Carboxylate metabolism in sugar beet plants grown with excess Zn

The effects of Zn excess on carboxylate metabolism were investigated in sugar beet (Beta vulgaris L.) plants grown hydroponically in a growth chamber. Root extracts of plants grown with 50 or 100μM Zn in the nutrient solution showed increases in several enzymatic activities related to organic acid metabolism, including citrate synthase and phosphoenolpyruvate carboxylase, when compared to activities in control root extracts. Root citric and malic acid concentrations increased in plants grown with 100μM Zn, but not in plants grown with 50μM Zn. In the xylem sap, plants grown with 50 and 100μM Zn showed increases in the concentrations of citrate and malate compared to the controls. Leaves of plants grown with 50 or 100μM Zn showed increases in the concentrations of citric and malic acid and in the activities of citrate synthase and fumarase. Leaf isocitrate dehydrogenase increased only in plants grown with 50μM Zn when compared to the controls. In plants grown with 300μM Zn, the only enzyme showing activity increases in root extracts was citrate synthase, whereas the activities of other enzymes decreased compared to the controls, and root citrate concentrations increased. In the 300μM Zn-grown plants, the xylem concentrations of citric and malic acids were higher than those of controls, whereas in leaf extracts the activity of fumarase increased markedly, and the leaf citric acid concentration was higher than in the controls. Based on our data, a metabolic model of the carboxylate metabolism in sugar beet plants grown under Zn excess is proposed.

Stomatal and mesophyll conductances to CO2 are the main limitations to photosynthesis in sugar beet (Beta vulgaris) plants grown with excess zinc

*The effects of zinc (Zn) toxicity on photosynthesis and respiration were investigated in sugar beet (Beta vulgaris) plants grown hydroponically with 1.2, 100 and 300 microM Zn. *A photosynthesis limitation analysis was used to assess the stomatal, mesophyll, photochemical and biochemical contributions to the reduced photosynthesis observed under Zn toxicity. *The main limitation to photosynthesis was attributable to stomata, with stomatal conductances decreasing by 76% under Zn excess and stomata being unable to respond to physiological and chemical stimuli. The effects of excess Zn on photochemistry were minor. Scanning electron microscopy showed morphological changes in stomata and mesophyll tissue. Stomatal size and density were smaller, and stomatal slits were sealed in plants grown under high Zn. Moreover, the mesophyll conductance to CO(2) decreased by 48% under Zn excess, despite a marked increase in carbonic anhydrase activity. Respiration, including that through both cytochrome and alternative pathways, was doubled by high Zn. *It can be concluded that, in sugar beet plants grown in the presence of excess Zn, photosynthesis is impaired due to a depletion of CO(2) at the Rubisco carboxylation site, as a consequence of major decreases in stomatal and mesophyll conductances to CO(2).

Effects of zinc toxicity on sugar beet (Beta vulgaris L.) plants grown in hydroponics

The effects of high Zn concentration were investigated in sugar beet (Beta vulgaris L.) plants grown in a controlled environment in hydroponics. High concentrations of Zn sulphate in the nutrient solution (50, 100 and 300 microm) decreased root and shoot fresh and dry mass, and increased root/shoot ratios, when compared to control conditions (1.2 microm Zn). Plants grown with excess Zn had inward-rolled leaf edges and a damaged and brownish root system, with short lateral roots. High Zn decreased N, Mg, K and Mn concentrations in all plant parts, whereas P and Ca concentrations increased, but only in shoots. Leaves of plants treated with 50 and 100 microm Zn developed symptoms of Fe deficiency, including decreases in Fe, chlorophyll and carotenoid concentrations, increases in carotenoid/chlorophyll and chlorophyll a/b ratios and de-epoxidation of violaxanthin cycle pigments. Plants grown with 300 microm Zn had decreased photosystem II efficiency and further growth decreases but did not have leaf Fe deficiency symptoms. Leaf Zn concentrations of plants grown with excess Zn were high but fairly constant (230-260 microg.g(-1) dry weight), whereas total Zn uptake per plant decreased markedly with high Zn supply. These data indicate that sugar beet could be a good model to investigate Zn homeostasis mechanisms in plants, but is not an efficient species for Zn phytoremediation.

