Relationship between leaf apoplast pH and iron chlorosis of sunflower (Helianthus annuus L.)

A hypothesis has been presented and tested that bicarbonate (HCO₃) and nitrate (NO₃) are the most important anions inducing iron (Fe) chlorosis because these anions increase the pH of leaf apoplast which in turn depresses ferric‐iron [Fe(III]) reduction, and hence, the uptake of Fe into the symplasm. Experiments with young sunflower (Helianthus annuus) plants showed that nutrition with NO₃ as the sole nitrogen (N) source induced chlorosis whereas ammonium nitrate (NH₄NO₃) did not. Monohydrogen carbonate (bicarbonate) also favoured the development of chlorosis. The degree of chlorosis was not related to the Fe concentration in the leaves. Both anion species, NO₃ and HCO₃, increased the pH of the leaf apoplast which was measured by means of the fluorescence dye 5‐carboxyfluorescein. A highly significant negative correlation between leaf apoplast pH and chlorophyll concentration in the leaves (r = ‐0.97) was found. Ferric‐Fe reduction in the apoplast—measured by means of ferrocene—provided evidence that a low leaf apoplast pH, obtained with ammonium (NH₄) supply, favoured the reduction of Fe(III) as compared with a higher leaf apoplast pH obtained with NO₃ supply. These results support the hypothesis tested.     

Effect of Various Nitrogen Forms on the pH in Leaf Apoplast and on Iron Chlorosis of Glycine max L.

The effect of NH₄NO₃ (control) and increasing NO₃- levels in nutrient solutions containing no and 100 μM Fe respectively on iron chlorosis of Glycine max was investigated. After two weeks of growth apoplastic pH in excised leaves was measured by means of fluorescence. In plants growing without Fe supply increasing concentrations of NO₃- in the nutrient solution which also was applied to the cut end of the petiole, resulted in a pH increase in the leaf apoplast from 5.34 (NH₄NO₃) to 5.50 (NO₃-) associated with chlorosis observed with intact plants. A close negative correlation was found between chlorophyll concentration and pH in the apoplast (r = ?0.97). While leaves in the treatment exclusively fed with NO₃- were strongly chlorotic, those in the NH₄NO₃ treatment were green. With exception of the plants only fed with NO₃- the Fe concentration in the leaves was not affected by the type of N nutrition. It is therefore assumed that some Fe is immobilized in the leaf tissue by high apoplast pH induced by an increase in the proportion of nitrate in the nutrient solution. Plants fed with Fe (100 μM) showed no chlorosis, regardless of the form of N nutrition and hence regardless of apoplast pH. The Fe concentration in leaves of Fe fed plants was approximately twice those in the leaves not supplied with Fe.     

Depressed growth of non‐chlorotic vine grown in calcareous soil is an iron deficiency symptom prior to leaf chlorosis

The development of iron deficiency symptoms (growth depression and yellowing of the youngest leaves) and the distribution of iron between roots and leaves were investigated in different vine cultivars (Silvaner, Riparia 1G and SO4) grown in calcareous soils. As a control treatment all cultivars were also grown in an acidic soil. Only the cultivars Silvaner and Riparia 1G showed yellowing of the youngest leaves under calcareous soil conditions at the end of the cultivation period. All cultivars including SO4 showed severe shoot growth depression, by 50 % and higher, before yellowing started or without leaf yellowing in the cultivar SO4. Depression of shoot growth occurred independently from that of root growth. In a further treatment the effect of Fe‐EDDHA spraying onto the shoot growth of the cultivar Silvaner after cultivation in calcareous soil was investigated. Prior to Fe application plants were non‐chlorotic, but showed pronounced shoot growth depression. Spraying led to a significant increase in shoot length, though leaf growth was not increased. Accordingly, depression of shoot growth of non‐chlorotic plants under calcareous soil conditions and with ample supply of nutrients and water has been evidenced to be at least partly an iron deficiency symptom. We suggest that plant growth only partially recovered because of dramatic apoplastic leaf Fe inactivation and/ or a high apoplastic pH which may directly impair growth. Since growth was impaired before the youngest leaves showed chlorosis we assume that meristematic growth is more sensitively affected by Fe deficiency than is chlorophyll synthesis and chloroplast development. In spite of high Fe concentrations in roots and leaves of the vines grown in calcareous soils plants suffered from Fe deficiency. The finding of high Fe concentrations also in young, but growth retarded green leaves is a further indication that iron deficiency chlorosis in calcareous soils is caused by primary leaf Fe inactivation. However, in future, only a rigorous study of the dynamic changes of iron and chlorophyll concentration, leaf growth and apoplastic pH at the cellular level during leaf development and yellowing will provide causal insights between leaf iron inactivation, growth depression, and leaf chlorosis.<?show $6#>     

