Autoradiographic Examination of Meristems of Intact Boron-deficient Squash Roots Treated with Tritiated Thymidine

Intact roots of boron-sufficient squash (Cucurbita pepo L.) plants, plants entering boron deficiency, and plants recovering from boron deficiency were exposed to tritiated thymidine at the end of the treatment period to label the replicating DNA of root tip cells. Using histological sections, autoradiographs of intact root meristems were prepared. The labeling pattern in +B root tips revealed the presence of a well defined quiescent center. The ability of root tip cells to incorporate label is correlated with the total root elongation during the −B treatment period. A greater amount of total root elongation during boron deficiency and recovery reflects the fact that root tip cells have retained their ability to synthesize DNA and enter mitosis for a longer time. In roots recovering from boron deficiency, cells of the quiescent center were seen to play no part in the recovery process in roots treated for as long as 20 hours in a −B nutrient solution. They were inactive before, during, and after the −B treatment. Cessation of mitosis occurs as early as 6.5 hours after boron is withheld from the nutrient solution while DNA synthesis can occur for as long as 20 hours after withholding boron. It was concluded that boron is essential for continued DNA synthesis and mitotic activity. The absence of boron results in the cessation of mitosis and DNA synthesis within 20 hours from the time boron is withheld.     

Effect of light intensity and plant size on rate of development of early boron deficiency symptoms in tomato root tips

Young tomato plants (Lycopersicon esculentum Miller, cultivar Rutgers) grown in solution culture at 27° at 2 light intensities with adequate boron (0.1 mg/l) and treated with these 2 intensities in the absence of adequate boron developed root boron deficiency symptoms. The typical deficiency symptoms of decreased root elongation, increased depth of brown color and decreased RNA content of tips developed more rapidly at high than at low light intensity, and plant size influenced results. Plants supplied with adequate boron did not exhibit deficiency symptoms.     

Chemistry and biology of boron.

Boron is an essential nutrient for certain organisms, notably vascular plants and diatoms. Cyanobacteria require boron for formation of nitrogen-fixing heterocysts and boron may be beneficial to animals. Boron deficiency in plants produces manifold symptoms: many functions have been postulated. Deficiency symptoms first appear at growing points, within hours in root tips and within minutes or seconds in pollen tube tips, and are characterized by cell wall abnormalities. Boron-deficient tissues are brittle or fragile, while plants grown on high boron levels may have unusually flexible or resilient tissues. Borate forms cyclic diesters with appropriate diols or polyols. The most stable are formed with cis-diols on a furanoid ring. Two compounds have this structure physiologically: ribose in ribonucleotides and RNA, and apiose in the plant cell wall. Germanium can substitute for boron in carrot cell cultures. Both boron and germanium are localized primarily in the cell wall. We postulate that borate-apiofuranose ester cross-links are the auxin-sensitive acid-growth link in vascular plants, that the cyanobacterial heterocyst envelope depends on borate cross-linking of mannopyranose and/or galactopyranose residues in a polysaccharide-lipid environment, and that boron in diatoms forms ester cross-links in the polysaccharide cell wall matrix rather than boron-silicon interactions. Complexing of ribonucleotides is probably a factor in boron toxicity.     

Effect of boron on cell elongation and division in squash roots

This work establishes that cessation of root elongation of intact squash (Cucurbita pepo L.) plants is an early result of boron deficiency. Root elongation is slowed by 6 hours and is virtually stopped as early as 24 hours after boron is first withheld from the nutrient solution. As root elongation ceased, cell elongation progressed distally into the region normally occupied by the apical meristem and eventually the meristem became indistinguishable. Differentiation was determined by use of an elongation index in which cell length was compared to cell width. This index ranged from a low of 0.8 in boron-sufficient root meristems to a high of 3 in root meristems grown in a boron-deficient nutrient solution for 98 hours. It is concluded that a continuous supply of boron is not essential for cell elongation but is required for maintenance of meristematic activity. Boron may act as a regulator of cell division in this tissue.     

Micro-Nutrient Deficiency and Transpiration in Tropaeolum majus L.

The transpiration of Tropaeolum plants, grown in water culture,on full nutrient solutions and on solutions deficient in oneof the following, i.e. boron, copper, manganese, and zinc, wasstudied. The transpiration rate of the control plants grownon the full nutrient solution was significantly higher thanthat of the deficient ones. The more acute the deficiency, thegreater was the disparity. Under the condition of the experiment, the control plants weresmaller in size, having fewer and smaller leaves than the deficientones, but the foliage was much greener and healthier in appearance.     