Effects of zinc toxicity on sugar beet (Beta vulgaris L.) plants grown in hydroponics

The effects of high Zn concentration were investigated in sugar beet (Beta vulgaris L.) plants grown in a controlled environment in hydroponics. High concentrations of Zn sulphate in the nutrient solution (50, 100 and 300 microm) decreased root and shoot fresh and dry mass, and increased root/shoot ratios, when compared to control conditions (1.2 microm Zn). Plants grown with excess Zn had inward-rolled leaf edges and a damaged and brownish root system, with short lateral roots. High Zn decreased N, Mg, K and Mn concentrations in all plant parts, whereas P and Ca concentrations increased, but only in shoots. Leaves of plants treated with 50 and 100 microm Zn developed symptoms of Fe deficiency, including decreases in Fe, chlorophyll and carotenoid concentrations, increases in carotenoid/chlorophyll and chlorophyll a/b ratios and de-epoxidation of violaxanthin cycle pigments. Plants grown with 300 microm Zn had decreased photosystem II efficiency and further growth decreases but did not have leaf Fe deficiency symptoms. Leaf Zn concentrations of plants grown with excess Zn were high but fairly constant (230-260 microg.g(-1) dry weight), whereas total Zn uptake per plant decreased markedly with high Zn supply. These data indicate that sugar beet could be a good model to investigate Zn homeostasis mechanisms in plants, but is not an efficient species for Zn phytoremediation.

Effects of two iron sources on iron and cadmium allocation in poplar (Populus alba) plants exposed to cadmium

Effects of 10 microM cadmium (supplied as Cd nitrate) on the utilization and allocation of iron (Fe) were investigated in poplar (Populus alba L.) plants grown in nutrient solution with Fe(III)-EDTA or Fe(III)-citrate as the Fe source. The effects of Cd were also compared with those of Fe deprivation. The accumulation of Fe in roots was 10-fold higher in plants grown with Fe-citrate than with Fe-EDTA. Cadmium decreased leaf chlorophyll concentrations and photosynthetic rates, and these decreases were more marked in plants grown with Fe-citrate than with Fe-EDTA. In both Fe treatments, addition of Cd caused large increases in root and shoot apoplasmic and non-apoplasmic Cd contents and increases in root Fe content; however, Cd decreased shoot Fe content, especially in plants grown with Fe-citrate. New leaves of plants grown with Fe-citrate had small cellular (non-apoplasmic) Fe pools, whereas these pools were large in new leaves of plants grown with Fe-EDTA. Non-apoplasmic Cd pools in new leaves were smaller in plants grown with Fe-citrate than with Fe-EDTA, indicating that inactivation of non-apoplasmic Cd pools is facilitated more by Fe-EDTA than by Fe-citrate. In the presence of Cd, Fe-EDTA was also superior to Fe-citrate in maintaining an adequate Fe supply to poplar shoots. Differences in plant responses to Fe-EDTA and Fe-citrate may reflect differences in long-distance transport of Fe rather than in acquisition of Fe by roots.

Biochemical responses to iron deficiency in kiwifruit (Actinidia deliciosa)

A comparative study of two kiwifruit genotypes (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson var. deliciosa) with different tolerance to iron (Fe) deficiency was conducted to identify biochemical features associated with tolerance to Fe deficiency. After 14 days of growth in hydroponic culture under Fe-deficient and Fe-sufficient conditions, leaf chlorophyll concentration, activities of ferric chelate reductase (FCR), phosphoenolpyruvate carboxylase (PEPC) and citrate synthase in root extracts, concentrations of organic acids in roots, leaves and xylem sap, and xylem sap pH were measured. In response to Fe deficiency, the tolerant genotype D1 showed: (i) higher FCR activity associated with a longer lasting induction of FCR; (ii) higher PEPC activity; (iii) higher concentrations of citric acid in roots; and (iv) lower xylem sap pH compared with the susceptible genotype Hayward. These findings imply that induction of FCR and PEPC activities in roots in response to Fe deficiency are important physiological adaptations enabling Fe-efficient kiwifruit plants to tolerate Fe deficiency.

