Effects of 5‐aminolevulinic acid on water uptake,ionic toxicity,and antioxidant capacity of Swiss chard (Beta vulgaris L.) under sodic‐alkaline conditions

Sodic‐alkalinity may be more deleterious to plant growth than salinity. The objectives of this study were to determine whether 5‐aminolevulinic acid (ALA: an essential precursor for chlorophyll biosynthesis) foliar application could improve the sodic‐alkaline resistance of Swiss chard (Beta vulgaris L. subsp. cicla ) by regulating water uptake, ionic homeostasis, photosynthetic capacity, and antioxidant metabolism. Eight‐week‐old uniform plants were grown in nutrient medium without and with a sodic‐alkaline regime generated by a mixture of NaHCO₃ and Na₂CO₃ (NaHCO₃ : Na₂CO₃ = 9:1 molar ratio) for 12 d, and leaves were sprayed daily with water or ALA. The Na⁺ and ALA concentrations were gradually increased to 60 mM and 120 μM, respectively. ALA foliar application alleviated the physiological damage from sodic‐alkalinity, as reflected by the increases in plant dry weight, relative growth rate, chlorophyll, Mg²⁺ concentration, and the decrease in Na⁺ concentration. However, ALA foliar application did not change the water uptake capacity or the concentration of K⁺, Fe³⁺, and endogenous ALA in leaf tissues under sodic‐alkaline conditions. ALA foliar application effectively mitigated damage from sodic‐alkalinity because of the increased activity of antioxidant enzymes (catalase and guaiacol peroxidase), particularly superoxide dismutase activity, which was maintained at the same level as for control plants. These results suggest that ALA foliar application alleviated sodic‐alkaline stress mainly owing to its antioxidant capacity, and superoxide dismutase has the main responsibility for reducing oxidative stress in Swiss chard.     

Zinc‐biofortified seeds improved seedling growth under zinc deficiency and drought stress in durum wheat

High zinc (Zn) concentration of seeds has beneficial effects both on seed vigor and human nutrition. This study investigated the effect of Zn biofortification on growth of young durum wheat (Triticum durum cv. Yelken) seedlings under varied Zn and water supply. The seeds differing in Zn concentrations were obtained by spraying ZnSO₄ to durum wheat plants at different rates under field conditions. Three groups of seeds were obtained with the following Zn concentrations: 9, 20, and 50 mg Zn kg^?1. The seeds differing in Zn were tested for germination rate, seedling height, shoot dry matter production, and shoot Zn concentration under limited and well irrigated conditions in a Zn‐deficient soil with and without Zn application. In an additional experiment carried out in solution culture, root and shoot growth and superoxide dismutase activity (SOD) of seedlings were studied under low and adequate Zn supply. Low seed Zn concentration resulted in significant decreases in seedling height both in Zn‐deficient and sufficient soil, but more clearly under water‐limited soil condition. Decrease in seed germination due to low seed Zn was also more evident under limited water supply. Increasing seed Zn concentration significantly restored impairments in seedling development. Drought‐induced decrease in seedling growth at a given seed Zn concentration was much higher when soil was Zn‐deficient. Increasing seed Zn concentration also significantly improved SOD activity in seedlings grown under low Zn supply, but not under adequate Zn supply. The results suggest that using Zn‐biofortified seeds assures better seed vigor and seedling growth, particularly when Zn and water are limited in the growth medium. The role of a higher antioxidative potential (i.e., higher SOD activity) is discussed as a possible major factor in better germination and development of seedlings resulting from Zn‐biofortified seeds.     

Zinc‐deficiency resistance and biofortification in plants

Zinc (Zn) deficiency is a well‐documented problem in plants, causing decreased yields and nutritional quality. When facing a shortage in Zn supply, plants acclimatize by enhancing the Zn acquisition. In this review, we highlight recent progress in understanding of plant resistance to Zn deficiency and discuss the future challenges to fully unravel its molecular basis of regulation. Emphasis is given on the physiological and molecular basis of Zn acquisition, the long‐distance transport of Zn and the genotypic variations in Zn use efficiency of plants. Prospects of Zn biofortification strategies as well as further efforts for crop improvement to overcome Zn deficiency are also addressed.     

