Effect of Boron on the Incorporation of Glucose from UDP-Glucose into Cotton Fibers Grown in Vitro

Boron is required for fiber growth and development in cotton ovules cultured in vitro. Incorporation of [¹⁴C]glucose by such fiber from supplied UDP-[¹⁴C]glucose into the hot alkali-insoluble fraction is rapid and linear for about 30 minutes. Incorporation of [¹⁴C]glucose from such substrate by fibers grown in boron-deficient ovule cultures is much less than in the case with fibers from ovules cultured with boron in the medium. Total products (alkali-soluble plus alkali-insoluble fractions) were also greater in fibers from ovules cultured with boron. The fraction insoluble in acetic-nitric reagent was a small part of the total glucans; however, in the boron-sufficient fibers, there was significantly more of this fraction than in fibers from boron-deficient ovule cultures. The hot water-soluble glucose polymers from the labeled fibers had a significant fraction of the total [¹⁴C]glucose incorporated from UDP-[¹⁴C]glucose. Both β-1,4- and β-1,3- water-soluble polymers were formed in the boron-sufficient fibers, whereas the same water-soluble fraction from the boron-deficient fibers was predominantly β-1,3-polymers. The incorporation of [¹⁴C]glucose from GDP-[¹⁴C]glucose by the fibers attached to the ovules was insignificant.     

Metabolic Requirement of Cucurbita pepo for Boron

Lateral roots of intact summer squash seedlings (Cucurbita pepo L.) were used to quantify the effects of boron deficiency on DNA synthesis, protein synthesis, and respiration. The temporal relationship between changes in these metabolic activities and the cessation of root elongation caused by boron deprivation was determined. Transferring 5-day-old squash seedlings to a hydroponic culture medium without boron for 6 hours resulted in a 62% reduction in net root elongation and a 30% decrease in the incorporation of [³H]thymidine into DNA by root tips (apical 5-millimeter segments). At this time, root tips from both boron-deficient and boron-sufficient plants exhibited nearly identical rates of incorporation of [¹⁴C]leucine into protein and respiration as measured by O₂ consumption. After an additional 6 hours of boron deprivation, root elongation had nearly ceased. Concomitantly, DNA synthesis in root apices was 66% less than in the boron-sufficient control plants and protein synthesis was reduced 43%. O₂ consumption remained the same for both treatments. The decline and eventual cessation of root elongation correlated temporally with the decrease in DNA synthesis, but preceded changes in protein synthesis and respiration. These results suggest that boron is required for continued DNA synthesis and cell division in root meristems.     

Interaction of boron with components of nucleic Acid metabolism in cotton ovules cultured in vitro

Cotton (Gossypium hirsutum L.) ovules grown in a defined nutrient medium undergo normal morphogenesis, including fiber production. In identical medium lacking boron, ovules callus and accumulate brown substances. Boron deficiency-like symptoms were induced by 6-azauracil and 6-azauridine in ovules growing in boron-sufficient media. Other nucleoside base analogs either reduced or had no effect on over-all growth, but did not cause typical boron-deficient callus growth of cotton ovules. Orotic acid and uracil countered the effects of 6-azauracil. Actinomycin D, fluorodeoxyuridine, and ethidium bromide reduced not only fiber production on ovules growing in boron-sufficient media but also callusing of ovules in boron-deficient media.     

Boron deficiency increases putrescine levels in tobacco plants

Polyamine concentrations were determined in leaves and roots of tobacco plants (Nicotiana tabacum L.) subjected to a short-term boron deficiency. A decrease in the growth of shoots and, especially, roots was found under this mineral deficiency. Boron deficiency did not lead to a significant decrease in leaf or root ion concentrations when compared to control treatment; however, as expected, leaf boron concentration was lower in boron-deficient plants in comparison to the control. In leaves, the levels of free putrescine and spermidine were similar in both treatments. In roots, a short-term boron deficiency caused an increase in free putrescine. Moreover, boron-deficient plants had higher conjugated polyamine concentration than boron-sufficient plants, which was especially evident for conjugated putrescine in leaves. A possible link between boron and polyamine levels is proposed and discussed.     