Comparative investigation on the susceptibility of faba bean (Vicia faba L.) and sunflower (Helianthus annuus L.) to iron chlorosis

Comparative physiological studies on iron (Fe) chlorosis of Vicia faba L. and Helianthus annuus L. were carried out. High internal Fe contents in Vicia cotyledons (16–37 μg) were completely used for plant growth and Fe chlorosis was not inducible by the application of nitrate (with or without bicarbonate). In Helianthus, low quantities of Fe in the seeds (4 μg) were insufficient for normal growth and without Fe in the nutrient solution, Fe chlorosis was obtained in all treatments. This chlorosis was an absolute Fe deficiency. Also, the treatment with 1 μM Fe in the nutrient solution and nitrate (with or without bicarbonate) led to severe chlorotic symptoms associated with low leaf Fe concentrations and high Fe concentrations in the roots. In contrast, Helianthus grown with NH₄NO₃ and 1 μM Fe had green leaves and high leaf Fe concentrations. However, with NO₃ supply (with or without bicarbonate), Fe translocation from the roots to the upper plant parts was restricted and leaves were chlorotic. Chlorotic and green sunflower leaves may have the same Fe concentrations, leaf Fe concentration being dependent on Fe translocation into the leaf at the various pH levels in the nutrient solution. At low external pH levels (controlled conditions) more Fe was translocated into the leaf leading to similar leaf Fe concentrations with higher chlorophyll concentrations (NH₄NO₃) and with lower chlorophyll concentrations (NO₃). This indicates a lower utilization of leaf Fe of NO₃ grown sunflower plants. Utilization of Fe in faba bean leaves is presumably higher than in sunflower leaves. In Vicia xylem sap pH was not affected by nitrate. In contrast, the xylem sap pH in Helianthus was permanently increased by about 0.4 pH units when fed with nitrate (with or without bicarbonate) compared with NH₄NO₃ nutrition. The xylem sap pH is indicative of leaf apoplast pH. From our earlier work (Mengel et al., 1994; Kosegarten und Englisch, 1994) we therefore suppose that in Helianthus, Fe immobilization occurs in the leaf apoplast due to high pH levels when grown with nitrate (with or without bicarbonate).     

Effect of K+ nutrition,leaf age and light intensity on apoplastic K+ in leaves of Vicia faba

The infiltration-centrifugation method is valuable for the estimation of ion concentrations within the leaf apoplast because it opens up the possibility of differentiating between free and bound cations. The K⁺ concentration in the apoplast of intact leaves of Vicia faba reflects the nutritional status with respect of this ion and can be monitored by the infiltration-centrifugation technique. Transfer into a nutrient solution without K⁺ led to a significant decrease in apoplastic K⁺ within two days. Under K⁺ deficiency, the K⁺ concentration in the leaf apoplast was maintained at 2 mM. Regardless of K⁺ supply, young leaves had higher K⁺ concentrations in the apoplast than older ones. Data on diurnal variations in apoplastic K⁺ suggest that the leaf apoplast is a transient pool for K⁺ accumulation within the leaf prior to absorption into the symplast or retranslocation to other growing tissues.     

INFLUENCE OF CHEMICAL FORM AND CONCENTRATION OF NITROGEN ON APOPLASTIC pH OF LEAVES

The objective of this study was to investigate the effects of various forms of nitrogen (NO^? ₃, NH⁺ ₄) supplied to the roots via a nutrient solution on the apoplastic pH in intact leaves determined by fluorescence ratio imaging. In contrast to NH⁺ ₄, higher apoplastic pH values in leaves of Phaseolus vulgaris and Helianthus annuus were measured with NO^? ₃ nutrition. In this context no significant differences were found in leaves of Vicia faba and Zea mays supplied with the various forms of N. Comparative studies on apoplastic pH in leaves of Vicia faba, Zea mays and Helianthus annuus demonstrated that NO^? ₃ reductase activity in roots was responsible for the differences in NO^? ₃ concentration and pH in the leaf apoplast. Light-induced pH changes in the leaf apoplast also occur and may overlap the effects of various forms of N. Increasing concentrations of NO^? ₃ supply to the roots did not significantly affect apoplastic pH in leaves of Helianthus annuus. Depletion of NO^? ₃ in the nutrient solution led to lower apoplastic pH in leaves of Zea mays. Leaf fertilization with NH⁺ ₄ led to a decline in apoplastic pH of leaves whereas NH₃ gas exposure caused a biphasic response in apoplastic pH.     

