Uptake of benzyladenine by excised watermelon cotyledons

The uptake of 8-[¹⁴C]N⁶-benzyladenine (BA) was studied in excised watermelon (Citrullus vulgaris Schrad.) cotyledons 24 hours after the start of imbibition. The passive nature of this uptake is suggested by the following evidence: (a) no sign of saturation on increasing external concentration of BA; (b) no decrease in uptake under conditions that inhibit ATP synthesis; (c) no change in amount of radioactivity absorbed when cotyledons are frozen and thawed before the uptake test. About two-thirds of the radioactivity taken up is released after 12 hours of washing. If the washing is performed at 2 C very little radioactivity is released.     

Sucrose Concentration at the Apoplastic Interface between Seed Coat and Cotyledons of Developing Soybean Seeds

The apoplastic sucrose concentration at the interface between cotyledons and surrounding seed coats of developing soybeans (Glycine max L. Merr. cv Wye) was found by three indirect methods to be in the range of 150 to 200 millimolar. This is an order of magnitude higher than has been reported elsewhere for soybean. It was also higher than the overall sucrose concentrations in the cotyledons and seed coats, each of which was approximately 90 millimolar. By defoliating plants 24 hours before measurement, both the overall sucrose concentration in the cotyledons and the interfacial apoplastic sucrose concentration were reduced by three-fourths. However, there was no day/night difference in overall tissue sucrose concentration of cotyledons or seed coats from intact plants suggesting the existence of a homeostatic mechanism compensating for the diurnal photosynthetic cycle. About 7 hours were required for a tritiated polyethylene glycol-900 solution to fully permeate developing cotyledons (from ~220 milligram fresh weight embryos), implying high diffusion resistance through the tissue.

These results indicate that a high interfacial sucrose concentration may exist in vivo. They suggest that the saturable carrier-mediated component of sucrose uptake may be of little physiological significance in the outermost cell layers of the cotyledons.

     

Transport of Benzyladenine and Gibberellin A(1) from Roots in Relation to the Dominance between the Axillary Buds of Pea (Pisum sativum L.) Cotyledons

In etiolated, 5-day-old pea (Pisum sativum L.) seedlings a significantly more intensive growth of buds situated in the axil of the excised cotyledons was observed as early as 4 hours after decapitation and excision of one cotyledon of each pair. If [8-¹⁴C]benzyladenine ([¹⁴C]BA) was applied to roots of intact plants 10 hours prior to such decapitation and excision, significantly higher both total and specific ¹⁴C activities were observed in buds situated on the side of the excised cotyledons as early as 4 hours after decapitation and excision. Although the removal of a substantial part of the root system carried out simultaneously with decapitation and excision of one cotyledon resulted in a decrease in total ¹⁴C activity of buds, nevertheless a higher accumulation of ¹⁴C activity was maintained in buds situated on the side of excised cotyledon. If [¹⁴C]BA was applied to roots of seedlings after they were decapitated and deprived of one cotyledon, both total and specific ¹⁴C activities of buds situated on the side of excised cotyledons were significantly higher as early as the end of uptake of [¹⁴C]BA by roots, i.e. after 10 hours. On the other hand, [1,2-³H]gibberellin A₁ applied to roots of intact and/or decapitated and one-cotyledon-deprived seedlings in the same way as [¹⁴C]BA did not appear in the buds until very much later and only in negligible amounts (i.e.³H activity). This indicates that the release of buds from apical dominance represents an active and selective process which can result from the ability of buds to utilize and/or synthesize only certain growth substances within a certain time interval.     

Correlative Studies on Plant Growth and Metabolism. III. Metabolic Changes Accompanying Inhibition of the Longitudinal Growth of Stem and Root by Kinetin

Kinetin-induced expansion of lettuce (Lactuca sativa) cotyledons and inhibition of root are accompanied by parallel changes in protein nitrogen. However, during its inhibition of the longitudinal growth and water uptake of hypocotyl and pea (Pisum sativum) epicotyl sections kinetin markedly stimulates protein synthesis. Kinetin seems to separate auxin induced effects on protein synthesis and water uptake and indicates that water uptake and protein synthesis may not necessarily be correlated.