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.     

Effects of Boron Deficiency on Rubidium Uptake and Photosynthesis in the Diatom Cylindrotheca fusiformis

Culturing the diatom Cylindrotheca fusiformis under boron-deficient conditions leads to changes in ⁸⁶Rb uptake and photosynthesis prior to any effect on the rate of cell division. The influx rate of ⁸⁶Rb into boron-deficient cells was 79% of the control rate after 5 to 5.5 hours culture. Despite lowered ⁸⁶Rb influx, however, boron-deficient diatoms accumulated more ⁸⁶Rb than did control cells; this was due to the deficient cells' lower efflux rate. After 24 hours culture, boron-deficient cells had accumulated 30% more ⁸⁶Rb than had control cells, while releasing ⁸⁶Rb at only one-half the control rate. Increased photosynthetic rates were another effect of boron deficiency during this early stage of culture. Prior to 20 hours boron-deficient culture, diatoms had photosynthetic rates 37% greater than those of control cells. Corresponding to the increase in photosynthesis, boron-deficient diatoms had 12% more carbohydrate than control cells after 16 hours culture.     

Early effects of boron deficiency on indoleacetic Acid oxidase levels of squash root tips

The indoleacetic acid (IAA) oxidase activity of root tips of boron-sufficient, -deficient, recovering, and IAA-treated boron-sufficient squash plants (Cucurbita pepo L.) was determined. Apical and subapical root sections displayed an increase in IAA oxidase activity between 6 and 9 hours after boron was withheld, and after 24 hours the activity of the apical sections showed a 20-fold increase over +B controls. Root elongation of -B plants was inhibited before an increase in oxidase activity could be detected. Roots of plants subjected to 12 hours of -B treatment and then transferred to +B treatment for recovery regained normal elongation rates and oxidase activity within 18 to 20 hours. IAA treatment of +B plants increased IAA oxidase activity of apical and subapical root sections and also inhibited root elongation and caused symptoms similar to -B treatments.     

Inorganic carbon fixation and metabolism in maize roots as affected by nitrate and ammonium nutrition

The effects of NO^?₃ and NH⁺₄ nutrition on the rates of dark incorporation of inorganic carbon by roots of hydroponically grown Zea mays L. cv. 712 and on the metabolic products of this incorporation, were determined in plants supplied with NaH¹⁴CO₃ in the nutrient solution. The shoots and roots of the plants supplied with NaH¹⁴CO₃ in the root medium for 30 min were extracted with 80%; (v/v) ethanol and fractionated into soluble and insoluble fractions. The soluble fraction was further separated into the neutral, organic acid, amino acid and non-polar fractions. The amino acid fraction was then analyzed to determine quantities and the ¹⁴C content of its individual components. The rates of dark incorporation of inorganic carbon calculated from H¹⁴CO^?₃ fixation and attributable to the activity of phosphoenolpyuvate carboxylase (EC 4.1.1.31), were 5-fold higher in ammonium-fed plants than in nitrate-fed plants after a 30-min pulse of ¹⁴C. This activity forms a small, but significant component of the carbon budget of the root. The proportion of ¹⁴C located in the shoots was also significantly higher in ammonium-fed plants than in nitrate-fed plants, indicating more rapid translocation of the products of dark fixation to the shoots in plants receiving NH⁺/sp₄ nutrition. Ammonium-fed plants favoured incorporation of ¹⁴C into amino acids, while nitrate-fed plants allocated relatively more ¹⁴C into organic acids. The amino acid composition was also dependent on the type of nitrogen supplied, and asparagine was found to accumulate in ammonium-fed plants. The ¹⁴C labelling of the amino acids was consistent with the diversion of ¹⁴C-oxaloacetate derived from carboxlyation of phosphoenolpyruvate into the formation of both asparatate and glutamate. The results support the conclusion that inorganic carbon fixation in the roots of maize plants provides an important anaplerotic source of carbon for NH⁺₄ assimilation.     

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.     

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.     