Einfluß von Bikarbonat auf die subzelluläre Verteilung von blatt- und wurzelappliziertem Eisen bei Sonnenblumen (Helianthus annuus L.)

Influence of bicarbonate on the subcellular distribution of iron applied to roots or leaves of sunflower (Helianthus annuus L.) 18 days old sunflower seedlings were transferred and cultivated for 9 days ( untill chlorosis appeared) in nutrient solutions. After that Fe concentration of roots and shoots and the subcellular distribution of Fe in the cytoplasm of the young leaves was determined. Bicarbonate in the nutrient solution with Fe reduced the concentration of Fe and chlorophyll in the young leaves of the plants, also the concentration of Fe and protein in the chloroplast fraction of the cytoplasm, but the subcellular distribution for Fe remained unchanged compared with the control. Leaf spray with Fe-EDTA to plants in nutrient solution without Fe + bicarbonate resulted in higher Fe but unchanged chlorophyll concentrations in the young leaves, while the cytoplasm fractions of these leaves had higher concentrations of iron and protein compared with the control. An inactivation of leaf iron by bicarbonate in the nutrient medium could not be demonstrated. There was no significant lowering of the concentration of disolved Fe in the nutrient solution by bicarbonate, indicating a disturbance of Fe-up-take rather than an insufficient Fe-supply as a factor for iron chlorosis. The physiological activity of leaf applied Fe was not diminished by bicarbonate in the nutrient solution. This observation too points to a primary effect of bicarbonate in the root area. The pH of the cytoplasm from young leaves remained unchanged after leaf spraying with Fe-EDTA. In spite of this there might be a local effect of sprayed solution (with pH 5,1) on the pH of solutes in the apoplast, influencing the mobility of leaf applied Fe.     

Iron chlorosis and nitrate reductase activity in response to Fe stress and additional nitrate in sorghum

Effect of additional nitrate supply on chlorosis development at different levels of Fe stress was examined in iron efficient and inefficient sorghum cultivars. Nitrate reductase, the enzyme primarily responsible for nitrate reduction was estimated in roots and leaves as affected by Fe stress and additional nitrate.

Young leaves showed differences in chlorosis progression at Fe stress levels. Nitrate reductase activity was depressed in roots and young leaves when iron supply was reduced beyond certain level (<0.0015 gm FeSO₄/1). Addition of extra nitrate was not able to enhance NR activity when iron supply was lower than ½ level. It was inferred that nitrate utilisation and iron nutrition influence each other and some minimu level of iron supply is necessary for efficient enzyme activity.     

Uptake of Iron (59Fe) Complexed to Water‐Extractable Humic Substances by Sunflower Leaves

Abstract

A research was carried out to evaluate the leaves' ability to utilize Fe supplied as a complex with water‐extractable humic substances (WEHS) and the long‐distance transport of ⁵⁹Fe applied to sections of fully expanded leaves of intact sunflower (Helianthus annuus L.) plants. Plants were grown in a nutrient solution containing 10 µM Fe(III)‐EDDHA (Fe‐sufficient plants), with the addition of 10 mM NaHCO₃ to induce iron chlorosis (Fe‐deficient plants). Fe(III)‐WEHS could be reduced by sunflower leaf discs at levels comparable to those observed using Fe(III)‐EDTA, regardless of the Fe status. On the other hand, ⁵⁹Fe uptake rate by leaf discs of green and chlorotic plants was significantly lower in Fe‐WEHS‐treated plants, possibly suggesting the effect of light on photochemical reduction of Fe‐EDTA. In the experiments with intact plants, ⁵⁹Fe‐labeled Fe‐WEHS or Fe‐EDTA were applied onto a section of fully expanded leaves. Irrespective of Fe nutritional status, ⁵⁹Fe uptake was significantly higher when the treatment was carried out with Fe‐EDTA. A significant difference was found in the amount of ⁵⁹Fe translocated from treated leaf area between green and chlorotic plants. However, irrespective of the Fe nutritional status, no significant difference was observed in the absolute amount of ⁵⁹Fe translocated to other plant parts when the micronutrient was supplied either as Fe‐EDTA or Fe‐WEHS. Results show that the utilization of Fe complexed to WEHS by sunflower leaves involves an Fe(III) reduction step in the apoplast prior to its uptake by the symplast of leaf cells and that Fe taken up from the Fe‐WEHS complexes can be translocated from fully expanded leaves towards the roots and other parts of the shoot.     

