Incidence of lime-induced chlorosis: plant response mechanisms and role ofwater soluble humic substances

Fe deficiency-induced chlorosis is a frequent problem in calcareous soils a nd represents a severe limitation to the productivity of many crops in the Mediterranean regions. Activation of plant; response mechanisms and the presence of humic substanc es may in part minimize or prevent the effects of this micronutritional dis order. These aspects were investigated by growing cucumber plants in nutrie nt solutions in the presence or absence of Fe-EDTA at pH 6.0 (unbuffered) o r pH 7.2 (buffered with 10 mM MES-NaOH or 1 g.L-1 CaCO3). Fe-deficient plan ts developed a physiological response typical of strategy I species (increa sed Fe-III-chelate reductase activity and acidifying capacity of roots) whi ch proved to be less efficient at high pH values and in the presence of CaC O3. To evaluate if a water soluble humic fraction (WEHS) was a suitable sou rce of Fe-III, 10-day-oId Fe-deficient plants were supplied daily with Fe-W EHS (0.1 mu M Fe and 2.5 mu g organic C.mL(-1)) or 0.1 mu M Fe-EDTA for a p eriod of 5 days. Fe-WEHS was more effective than Fe-EDTA in inducing the re covery of chlorophyll content and visible re-greening of the leaves. Plants supplied with Fe-complexes had a higher Fe-III-chelate reductase activity than Fe starved plants, in particular when the latter were treated with CaC O3 or MES-NaOH (pH 7.2). Results show that the presence of CaCO3 and WEHS c an be important factors in determining Fe acquisition from the soil and sho uld be taken into account when studying Fe deficiency under natural conditi ons.