Response of carboxylate metabolism to zinc deficiency in Lactuca sativa and Brassica oleracea plants

Species or genotypes differ in their zinc use efficiency (ZnUE) under low Zn availability in the soil. Organic acids (OAs) synthetized by plant carboxylate metabolism may play a role in Zn‐deficiency tolerance. The main objective of the present work was to assess the response of two species of great agronomic interest such as Lactuca sativa and Brassica oleracea to Zn deficiency focusing on OAs and carboxylate metabolism. For this, L. sativa and B. oleracea plants were grown in hydroponic culture with two different Zn‐application rates: 10 µM Zn as control and 0.1 µM Zn as deficiency treatment. ZnUE parameters, concentrations of OAs and enzymes of carboxylate metabolism were analyzed. L. sativa showed better Zn uptake efficiency (ZnUpE), while B. oleracea demonstrated better Zn utilization efficiency (ZnUtE). In L. sativa, citrate and oxaloacetate concentrations and phosphoenolpyruvate carboxylase and citrate synthase activities increased, while fumarase and malate dehydrogenase activities declined. In B. oleracea no significant response was found in concentrations of carboxylate metabolism or enzyme activity except for a decrease in fumarase activity. These results suggest that a possible factor that induces the tricarboxylic acid cycle could be the low ZnUtE rather than the low Zn concentration under Zn‐deficiency conditions. In L. sativa citrate, oxaloacetate, phosphoenolpyruvate carboxylase, and citrate synthase may play a key role to face Zn deficiency, while in B. oleracea the higher ZnUtE cannot be explained in terms of a rise in OAs synthesis.     

Cadmium‐ and barium‐toxicity effects on growth and antioxidant capacity of soybean (Glycine max L.) plants,grown in two soil types with different physicochemical properties

The pollution of agricultural soils by metals is of growing concern worldwide, and is increasingly subject to regulatory limits. However, the effect of metal pollutants on the responses of plants can vary with soil types. In this study, we examined the growth and antioxidant responses of soybean plants exposed to contrasting soils (Oxisol and Entisol), which were artificially contaminated with cadmium (Cd) or barium (Ba). Cadmium reduced plant growth at concentrations higher than 5.2 mg (kg soil)^–1, while Ba only affected plant growth at 600 mg kg^–1. Such levels are higher than the limits imposed by the Brazilian environmental legislation. Lipid peroxidation was increased only at a Cd concentration of 10.4 mg kg^–1 in the Oxisol, after 30 d of exposure. Twelve superoxide dismutase (SOD; EC 1.15.1.1) isoenzymes were evaluated, most of which were classified as Cu/Zn forms. The SOD activity in the leaves of plants grown in the Oxisol decreased over time, whilst remaining high in the Entisol. Catalase (CAT; EC 1.11.1.6) activity in the leaves exhibited little response to Cd or Ba, but increased over time. Glutathione reductase (GR; EC 1.6.4.2) activity was reduced over time when exposed to the higher Cd concentrations, but increased following Ba exposure in the Oxisol. The enzyme‐activity changes were mainly dependent on soil type, time of exposure and, to a lesser extent, the metal concentration of the soil. Soybean plants grown in a sandy soil with a low buffering capacity, such as Entisol, suffer greater oxidative stress than those grown in a clay soil, such as Oxisol.     

Irrigation and Zn fertilizer management improves Zn phyto‐availability in various rice production systems

Zinc (Zn) is an important micronutrient for rice (Oryza sativa L.) production and its deficiency has been observed in various production systems. High grain Zn concentration is equally important for high rice yield and human health. In this work, the effects of Zn fertilization on seedling growth, grain yield, grain Zn concentration, and their association with root traits were studied under alternate wetting and drying (AWD), aerobic rice (AR), system of rice intensification (SRI), and continuous flooding (CF). Zinc fertilization (15 kg ha^?1) improved nursery seedlings chlorophyll and Zn concentrations, root length, and number of roots with highest values observed in CF. At harvesting, maximum plant height, panicle length, total and panicle bearing tillers, and kernel yield were found with Zn addition in AWD and CF rice systems. Mid season drainage provided at maximum tillering and Zn fertilization increased its concentration in leaves, culms, panicles, and grains under CF and AR at physiological maturity. Most of Zn applied was allocated into culms and panicles, nevertheless, a significant increase in grain Zn concentration was also observed in all production systems. Association of leaf Zn with grain Zn concentration was stronger than with culm and panicle Zn. The results indicate that Zn application after rice nursery transplanting is more important for grain Zn enrichment in all rice systems than for increase in grain yield in all systems except AWD where grain yield was also increased. More grain yield in CF and AWD as compared to SRI and AR can also be attributed to decreased spikelet sterility and to better Zn phyto‐availability in these rice systems at physiological maturity.     