Physiology of manganese toxicity and tolerance in Vigna unguiculata (L.) Walp.

In cowpea (Vigna unguiculata (L.) Walp.) tolerance of manganese (Mn) excess depends on genotype, silicon (Si) nutrition, form of nitrogen (N) supply, and leaf age. The physiological mechanisms for improved Mn leaf-tissue tolerance are still poorly understood. On the basis of the density of brown spots per unit of leaf area and the callose content which are sensitive indicators of Mn toxicity, it was confirmed that cultivar (cv.) TVu 1987 was more Mn-tolerant than cv. TVu 91, young leaves were more Mn-tolerant, Si improved Mn tolerance, and NO₃^—-grown plants were more Mn-tolerant than NH₄⁺-grown plants. A close positive relationship existed between the bulk-leaf Mn content and the vacuolar Mn concentration from the same leaves. Since no clear and consistent differences existed between leaf tissues differing in Mn tolerance, the results suggest that accumulation of Mn in the vacuoles and its complexation by organic anions do not play a role in Mn leaf-tissue tolerance in cowpea. A near linear relationship was found between leaf Mn contents and concentrations of free (H₂O-soluble) and exchangeable-bound (BaCl₂-extractable) Mn in the apoplastic washing fluid (AWF) extracted from whole leaves by an infiltration and centrifugation technique. There were no differences in apoplastic Mn concentrations owing to genotype and form of nitrogen nutrition. However, Si decreased the Mn concentration in the AWF. With increasing bulk-leaf Mn contents, concentrations of organic anions in the AWF also increased. The results suggest that complexation of Mn by organic anions in the leaf apoplast contribute to Mn tolerance due to genotype and more clearly due to NO₃-N nutrition. Cell wall-bound peroxidase activity increased with leaf age and was higher in the Mn-sensitive cv. TVu 91 than in cv. TVu 1987. This was in agreement with a higher H₂O₂ production rate in cv. TVu 91. Also, a lower ratio of reduced to oxidized ascorbic acid in the AWF revealed that in Mn-sensitive leaf tissue, the apoplastic reduction capacity was lower than in Mn-tolerant leaf tissue when genotypes and leaves of different age were compared. We interpret our results as strong circumstantial evidence that Mn tolerance depends on the control of the free Mn²⁺concentration and of Mn²⁺-mediated oxidation/reduction reactions in the leaf apoplast.     

Use of Minolta SPAD‐502 chlorophyll meter to quantify the effectiveness of mid‐summer trunk injection of iron on chlorotic pear trees

The severity of leaf chlorosis in iron (Fe)‐deficient fruit trees is often characterized using a semi‐quantitative visual rating index that is subject to evaluator bias. Analytical instruments are now available that provide a quantitative measure of leaf green color that could substitute for visual ratings. We injected limbs of mature chlorotic pear trees (Pyrus communis L. cv. Bartlett) with distilled water or a solution of 0.1% Fe (w/v) as FeSO₄‐7H₂O on 17 July 1995. Treatments were replicated eight‐fold. On 18 August 1995, a Minolta SPAD‐502 chlorophyll meter was used to measure the green color of 30 randomly sampled leaves located above the point of injection on each injected limb. Average leaf green color was higher in the Fe‐injected tree than in the water‐injected tree of each experimental block. Leaf green color (mean±SD) averaged 34.7±3.8 SPAD units for the Fe‐injected trees and 27.3±3.8 SPAD units for the water‐injected trees. The absolute increase in mean leaf color of 7.4 SPAD units was equivalent to a relative increase of 27%. Iron injection also induced more negative skewness and increased kurtosis in the frequency distribution curve for leaf SPAD meter readings. These results suggest that the SPAD meter can provide an unbiased quantitative measure of the severity of leaf chlorosis associated with Fe deficiency, and confirm that mid‐summer trunk injection of Fe can partially ameliorate Fe‐chlorosis symptoms.     