In contrast to gibberellic acid, kinetin restricts in lettuce seedlings, the mobilization of nitrogen reserves from the cotyledons, and kinetin induced growth is accompanied by a high protein nitrogen/soluble-nitrogen ratio which is characteristic of growth in light. Growth in light may be under the dominant control of kinins.

     

Uptake of [2-14C]abscisic acid and distribution of 14C in apple embryos

Pyrus malus L. var. Golden delicious embryos were incubated with (±)-[2-¹⁴C]abscisic acid (ABA) (10^-5 M, 355 kBq mol^-1). After incubations of various durations, the radioactivity was measured in whole embryos, cotyledons, and embryonic axes.With either 48-h or 16-d incubation periods, the uptake of [¹⁴C]ABA depended upon the mode of culture used. The lowest values corresponded to the absorption by the embryonic axis, the highest to the absorption by the distal parts of the two cotyledons. The cotyledons accumulated the main part of the radioactivity under all conditions. Dormant and almost completely after-ripened embryos cultivated for 4 d showed no significant differences in the radioactivity uptake for identical modes of culture. There was a linear relationship between exogenous ABA concentrations (0.5 to 3.10^-5 M) and ABA uptake for embryos cultivated for 4 d with the distal parts of the cotyledons immersed in the medium.Abbreviations ABA abscisic acid. RM, RM⁺, C/2 M, and CM are different modes of embryo cultures: embryonic axis immersed alone (RM), together with the proximal parts of the cotyledons (RM⁺); distal parts of the cotyledons immersed alone (CM); embroyo flat on the medium, the root and the external surface of one cotyledon being in contact with the medium (C/2 M) - PP proximal parts of the cotyledons - DP distal parts of the cotyledons     

Relationships between Photosynthetically Active Radiation, Nocturnal Acid Accumulation, and CO(2) Uptake for a Crassulacean Acid Metabolism Plant, Opuntia ficus-indica

The influences of photosynthetically active radiation (PAR) and water status on nocturnal Crassulacean acid metabolism (CAM) were quantitatively examined for a widely cultivated cactus, Opuntia ficus-indica (L.) Miller. When the total daily PAR was maintained at 10 moles photons per square meter per day but the instantaneous PAR level varied, the rate of nocturnal H⁺ accumulation (tissue acidification) became 90% saturated near 700 micromoles per square meter per second, a PAR level typical for similar light saturation of C₃ photosynthesis. The total nocturnal H⁺ accumulation and CO₂ uptake reached 90% of maximum for a total daily PAR of about 22 moles per square meter per day. Light compensation occurred near 0 moles per square meter per day for nocturnal H⁺ accumulation and 4 moles per square meter per day for CO₂ uptake. Above a total daily PAR of 36 moles per square meter per day or for an instantaneous PAR of 1150 micromoles per square meter per second for more than 6 hours, the nocturnal H⁺ accumulation actually decreased. This inhibition, which occurred at PAR levels just above those occurring in the field, was accompanied by a substantial decrease in chlorophyll content over a 1-week period.

A minimum ratio of H⁺ accumulated to CO₂ taken up of 2.5 averaged over the night occurred for a total daily PAR of 31 moles per square meter per day under wet conditions. About 2 to 6 hours into the night under such conditions, a minimum H⁺-to-CO₂ ratio of 2.0 was observed. Under progressively drier conditions, both nocturnal H⁺ accumulation and CO₂ uptake decreased, but the H⁺-to-CO₂ ratio increased. A ratio of two H⁺ per CO₂ is consistent with the H⁺ production accompanying the conversion of starch to malic acid, and it apparently occurs for O. ficus-indica when CAM CO₂ uptake is strongly favored over respiratory activity.

     

Specific Effects of Kinetin on the Uptake of Monovalent Cations by Sunflower Cotyledons