荔枝铜锌超氧物歧化酶基因的克隆与序列分析

以"三月红"荔枝基因组DNA为模板,用Cu*ZnSOD基因特异寡聚核苷酸为引物,进行PCR扩增,得到特异基因片段,将其克隆到pGEMT载体上,转化感受态大肠杆菌TG1中,对转化子中重组pGEMT上的Cu*ZnSOD基因片段的PCR扩增检测和序列分析,表明克隆成功;该基因片段有478个核苷酸,由4个外显子和3个内含子组成;外显子由234个核苷酸组成,编码78个氨基酸;该基因片段编码的氨基酸序列与水稻、玉米、番茄、大白菜和松树的Cu*ZnSOD基因编码的氨基酸序列的同源性分别为79.5%、73.1%、73.1%、71.8%和75.7%.     

Effect of zinc‐biofortified seeds on grain yield of wheat,rice, and common bean grown in six countries

Seeds enriched with zinc (Zn) are ususally associated with better germination, more vigorous seedlings and higher yields. However, agronomic benefits of high‐Zn seeds were not studied under diverse agro‐climatic field conditions. This study investigated effects of low‐Zn and high‐Zn seeds (biofortified by foliar Zn fertilization of maternal plants under field conditions) of wheat (Tritcum aestivum L.), rice (Oryza sativa L.), and common bean (Phaseolus vulgaris L.) on seedling density, grain yield and grain Zn concentration in 31 field locations over two years in six countries. Experimental treatments were: (1) low‐Zn seeds and no soil Zn fertilization (control treatment), (2) low‐Zn seeds + soil Zn fertilization, and (3) Zn‐biofortified seeds and no soil Zn fertilization. The wheat experiments were established in China, India, Pakistan, and Zambia, the rice experiments in China, India and Thailand, and the common bean experiment in Brazil. When compared to the control treatment, soil Zn fertilization increased wheat grain yield in all six locations in India, two locations in Pakistan and one location in China. Zinc‐biofortified seeds also increased wheat grain yield in all four locations in Pakistan and four locations in India compared to the control treatment. Across all countries over 2 years, Zn‐biofortified wheat seeds increased plant population by 26.8% and grain yield by 5.37%. In rice, soil Zn fertilization increased paddy yield in all four locations in India and one location in Thailand. Across all countries, paddy yield increase was 8.2% by soil Zn fertilization and 5.3% by Zn‐biofortified seeds when compared to the control treatment. In common bean, soil Zn application as well as Zn‐biofortified seed increased grain yield in one location in Brazil. Effects of soil Zn fertilization and high‐Zn seed on grain Zn density were generally low. This study, at 31 field locations in six countries over two years, revealed that the seeds biofortfied with Zn enhanced crop productivity at many locations with different soil and environmental conditions. As high‐Zn grains are a by‐product of Zn biofortification, use of Zn‐enriched grains as seed in the next cropping season can contribute to enhance crop productivity in a cost‐effective manner.     

The role of potassium in alleviating detrimental effects of abiotic stresses in plants