Characterization of the responses of cork oak (Quercus suber) to iron deficiency

We studied responses of cork oak (Quercus suber L.) to iron (Fe) deficiency by comparing seedlings grown hydroponically in nutrient solution with and without Fe. Seedlings grown without Fe developed some responses typical of the Strategy I group of Fe-efficient plants, including two- and fourfold increases in plasma membrane ferric chelate reductase activity of root tips after 2 and 4 weeks of culture in the absence of Fe, respectively. Moreover, seedlings grown hydroponically for 2 weeks without Fe caused marked decreases in the pH of the nutrient solution, indicating that root plasma membrane ATPase activity was induced by Fe deficiency. Iron deficiency also caused marked decreases in leaf chlorophyll and carotenoid concentrations, and chlorophyll concentrations were decreased more than carotenoid concentrations. Iron deficiency resulted in an 8% decrease in the dark-adapted efficiency of photosystem II and a 43% decrease in efficiency of photosystem II at steady-state photosynthesis. No major root morphological changes were observed in seedlings grown without Fe, although seedlings grown in Fe-deficient nutrient solution had light-colored roots in contrast to the dark brown color of control roots.

Iron deficiency-associated changes in the composition of the leaf apoplastic fluid from field-grown pear (Pyrus communis L.) trees

Experiments have been carried out with field-grown pear trees to investigate the effect of iron chlorosis on the composition of the leaf apoplast. Iron deficiency was associated with an increase in the leaf apoplastic pH from the control values of 5.5-5.9 to 6.5-6.6, as judged from direct pH measurements in apoplastic fluid obtained by centrifugation and fluorescence of leaves incubated with 5-CF. The major organic acids found in leaf apoplastic fluid of iron-deficient and iron-sufficient pear leaves were malate, citrate and ascorbate. The total concentration of organic acids was 2.9 mM in the controls and increased to 5.5 mM in Fe-deficient leaves. The total apoplastic concentration of inorganic cations (Ca, K and Mg) increased with Fe deficiency from 15 to 20 mM. The total apoplastic concentration of inorganic anions (Cl-, NO3-, SO4(2-) and HPO4(2-)) did not change with Fe deficiency. Iron concentrations decreased from 4 to 1.6 microM with Fe deficiency. The major Fe species predicted to exist in the apoplast was [FeCitOH](-1) in both Fe-sufficient and deficient leaves. Organic acids in whole leaf homogenates increased from 20 to 40 nmol x m(-2) with Fe deficiency. The accumulation of organic anions in the Fe-deficient leaves does not appear to be associated to an increased C fixation in leaves, but rather it seems to be a consequence of C transport via xylem.

Technical advance: reduction of Fe(III)-chelates by mesophyll leaf disks of sugar beet. Multi-component origin and effects of Fe deficiency

The characteristics of the Fe(III)-chelate reductase activity have been investigated in mesophyll disks of Fe-sufficient and Fe-deficient sugar beet leaves. The Fe(III)-chelate reductase activity of mesophyll disks was light dependent and increased markedly when the epidermis was removed. Iron(III)-citrate was photo-reduced directly by light in the absence of plant tissue. Total reductase activity was the sum of enzymatic mesophyll reduction, enzymatic reduction carried out by organelles exposed at the disk edge and reduction caused by the release of substances both by exposed mesophyll cells and at the disk edge. Compounds excreted were shown by HPLC to include organic anions, mainly oxalate, citrate and malate. When expressed on a leaf surface basis, Fe deficiency decreased the total mesophyll Fe(III)-chelate reductase activity. However, Fe-sufficient disks reduced less Fe than the Fe-deficient ones when expressed on a chlorophyll basis. The optimal pH values for Fe(III) reduction were always in the range 6.0-6.7. In control leaves Fe(III)-citrate and Fe(III)-malate were the substrates that led to the highest Fe reduction rates. In Fe-deficient leaves Fe(III)-malate led to the highest Fe reduction rates, followed by Fe(III)-EDTA and then Fe(III)-citrate. K:(m) values for the total reductase activity, enzymatic mesophyll reduction and enzymatic reduction carried out by organelles at the disk edge were obtained.

Iron deficiency interrupts energy transfer from a disconnected part of the antenna to the rest of Photosystem II

