植物应答缺铁胁迫的分子生理机制及其调控

铁是植物生长发育中所必需的微量营养元素。虽然土壤中铁的丰度很高,但其生物有效性非常低,特别是在碱性石灰性土壤上,高pH和高重碳酸盐含量严重降低了土壤中铁的有效性。因此如何有效地提高植物对铁的利用效率及增强植物对缺铁胁迫的响应已成为目前该领域的研究热点。本文重点阐述了植物两种不同的铁吸收机制,以及对缺铁胁迫的应答反应;对目前所发现的植物中调控缺铁胁迫的相关基因进行了全面的综述,包括新发现的吞噬机理中所涉及的NRAMP基因;同时也介绍了感应铁缺乏的众多相关信号,包括植物激素、气体信号分子及microRNAs等;此外,还提出利用铁吸收相关基因的转导、控制铁吸收相关因子以及各种农艺措施的实施来提高植物铁的生物有效性从而有效缓解缺铁胁迫。最后对未来有关植物吞噬机制、铁缺乏感应信号及改善植物铁营养新途径等研究方向作了初步展望。     

Modified cyclodextrins are chemically defined glucan inducers of defense responses in grapevine cell cultures

In grapevine (Vitis vinifera L.), defense responses after microbial infection or treatment with elicitors involve accumulation of phytoalexins, oxidative burst, and the synthesis of pathogenesis-related proteins. Oligosaccharide fractions from fungal or algal cell walls efficiently induce the defense responses, but a detailed analysis of the elicitor-plant cell surface interaction at the molecular level is precluded by the lack of chemically pure oligosaccharide elicitors. A grapevine liquid cell culture system was used to examine the properties of cyclodextrins (CDs) as inducers of defense responses. This work shows that the chemically pure heptakis(2,6-di-O-methyl)-betaCD caused a dramatic extracellular accumulation of the phytoalexin resveratrol and changes in peroxidase activity and isoenzymatic pattern. Other modified CDs tested on several grapevine cell lines resulted in different eliciting capacities of CDs and different sensibilities of the cell lines. The spent medium of elicited cultures was shown to disturb Botrytis cinerea growth in a plate assay.     

Physiological and genetic responses to boron deficiency in Brassica napus: A review

Abstract

Boron (B) is an essential microelement for the growth and development of plants, and B deficiency affects many biochemical and physiological processes. Brassica napus L. has a high demand for B and is extremely sensitive to B deficiency. Seed yields and oil quality of B. napus are often limited by the low availability of B in soils. Developing new cultivars of B. napus with high B efficiency is therefore required, which requires a greater understanding of responses to B deficiency. Significant genotypic differences in response to low soil B have been observed among varieties of B. napus. B-efficient genotypes can grow and yield normally and usually have a larger root system than B-inefficient genotypes at low B conditions. The mechanisms for B efficiency in B. napus are attributed to B absorption, transportation and utilization. In addition, the cell wall component plays an important role in the tolerance of B. napus to B deficiency, and the B-efficient line presents fewer B-binding sites in the cell walls compared with the B-inefficient line. Genetic and proteomic analyses in B. napus revealed the modulation of a complex network in response to B deficiency. This review gives a comparative overview of the physiological and genetic responses to B deficiency in B. napus and discusses the possible underlying mechanisms of B efficiency.     

Manganese deficiency in wheat genotypes: Physiological responses and manganese deficiency tolerance index

Manganese (Mn) deficiency limits wheat productivity on sandy loam, calcareous and alkaline soils cropped with rice. Variation of wheat genotypes to sustain production and Mn use from Mn deficient condition was investigated to screen efficient genotypes. Forty-seven diverse wheat genotypes were evaluated on Mn sufficient (0.195 µM) and Mn deficient (0 µM) nutrient solution to elucidate physiological basis of Mn deficiency tolerance and to develop manganese deficiency tolerance index (MDTI). Shoot dry weight and mean Mn accumulation was 136.7% and 76.5% enhanced when Mn nutrition was improved, respectively. Efficient genotypes under limited Mn had lower root length/shoot weight ratio but higher relative shoot growth rate with higher shoot demand on root which reflected higher Mn influx. Genotypes were classified as tolerant (>0.66), semi-tolerant (0.33–0.66) and sensitive (<0.33) on the basis of MDTI (0–1 scale). Manganese efficient genotypes are most desirable for sustainable production of wheat under low Mn.     

