George W. Welkie

Iron chlorosis, a world wide problem, the relation of chlorophyll biosynthesis to iron

Abstract Iron‐Chlorosis is a world‐wide problem, particularly in semi‐arid regions containing calcareous soils. Although most soils contain adequate total iron, the amount available to the plant is dependent on factors such as iron species and plant genotype. Iron is reduced to ferrous form at the root surface if necessary and transported to the leaf through the xylem in a combined form. In the leaf it is used for chlorophyll (Chl) formation as well as for the functioning of various iron‐containing enzymes. Chlorophyll is formed in higher plants from the precursor δ‐amino‐levulinic acid (ALA). This intermediate, ALA, functions for both heme and chlorophyll formation and may be formed through a primary biosynthesis pathway in higher plants requiring a 5‐carbon substrate, such as a ketoglutarate or glutamate. This pathway may be activated by iron, related to aconitase ativity and/or the formation of ferredoxin (Fd). Ferredoxin could be necessary to activate the ALA‐synthesizing enzyme. With plants that are ...

Potassium effect on iron stress in tomato. II. The effects on root CO2-fixation and organic acid formation.

Abstract Tomato plants, two Fe‐efficient cultivars and one Fe‐inefficient cultivar, were grown in treatment combinations of normal and low Fe and K concentrations (Normal Fe, Normal K; Low Fe, Normal K; Low Fe, Low K). Iron‐stressed plants with normal K levels were the first to show elevation of root proton excretion and an enhanced ability to regreen. Roots from the Fe‐efficient cultivars had a marked increase in the rate of CO2 dark fixation that was most prominent with a Normal K, Low Fe. The amount of 14C labeling of root organic acids, particularly malate, citrate, lactate and oxaloacetate increased under Fe‐stress conditions in the Fe‐efficient cultivars; Fe stress with low K resulted in less 14C labeling. A scheme for Fe reduction, uptake and transport is discussed related to CO2 fixation and organic acids (malate, citrate and oxaloacetate.) 1Part of the research done by the senior author while on leave at the Institute for Whole Body Metabolism, Inba, Chiba, Japan. The helpful assistance of Mr. Y....

Taxonomic distribution of dicotyledonous species capable of root excretion of riboflavin under iron deficiency

Abstract Iron efficiency of some dicotyledonous species and cultivars have been reported to be associated with their ability to release riboflavin from their roots when plants are iron deficient. The biochemical relationship of this excretion to increased Fe reduction and proton excretion with deficiency is not yet established, but may involve a transmembrane oxidation of reduced riboflavin. This presentation is an attempt to examine the distribution of species with respect to genus and family that have been reported or are now known to excrete or not excrete abundant riboflavin when iron deficient. A rational explanation for species distribution of the capacity for the Fe‐deficiency stress induced riboflavin excretion is desirable. The property may be one of a number of mechanisms that random species or groups of species developed to adapt to Fe‐stress conditions. Response of 57 species in twelve families are reported and there are those in which no species excrete riboflavin, those in which some species excrete riboflavin, and others in which all species tested excrete riboflavin. More species need to be tested in these and other families to understand the basis for the distribution of the positive or negative riboflavin response. Some tentative explanations for the observed distributions are presented.

Potassium effect on iron stress in tomato. I. The effect on pH, Fe‐reductase and chlorophyll

Abstract Seedlings of Fe‐efficient tomato plants were cultured in nutrient solutions with two concentrations of Fe and K. The pH of the nutrient solution increased initially in all treatments. A subsequent drop in pH of the nutrient solution to 4.4 occurred only in the low Fe, normal K treatment. Ferric iron reduction by root samples was markedly increased only in those producing a decrease in pH of nutrient solution. Most “Fe‐reductant” was released from the swollen elongation zones of young roots. Chlorophyll content in young leaves (after a pH Fe‐stress response) was greater with normal K versus low K for both normal and low iron treatments. Iron‐stressed plants accumulated more K/g dry weight, especially in the roots, than with normal Fe supply.

Riboflavin excretion from roots of iron‐stressed and reciprocally grafted tobacco and tomato plants

Abstract Seedlings of tomato and tobacco were V‐grafted reciprocally and intraspecifically and cultured for 22 days in 2‐liter containers of nutrient solution with normal or no iron. The pH of solutions containing iron increased slightly, while the pH of solutions with no iron decreased. There was little decrease in the pH of tomato grafts on tomato, while the pH of other combinations decreased from 1.5 to 3.1 pH units. Riboflavin concentrations in nutrient solution were highest with tobacco on tobacco, and were about one‐third less with tomato on tobacco. The riboflavin values for tobacco on tomato and tomato on tomato were barely detectable like those for plants supplied with iron. Roots of iron‐deficient tobacco plants at harvest contained much more total riboflavin than roots of iron‐deficient tomato plants and roots of iron‐supplied plants contained less total riboflavin than iron‐deficient plants. Total riboflavin in roots of tobacco plants exceeded that in roots of tomato plants. The total riboflav...