Iron deficiency changed markedly the shape of the leaf chlorophyll fluorescence induction kinetics during a dark-light transition, the so-called Kautsky effect. Changes in chlorophyll fluorescence lifetime and yield were observed, increasing largely the minimal and the intermediate chlorophyll fluorescence levels, with a marked dip between the intermediate and the maximum levels and loss of the secondary peak after the maximum. During the slow changes, the lifetime-yield relationship was found to be linear and curvilinear (towards positive lifetime values) in control and Fe-deficient leaves, respectively. These results suggested that part of the Photosystem II antenna in Fe-deficient leaves emits fluorescence with a long lifetime. In dark-adapted Fe-deficient leaves, measurements in the picosecond-nanosecond time domain confirmed the presence of a 3.3-ns component, contributing to 15% of the total fluorescence. Computer simulations revealed that upon illumination such contribution is also present and remains constant, indicating that energy transfer is partially interrupted in Fe-deficient leaves. Photosystem II-enriched membrane fractions containing different pigment-protein complexes were isolated from control and Fe-deficient leaves and characterized spectrophotometrically. The photosynthetic pigment composition of the fractions was also determined. Data revealed the presence of a novel pigment-protein complex induced by Fe deficiency and an enrichment of internal relative to peripheral antenna complexes. The data suggest a partial disconnection between internal Photosystem II antenna complexes and the reaction center, which could lead to an underestimation of the Photosystem II efficiency in dark-adapted, low chlorophyll Fe-deficient leaves, using chlorophyll fluorescence.

Effects of iron deficiency on the composition of the leaf apoplastic fluid and xylem sap in sugar beet. Implications for iron and carbon transport

The effects of iron deficiency on the composition of the xylem sap and leaf apoplastic fluid have been characterized in sugar beet (Beta vulgaris Monohil hybrid). pH was estimated from direct measurements in apoplastic fluid and xylem sap obtained by centrifugation and by fluorescence of leaves incubated with 5-carboxyfluorescein and fluorescein isothiocyanate-dextran. Iron deficiency caused a slight decrease in the pH of the leaf apoplast (from 6.3 down to 5.9) and xylem sap (from 6.0 down to 5.7) of sugar beet. Major organic acids found in leaf apoplastic fluid and xylem sap were malate and citrate. Total organic acid concentration in control plants was 4.3 mM in apoplastic fluid and 9.4 mM in xylem sap and increased to 12.2 and 50.4 mM, respectively, in iron-deficient plants. Inorganic cation and anion concentrations also changed with iron deficiency both in apoplastic fluid and xylem sap. Iron decreased with iron deficiency from 5.5 to 2.5 microM in apoplastic fluid and xylem sap. Major predicted iron species in both compartments were [FeCitOH](-1) in the controls and [FeCit(2)](-3) in the iron-deficient plants. Data suggest the existence of an influx of organic acids from the roots to the leaves via xylem, probably associated to an anaplerotic carbon dioxide fixation by roots.

Responses of sugar beet roots to iron deficiency. Changes in carbon assimilation and oxygen use

Different root parts with or without increased iron-reducing activities have been studied in iron-deficient and iron-sufficient control sugar beet (Beta vulgaris L. Monohil hybrid). The distal root parts of iron-deficient plants, 0 to 5 mm from the root apex, were capable to reduce Fe(III)-chelates and contained concentrations of flavins near 700 microM, two characteristics absent in the 5 to 10 mm sections of iron-deficient plants and the whole root of iron-sufficient plants. Flavin-containing root tips had large pools of carboxylic acids and high activities of enzymes involved in organic acid metabolism. In iron-deficient yellow root tips there was a large increase in carbon fixation associated to an increase in phosphoenolpyruvate carboxylase activity. Part of this carbon was used, through an increase in mitochondrial activity, to increase the capacity to produce reducing power, whereas another part was exported via xylem. Root respiration was increased by iron deficiency. In sugar beet iron-deficient roots flavins would provide a suitable link between the increased capacity to produce reduced nucleotides and the plasma membrane associated ferric chelate reductase enzyme(s). Iron-deficient roots had a large oxygen consumption rate in the presence of cyanide and hydroxisalycilic acid, suggesting that the ferric chelate reductase enzyme is able to reduce oxygen in the absence of Fe(III)-chelates.

Iron deficiency decreases the Fe(III)-chelate reducing activity of leaf protoplasts

The ferric-chelate reductase (FC-R) activity of mesophyll protoplasts isolated from Fe-sufficient (control) and Fe-deficient sugar beet (Beta vulgaris L.) leaves has been characterized. Measurements were made in an ionic environment similar to that in the apoplastic space of the sugar beet mesophyll cells. The FC-R activity of Fe-sufficient and Fe-deficient protoplasts was dependent on light. Fe deficiency decreased markedly the FC-R activity per protoplast surface unit. The optimal pH for the activity of the FC-R in mesophyll protoplasts was in the range 5.5 to 6.0, typical of the apoplastic space. Beyond pH 6.0, the activity of the FC-R in mesophyll protoplasts decreased markedly in both Fe-sufficient and Fe-deficient protoplasts. These data suggest that both the intrinsic decrease in FC-R activity per protoplast surface and a possible shift in the pH of the apoplastic space could lead to the accumulation of physiologically inactive Fe pools in chlorotic leaves.