Response of five citrus rootstocks to iron deficiency

Citrus established in calcareous soils can be affected by iron (Fe)‐deficiency chlorosis which limits yield and the farmers' income. The degree of deficiency depends on the rootstock, but the resistance to Fe chlorosis still requires further investigation. To study physiological parameters of citrus rootstocks that could be used to evaluate resistance to Fe deficiency, plants of Troyer citrange (Citrus sinensis L. Osb. × Poncitrus trifoliata L. Raf.), Carrizo citrange, Volkamer lemon (Citrus volkameriana Ten. & Pasq.), alemow (Citrus macrophylla Wester), and sour orange (Citrus aurantium L.) were grown in nutrient solutions with 0, 5, 10, 15, or 20 μM Fe. For each rootstock, plant height, root and shoot dry weights, and concentration of Fe in the shoots and roots were measured at the end of the experiment. Chlorophyll (CHL) concentration was estimated throughout the experimental period using a portable CHL meter (SPAD‐502) calibrated for each rootstock. At the end of the experiment, CHL fluorescence parameters were measured in each rootstock with a portable fluorimeter. Maximal and variable fluorescence values indicated that the photochemistry of Troyer was more affected by a low concentration of Fe in the nutrient solution than that of other rootstocks. To compare rootstocks, the absolute CHL concentration was converted into relative yield by employing a scaling divisor based on the maximum value of total CHL in plants without Fe‐deficiency symptoms. Exponential models were developed to determine the minimum Fe concentration in nutrient solution required to maintain leaf CHL at 50% of the maximum CHL concentration (IC50). Models were also developed to assess the period of time the rootstocks were able to grow under Fe‐stress conditions before they reached IC50. Volkamer lemon and sour orange needed the lowest Fe concentration (between 4 and 5 μM Fe) to maintain IC50, and Troyer citrange had the highest Fe requirement (14 μM Fe). Citrus macrophylla and Carrizo citrange required 7 and 9 μM of Fe, respectively. Similarly, Volkamer lemon and sour orange rootstocks withstood more days under total Fe depletion or with a low concentration of Fe (5 μM Fe in nutrient solution) until they reached IC50, compared to the other rootstocks. The approach used led to a classification of the rootstocks into three categories, regarding their internal tolerance to Fe chlorosis: resistance (sour orange and Volkamer lemon), intermediate resistance (C. macrophylla and Carrizo citrange), and reduced resistance (Troyer citrange).     

Differential response of foliage plants to iron deficiency

Eight species of tropical foliage plants were screened to determine their response to Fe‐stress conditions. Plants were grown for 120 days in modified Hoagland's nutrient solution at pH 6.3 containing either 0, 0.22 or 5.52 mg/liter Fe (as Fe⁺³‐HEEDTA). Araucaria heterophylla and Dracaena marginata showed leaf chlorosis and decreased growth at 0 and 0.22 mg/liter Fe. Ficus benjamina and Nephrolepis exaltata ’Bostoniensis’ showed little or no chlorosis or growth differences at either 0 or 0.22 mg/liter Fe. Over a 3 week period, F. benjamina and N. exaltata ’Bostoniensis’ decreased nutrient solution pH approximately 1 to 1.5 units lower than either D. marginata or A. heterophylla at all Fe levels. Codiaeum variegatum var. pictum, Dieffenbachia maculata ’Camille’, Epipremnum aureum, and Philodendron scandens oxycardium were intermediate in growth, chlorosis and lowering of the nutrient solution pH. One explanation for the differential Fe response between these species may be their ability to lower pH of the rhizosphere.     

