDTA were clearly less effective than Fe-M-C-S. In leaves moderately affected by chlorosis the highest re-greening effect was induced by Fe-II-sulphate. The relative re-greening effect of organic acids applied with Fe-III-sulphate, particularly in case of moderately chlorotic leaves, is likely due to an increased availability of Fe to leaf FCR. Organic acids applied alone and their combinations were not effective, a fact likely explained by the pH equal to 6.0. Sorbitol had no effect on leaf chlorophyll neither when applied alone, nor when supplied with Fe-III or with the organic acids.
