Incidence of lime-induced chlorosis: plant response mechanisms and role of water soluble humic substances
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Fe deficiency-induced chlorosis is a frequent problem in calcareous soils and represents a severe limitation to the productivity of many crops in the Mediterranean regions. Activation of plant response mechanisms and the presence of humic substances may in part minimize or prevent the effects of this micronutritional disorder. These aspects were investigated by growing cucumber plants in nutrient solutions in the presence or absence of Fe-EDTA at pH 6.0 (unbuffered) or pH 7.2 (buffered with 10 mM MES-NaOH or 1 g · L-1 CaCO3). Fe-deficient plants developed a physiological response typical of strategy I species (increased FeIII-chelate reductase activity and acidifying capacity of roots) which proved to be less efficient at high pH values and in the presence of CaCO3. To evaluate if a water soluble humic fraction (WEHS) was a suitable source of FeIII, 10-day-old Fe-deficient plants were supplied daily with Fe-WEHS (0.1 μM Fe and 2.5 μg organic C · mL-1) or 0.1 μM Fe-EDTA for a period of 5 days. Fe-WEHS was more effective than Fe-EDTA in inducing the recovery of chlorophyll content and visible re-greening of the leaves. Plants supplied with Fe- complexes had a higher FeIII-chelate reductase activity than Fe-starved plants, in particular when the latter were treated with CaCO3 or MES-NaOH (pH 7.2). Results show that the presence of CaCO3 and WEHS can be important factors in determining Fe acquisition from the soil and should be taken into account when studying Fe deficiency under natural conditions.