Iron (Fe) deficiency is a common nutritional disorder in several crops grown in calcareous soils, but
some species are well adapted to these conditions. A hydroponic experiment was conducted to compare
the response of a calcicole species Ceratonia siliqua L. (carob) and of Poncirus trifoliata (L.) Raf., a citrus
rootstock very sensitive to Fe deficiency. Rootstocks from both species were grown in nutrient solutions
without Fe (0 M Fe), with 1 M Fe, and with 10 M Fe (carob) or 40 M Fe (P. trifoliata). A low level of
Fe or its absence in the nutrient solution led to a significant decrease in P. trifoliata vegetative growth
and in SPAD readings. The root activity of ferric-chelate reductase (FC-R), a key enzyme in Fe uptake, was
low in the absence or with high levels of Fe. Its highest values were in roots exposed to a low level of Fe
as described in several sensitive species. In contrast, the activity of FC-R was very high in carob in the
absence of Fe and was decreased sharply even when only a low level of Fe was present in the nutrient
solution. Plant growth and SPAD readings in the leaves of carob were similar in all treatments. Carob
seems to maintain a large activity of root FC-R that may ensure enough Fe to satisfy plant demand. The
fact that it presents a slow growing pattern may also contribute to the tolerance of this species to low
levels of external Fe