小麦幼苗对氧乐果胁迫的生理学响应

通过溶液培养研究了不同浓度(0、0.1、1.0、5.0和10.0 g/L)氧乐果处理后小麦幼苗叶绿素含量、类胡萝卜素含量、可溶性糖含量及保护酶活性的动态变化。结果表明:在氧乐果胁迫下,高浓度的氧乐果(5.0和10.0 g/L)处理显著降低了小麦幼苗的叶绿素含量和类胡萝卜素含量,且随着处理时间的延长其差异尤为显著;小麦叶片中可溶性糖含量随着氧乐果浓度和处理天数的增加而显著增加。超氧化物歧化酶(SOD)、过氧化物酶(POD)和过氧化氢酶(CAT)的活性均先上升后下降;抗坏血酸过氧化物酶(APX)和谷胱甘肽还原酶(GR)活性在0.1 g/L氧乐果处理的第1d时略有上升,然后下降。小麦响应氧乐果胁迫并上调SOD、POD、CAT等保护酶的活性和可溶性糖的含量,降低叶片光合作用,籍此维持小麦免受低浓度氧乐果的胁迫以维持小麦的正常生长;但高浓度氧乐果处理对小麦根系产生了明显的毒害作用,致使5.0 g/L氧乐果胁迫的小麦根系SOD、POD、CAT活性显著下降。APX和GR可能在低浓度氧乐果处理初期起主要保护作用,而在高浓度氧乐果胁迫下则受到明显抑制作用。     

Construction of artificial promoters highly responsive to iron deficiency

Iron deficiency-responsive element 1 (IDE1) and IDE2 are cis-acting elements that are responsible for Fe-deficiency-inducible and root-specific expression of the barley (Hordeum vulgare L.) gene IDS2 (Fe-deficiency-specific clone no. 2). Using these cis-acting elements, we aimed to construct super-promoters that would induce prominent gene expression in the roots of Fe-deficient rice plants (Oryza sativa L.). Modules containing IDE1 and IDE2 of the IDS2 promoter were used as repeats or were linked to the Fe-deficiency-responsive promoter of barley IDS3, and were connected to known enhancer-like sequences. Five artificial promoters, as well as the native promoters of barley IDS2 or IDS3, were connected individually upstream of β-glucuronidase (GUS) and were introduced into rice. Transgenic rice plants were grown under control or Fe-deficient conditions, and GUS expression was analyzed. The artificial promoter that contained one module of IDE1 and IDE2 conferred strong Fe-deficiency-inducible GUS expression to the roots of rice plants. Each of the five artificial promoters induced a similar level of GUS expression in Fe-deficient roots, which did not exceed the GUS expression driven by the native IDS2 or IDS3 promoter. Artificial and native promoters induced GUS expression in response to Fe-deficiency in leaves, although the level of expression was lower than that in roots. Histochemical observations revealed that GUS expression driven by artificial and native promoters was spatially similar, and expression was dominant within vascular bundles and root exodermis. These findings suggest that there is coordinated expression of the genes that are involved in Fe-deficiency-induced Fe uptake in rice.     

Physiological and biochemical changes in young maize plants under iron deficiency I. Growth and photosynthesis

Young maize plants, grown hydroponically, were supplied with different amounts (7.5, 0.75, 0.15, 0.075, and 0 mg Fe/L) of iron (Fe). At 14, 21, and 28 days, parameters characterizing growth and photosynthesis were determined. Iron‐deficiency resulted in significant changes in biomass accumulation and distribution between vegetative organs as well as changes in the content of chlorophyll a, chlorophyll b, and the carotenoids. The photosynthetic rate per leaf area was decreased. Part of ¹⁴C incorporated in low molecular compounds was increased and the share of amino acids and organic acids in them was increased. Plants supplied with 1/10th of the optimum Fe required partially adapted to the Fe deficiency. Plants with visual symptoms of Fe deficiency showed some peculiarities as compared to those plants severely Fe‐deficient.     