Iron deficiency and zinc toxicity in soybean grown in nutrient solution with different levels of sulfur

A typical symptom of iron (Fe) deficiency in plants is yellowing or chlorosis of leaves. Heavy metal toxicity, including that of zinc (Zn), is often also expressed by chlorosis and may be called Fe chlorosis. Iron deficiency and Zn toxicity were evaluated in soybean (Glycine max [L.] Merr.) at two levels each of Zn (0.8 and 40 μM), Fe (0 and 20 μM), and sulfur (S) (0.02 and 20 mM). Reduction in dry matter yield and leaf chlorosis were observed in plants grown under the high level of Zn (toxic level), as well as in the absence of Fe. Zinc toxicity, lack of Fe, and the combination of these conditions reduced dry matter yield to the same extent when compared to the yield of the control plants. The symptoms of Zn toxicity were chlorosis in the trifoliate leaves and a lack of change in the orientation of unifoliate leaves when exposed to light. The main symptoms of Fe deficiency were chlorosis in the whole shoot and brown spots and flaccid areas in the leaves. The latter symptom did not appear in plants grown with Fe but under Zn toxicity. It seems that Fe deficiency is a major factor impairing the growth of plants exposed to high levels of Zn. Under Zn toxicity, Fe and Zn translocation from roots to shoots increased as the S supply to the plants was increased.     

Lead uptake,distribution, and remobilization in cucumber

Mobility and distribution of lead (Pb) were investigated in cucumber (Cucumis sativus L.) grown in solution culture. Based on the observation that Pb uptake is influenced by the chemical form of iron (Fe) (complexed or ionic) in the solution, Fe distribution was also determined. Iron concentration was even in the stem (separated to internodes) and petioles but slightly increased at the shoot tip while Pb concentration decreased upwards. In the leaves Fe concentration decreased upwards (whereas in the control it increased a little at the youngest leaves) while Pb concentration increased to the largest leaves then decreased towards the shoot tip. It is suggested that the distribution of Pb in cucumber is passive process and it accumulates in the apoplast while Fe distribution is determined by the requirements of synthetic processes in young or photosynthesizing tissues. Lead probably lowers Fe transport into the symplast in the leaves causing slight chlorosis at the youngest leaves. In the plants that were loaded with Pb, decapitated and rooted again in Pb‐free culture solution we found no significant remobilization of Pb which supports that Pb binds strongly to the binding sites in the apoplast or is detoxified in an immobile form.     

叶面追施硝态氮肥抑制菠萝营养生长效应

采用叶面淋施的盆栽试验方法,以我国菠萝主栽品种-巴厘为试材,研究不同形态氮素对盆栽菠萝营养生长和菠萝叶片黄化的影响,为菠萝氮肥合理施用提供参考。试验结果表明,叶面淋施硝态氮处理的菠萝根、茎叶生物量显著低于叶面淋施铵态氮、酰胺态氮,黄叶数显著高于叶面淋施铵态氮、酰胺态氮。与铵态氮相比,硝态氮处理的菠萝总叶数、根数目、根重、茎叶重分别减少18.7%、26.5%、49.7%、43.5%,黄叶数增加192.7%。叶面淋施硝态氮抑制菠萝营养生长主要机理是硝态氮提高了土壤p H值,减少了铁吸收,降低菠萝叶片中全铁、活性铁、叶绿素含量(与铵态氮相比,分别减少25.9%、66.9%、23.2%)。     

The central role of microbial activity for iron acquisition in maize and sunflower

 Maize (Zea mays L.) and sunflower (Helianthus annuus L.) grown on a calcareous soil showed poor growth and/or were chlorotic in spite of abundant Fe in the roots. It has been hypothesized that microbial siderophores chelate Fe (III) in the soil, and that in this form Fe is transported towards the root apoplast. On the calcareous soil, total and apoplastic root Fe concentrations were high, probably because of a high apoplastic pH depressing Fe (III)-reductase activity and thus the Fe²⁺ supply to the cytoplasm. On the acidic soil, total and apoplastic root Fe concentrations were low, probably because of a low apoplastic pH favouring Fe (III) reduction, hence plants showed no Fe-deficiency symptoms. The main objective of the present work was to investigate the role of microbial soil activity in plant Fe acquisition. For this purpose, plants were grown under sterile and non-sterile conditions on a loess loam soil. Plants cultivated under non-sterile conditions grew well, showed no Fe-deficiency symptoms and had fairly high Fe concentrations in the roots in contrast to plants grown in the sterile medium. Low root and leaf Fe concentrations in the axenic treatments indicated that the production of microbial siderophores was totally suppressed. Accordingly, sunflowers were severely chlorotic and this was associated with very poor growth, whereas in maize only growth was drastically reduced. In maize under sterile conditions, root apoplastic and total Fe concentrations were not as low as in sunflowers, which may have indicated that phytosiderophores produced in maize partly sustained Fe acquisition, but due to poor growth were not as efficient in supplying Fe as microbial activity under natural conditions. It may be therefore assumed that in natural habitats soil microbial activity is of pivotal importance for plant Fe acquisition. Received: 11 March 1999     