Sugar beet responses to iron nutrition and stress

Abstract Sugar beets were grown at four concentrations of added Fe (0.00, 0.02, 0.2 and 2.0 mg L‐1) in nutrient solutions. The pH increased similarly in solutions at the two higher Fe concentrations and decreased moderately and negligibly respectively at the lowest added Fe and no Fe concentrations. Chlorosis was pronounced in plants of the latter two treatments and was accompanied by marked reductions in the chlorophyll content of apical leaves, in the size of young expanded leaves, and in the fresh and dry mass of roots and shoots. The decreases in these parameters were greater for plants at no Fe than at 0.02 mg Fe L‐1 The riboflavin content in the nutrient solutions increased only negligibly with maximum iron, increased slightly with 0.2 mg Fe L‐1, and increased substantially with 0.02 mg Fe L‐1 and no Fe, but was lower in the latter. The cumulative amounts of riboflavin excreted and the times of maximum excretion varied within replicates of solutions containing both low Fe and no Fe. The maximum ribo...

Iron‐deficiency stress responses of a chlorosis‐susceptible and a chlorosis‐resistant cultivar of muskmelon as related to root riboflavin excretion

Abstract Cultivar Edisto and an Fe‐inefficient mutant (fefe) muskmelon were noted by Jolley et al. (1991) to differ with Fe‐deficiency stress in their ability to lower their nutrient solution pH, to increase their root Fe‐reductive capacity, to transport Fe to leaves, and to develop chlorosis. These two muskmelon cultivars were Fe‐stressed along with controls, and daily measurements for nutrient solution pH and riboflavin concentrations were taken. Root Fe‐reductive activity and apical leaf chlorophyll content was measured on selected days. Riboflavin excretion by Fe‐deficiency stressed Edisto plants increased by the second day, reached a peak after seven days and declined slightly. Edisto plants with Fe excreted 100 to 300 times less riboflavin than those without Fe. The fefe Fe‐deficiency stressed plants and those with Fe excreted similar very low amounts of riboflavin several fold less than the Edisto plants with Fe. The nutrient solution pH was decreased by Fe‐stressed Edisto plants from pH 5.0 to 3.7...

Iron stress and salt stress responses of lettuce (Lactuca sativa L.)

Abstract 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 tha...

Effects of root temperature on iron stress responses

Pepper plants (Capsicum annuum L.) were transferred to all NO3 - N nutrient solution, lacking Fe, in tubes maintained in water baths held at 15, 20, 25, 30 and 35°C with air temperature at 28°C day and 25°C night. Nutrient solution pH incresed initially at all temperatures as it did in plants having iron, but did so slightly more rapidly at 30° and least rapidly at 15°C. With the onset of iron stress, decreases in nutrient pH were most pronounced at 30°C, moderate at 25°C and not evident at 15, 20, and 35°C over 12 days. Riboflavin excretion from roots preceded pH decreases by several days, but amounts increased greatly with pH decreases. The maximum riboflavin from plants at 30°C root temperature exceeded that for plants at the lowest root temperature by more than 2000 fold. Root iron reduction increases were pronounced and moderate at root temperature of 30°C and 25°C, respectively, and negligible at other root temperatures. These reduction increases paralleled initial riboflavin excretion. Root and shoot fresh mass were maximal at 30°C root temperature and total mass exceeded that from plants at 15°C root temperature by about 2 fold, whereas the differential in secondary root mass was larger. Chlorosis was most pronounced at 25, 30, and 35°C root temperatures.

Iron stress responses of a chlorosis-susceptible and chlorosis-resistant cultivars of pepper (Capsicum annuum L.)

A cultivar of pepper (Zehavi) known to produce Fe chlorosis in calcareous soils in Israel was compared to two ‘normal’ pepper cultivars in Hoagland’s I nutrient solutions with no Fe and several concentrations of FeEDDHA. A number of parameters were monitored to establish to what degree they might be linked in Strategy I plant responses to Fe stress. These included apical leaf chlorophyll, nutrient solution pH, riboflavin excretion, root reductive capacity and tissue mass. The Zehavi cultivar required more FeEDDHA to prevent chlorosis of apical leaves, but the chlorophyll content of its apical leaves of plants supplied with 2 mg L−1 was almost equal to that of the other cultivars: Iron stressed Zehavi plants were unable to lower the nutrient solution pH as did plants of the other cultivars. Riboflavin excretion by Fe-stressed roots of Zehavi plants occurred, but concentrations were much lower than those of the other cultivars. Riboflavin concentrations were independent of nutrient solution pH decreases. Root Fe reductive activity for Fe-stressed Zehavi plants harvested over 14 days were mostly less than that of the other cultivars, but activity of all plants fluctuated with time. Use of some NH 4 + in solutions to maintain low pH prevented chlorosis of Zehavi plants supplied with an intermediate Fe concentration.