Plants exposed to environmental stress factors, such as drought, chilling, high light intensity, heat, and nutrient limitations, suffer from oxidative damage catalyzed by reactive oxygen species (ROS), e.g., superoxide radical (O₂equation/tex2gif-sup-1.gif^–), hydrogen peroxide (H₂O₂) and hydroxyl radical (OHequation/tex2gif-sup-4.gif). Reactive O₂ species are known to be primarily responsible for impairment of cellular function and growth depression under stress conditions. In plants, ROS are predominantly produced during the photosynthetic electron transport and activation of membrane‐bound NAD(P)H oxidases. Increasing evidence suggests that improvement of potassium (K)‐nutritional status of plants can greatly lower the ROS production by reducing activity of NAD(P)H oxidases and maintaining photosynthetic electron transport. Potassium deficiency causes severe reduction in photosynthetic CO₂ fixation and impairment in partitioning and utilization of photosynthates. Such disturbances result in excess of photosynthetically produced electrons and thus stimulation of ROS production by intensified transfer of electrons to O₂. Recently, it was shown that there is an impressive increase in capacity of bean root cells to oxidize NADPH when exposed to K deficiency. An increase in NADPH oxidation was up to 8‐fold higher in plants with low K supply than in K‐sufficient plants. Accordingly, K deficiency also caused an increase in NADPH‐dependent O₂equation/tex2gif-sup-6.gif^– generation in root cells. The results indicate that increases in ROS production during both photosynthetic electron transport and NADPH‐oxidizing enzyme reactions may be involved in membrane damage and chlorophyll degradation in K‐deficient plants. In good agreement with this suggestion, increases in severity of K deficiency were associated with enhanced activity of enzymes involved in detoxification of H₂O₂ (ascorbate peroxidase) and utilization of H₂O₂ in oxidative processes (guaiacol peroxidase). Moreover, K‐deficient plants are highly light‐sensitive and very rapidly become chlorotic and necrotic when exposed to high light intensity. In view of the fact that ROS production by photosynthetic electron transport and NADPH oxidases is especially high when plants are exposed to environmental stress conditions, it seems reasonable to suggest that the improvement of K‐nutritional status of plants might be of great importance for the survival of crop plants under environmental stress conditions, such as drought, chilling, and high light intensity. Several examples are presented here emphasizing the roles of K in alleviating adverse effects of different abiotic stress factors on crop production.     

The Cd‐tolerant rice mutant cadH‐5 is a high Cd accumulator and shows enhanced antioxidant activity

To investigate the mechanism of cadmium (Cd) detoxification in rice (Oryza sativa L.), a Cd‐tolerant mutant cadH‐5, obtained by an Agrobacterium tumefaciens‐based gene‐delivery system, was used for a Cd‐tolerance and accumulation study. After 15 d of exposure to 0.75 mM CdCl₂, significant phenotypic differences were observed between the wild type (WT) and cadH‐5. When exposed to 0.5 mM CdCl₂, higher Cd levels were accumulated in cadH‐5 root cell wall, root cytosol, and membranes than those in WT. However, Cd concentrations in root tissues varied in both WT and cadH5. No significant difference of hydrogen peroxide (H₂O₂) concentrations was observed between WT and cadH‐5, while contents of cell‐wall polysaccharides and phytochelatins (PCs) in the mutant were higher compared to WT. The ratios of reduced glutathione to oxidized glutathione (GSH : GSSG) and ascorbate to dehydroascorbate (ASC : DHA) were lower in WT than in cadH‐5, while the NADPH : NADP⁺ ratio was different to the ratios of GSH : GSSG and ASC : DHA; the ascorbate peroxidase (APX, EC 1.11.1.11), glutathione peroxidase (GR, EC 1.6.4.2), dehydroascorbate reductase (DHAR, EC 1.8.5.1), and monodehydroascorbate reductase (MDHAR, EC 1.6.5.4) activities were lower in WT compared to cadH‐5. Our results indicate that under long‐term Cd stress, cadH‐5 plants can accumulate more Cd with more PC. Also, the redox status of ASC‐GSH cycle was more inhibited in WT than in cadH‐5 plants, rendering WT less able to scavenge reactive oxygen species (ROS). The cadH‐5 mutant maintains relatively high ASC, GSH, and NADPH concentrations, ratios of ASC : DHA, GSH : GSSG, and NADPH : NADP⁺, as well as antioxidative enzymatic activities and PC concentrations. Thus, it is tolerant of relatively high Cd accumulation.     

Effect of lime and phosphorus on zinc uptake from four soils of Brazil

Abstract

Four soils from southern Brazil were treated with four levels each of lime and P and cropped with corn (Zea mays L.) in the greenhouse. Zinc deficiencies occurred at soil pH values of 6.3 or greater and independently of the rate of P application. Application of P alone did not induce Zn deficiency.     

