Root hairs on chlorotic tomatoes are an effect of chlorosis rather than part of the adaptive Fe‐stress‐response

Abstract

Extensive research has been reported from studies of the Fe‐deficiency‐stress responses of chlorotic‐Fe plants, but little has been done to evaluate the more relevant Fe‐stress‐responses of plants that exhibit no chlorosis. The present work examined the stress responses using a novel, chelator‐buffered hydroponic solution able to induce a wide range of Fe‐deficiency and stress response even in green plants. The solution contains DTPA (diethylentriamine‐pentaacetate), a chelator that limits the availability of Fe²⁺ (formed at the root by reduction of Fe³⁺DTPA) to plants by catalyzing Fe²⁺ oxidation back to the substrate Fe³⁺TPA. This produces Fe stress and induces the adaptive Fe‐stress‐response in tomatoes (Lycopersicon esculentum) and other non‐Poaceae. At 18–31.6 μM FeDTPA (with 100 μM free DTPA), tomatoes had increased rates of Fe³⁺‐chelate reduction and proton secretion, but remained green because the adaptive Fe‐stress‐response allowed the roots to obtain adequate Fe. Below 18 μM FeDTPA, the plants become increasingly chlorotic as FeDTPA was decreased. However, plants remained at intennediate levels of chlorosis severity at intermediate FeDTPA levels rather than simply becoming increasingly chlorotic with time as occurs when Fe is totally omitted from the solution.

Because of our earlier work on root hairs which appeared to be induced by Fe‐stress on tomato roots, we studied the change in root hairs and lateral roots over a 7 day period in plants grown with varied Fe in DTPA‐buffered 0.5 strength Hoagland solutions, compared with traditional +FeEDDHA and ‐Fe controls without DTPA‐buffering. At levels of FeDTPA which allowed the leaves to stay green, the roots had increased FeHEDTA (hydroxyethyl‐ethylenediaminetriacetate) reduction rate and proton secretion rate, but the dense root hairs on frequent lateral roots found on ‐Fe plants were not observed in the FeDTPA plants. At lower FeDTPA, chlorosis became more severe; only when the plants became severely chlorotic was the root hair density and lateral frequency increased. Further, chlorosis appeared about 1–1.5 days before the appearance of dense root hairs. Fe³⁺‐chelate reduction rate was as high as found in ‐Fe plants, and not significantly different from 2–32 μM FeDTPA. In contrast, the reduction rate of tomatoes grown under Fe‐sufficient conditions (20 μM FeEDDHA) was one tenth that of ‐Fe or FeDTPA grown plants. Yield of reduction activity (rate times root yield) was greater with 12 μM FeDTPA than with 0 or 2 μM FeDTPA in DTPA‐buffered solutions and greater than in the ‐Fe treatment. However, the quantity of plasma membrane Fe⁺‐chelate reductase activity from ‐Fe plants was about 3 times greater than from plants grown with 10 μM FeDTPA. We conclude that the Fe‐stress induced root hairs on tomato are an effect of chlorosis rather than part of the adaptive response to Fe‐stress. This work illustrates the utility of chelator‐buffered nutrient solutions in research on Fe nutrition. The ability to buffer Fe at continuing intermediate levels of Fe‐stress severity provided results to clarify the role of Fe‐stress in root hair development which were not obtainable using the all‐or‐none approach of ‐Fe solutions.

Notes

Hebrew University of Jerusalem.

USDA‐ARS, Frederick, MD.

Department of Agronomy, University of Maryland, College Park, MD.