IRON STRESS AND SALT STRESS RESPONSES OF LETTUCE (LACTUCA-SATIVA L)

A flavin-like compound that accumulates in nutrient solutions of lettuce under osmotic stress is reportedly not a response associated with Fe stress [Cramer and Spurr, J. Plant Nutr. 8:709-719 (1985)]. We re-examined both salt stress and iron stress in lettuce (Buttercrunch cv.) using four concentrations of chelated and non-chelated iron. Lettuce was quite Fe efficient and plants grown with no Fe or very low Fe concentrations markedly decreased the pH of their nutrient solutions and experienced only a transient chlorosis. A yellow compound that these plants excreted in the nutrient solution was chromatographed on thin layer plates. The Rf value and fluorescence spectral characteristics of this compound corresponded to those of riboflavin. When plants received adequate Fe, NaCl or KCl in the nutrient solution did not cause roots to excrete riboflavin nor did these salts increase the excretion of riboflavin from Fe-deficient plants. Salt-stressed plants were smaller and excreted less riboflavin than those that were not salt stressed, regardless of Fe level. Plants that received Fe as FeSO4 (as in previous report) did not develop any obvious signs of iron chlorosis, but were not as green as those that received an equivalent concentration of Fe as Chel 138. Several factors may account for the discrepancy in the results of the two reports.