缺锌和高锌胁迫下苹果根系有机酸对锌的内稳性的响应

To elucidate the mechanisms of tolerance to zinc (Zn) deficiency and Zn toxicity in the root of apple trees, the apple rootstock Malus hupehensis (Pamp.) Rehd seedlings were selected to study the responses of organic acids to Zn homeostasis in roots under low Zn (0 μmol L-1 ), adequate Zn (as control, 4 μmol L-1 ) and toxic Zn (100 μmol L-1 ) treatments. The differences of Zn concentrations and accumulations in the roots were highest, compared with those in the stems and leaves, when apple seedlings were subjected to low and toxic Zn treatments for 1 d. The concentrations and accumulations of oxalic and malic acids in the roots in the low and toxic Zn treatments increased by 20% to 60% compared with those of the control treatment. Significantly negative correlations were found between the total Zn concentrations and the concentrations of oxalic and malic acids in the roots under 1 d of low Zn treatment. However, contrary correlations were found for the toxic Zn treatment. Meanwhile, the maximum influx rates of Zn 2+ under low and toxic Zn treatments increased by 30% and 20%, respectively, compared with the rate of the control treatment. Both Zn deficiency and Zn toxicity increased the concentrations of organic acids in root after short-time Zn treatment, which could resist Zn stress through balanding Zn homeostasis in M. hupehensis Rehd.     

Modulation of the ROS‐scavenging system in salt‐stressed wheat plants inoculated with arbuscular mycorrhizal fungi

Although there is evidence for a positive involvement of the antioxidant defense system in plant response to salt stress, there is poor information regarding the influence of mycorrhizal symbiosis on enzymatic and nonenzymatic antioxidant defense in wheat under saline conditions. The present article focuses on the contribution of mycorrhizae to antioxidant defense in salt‐stressed wheat plants. Two wheat (Triticum aestivum L.) cultivars, Sids 1 and Giza 168, were grown under nonsaline or two saline conditions (4.7 and 9.4 dS m^–1) with and without arbuscular mycorrhizal fungi (AMF) inoculation. Salt stress considerably decreased root colonization and plant productivity, particularly in Giza 168. Interestingly, mycorrhizal colonization alleviated the adverse effect of salt stress and significantly enhanced plant productivity, especially in Sids 1. The concentration of glycinebetaine, the activities of antioxidative enzymes (superoxide dismutase, peroxidase, catalase, and glutathione reductase) and the concentrations of antioxidant molecules (glutathione and ascorbate) were increased under saline conditions; these increases were more significant in salt‐stressed mycorrhizal plants, especially in Sids 1. Salt stress induced oxidative damage through increased lipid peroxidation, electrolyte leakage, and hydrogen peroxide concentration, particularly in Giza 168. Mycorrhizal colonization altered plant physiology and significantly reduced oxidative damage. Elimination of reactive oxygen species (ROS) can be one of the mechanisms how AMF improve wheat adaptation to saline soils and increase its productivity.     

Zinc uptake and distribution in tomato plants in response to foliar supply of Zn hydroxide‐nitrate nanocrystal suspension with controlled Zn solubility

The present study investigated the foliar uptake rate, distribution, and retranslocation patterns of novel, synthesized zinc hydroxide‐nitrate nanocrystals (ZnHN; solubility 30–50 mg Zn L^?1) applied on to the adaxial surface of tomato leaves (Solanum lycopersicum L. cv. Roma). The total Zn absorption from ZnHN suspension positively increased with ZnHN application rates, but the relative efficacy started to decline at > 400 mg Zn L^?1. Within the 3 weeks, total Zn recovery in the ZnHN‐treated plants was 16% of the total ZnHN‐Zn applied, compared to the near 90% total Zn recovery in the Zn(NO₃)₂‐plants at the same Zn rate. Foliar‐absorbed ZnHN‐Zn was distributed from the treated leaves into other plant parts and preferentially translocated into the roots. Distribution of Zn from ZnHN‐treated leaves to apical parts was not limited by Zn deficiency. These results demonstrate that ZnHN crystals with controlled solubility provided some sort of slow‐release Zn over a certain growth period at a rate slower (but quantitatively effective) than the soluble Zn(NO₃)₂. The efficacy of the prolonged foliar Zn supply could be enhanced if the ZnHN suspension is sprayed over a large leaf surface area at the peak vegetative or early flowering stage.     