Synthesis, salvage, and catabolism of uridine nucleotides in boron-deficient squash roots

Previous work has provided evidence that plants may require boron to maintain adequate levels of pyrimidine nucleotides, suggesting that the state of boron deficiency may actually be one of pyrimidine starvation. Since the availability of pyrimidine nucleotides is influenced by their rates of synthesis, salvage, and catabolism, we compared these activities in the terminal 3 centimeters of roots excised from boron-deficient and -sufficient squash plants (Cucurbita pepo L.). Transferring 5-day-old squash plants to a boron-deficient nutrient solution resulted in cessation of root elongation within 18 hours. However, withholding boron for up to 30 hours did not result in either impaired de novo pyrimidine biosynthesis or a change in the sensitivity of the de novo pathway to regulation by end product inhibition. Boron deprivation had no significant effect on pyrimidine salvage or catabolism. These results provide evidence that boron-deficient plants are not starved for uridine nucleotides collectively. Whether a particular pyrimidine nucleotide or derivative is limiting during boron deprivation remains to be examined.     

Dependence of photosynthesis of sunflower and maize leaves on phosphate supply, ribulose-1,5-bisphosphate carboxylase/oxygenase activity, and ribulose-1,5-bisphosphate pool size

Sunflower (Helianthus annuus L. cv Asmer) and maize (Zea mays L. cv Eta) plants were grown under controlled environmental conditions with a nutrient solution containing 0, 0.5, or 10 millimolar inorganic phosphate. Phosphate-deficient leaves had lower photosynthetic rates at ambient and saturating CO₂ and much smaller carboxylation efficiencies than those of plants grown with ample phosphate. In addition, phosphate-deficient leaves contained smaller quantities of total soluble proteins and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) per unit area, although the relative proportions of these components remained unchanged. The specific activity of Rubisco (estimated in the crude extracts of leaves) was significantly reduced by phosphate deficiency in sunflower but not in maize. Thus, there was a strong dependence of carboxylation efficiency and CO₂-saturated photosynthetic rate on Rubisco activity only in sunflower. Phosphate deficiency decreased the 3-phosphoglycerate and ribulose-1,5-bisphosphate (RuBP) contents of the leaf in both species. The ratio of 3-phosphoglycerate to RuBP decreased in sunflower but increased in maize with phosphate deficiency. The calculated concentrations of RuBP and RuBP-binding sites in the chloroplast stroma decreased markedly with phosphate deficiency. The ratio of the stromal concentration of RuBP to that of RuBP-binding sites decreased in sunflower but was not affected in maize with phosphate deficiency. We suggest that a decrease in this ratio made the RuBP-binding sites more vulnerable to blockage or inactivation by tight-binding metabolites/inhibitors, causing a decrease in the initial specific activity of Rubisco in the crude extract from phosphate-deficient sunflower leaves. However, the decrease in Rubisco specific activity was much less than the decrease in the RuBP content in the leaf and its concentration in the stroma. A large ratio of RuBP to RuBP-binding sites may have maintained the Rubisco-specific activity in phosphate-deficient maize leaves. We conclude that the effect of phosphate deficiency is more on RuBP regeneration than on Rubisco activity in both sunflower and maize.     

A Study on Selected Physiological Parameters of Plants Grown Under Lithium Supplementation

Exposure of sunflower and maize plants to increasing concentrations of lithium (0?C50?mg Li dm^?3) in a nutrient solution induced changes in biomass, leaf area and photosynthetic pigment accumulation, as well as levels of lipid peroxidation. The highest applied lithium dose (50?mg Li dm^?3) evoked a significant reduction in the shoot biomass for both examined species, as well as necrotic spots and a reduction of the leaf area in sunflower plants. An enrichment of a nutrient solution with 5?C50?mg Li dm^?3 did not significantly affect chlorophylls a and b and the carotenoid content in sunflower plants. However, in maize, a significant decrease in all pigment content under highest used lithium concentration was noted. The levels of lipid peroxidation of the cell membranes in leaves of sunflower plants and the roots of maize increased significantly in the presence of 50?mg Li dm^?3, which suggests disturbances of the membrane integrity and pro-oxidant properties of the excess lithium ions. Nonetheless, in maize, an increase of shoot biomass and leaf area in the presence of 5?mg Li dm^?3 was found. An analysis of the metal content indicated that lithium accumulated significantly in sunflower and maize shoots in a dose-dependent manner, but differences occurred between species. The sunflower plants accumulated considerably greater amounts of this metal than maize. The potassium content in shoots remained unchanged under lithium treatments, except for a significant increase in the potassium levels for sunflower plants grown in the presence of 50?mg Li dm^?3. These results suggest that lithium at 50?mg Li dm^?3 is toxic to both plant species, but the symptoms of toxicity are species-specific. Moreover, the lithium influence on plants is dose-dependent and its ions can exert toxicity at high concentrations (50?mg Li dm^?3) or stimulate growth at low concentrations (5?mg Li dm^?3).     