Sugar beet responses to iron nutrition and stress

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 riboflavin concentrations were not consistently associated with the lowest solution pH values of single plants within a treatment, nor of plants at different iron concentrations at all sampling times. The release of riboflavin from roots may depend more upon the amount formed than upon root acidification.     

Screening for resistance to iron deficiency chlorosis in dry bean using iron reduction capacity

Identifying cultivars resistant to iron (Fe) deficiency chlorosis so prevalent in calcareous soils is a more economical solution than fertilizer application in field crops. The current method of screening for resistance using chlorosis ratings in field trials is time consuming and highly variable. Root Fe reduction successfully separated cultivars or rootstocks, varying widely in resistance, of soybean (Glycine max L.), peach (Prunus persica L.), and grape (Vitis spp.), but was unsuccessful in sub‐clover (Trifolium subterraneum L.). Dry bean (Phaseolus vulgaris L.) exhibits Fe deficiency chlorosis in calcareous soils and initiates Fe reduction by the roots in response to such stress. The resistance of 24 dry bean cultivars to Fe deficiency chlorosis was assessed by measuring and summing daily Fe reduction by the roots. The cultivars were grown both hydroponically in an environmental chamber in low Fe solutions (0.05 mg‐L^‐1) and at three field sites in both 1995 and 1996. A significant relationship (P<0.01) between field chlorosis scores made 36 days after planting and root Fe reduction summations was observed for all sites in 1995 and 1996 (r = ‐0.42 to ‐0.71). The variability of chlorosis scores among sites, especially in 1996, points out the difficulty of using field chlorosis scores for screening. These results indicate that measurements of root Fe reduction can be used to predict resistance to Fe deficiency chlorosis in dry bean. Successful implementation of this technique should reduce if not eliminate field trials for screening resistance to Fe deficiency chlorosis.     

氮素亏缺对苹果愈伤组织硝态氮吸收及同化的影响

【目的】研究硝态氮亏缺对苹果叶片愈伤组织生长及硝态氮吸收同化的影响,了解苹果愈伤组织对硝态氮亏缺的响应机制,为进一步研究缺氮处理影响愈伤组织生长发育的分子机理提供理论依据。【方法】以‘嘎拉3’组培苗叶片愈伤组织为试材进行组培试验,设置培养基中NO₃–-N亏缺和适宜两个水平 (NO₃–-浓度分别为0 mol/L和0.039 mol/L)。选取叶龄一致的功能性叶片,用灭菌手术刀片沿垂直叶脉方向划伤叶片并切除叶柄和叶尖,叶背向上平铺于MS分化培养基,暗培养3天然后转至光下7天,将长出愈伤组织的叶片分别转移至MS正常分化培养基 (CK) 和MS NO₃–-N亏缺分化培养基 (T,用NH₄Cl、KCl分别代替MS中的NH₄NO₃、KNO₃),培养3周。在转板第0、1、3、7、14、21天分别取叶片伤口处的愈伤组织,观察其细胞形态,测定硝态氮含量、NO₃–流速、氮素同化酶活性和氮素同化酶基因相对表达量。【结果】苹果愈伤组织经NO₃–-N亏缺处理1天后,细胞体积变小,间隙变大,排列疏松,7天后细胞变形,排列无规则。愈伤组织中硝态氮含量在处理7天时达到峰值,为1.54 mg/g,显著高于对照,最大降幅出现在7天后,为13.64%。NO₃–-N亏缺处理前,NO₃–吸收速率最大,为22.38 pmol/(cm2·s),处理1天后降幅为84.1%,处理至7天时,NO₃–已经由吸收变为外排,逆差为24.45 pmol/(cm2·s)。NR活性在处理至7天时无显著变化,7天后快速增加,增幅为19.26%。NiR活性在处理至14天时,无显著性差异,14天后上升幅度为21.83%,缺氮处理1天后,GS活性最低,为0.22 U/g,7天后稍有增加,增幅为22.9%。处理组GOGAT活性在第3天时最低,为0.088 U/g,随后酶活性增加并保持稳定,但是仍低于对照组。处理组氮代谢关键酶基因MdNR2、MdNIR、MdGS2、MdGOGAT的表达量在处理至21天时达到峰值,分别为对照组表达量的3.36、2.52、11.37和2.29倍。【结论】苹果愈伤组织对缺氮非常敏感,从第一天起就可以观测到细胞间隙变大且体积变小,对NO₃–的吸收速率逐渐降低,氮素同化酶活性基本呈逐渐降低的趋势,氮素同化酶基因表达量逐渐升高。缺氮7天后,苹果愈伤组织硝态氮含量趋于稳定,并开始外排NO₃–;氮素同化酶活性基本呈逐渐升高的趋势,氮素同化酶基因表达量进一步升高。总之,氮素亏缺处理前期提高了苹果愈伤组织对NO₃–的吸收,随着处理时间的延长,氮素代谢失衡,严重影响了细胞的形态结构,导致愈伤组织生长发育异常。     