Effect of FE level and solution culture PH on severity of chlorosis and elemental content of apple seedlings

The development of chlorosis and subsequent growth of apple seedlings grown in solution cultures containing various levels of Fe under a range of solution pH regimes were examined. Initial solution pHs were 5.5, 6.5 and 7.8 respectively, with Fe levels of 0.0, 0.13 and 1.3 ppm in a 3x3 factorial arrangement. Leaf chlorosis increased with a decrease in Fe levels and with higher solution pH. Nutrient solutions were changed weekly and during each weekly cycle solution pH levels were monitored. Independent of Fe level, the lower the initial solution pH the greater the change in solution pH during each weekly cycle. Decreasing solution Fe levels decreased both leaf and root Fe concentrations but both parameters were relatively unaffected by solution pH suggesting a solution pH by Fe supply interaction at the root surface.     

Morphological Changes in Leaves of Mexican Lime Affected by Iron Chlorosis

Iron (Fe) chlorosis reduces the concentration of photosynthetic pigments, photosynthates, and crop yield. The effect of Fe chlorosis on leaf composition and cell structure was evaluated in Mexican lime (Citrus aurantifolia) with different degrees of Fe chlorosis. Iron chlorosis significantly reduced concentrations of chlorophylls a, b, and a + b, and caused thickening of leaves, due to the increase in palisade and spongy parenchyma cells. The chloroplasts of the chlorotic and albino leaves showed a disorganized ultrastructure; they had an elongated shape with disarrayed thylakoids, underdeveloped grana, scarce starch granules, and hole-like folds in the thylakoid membranes. The accumulation of calcium oxalate crystals in the upper and lower sides of the epidermis, crystal length, and total crystal content increased with Fe chlorosis severity. The green leaves, in contrast, had chloroplasts with typical ultrastructure. The degree of Fe chlorosis in the leaves significantly affected the concentrations of potassium (K); Fe, manganese (Mn), Fe²⁺, and the phosphorus (P)/Fe and K/calcium (Ca) ratios.     

椪柑叶片硼毒害症状及光合生理响应研究

  【目的】  准确及时诊断硼毒害,了解硼毒害对叶片造成的生理影响,为硼毒害的有效防治提供理论依据。  【方法】  通过田间调查和叶片养分含量测定,明确福建省安溪县椪柑叶片黄化和脱落是由硼中毒引起的。分别采集不同程度硼毒害椪柑叶片,测定叶片光合作用速率、叶绿素荧光特性和细胞膜透性。  【结果】  在正常、中度黄化和重度黄化叶片中,钾、镁、锌含量均处于椪柑适宜范围内,而中度黄化和重度黄化叶片的硼含量比正常叶片分别提高了11.11和19.71倍,显示硼毒害是造成椪柑叶片黄化的原因。椪柑叶片硼毒害症状有两种表现形式:一是症状由叶尖沿主脉向下发展,叶肉和叶脉均褪绿黄化,黄化部位可见棕褐色的坏死斑点;二是症状由叶缘向主脉发展,主脉保持绿色,叶片呈不规则的黄、绿斑驳黄化。硼毒害椪柑叶片的光合色素含量、有效光化学效率 (Fv'/Fm')、电子传递效率 (ETR)、有效量子产额 (ΦPS II) 和光化学淬灭系数 (qP) 随症状的加重而下降,而非光化学淬灭系数 (NPQ)、过剩激发能 (E) 和天线热耗散 (D) 则随症状的加重而提高,引起光合作用速率、淀粉和可溶性糖含量下降,硼毒胁迫下椪柑叶片细胞膜透性明显提高。  【结论】  过量喷施硼肥所造成的硼毒害会导致椪柑叶片黄化、异常落叶,硼毒害使椪柑光合作用受抑、光合产物合成受阻,细胞膜受到伤害。柑橘生产上应重视含硼叶面肥的合理使用,以免造成硼中毒现象。有关椪柑硼毒害的防治措施还有待进一步研究。     

CHLOROSIS IN WILD PLANTS: IS IT A SIGN OF IRON DEFICIENCY?