Decrease in nitrate uptake and increase in proton release in zinc deficient cotton,sunflower and buckwheat plants

The effect of varied Zn supply on the pH of the nutrient solution and uptake of cations and anions was studied in cotton (Gossypium hirsutum L.), sunflower (Helianthus annuus L.) and buckwheat (Fagopyrum esculentum Moench) plants grown under controlled environmental conditions in nutrient solutions with nitrate as source of nitrogen. With the appearance of visual Zn deficiency symtoms, the pH of the nutrient solutions decreased from 6 to about 5 whereas the pH increased to about 7 when the plants were adequately supplied with Zn. In Zn deficient plants the pH decrease was associated with a shift in the cation-anion uptake ratio in favour of cation uptake. Of the major ions, uptake of Ca²⁺ and K⁺ was either not affected or only slightly lowered whereas NO₃ ^- uptake was drastically decreased in Zn deficient plants. Although the Zn nutritional status of plants hardly affected the NO₃ ^- concentrations in the plants, the leakage of NO₃ ^- from roots of Zn deficient plants into a diluted CaCl₂ solution was nearly 10 times higher than that of plants adequately supplied with Zn. In contrast to Zn deficiency, Mn deficiency in cotton plants neither affected NO₃ ^- uptake nor the pH of the nutrient solution.The results indicate that, probably as a consequence of the role of Zn in plasma membrane integrity and nitrogen metabolism, when Zn is deficient in dicotyledonous species net uptake of NO₃ ^- is particularly depressed which in turn results in an increase in cation-anion uptake ratio and a corresponding decrease in external pH. The ecological relevance of this rhizosphere acidification is discussed.     

2,6-Dichlorobenzonitrile and Boron Deficiency

Symptoms of 2,6-dichlorobenzonitrile poisoning in plants aredescribed and compared with the symptoms produced by phenylboronicacid and boron deficiency. The effects on the macroscopic andmicroscopic appearance of plants and on their ability to translocategrowth regulators are so alike that it is thought that both2,6-dichlorobenzonitrile and boron deficiency affect the samebasic process.     

Essentiality of Boron for Dinitrogen Fixation in Anabaena sp. PCC 7119

The relationship between the requirement for boron and the form of N supplied in nutrient media to cyanobacterium Anabaena sp. PCC 7119 was investigated. When cells were grown in a medium which contained nitrate or ammonium-N, boron deficiency in the nutrient media did not inhibit growth or change cell composition. However, when cells were dependent on N₂ fixation, the lack of boron inhibited growth (i.e. growth ceased after 96 hours under these conditions). Additionally, boron-deficient cells showed a significant decrease in their content of phycobiliproteins and chlorophyll and accumulated carbohydrates within 24 hours of removing boron from the nutrient media. Inhibition of photosynthetic O₂ evolution accompanied the decrease in photosynthetic pigments. Boron deficiency symptoms were relieved when either boron or combined N was added to boron-deficient cultures. The degree of recovery depended upon the age of the cultures. Assays of nitrogenase activity showed that, after 2 hours of growth, nitrogenase activity of boron-deficient cells was inhibited by 40%. After 24 hours a total inactivation of nitrogenase activity was observed in boron-deficient cells. These results strongly suggest an involvement of boron in N₂ fixation in cyanobacteria.     