Physiological responses of lupinus mutabilis to phosphorus nutrition and season of growth

The response to phosphorus (P) concentration in the nutrient solution (0–0.5 mol P m^‐3) was studied in Lupinus mutabilis Sweet cv. Potosi in two different seasons (winter and spring). Phosphorus deficiency was more severe on growth than on photosynthesis and the season of growth dramatically influenced the optimal concentration of P for plant growth; root biomass was proportionally less affected than shoot biomass. During winter, growth and photosynthesis of plants supplied with 0.02–0.5 mol P m^‐3 were not significantly different, whereas in spring, rates of growth and photosynthesis were faster at the 0.5 mol P m^‐3 level. Stomatal conductance decreased with deficient P independently of leaf water relations. Severe P deficiency limited carbon (C) assimilation rates due to reduction in stomatal conductance and mesophyll photosynthetic capacity. Decreased sucrose/starch in P‐deficient leaves was a consequence of the observed source/sink imbalance which was more marked in winter. Hydraulic conductance was not a limiting factor for leaf expansion under low P. In conclusion, growth and metabolic changes observed in lupins grown at low P supply can be ascribed to an adjustment at the whole plant level, preventing a large drop in leaf P, reducing shoot growth and facilitating P uptake through higher root biomass.     

Alfalfa cultivar responses to phosphorus and potassium deficiency: Biomass

Soil phosphorus (P) fertility needs for alfalfa (Medicago sativa L.) in the intermountain western United States have been a concern since the inception of irrigation during the late 19th century. Soil potassium (K) fertility needs have received much less attention because K deficiencies, related to long term use of high quality, low‐K irrigation water, have developed only in recent years. The objectives of this research were to compare the responses of five commercial alfalfa cultivars (CVs) to phosphorus (P) and K deficiency and to examine the interactions between CVs and P and K soil fertility. An experiment was established on a highly calcareous Redfield silt loam soil (Fine‐silty carbonatic, mesic Xeric Torrifluvents) that was low in both P and K. The CVs, planted in 1988, were studied through 1993. A 3x3 complete factorial combination of P and K fertilizer rates was established in a split plot design where CV was the whole plot. There was a significant CV x year interaction in biomass production that was not related to diseases or insects. There was also a significant CV x P interaction within and among years. Response to K fertility, though significant, was not prominent in this trial. Alfalfa CV performance trials should segregate CVs best adapted to short term (2–3 year) rotations from those best adapted to long term (4–5 year) rotations. Fertilizer effects on low‐K calcareous soils are short lived; moderate rates of K fertilizer should be applied each spring. Soil P requirements of alfalfa can be met by applying small rates annually or by applying enough for a 4‐ to 5‐year rotation. Phosphorus‐use efficiency by alfalfa may be improved by plant breeding and selection for this trait.     