Kinetin promoted the uptake of K⁺ and Rb⁺ into detached sunflower cotyledons. This action was concomitant with an acceleration of growth. A slighter promotion of Li⁺ uptake was also noted, but there was no consistent influence on that of Na⁺. A small inhibitory effect on NH₄⁺₄ uptake was apparent when the latter was computed per average weight of sample during the course of incubation. Light also promoted the growth of the cotyledons, but depressed their capacity to absorb potassium. The action of kinetin on cotyledons removed from 5–7 day old seedlings was weaker than on those removed from 2–4 day old seedlings with regard to growth but stronger with regards to K⁺ uptake. When K⁺ uptake by cotyledons taken from 7-day old seedlings was followed with time the kinetin effect was already detectable within a few hours, but it became more pronounced after 10 hours' incubation. Kinetin did not accelerate growth or K⁺ uptake in hypo-cotyl segments. IAA, which was previously shown to promote these processes in hypocotyl segments, inhibited them in cotyledons. A working hypothesis is suggested according to which endogenous auxins and cytokinins regulate the absorption of K⁺ in shoot cells of the intact plant in a manner similar to that in which they act in excised tissues and in this way affect the distribution and redistribution of K⁺ in the shoot; and that they are among the factors which determine the selectivity of ion uptake in the intact plant.     

Estimation of osmotic parameters accompanying zeatin-induced growth of detached cucumber cotyledons

Water potential (ψ), the osmotic potential (ψ_π), and the pressure potential (ψ_p) of detached cotyledons isolated from Cucumis sativus L. cv Marketer seedlings after 0, 1.5, and 3 days growth with and without zeatin were determined. From zero time to 3 days, cotyledons incubated without exogenous zeatin exhibited a slight decrease in ψ (from −0.4 to −1.0 bars), while those grown with zeatin developed even more negative values (about −4 bars). Both groups showed rising ψ_π values (decreases in solutes per unit volume), but this rise was more dramatic in those treated with zeatin. These data indicate that the capacity of zeatin-treated cotyledons to take up water more rapidly than controls and thus expand faster must be due to wall loosening, as reflected in ψ_p values which declined during 3 days from about +11 bars to about +1.4 bars.

It was also found that freshly detached cotyledons or those grown without exogenous zeatin exhibited osmoregulation in polyethylene glycol (PEG) solutions. That is, while cotyledons initially lost H₂O into certain PEG solutions, their ψ values decreased over time and they began absorbing water after 1 to 4 hours. After 3 days growth, zeatin-treated cotyledons had lost most of this capacity of osmoregulate. It seems likely that osmoregulation in cotyledons not treated with zeatin is due to wall loosening rather than changes in ψ_π. Zeatin-treated cotyledons with already loosened walls may not have this option to deal with water stress and thus simply come to equilibrium with external PEG solutions.

     

Germination of phaseolus vulgaris I. Resumption of axis growth

Growth of the excised axis of Phaseolus vulgaris L. (var. White Marrowfat) begins after a 7-hour incubation in buffer or water at 26°. Growth, as measured by axis elongation or fresh weight increase, is linear for at least 8 hours with a resultant fresh weight increase of approximately 65%. Cell elongation begins 4 or 5 hours prior to cell division and 5 or 6 hours prior to radicle protrusion in the intact seed.

The initiation of axis elongation is apparently dependent on synthesis of RNA and protein. Both actinomycin D and puromycin inhibit the initiation of elongation. Actinomycin I) inhibits the incorporation of ATP-8-C¹⁴ into axis RNA and C¹⁴-leucine into protein, while puromycin inhibits the incorporation of C¹⁴-leucine into axis protein.

The respiratory rate of the axes increases sharply at about the time of initiation of cell elongation. Dinitrophenol initially increases O₂ uptake by the axes, but at the end of 15 hours the rates of O₂ uptake by control or dinitrophenol-treated axes are approximately the same.

     

Effects of Benzyladenine on Chlorophyll, DNA, RNA and Protein Content of Attached Young Bean (Phaseolus vulgaris L.) Leaves

N⁶-Benzyladenine (BA) was applied to intact bean (Phaseolusvulgaris L.) primary leaves at 2 and 6 days after imbibition,when they were in the cell division and post-cell division stages,respectively. BA treatment at day 2 temporarily inhibited an increase in chlorophyllcontent in the following day, but stimulated it in later days.No such inhibition by BA was observed for changes with timein DNA, RNA, and protein content and f. wt. On the other hand,BA treatment at day 6 enhanced RNA and protein content, withoutsignificant influence on DNA and chlorophyll content and f.wt. The mode of cytokinin action on greening in leaves during cell-divisiongrowth seems to be different from that in etiolated cotyledons. Phaseolus vulgaris L., bean, greening, benzyladenine, DNA, RNA, protein     