Photosystem II efficiency and mechanisms of energy dissipation in iron-deficient, field-grown pear trees (Pyrus communis L.)

The dark-adapted Photosystem II efficiency of field-grown pear leaves, estimated by the variable to maximum chlorophyll fluorescence ratio, was little affected by moderate and severe iron deficiency. Only extremely iron-deficient leaves showed a decreased Photosystem II efficiency after dark adaptation. Midday depressions in Photosystem II efficiency were still found after short-term dark-adaptation in iron-deficient leaves, indicating that Photosystem II down-regulation occurred when the leaves were illuminated by excessive irradiance. The actual Photosystem II efficiency at steady-state photosynthesis was decreased by iron deficiency both early in the morning and at midday, due to closure of Photosystem II reaction centers and decreases of the intrinsic Photosystem II efficiency. Iron deficiency decreased the amount of light in excess of that which can be used in photosynthesis not only by decreasing absorptance, but also by increasing the relative amount of light dissipated thermally by the Photosystem II antenna. When compared to the controls, iron-deficient pear leaves dissipated thermally up to 20% more of the light absorbed by the Photosystem II, both early in the morning and at midday. At low light iron-deficient leaves with high violaxanthin cycle pigments to chlorophyll ratios had increases in pigment de-epoxidation, non-photochemical quenching and thermal dissipation. Our data suggest that DeltapH could be the major factor controlling thermal energy dissipation, and that large (more than 10-fold) changes in the zeaxanthin plus antheraxanthin to chlorophyll molar ratio caused by iron deficiency were associated only to moderate increases in the extent of photoprotection.

Riboflavin 3'- and 5'-sulfate, two novel flavins accumulating in the roots of iron-deficient sugar beet (Beta vulgaris)

Roots from iron-deficient sugar beet grown in the presence of calcium carbonate exhibit a yellow color and autofluorescence typical of flavin-like compounds, whereas roots of control, iron-sufficient plants exhibited no yellow color and extremely low autofluorescence. The two major flavins whose accumulation is induced by iron deficiency have been shown to be different from riboflavin, FMN, and FAD by reversed-phase high performance liquid chromatography. These flavins, accounting for 82 and 15% of the total flavin concentration in deficient roots, have been shown unequivocally to be riboflavin 3'-sulfate and riboflavin 5'-sulfate, respectively, by electrospray-mass spectrometry, inductively coupled plasma emission spectroscopy, infrared spectrometry, and 1H nuclear magnetic resonance. These flavin sulfates have not been found previously in biological systems. The localization of riboflavin sulfates in deficient roots is similar, but not identical, to that of high iron reductase activity. The concentration of riboflavin sulfates has been estimated from root extracts to be at least 1 mM. We hypothesize, based on the similar localization of flavin and that of iron reduction, that the accumulation of riboflavin sulfates induced by iron deficiency may be an integral part of the turbo iron-reducing system in sugar beet roots.

Chlorophyll Fluorescence and Photon Yield of Oxygen Evolution in Iron-Deficient Sugar Beet (Beta vulgaris L.) Leaves

The response of sugar beet (Beta vulgaris L.) leaves to iron deficiency can be described as consisting of two phases. In the first phase, leaves may lose a large part of their chlorophyll while maintaining a roughly constant efficiency of photosystem II photochemistry; ratios of variable to maximum fluorescence decreased by only 6%, and photon yields of oxygen evolution decreased by 30% when chlorophyll decreased by 70%. In the second phase, when chlorophyll decreased below a threshold level, iron deficiency caused major decreases in the efficiency of photosystem II photochemistry and in the photon yield of oxygen evolution. These decreases in photosystem II photochemical efficiency were found both in plants dark-adapted for 30 minutes and in plants dark-adapted overnight, indicating that photochemical efficiency cannot be repaired in that time scale. Decreases in photosystem II photochemical efficiency and in the photon yield of oxygen evolution were similar when measurements were made (a) with light absorbed by carotenoids and chlorophylls and (b) with light absorbed only by chlorophylls. Leaves of iron-deficient plants exhibited a room temperature fluorescence induction curve with a characteristic intermediate peak I that increases with deficiency symptoms.