Melatonin alleviates cold‐induced oxidative damage in maize seedlings by up‐regulating mineral elements and enhancing antioxidant activity

Melatonin, known as an animal hormone and an antioxidant with a low molecular weight, is one of the most commonly used substances to improve plant resistance against various environmental stresses. However, there are no studies explaining the effects of melatonin on the relationship between defense system and mineral composition of plants under stressed and unstressed‐conditions. The present study was conducted to investigate whether the mitigating effect of melatonin is associated with its modulating influence on the mineral elements of cold‐stressed maize seedlings. The seedlings were treated with melatonin (1 mM) and cold stress (10/7°C) for 3 d separately and in combination. After 3 d, the seedlings were harvested to determine several physiological, biochemical, and molecular parameters. Melatonin application effectively mitigated the damages from cold stress, as demonstrated by higher relative water concentration, chlorophyll concentration and antioxidant enzyme activities (superoxide dismutase, guaiacol peroxidase, catalase, ascorbate peroxidase, and glutathione reductase), as well as lower superoxide, hydrogen peroxide, and malondialdehyde concentrations. Similarly, melatonin significantly ameliorated cold‐induced reductions in the concentrations of potassium, phosphorus, sulfur, magnesium, iron, copper, manganese, and zinc. Besides, it further increased calcium and boron concentrations compared to cold stress alone. Our results reveal that melatonin has an important modulating influence on the mineral element composition of plants and mitigates cold stress through up‐regulation of these elements and simultaneously enhanced antioxidant activity.     

铁、镁、锌营养胁迫对植物体内活性氧代谢影响机制

活性氧是植物体内常见的一类自由基,对植物有很强的伤害。本文总结了铁、镁、锌元素胁迫影响植物体内活性氧代谢机制。铁对于催化植物体内的Haber-Weiss反应产生活性氧具有重要作用。镁诱导植物体内活性氧代谢失调与光氧化有密切关系。缺锌条件下,植物体内活性氧含量升高,其机制是多方面的:NADPH氧化酶氧化能力提高,O2产生增多;体内铁浓度升高,增强了铁诱导的活性氧的产生;光氧化伤害加重;清除系统活性降低。     

Quantitative methods for estimating foliar uptake of zinc from suspension‐based Zn chemicals

The present study developed methods for quantifying foliar Zn uptake from suspension‐based Zn chemicals of low solubility, which were ZnO (particle size: 0.15^–1.34 μm) and a newly synthesized Zn hydroxide nitrate crystal (Zn‐HNC) (50^–100 nm thickness and 0.2^–1 μm lateral dimension). Recently matured leaves of citrus (Citrus aurantium L. cv. Valencia), capsicum (Capsicum annume L. cv. Giant Bell), and/or tomato (Solanum lycopersicum L. cv. Roma) were in vitro–treated with microdroplets (5 μL per droplet) of Zn‐HNC‐ and ZnO‐suspension solutions on the adaxial surface and incubated under controlled conditions for up to 72 h. Leaf‐washing protocols were compared, including: dilute ethanol (3%), dilute nitric acid (2%), and their combination. The methods for quantifying Zn uptake were: (1) whole‐leaf loading by which droplets of the Zn suspension solutions were loaded onto central regions of both left and right sides of leaf blades and (2) half‐leaf loading by which soluble‐Zn (ZnSO₄) droplets were loaded onto only one side of leaf blades while the other was used as the background Zn control. Foliar‐surface characteristics of the plant species affected the effectiveness of the washing methods. The dilute nitric acid (2%; ± 3% ethanol) was required to remove residue particles of the ZnO and Zn‐HNC suspensions from foliar surfaces of capsicum and tomato (highly trichomatic), but the residue Zn chemicals on citrus leaves (nontrichomatic and highly waxy) were similarly and effectively removed by the three washing methods. For quantifying Zn uptake by the leaves, the half‐leaf loading method showed its advantages over the whole‐leaf loading method, because it did not stringently require similar background Zn concentrations in the control and treated leaves at the start and had little risks of secondary absorption of soluble Zn in the washing solution.     