ABSTRACT

Chlorosis in crops grown on calcareous soil is mainly due to iron (Fe) deficiency and can be alleviated by leaf application of soluble Fe²⁺ or diluted acids. Whether chlorosis in indigenous plants forced to grow on a calcareous soil is also caused by Fe deficiency has, however, not been demonstrated. Veronica officinalis, a widespread calcifuge plant in Central and Northern Europe, was cultivated in two experiments on acid and calcareous soils. As phosphorus (P) deficiency is one of the major causes of the inability of many calcifuges to grow on calcareous soil we added phosphate to half of the soils. Leaves in pots with the unfertilized and the P-fertilized soil, respectively, were either sprayed with FeSO₄ solution or left unsprayed. Total Fe, P, and manganese (Mn) in leaves and roots and N remaining in the soil after the experiment were determined. In a second experiment, no P was added. Leaves were either sprayed with FeSO₄ or with H₂SO₄ of the same pH as the FeSO₄ solution. Degree of chlorosis and Fe content in leaves were determined. Calcareous soil grown plants suffered from chlorosis, which was even more pronounced in the soils supplied with P. Newly produced leaves were green with Fe spray but leaves that were chlorotic before the onset of spraying did not totally recover. H₂SO₄ spray even increased chlorosis. This demonstrated that chlorosis was due to Fe deficiency. As total leaf Fe was similar on acid and calcareous soil, it was a physiological Fe deficiency, caused by leaf tissue immobilization in a form that was not metabolically “active”. Iron in the leaves was also extracted by 1,10-phenanthroline, an Fe chelator. In both experiments, significant differences between leaves from acid and calcareous soil were found in 1,10-phenanthroline extractable Fe but not in total leaf Fe, when calculated on a dry weight basis. Differences in 1,10-phenanthroline extractable Fe were more pronounced when calculated per unit dry weight than calculated per leaf area, whereas the opposite condition was valid for total leaf Fe.     

Activities of Iron‐Containing Enzymes in Leaves of Two Tomato Genotypes Differing in Their Resistance to Fe Chlorosis

Abstract

Two tomato (Lycopersicon esculentum Mill., cvs. Pakmor and Target) genotypes differing in resistance to iron (Fe) deficiency were grown in nutrient solution under controlled environmental conditions over 50 days to study the relationships between severity of leaf chlorosis, total concentration of Fe, and activities of Fe‐containing enzymes in leaves. The activities of Fe‐containing enzymes ascorbate peroxidase, catalase, and guaiacol peroxidase, and additionaly the activity of glutathione reductase, an enzyme that does not contain Fe, were measured. Plants were supplied with 2 × 10^?7 M (Fe deficient) and 10^?4 M (Fe sufficient) FeEDTA, respectively. Leaf chlorosis appeared more rapidly and severely in Target (Fe deficiency senstive genotype) than Pakmor (Fe deficiency resistant genotype). On day 50, Pakmor had 2‐fold more chlorophyll than Target under Fe deficiency, while at adequate supply of Fe the two genotypes were very similar in chlorophyll concentration. Despite distinct differences in development of leaf chlorosis and chlorophyll concentrations, Pakmor and Target were very similar in concentrations of total Fe under Fe deficiency. In contrast to Fe concentration, activities of Fe‐containing enzymes were closely related to the severity of leaf chlorosis. The Fe‐containing enzymes studied, especially catalase, showed a close relationship with the concentration of chlorophyll and thus differential sensitivity of tomato genotypes to Fe deficiency. Glutathione reductase did not show relationship between Fe deficiency chlorosis and enzyme activity. The results confirm that measurement of Fe‐containing enzymes in leaves is more reliable than the total concentration of Fe for characterization of Fe nutritional status of plants and for assessing genotypical differences in resistance to Fe deficiency. It appears that Fe deficiency‐resistant genotype contains more physiologically available Fe in tissues than the genotype with high sensitivity to Fe deficiency.