Mechanism of Fe uptake by the leaf symplast: Is Fe inactivation in leaf a cause of Fe deficiency chlorosis?

The mechanism of iron (Fe) uptake from the leaf apoplast into leaf mesophyll cells was studied to evaluate the putative Fe inactivation as a possible cause of Fe deficiency chlorosis. For this purpose, sunflower (Helianthus annuus L.) and faba bean plants (Vicia faba L.) were precultured with varied Fe and bicarbonate (HCO3-) supply in nutrient solution. After 2-3 weeks preculture, Fe-III reduction and Fe-59 uptake by leaf discs were measured in solutions with Fe supplied as citrate or synthetic chelates in darkness. The data clearly indicate that Fe-III reduction is a prerequisite for Fe uptake into leaf cells and that the Fe nutritional status of plants does not affect either process. In addition, varied supply of Fe and HCO3- to the root medium during preculture had no effect on pH of the xylem sap and leaf apoplastic fluid. A varied pH of the incubation solution had no significant effect on Fe-III reduction and Fe uptake by leaf discs in the physiologically relevant pH range of 5.0-6.0 as measured in the apoplastic leaf fluid. It is concluded that Fe inactivation in the leaf apoplast is not a primary cause of Fe deficiency chlorosis induced by bicarbonate

Does high bicarbonate supply to roots change availability of iron in the leaf apoplast?

The role of the leaf apoplast in iron (Fe) uptake into the leaf symplast is insufficiently understood, particularly in relation to the supposed inactivation of Fe in leaves caused by elevated bicarbonate in calcareous soils. It has been supposed that high bicarbonate supply to roots increases the pH of the leaf apoplast which decreases the physiological availability of Fe in leaf tissues. The study reported here has been carried out with sunflower plants grown in nutrient solution and with grapevine plants grown on calcareous soil under field conditions. The data obtained clearly show that the pH of the leaf apoplastic fluid was not affected by high bicarbonate supply in the root medium (nutrient solution and field experiments). The concentrations of total, symplastic and apoplastic Fe were decreased in chlorotic leaves of both sunflower (nutrient solution experiment) and grapevine plants in which leaf expansion was slightly inhibited (field experiment). However, in grapevine showing severe inhibition of leaf growth, total Fe concentration in chlorotic leaves was the same or even higher than in green ones, indicative to the so-called `chlorosis paradox'. The findings do not support the hypothesis of Fe inactivation in the leaf apoplast as the cause of Fe deficiency chlorosis since no increase was found in the relative amount of apoplastic Fe (% of total leaf Fe) either in the leaves of sunflower or grapevine plants. It is concluded that high bicarbonate concentration in the soil solution does not decrease Fe availability in the leaf apoplast

Physiological root responses of iron deficiency susceptible and tolerant tomato genotypes and their reciprocal F-1 hybrids

WOS: 000176415100012By using two tomato genotypes line 227/1 (Fe chlorosis susceptible) and Roza (Fe chlorosis tolerant) and their reciprocal F(1)hybrid, some root morphological changes, pH changes of nutrient solution, reduction capacity of Fe-III and uptake and root-to-shoot translocation of Fe-59 were studied under controlled environmental conditions in nutrient solution with 3 different Fe supplies as Fe EDDHA (i.e., 10(-7) M, severe Fe deficiency; 10(-6) M, intermediate Fe deficiency; 10(-4) M, adequate Fe supply). Tolerant parent `Roza' was less affected by low Fe supply than susceptible parent `line 227/1' as judged from the severity of leaf chlorosis. Under both Fe deficient conditions there were no differences between the reciprocal hybrids concerning the appearance of chlorosis. Under intermediate Fe deficiency, reciprocal F-1 hybrids (`line 227/1 x Roza' and `Roza x line' 227/1) showed an intermediate chlorosis between tolerant and susceptible parents. However, under severe Fe deficiency the reciprocal hybrids were more chlorotic than the tolerant parent irrespective of which parent was the cytoplasm contributor. A decreased Fe supply during preculture enhanced (FeII)-I-I reduction capacities of the parents and reciprocal hybrids. Differences in the tolerance to Fe deficiency always were better correlated with (FeII)-I-I reduction capacity of the genotypes than the Fe deficiency-induced release of H+ ions. Under both Fe deficient conditions the tolerant parent Roza had a much higher (FeII)-I-I reduction capacity than the susceptible parent line 227/1. The reduction capacity of the hybrids `Roza x line 227/1' was very similar to the capacity of the parent Roza, but higher than the capacity of the hybrids `line 227/1xRoza' at both Fe-deficient conditions. Under both Fe deficient conditions tolerant parent had higher number of lateral roots than the susceptible parent. Among the reciprocal hybrids `Roza x line 227/1' possessed more lateral roots than the `line 227/1 x Roza' under both Fe deficient conditions. Low Fe nutritional status resulted in marked increase in root uptake of (5)9Fe. At adequate Fe supply, reciprocal hybrids and their parents did not differ in uptake and root-to-shoot translocation of Fe. However, under Fe-deficient conditions uptake and root-to-shoot translocation of (5)9Fe were significantly higher in the Fe chlorosis tolerant than the susceptible parent. Based on the reduction capacity of (FeII)-I-I and uptake and root-to-shoot translocation of Fe, the F-1 hybrids obtained from the cross in which the maternal genotype was Roza appeared to be more tolerant than when the maternal genotype was the susceptible line 227/1. Uptake and translocation ratio of the F-1 hybrids obtained from `Roza x line 227/1' were similar to those of the parent Roza, but higher than the F-1 hybrids obtained from `line 227/1 x Roza', particularly under intermediate Fe deficiency. The results indicate that (FeII)-I-I reduction show a better relationship to Fe efficiency than Fe deficiency induced release of H+ ions. The inheritance of Fe deficiency tolerance of Roza seems not to be simple monogenic. It might be characterised by both, nuclear and extranuclear heredity. The intermediate responses of the reciprocal hybrids of the `line 227/1 x Roza' indicates that the Fe deficiency tolerance character of Roza is transferable by nuclear heredity. The better responses of the hybrids of `Roza x line 227/1' than the hybrids of `line 227/1 x R