Salinity‐induced changes in growth,superoxide dismutase activity,and ion content of two olive cultivars

Olive trees (Olea europaea L.) are considered moderately tolerant to salinity, with clear differences found among cultivars. One‐year‐old self‐rooted olive plants of the Croatian cv. Oblica and Italian cv. Leccino were grown for 90 d in nutrient solutions containing 0, 66, or 166 mM NaCl, respectively. The shoot length and the number of nodes and leaves for both cultivars were not affected by salinity up to 66 mM NaCl. However, at 166 mM NaCl, growth of Leccino was reduced earlier and to a higher extent than growth of Oblica. After 10 d of exposure to 66 and 166 mM NaCl, increased activity of superoxide dismutase (SOD) was observed in Leccino, whereas there was almost no response in Oblica. Reduced SOD activity in Leccino at 166 mM NaCl was observed after prolonged stress (90 d), whereas in Oblica SOD was increased at 66 mM compared to control or 166 mM NaCl. Electrolyte and K⁺ leakage were increased and relative water content decreased as NaCl concentration increased with similar intensity of response measured in both cultivars. Oblica exhibited an ability to keep a higher K⁺ : Na⁺ ratio at all salinity levels compared to Leccino, but since no difference was found in leaf K⁺ concentration, this was mainly achieved by less Na⁺ ions reaching the younger leaves. The antioxidative system represents a component of the complex olive salt‐tolerance mechanism, and it seems that the role of SOD in protection from oxidative stress depends on sodium accumulation in leaves.     

Studies on photosynthetic electron transport,photophosphorylation and CO2 fixation in zinc deficient leaf cells of Zea mavs. L.

The effect of Zn deficiency on rate of photosynthesis of leaf discs, isolated mesophyll and bundle sheath cells and chloroplasts of maize (Zea mays. L) was studied. The yield of mesophyll and bundle sheath cells obtained by enzymic digestion of the leaf tissues from Zn deficient plants is lower than the identical tissues from normal plants which suggests that Zn deficiency brings about some structural changes in the leaf cell. Photosynthetic oxygen evolution measured in the leaf discs is low due to Zn deficiency. Photosystem‐II dependent Hill reaction and non cyclic photophosphorylation of chloroplasts were also affected by Zn deficiency. Rate of photosynthetic carbon dioxide fixation by both bundle sheath and mesophyll cells obtained from Zn deficient leaf‐tissue waslower than the cells free from Zn deficiency. Addition of various metabolites like NADPH, ATP and PEP to Zn deficient mesophyll cells whowed marked enhancement in 14‐CO₂ fixation. However, addition of NADPH, ATP and RuBP to Zn deficient bundle sheath cells showed no or very little enhancement in the rate of 14‐Cu₂ fixation. Addition of exogenous Zn ions to isolated cells inhibited the CO₂ fixation both in the non‐deficient and Zn deficient cell types. It is suggested that Zn deficie ‐ncy affects the primary electron transport and phospho‐rvlation ability for chloroplasts which in turn affects CO₂ fixation in leaf cells.     

Formulation,synthesis and characterization of boron phosphate (BPO4) compounds as raw materials to develop slow‐release boron fertilizers

We investigated the use of boron phosphate (BPO₄) as a slow‐release boron (B) source. Boron phosphate compounds were synthesized by mixing boric acid (H₃BO₃) and phosphoric acid (H₃PO₄) and heating at temperatures of 25 to 1000°C for 1 or 24 h. X‐ray diffraction (XRD) patterns and chemical analysis confirmed the formation of BPO₄. The crystallinity of these compounds increased with increasing temperature and heating time. The compounds synthesized at 300°C or less were hygroscopic and clumped together, while those synthesized at 500 to 1000°C were non‐hygroscopic and free‐flowing. The solubility of these compounds was assessed at different pH and P concentrations, and compared to the solubility of ulexite and colemanite. The solubility of the BPO₄ compounds decreased with increasing synthesis temperature and with decreasing pH. The solubility and the kinetics of B release from BPO₄ compounds synthesized at 500 and 800°C were slower than for most commonly used B sources. Given their slow dissolution, the BPO₄ compounds may have potential to continuously supply B to crops in environments where B leaching is a problem. The compounds synthesized at 500 and 800°C show potential for co‐granulation with macronutrient fertilizers such as mono‐ammonium phosphate to produce slow‐release B‐enriched granules.