Purine biosynthesis and catabolism in soybean root nodules: incorporation of 14C from 14CO2 into xanthine

Nodulated root systems of soybean plants were exposed to ¹⁴CO₂ in the presence and absence of allopurinol. After 5 h about one-fifth of the label in the perchloric acid-soluble fraction of the nodules was found to be in xanthine in the allopurinol-treated plants. Control plants contained much lower levels of xanthine, but with similar specific activity. Hypoxanthine was not detected in either control or allopurinol-treated plants, even though it would be expected to accumulate in the latter. Degradation of labeled xanthine from allopurinol-treated plants using xanthine oxidase and uricase resulted in the loss of most of the label. The preferential incorporation and accumulation of ¹⁴C from ¹⁴CO₂ into C₆ of xanthine in allopurinol-treated plants is consistent with the involvement of phosphoribosylaminoimidazole carboxylase in the de novo synthesis of purines. The accumulation of xanthine and absence of hypoxanthine in nodules of allopurinol-treated plants confirms earlier observations. In addition, the similar specific activities of ¹⁴C in xanthine in allopurinol-treated and control plants indicate that the xanthine which accumulates in allopurinol-treated plants is the product of de novo purine biosynthesis.     

Metabolic fate of 3,4-dichloropropionanilide in plants: the metabolism of the propionic Acid moiety

3,4-Dichloropropionanilide-¹⁴C (propanil) labeled in either the C-1 or C-3 carbon atoms of the propionic acid moiety was applied to the roots of pea (Pisum sativum L.) and rice (Oryza sativa L.) plants in nutrient solution (0.1 mm-0.28 mm). Radioactivity was detected throughout the treated plants, but the greatest labeling was found in the roots. None of the products that contained aniline were radioactive, suggesting that the plants split the propionic acid moiety from propanil. The fate of the propionate moiety of propanil was determined by recovery of ¹⁴CO₂ from plants exposed to propanil-¹⁴C. The time-course of the ¹⁴CO₂ production demonstrated that the intact propionic acid was cleaved from the propanil and subsequently catabolized by the β-oxidation catabolic sequence. The appearance of radioactivity in the shoots was attributed to the incorporation of products of propionate metabolism. Both the susceptible pea plants and the tolerant rice plants converted a high percentage of the administered propanil-¹⁴C to ¹⁴CO₂.     

Role of sugars in nitrate utilization by roots of dwarf bean

Nitrate uptake and in vivo, nitrate reductase activity (NRA) in roots of Phaseolus vulgaris, L. cv. Witte Krombek were measured in nitrogen-depleted plants of varying sugar status, Variation in sugar status was achieved at the start of nitrate nutrition by excision, ringing, darkness or administration of sugars to the root medium. The shape of the apparent induction pattern of nitrate uptake was not influenced by the sugar status of the absorbing tissue. When measured after 6 h of nitrate nutrition (0.1 mol m^?3), steady state nitrate uptake and root NRA were in the order intact>dark>ringed>excised. Exogenous sucrose restored NRA in excised roots to the level of intact plants. The nitrate uptake rate of excised roots, however, was not fully restored by sucrose (0.03–300 mol m^?3). When plants were decapitated after an 18 h NO₃^? pretreatment, the net uptake rate declined gradually to become negative after three hours. This decline was slowed down by exogenous fructose, whilst glucose rapidly (sometimes within 5 min) stimulated NG^?₃ uptake. Presumably due to a difference in NO₃^? due to a difference in NO₃^? uptake, the NRA of excised roots was also higher in the presence of glucose than in the presence of fructose after 6 h of nitrate nutrition. The sugar-stimulation of, oxygen consumption as well as the release of ¹⁴CO₂ from freshly absorbed (U-¹⁴C) sugar was the same for glucose and fructose. Therefore, we propose a glucose-specific effect on NO₃^? uptake that is due to the presence of glucose rather than to its utilization in root respiration. A differential glucose-fructose effect on nitrate reductase activity independent of the effect on NO₃^? uptake was not indicated. A constant level of NRA occurred in roots of NO₃^? induced plants. Removal of nutrient nitrate from these plants caused an exponential NRA decay with an approximate half-life of 12 h in intact plants and 5.5 h in excised roots. The latter value was also found in roots that were excised in the presence of nitrate, indicating that the sugar status primarily determines the apparent rate of nitrate reductase decay in excised roots.     