Differential responses of soybean and dry bean to zinc deficiency 1

Whether a legume obtains its nitrogen (N) from the air, through dinitrogen fixation, or from the soil, as nitrate (NO₃), may influence its susceptibility to zinc (Zn) deficiency. The influence of N source [potassium nitrate (KNO₃)+ native soil N versus rhizobium‐inoculated seed + native soil N] and phosphorus (P) (0 and 200 mg P/kg), and Zn fertilizers (0, 1, and 8 mg Zn/kg) on growth and nutrient composition of soybean (Glycine max L. cv. McCall) and navy bean (Phaseolus vulgaris L. cv. Seafarer) grown on a calcareous soil were studied under greenhouse conditions. Inoculated plants, but not their KNO₃‐treated counterparts, had root nodules. However, due to N deficiency resulting from suboptimal N fixation, growth of these inoculated plants, especially of navy bean, was poorer than that of similarly treated KNO₃‐fed plants. As a consequence of this restricted growth, responses to P and Zn fertilizers were generally greater in KNO₃‐treated plants. Added P decreased the yield of KNO₃‐treated navy bean in the absence of added Zn, but P‐induced Zn deficiency had little effect on the growth of similarly treated inoculated plants. Plant excess bases (EB)/total plant N ratios [EB = 1/2 Ca + l/2Mg + Na + K ‐ Cl ‐ total S (S = divalent) ‐ total P (P = monovalent)] were less in KNO₃‐treated soybean than in correspondingly treated navy bean. Therefore, rhizosphere pH values around navy bean roots were probably less than those around soybean roots. Despite the hypothesized lower rhizosphere pH values, KNO₃‐treated navy bean was more susceptible to Zn deficiency than soybean. This greater susceptibility of navy bean to Zn deficiency was apparently at least partly due to poor translocation of Zn from the roots to the tops.     

Physiological responses of tomato roots grown in organ culture to iron-deficiency stress

Roots of the Fe-efficient tomato (Lycopersicon esculentum Mill., cultivar Floradel) were cultured in an inorganic medium supplemented with glycine, thiamine, pyridoxine, and nicotinic acid, with sucrose as an energy and carbon source. Iron was supplied as ferric hydroxyethylethylenediaminetriacetic acid (FeHEDTA) and the initial PH was 5.5. Root growth was limited when less than 40 μm FeHEDTA was supplied. Roots grown at lower Fe concentrations decreased the pH of the FCR assay medium to a greater extent than did roots grown at higher Fe concentrations. Cultured roots grown with 10 μm FeHEDTA had increased levels of ferric chelate reductase (FCR) activity compared to roots grown with either lower or higher concentrations of FeHEDTA. Low FCR activity of roots grown at 2.5 or 5 μm FeHEDTA was attributed either to impaired metabolism due to Fe-deficiency or the lack of sufficient Fe for enhanced FCR formation. Roots of hydroponically grown tomato plants exhibited typical increases in FCR activity with Fe-deficiency. Based on these preliminary results, cultured roots were found to exhibit similar Physiological responses to Fe-deficiency stress as intact root systems. Cultured roots should provide a useful system for the investigation of the role of the root in plant Fe-deficiency stress responses as previously suggested by Bienfait et al.(Plant Physiol., 83, 244–247, 1987).     