Characterization of the Xanthophyll Cycle and Other Photosynthetic Pigment Changes Induced by Iron Deficiency in Sugar Beet (Beta vulgaris L.)

In this work we characterize the changes induced by iron deficiency in the pigment composition of sugar beet (Beta vulgaris L.) leaves. When sugar beet plants were grown hydroponically under limited iron supply, neoxanthin and beta-carotene decreased concomitantly with chlorophyll a, whereas lutein and the carotenoids within the xanthophyll cycle were less affected. Iron deficiency caused major increases in the lutein/chlorophyll a and xanthophyll cycle pigments/chlorophyll a molar ratios. Xanthophyll cycle carotenoids in Fe-deficient plants underwent epoxidations and de-epoxidations in response to ambient light conditions. In dark adapted Fe-deficient plants most of the xanthophyll cycle pigment pool was in the epoxidated form violaxanthin. We show, both by HPLC and by in vivo 505 nanometers absorbance changes, that in Fe deficient plants and in response to light, the de-epoxidated forms antheraxanthin and zeaxanthin were rapidly formed at the expense of violaxanthin. Several hours after returning to dark, the xanthophyll cycle was shifted again toward violaxanthin. The ratio of variable to maximum chlorophyll fluorescence from intact leaves was decreased by iron deficiency. However, in iron deficient leaves this ratio was little affected by light conditions which displace the xanthophyll cycle toward epoxidation or de-epoxidation. This suggests that the functioning of the xanthophyll cycle is not necessarily linked to protection against excess light input.

A New Reversed Phase-HPLC Method Resolving All Major Higher Plant Photosynthetic Pigments

A new reversed phase-high performance liquid chromatography method has been developed to analyze the full complement of higher plant photosynthetic pigments (cis-neoxanthin, neoxanthin, violaxanthin, taraxanthin, anteraxanthin, lutein, zeaxanthin, cis-lutein, chlorophyll b, chlorophyll a, alpha- and beta-carotene). The separation is carried out on a C(18) column in about 10 minutes, using a single high-pressure pump and three different mobile phases in three isocratic steps. This method introduces a major improvement in higher plant photosynthetic pigment analysis, resolving in only 10 minutes all photosynthetic pigments while achieving good separation of lutein from its isomer zeaxanthin. Zeaxanthin is involved in the xanthophyll cycle, which recently has been proposed to play a significant role in the protection of the photosynthetic apparatus from photoinhibitory conditions (Demmig et al. [1987] Plant Physiol 84: 218-224).

Variability and correlations in muskmelon in relation to the cultivation method

Six fruit characters have been measured in 23 cultivars of Cucumis melo, representing a wide geographical range. Plants were grown both in the greenhouse and in the field. When the 23 cultivars were analyzed together, the largest component of variance was found between cultivars under both growth conditions, suggesting the existence of large genetic diversity for all the characters studied. Generally, variance between plants within cultivars was less than or equal to variance between fruits within plant. This indicates that environmental variation is the most important part of the variation within cultivars. Correlations between pairs of characters at cultivar, plant and fruit levels were calculated from the variance-covariance components. In the majority of paired traits, the correlation values indicated that genetic and environmental factors may act in the same direction.

Chlorophyll-Proteins and Electron Transport during Iron Nutrition-Mediated Chloroplast Development

Chlorophyll-protein complexes and electron transport activities were measured during iron nutrition-mediated chloroplast development in sugar beet (Beta vulgaris L. cv F58-554H1). Results showed that the chlorophyll-protein complexes associated with the reaction centers of photosystem I (CP1) and photosystem II (CPa) and the electron transport activities of these two photosystems per leaf area increased rapidly during the first 24 to 48 hours of iron resupply to iron-deficient sugar beet plants. Bulk chlorophyll and the amounts of light-harvesting chlorophyll-proteins increased after a lag period of 24 hours. The changes in chlorophyll-proteins with time were apparently the cause of an initial increase, then decrease, in the chlorophyll a/b ratio during iron resupply. There was evidence that iron deficiency diminished photosystem I more than photosystem II. We propose that there are two distinct phases in iron nutrition-mediated chloroplast development: (a) the commencement of the synthesis of the lipid matrix of the thylakoid membrane, including a fully functioning electron transport (and photosynthetic) system, during the first 24 hours of iron resupply; and (b) after 24 to 48 hours, the formation of the bulk of the thylakoid proteins, including the light-harvesting chlorophyll-proteins with which the large increase in total chlorophyll is associated.