Water-extractable humic substances enhance iron deficiency responses by Fe-deficient cucumber plants

The ability of Fe-deficient cucumber plants to use iron complexed to a wate r-extractable humic substances fraction (WEHS), was investigated. Seven-day -old Fe-deficient plants were transferred to a nutrient solution supplement ed daily for 5 days with 0.2 mu M Fe as Fe-WEHS (5 mu g org. C mL(-1)), Fe- EDTA, Fe-citrate or FeCl3. These treatments all allowed re-greening of the leaf tissue, and partial recovery of dry matter accumulation, chlorophyll a nd iron contents. However, the recovery was faster in plants supplied with Fe-WEHS and was already evident 48 h after Fe supply. The addition of 0.2 m u M Fe to the nutrient solution caused also a partial recovery of the dry m atter and iron accumulation in roots of Fe-deficient cucumber plants, parti cularly in those supplied with Fe-WEHS. The addition of WEHS alone (5 mu g org. C mL(-1), 0.04 mu M Fe) to the nutrient solution slightly but signific antly increased iron and chlorophyll contents in leaves of Fe-deficient pla nts; in these plants, dry matter accumulation in leaves and roots was compa rable or even higher than that measured in plants treated with Fe-citrate o r FeCl3. After addition of the different iron sources for 5 days to Fe-defi cient roots, morphological modifications (proliferation of lateral roots, i ncrease in the diameter of the sub-apical zones and amplified root-hair for mation) and physiological responses (enhanced Fe(III)-chelate reductase and acidification of the nutrient solution) induced by Fe deficiency, were sti ll evident, particularly in plants treated with the humic molecules. The pr esence of WEHS caused also a further acidification of the nutrient medium b y Fe-deficient plants. The Fe-WEHS complex (1 mu M Fe) could be reduced by intact cucumber roots, at rates of reduction higher than those measured for Fe-EDTA at equimolar iron concentration. Plasma membrane vesicles, purifie d by two-phase partition from root microsomes of Fe-deficient plants, were also able to reduce Fe-WEHS. Results show that Fe-deficient cucumber plants can use iron complexed to water soluble humic substances, at least in part via reduction of complexed Fe(III) by the plasma membrane Fe(III)-chelate reductase of root cells. In addition, the stimulating effect of humic subst ances on H+ release might be of relevance for the overall response of the p lants to iron shortage.