不同品种苹果幼树对氮磷亏缺的生理响应

【目的】探究不同苹果品种对低磷、低氮及低磷低氮胁迫的生理响应,为养分高效利用苹果品种的选育提供理论基础。【方法】采用沙培盆栽试验方法,供试材料为三年生‘嘎拉’/M9T337、‘富士’/M9T337和‘蜜脆’/M9T337矮化自根砧苹果(M9T337为矮化砧木)。以改良1/2Hoagland营养液为基础,设置正常、低氮两个氮水平(NO₃~–15、1.5 mmol/L)和正常、低磷两个磷水平(H₂PO₄~–1.0、0.1 mmol/L),共配置适氮适磷、适氮低磷、低氮适磷和低氮低磷4个处理。测定了苹果树体生长、叶片光合作用和叶绿素荧光参数,分析了苹果叶片氮、磷代谢相关酶活性,树体氮、磷累积量。【结果】与适氮适磷相比,适氮低磷和低氮适磷条件下,‘嘎拉’和‘蜜脆’的植株总干物质量均显著降低,‘富士’的植株总干物质量有显著增加;适氮低磷条件下的‘嘎拉’、‘富士’和‘蜜脆’叶绿素b含量均显著降低,F_o呈升高趋势,但‘嘎拉’和‘富士’的F_v/F_m显著升高且P_n...     

Physiological changes associated with potassium deficiency in cotton

Potassium (K) fertility recommendations based on cotton petiole diagnostic analysis results have been inconsistent in the past, partly because the lowest acceptable petiole K concentration is unknown. Therefore, cotton was grown in sand filled 8‐L pots under two K treatments in a growth chamber at the Altheimer Laboratory in Fayetteville, AR to determine the petiole K concentration that will impact leaf physiology. Chamber‐grown plants were watered every second day with nutrient solution and with deionized water on alternate days. At 14 days after planting two treatments were established consisting of (1) continued complete nutrient solution, and (2) nutrient solution containing no K. Measurements were taken 13, 19, and 26 days after treatment establishment (DATE). Organ K concentrations, leaf chlorophyll, photosynthesis, adenosine triphosphate (ATP), and nonstructural carbohydrate concentrations were monitored as plant K deficiencies developed. All organ K concentrations were much lower in the no‐K treatment on each analysis date. Visual K deficiencies were first observed at 19 DATE along with reductions reductions in leaf chlorophyll concentration. Leaf photosynthesis was greatly reduced in the no‐K treatment beginning at 19 DATE. However, leaf ATP and nonstructural carbohydrate concentrations were higher at 19 and 26 DATE in the no‐K treatment, which may have been the result of reduced utilization and translocation of these metabolites. Our studies show that reductions in leaf physiological processes and plant growth did not occur until the petiole K concentration fell below 0.88% on a dry weight basis. Therefore, reductions in lint yield and quality should not develop until this critical petiole level is attained.     

Responses of fees used for revegetation of desert areas to iron deficiency stress

We investigated the responses to iron deficiency of four major tree species, Prosopis cineraria (local name: Ghaf), Acacia tortilis (Samar), Zizyphus spina-christi (Sidr), and Leptadenia pyrotechnica (Markh), used for revegetation of desert areas in the United Arab Emirates (UAE). The responses to iron deficiency differed among the tree species. Ghaf, Samar, and Sidr decreased the nutrient solution pH in response to iron deficiency. Markh and iron-deficient Sidr did not release protons in the CaCl₂ solution. The Fe reducing capacity of the roots of Ghaf, Samar, and Sidr was significantly enhanced by iron deficiency, whereas a smaller increase in the reducing activity of the roots of Markh was observed. In all the tree species, the amount of reductant released from iron-deficient plants was higher than that from iron-sufficient ones. Markh released a small amount of reductant under irondeficient conditions. In the present study, the reductant released by all the trees was caffeic acid. Acidification treatment enhanced the amount of reductant released irrespective of iron treatments in Ghaf, Samar, and Sidr but had no effect on the amount of reductant released in iron-deficient Markh. The activity of p-coumarate hydroxylase was higher in Sidr and Ghaf than in the other trees. This activity decreased in iron-deficient Samar. Ghaf, Samar, and Sidr showed a high capacity to lower the pH of the nutrient solution, and Ghaf markedly enhanced the effectiveness of all the examined mechanisms, while Markh showed a lower ability compared to the other tree species.