Effects of boron foliar applications on vegetative and reproductive growth of sunflower

Foliar application may be used to supply boron (B) to a crop when B demands are higher than can be supplied via the soil. While B foliar sprays have been used to correct B deficiency in sunflower (Helianthus annuus L.) in the field, no studies have determined the amount of B taken up by sunflower plant parts via foliar application. A study was conducted in which sunflower plants were grown at constant B concentration in nutrient solution with adequate B (46 micro m) or with limited B supply (0.24, 0.40 and 1.72 micro m) using Amberlite IRA-743 resin to control B supply. At the late vegetative stage of growth (25 and 35 d after transplanting), two foliar sprays were applied of soluble sodium tetraborate (20.8 % B) each at 0, 28, 65, 120 and 1200 mm (each spray equivalent to 0, 0.03, 0.07, 0.13 and 1.3 kg B ha-1 in 100 L water ha-1). The highest rate of B foliar fertilization resulted in leaf burn but had no other evident detrimental effect on plant growth. Under B-deficient conditions, B foliar application increased the vegetative and reproductive dry mass of plants. Foliar application of 28-1200 mm B increased the total dry mass of the most B-deficient plants by more than three-fold and that of plants grown initially with 1.72 micro m B in solution by 37-49 %. In this latter treatment, the dry mass of the capitulum was similar to that achieved under control conditions, but in no instance was total plant dry mass similar to that of the control. All B foliar spray rates increased the B concentration in various parts of the plant tops, including those that developed after the sprays were applied, but the B concentration in the roots was not increased by B foliar application. The B concentration in the capitulum of the plants sprayed at the highest rate was between 37 and 93 % of that in the control plants. This study showed that B foliar application was of benefit to B-deficient sunflower plants, increasing the B status of plant tops, including that of the capitulum which developed after the B sprays were applied.     

Salt tolerance in crop plants monitored by chlorophyll fluorescence in vivo

The potential of measurements of chlorophyll fluorescence in vivo to detect cellular responses to salinity and degrees of salt stress in leaves was investigated for three crop plants. Sugar beet (Beta vulgaris L.) (salt tolerant), sunflower (Helianthus annuus L.) (moderately salt tolerant), and bean (Phaseolus Vulgaris L. cv Canadian Wonder) (salt intolerant) were grown in pots and watered with mineral nutrient solution containing 100 millimolar NaCl. The fast rise in variable chlorophyll fluorescence yield that is correlated with photoreduction of photosystem II acceptors increased in leaves of sugar beet plants treated with salt suggesting stimulation of photosystem II activity relative to photosystem I. In sunflower, this fast rise was depressed by approximately 25% and the subsequent slow rate of quenching of the chlorophyll fluorescence was stimulated. These differences were more marked in the older mature leaves indicating an increasing gradient of salt response down the plant. The salt effect in vivo was reversible since chloroplasts isolated from mature leaves of salt-treated and control sunflower plants gave similar photosystem II activities. Unlike in sugar beet and sunflower, leaves of salt-treated bean progressively lost chlorophyll. The rate of slow quenching of chlorophyll fluorescence decreased indicating development of a partial block after photosystem II and possible initial stimulation of photosystem II activity. With further loss of chlorophyll photosystem II activity declined. It was concluded that measurements of chlorophyll fluorescence in vivo can provide a rapid means of detecting salt stress in leaves, including instances where photosynthesis is reduced in the absence of visible symptoms. The possible application to screening for salt tolerance is discussed.     

Effect of nutrition on the distribution of potassium and phosphorus inCucurbita pepo L.

In young pumpkin plants, the phosphorus was found not to be affected substantially by potassium deficiency whereas the deficiency of calcium has marked effects. Potassium distribution is not affected substantially by either phosphorus or calcium deficiency. In short-term experiments, the presence of calcium affects markedly the total uptake of potassium but the potassium distribution remains unaltered. The stimulation of potassium uptake is far more pronounced in plants pre-cultivated in a complete nutrient solution than in those pre-cultivated in a calcium-deficient one; this is apparently due to profound metabolic changes taking place in plants grown without calcium. The reserves of phosphorus and potassium stored in the cotyledons are easily mobilized and translocated so that they can be used for growth in early stages of development; the appearance of pathological symptoms of phosphorus or potassium deficiency is thus prevented.     

Structural and Functional Damage Caused by Boron Deficiency in Sunflower Leaves

Photosynthetica - Boron deficiency induced a dramatic inhibition in sunflower plant growth, shown by a reduction in dry mass of roots and shoots of plants grown for 10 d in nutrient solution...     

Does high bicarbonate supply to roots change availability of iron in the leaf apoplast?