Translocation of sucrose by squash plants

Sucrose-¹⁴C was fed to the leaf blades of squash plants (Cucurbita pepo L. var. melopepo torticallis Bailey) for 30 or 60 minutes. Petioles of treated plants were cut into sections and extracted. The majority of the ¹⁴C within the petiole was in sucrose rather than stachyose, the sugar that is transported by the squash plants when ¹⁴CO₂ is supplied. This indicates that the phloem loading system of squash plants is not the system that specifies which sugar is transported.     

Relationship between Carbon and Nitrogen Metabolisms in Photosynthesis. The Role of Photooxidation Processes

¹⁴CO₂ uptake in leaves of wheat plants (Triticum aestivum L.) fertilized by urea or Ca(NO₃)₂ (25 mol m^-3) was investigated. The Warburg effect (inhibition of ¹⁴CO₂ uptake by oxygen) under 0.03 vol. % CO₂ concentration was observed only in non-fertilized plants. Under 0.03 vol. % CO₂, the Warburg antieffect (stimulation of ¹⁴CO₂ uptake by oxygen) was detected only in plants fertilized by Ca(NO₃)₂. Under saturating CO₂ concentration (0.30 vol. %), the Warburg antieffect was observed in all variants. Under limitation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity (0.30 vol. % CO₂ + 1 vol. % O₂), the rate of synthesis of glycollate metabolism products decreased in control and urea-fertilized plants but was enhanced in nitrate-fed plants. Hence, there was an activation of glycollate formation via transketolase reaction in fertilized plants, and the products of nitrate reduction function were oxidants in nitrate-fertilized plants whereas the superoxide radical played this role in urea-fertilized plants.     

The uptake of carbon dioxide by plant roots

Summary The uptake of C¹⁴O₂ by the roots of intact tomato plants from solution containing Na₂C¹⁴O₃ was studied at different light intensities as well as in darkness.Where plants had previously been starved for CO₂ for 12 hours, a higher rate of C¹⁴ uptake was observed than with plants which had been transferred directly from the soil to the radioactive solution.In general, the C¹⁴ content of the roots was slightly higher than that of the shoots. At light intensities under the compensation point and in darkness the C¹⁴ content of the shoots relative to the roots decreased. This was accompanied by release of C¹⁴O₂ during respiration, indicating that the absorbed C¹⁴ was readily translocated upwards and released as C¹⁴O₂ under these conditions. At light intensities above the compensation point no C¹⁴O₂ was released.     

Photorespiration studies in Cycas circinalis L. and Cycas beddomei Dyer

Photosynthetic carboxylating enzymes and the effects of light and temperature on ¹⁴CO₂ efflux in two species of gymnosperm leaves were studied. The activity of RuBP carboxylase was high and that of PEP carboxylase was very low when compared to C₄ plants. The CO₂ compensation point was high. ¹⁴CO₂ efflux was greater in light than in darkness and the ratio (L/D) increased with increase in temperature and light intensity. The inhibitors of glycolate metabolism showed decreased ¹⁴CO₂ evolution in light while dark respiration was unaffected. It is concluded that both Cycas circinalis, L. and Cycas beddomei Dyer are C₃ plants.     

Lateral movement out of the sieve tubes and its effect on the 14C translocation profile in Helianthus seedlings

Summary The profile of ¹⁴C, that is the plot of log (radioactivity) against distance, down the stem of Helianthus seedlings was studied over the course of the first hour after feeding ¹⁴C to a leaf. These seedlings are shown to accumulate a large proportion of the total ¹⁴C present in the stem in an ethanol insoluble form. The effect of this accumulation on the shape of the profile is considered. The steep advancing front is similar in shape and behaviour in Helianthus and in plants which show little accumulation of insoluble ¹⁴C but the later part of the profile is different in the two groups. This difference would be expected if lateral movement of ¹⁴C out of the translocation stream were mainly irreversible in these Helianthus plants but mainly reversible in plants of the other group.