The role of the leaf apoplast in iron (Fe) uptake into the leaf symplast is insufficiently understood, particularly in relation to the supposed inactivation of Fe in leaves caused by elevated bicarbonate in calcareous soils. It has been supposed that high bicarbonate supply to roots increases the pH of the leaf apoplast which decreases the physiological availability of Fe in leaf tissues. The study reported here has been carried out with sunflower plants grown in nutrient solution and with grapevine plants grown on calcareous soil under field conditions. The data obtained clearly show that the pH of the leaf apoplastic fluid was not affected by high bicarbonate supply in the root medium (nutrient solution and field experiments). The concentrations of total, symplastic and apoplastic Fe were decreased in chlorotic leaves of both sunflower (nutrient solution experiment) and grapevine plants in which leaf expansion was slightly inhibited (field experiment). However, in grapevine showing severe inhibition of leaf growth, total Fe concentration in chlorotic leaves was the same or even higher than in green ones, indicative to the so-called `chlorosis paradox'. The findings do not support the hypothesis of Fe inactivation in the leaf apoplast as the cause of Fe deficiency chlorosis since no increase was found in the relative amount of apoplastic Fe (% of total leaf Fe) either in the leaves of sunflower or grapevine plants. It is concluded that high bicarbonate concentration in the soil solution does not decrease Fe availability in the leaf apoplast.     

缺硼对大豆根瘤结构和功能的影响

在营养液培养条件下以普通结结瘤大豆Ｂｒａｇｇｃｖ．「Ｇｌｙｃｉｎｅｍａｘ（Ｌ．）Ｍｅｒｒ」及其超结瘤突变体ｎｔｓ３８２为实验材料,运用光学显微方法研究了硼对大豆根瘤结构的影响,并测定了根瘤固氮酶活性结果表明,缺硼使根瘤结构受到严重破坏,并使固氮酶活性显著下降,缺硼使根瘤结构受到破坏是导致固氮酶活性下降的可能原因。     

Structural and Functional Damage Caused by Boron Deficiency in Sunflower Leaves

Boron deficiency induced a dramatic inhibition in sunflower plant growth, shown by a reduction in dry mass of roots and shoots of plants grown for 10 d in nutrient solution supplied with 0.02 μM B. This low B supply facilitated the appearance of brown purple pigmentation on the plant leaves over the entire growth period. Compared to B-sufficient (BS) leaves, leakage from B-deficient (BD) leaves was 20 fold higher for potassium, 38 fold for sucrose, and 6 fold for phenolic compounds. High level of membrane peroxidation was detected by measuring peroxidase activities as well as peroxidative products in BD sunflower plants. Soluble and bound peroxidase activities measured in BD thylakoid membranes were accelerated two fold compared to those detected in BS-membranes. No detectable change in soluble peroxidase activity in roots whereas a 4 fold stimulation in bound peroxidase activity was detected. Thylakoid membranes subjected to low B supply showed enhancement in lipoxygenase activity and malondialdehyde (MDA) content in parallel with 40 and 30 % decrease of linoleic and linolenic acid contents (related to total unsaturated fatty acids). A slower rate of Hill reaction activity (40 %) and a suppressed flow of electron transfer of the whole chain (30 %) were detected in BD thylakoid membranes. This reduction was accompanied with a decline in the activity of photosystem 2 shown by a diminished rate of oxygen evolution (42 %) coupled with a quenching (27.5 %) in chlorophyll a fluorescence emission spectra at 685 nm (F₆₈₅). Thus B is an important element for membrane maintenance, protection, and function by minimizing or limiting production of free oxygen radicals in thylakoid membranes of sunflower leaves. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of Boron Deficiency on the Chemical Composition of a Marine Diatom

Cells of Cylindrotheca fusiformis, a marine pennate diatom,multiplied in complete nutrient medium (containing 0?5 mg l^–1B) with a generation time of 11 h; in a boron-deficient culturemedium (0?02 mg l^–1 B), the generation time was 90 h.The chemical composition of logarithmically growing cells harvestedfrom each of the two culture conditions was compared in orderto assess the effects of boron deficiency on the various chemicalfractions. Under conditions of boron deficiency, the concentrationof protein, carbohydrate, and RNA was depressed below that ofcontrol cells; DNA showed no change, while lipids, phenoliccompounds, and unaccounted-for organic fractions were all increasedin concentration. The greatest effect was on the phenolic fraction.These results are discussed and related to the published literatureon effects of boron deficiency on higher plants.