Abscisic Acid and its relationship to seed filling in soybeans

The effect of exogenous abscisic acid (ABA) on the rate of sucrose uptake by soybean (Glycine max L. Merr.) embryos was evaluated in an in vitro system. In addition, the concentrations of endogenous ABA in seeds of three soybean Plant Introduction (PI) lines, differing in seed size, were commpared to their seed growth rates. ABA (10⁻⁷ molar) stimulated in vitro sucrose uptake in soybean (cv `Clay') embryos removed from plants grown in a controlled environment chamber, but not in embryos removed from field-grown plants of the three PI lines. However, the concentration of ABA in seeds of the three field-grown PI lines correlated well with their in situ seed growth rates and in vitro [¹⁴C] sucrose uptake rates.

Across genotypes, the concentration of ABA in seeds peaked at 8.5 micrograms per gram fresh weight, corresponding to the time of most rapid seed growth rate, and declined to 1.2 micrograms per gram at physiological maturity. Seeds of the large-seeded genotype maintained an ABA concentration at least 50% greater than that of the small-seeded genotype throughout the latter half of seed filling. A higher concentration of ABA was found in seed coats and cotyledons than in embryonic axes. Seed coats of the large-seeded genotype always had a higher concentration of ABA than seed coats of the small-seeded line. It is suggested that this higher concentration of ABA in seed coats of the large-seeded genotype stimulates sucrose unloading into the seed coat apoplast and that ABA in cotyledons may enhance sucrose uptake by the cotyledons.

     

Osmosensitivity of Sucrose Uptake by Immature Pea Cotyledons Disappears during Development

Sucrose uptake was studied in isolated, immature pea cotyledons (Pisum sativum L. cv Marzia) in relation to their developmental stage. During the developmental period examined the water content of the cotyledons decreased from ≈80% “stage 1” to ≈55% “stage 2”. When assayed in an isotonic medium (400 osmoles per cubic meter) the influx capacity per gram fresh weight for sucrose was almost constant during this developmental period. The influx could be analyzed into a saturable component (K_m ≈ 9 moles per cubic meter; V_max ≈ 150 nanomoles per minute per gram fresh weight) and an unsaturable component (k_i ≈ 0.5 nanomoles per minute per gram fresh weight [per mole per cubic meter]). Incubation in a hypotonic medium reduced the sucrose influx in stage 1 cotyledons, up to 80% reduction at 0 milliosmole (medium without mannitol), but had no effect on sucrose uptake by stage 2 cotyledons. Reduced uptake in a hypotonic medium (100 osmoles per cubic meter) could be attributed to a lowering of the V_max from 150 to 36 nanomoles per minute per gram fresh weight. During incubation of stage 1 cotyledons and stage 2-cotyledons in a hypotonic medium (200 osmoles per cubic meter) their volume increased by 16% and 5.6%, respectively, while the calculated turgor pressure increased from 0.2 to 0.6 megapascal for cotyledons of both developmental stages. Reduced sucrose influx in hypotonic medium, therefore, seems to be related to cell swelling (membrane stretching) rather than to increased turgor pressure.     

Role of ethylene in chlorophyll degradation in radish cotyledons

The effect of age of radish seedlings on changes in chlorophyll concentration caused by ethylene was examined. Ethylene was produced at 2–4 nl g^?1 h^?1 following excision of cotyledons from 5-to 20-day-old seedlings. The youngest cotyledons maintained this rate, whereas ethylene synthesis declined by as much as 80% during a 24-h period in older cotyledons. The youngest cotyledons continued to accumulate chlorophyll in the dark, but after 7 days cotyledons lost chlorophyll and the proportion of chlorophyll lost increased with age. Ethylene promoted, and norbornadiene inhibited, this loss of chlorophyll; in combined treatments the effects of ethylene and norbornadiene were competitive. The maximal rate of chlorophyll loss occurred in 1μl L^?1 ethylene; extrapolation of the response to concentration indicated that half-maximum loss would occur at 0.005–0.01 μl L^?1 ethylene. In cotyledons from 20-day-old seedlings, chlorophyll degradation occurred mainly after 24 h from excision and transfer to the dark. Chlorophyll degradation during 48 h in the dark was affected by norbornadiene or ethylene applied from 0–24 h or from 24–48 h.     

Uptake and metabolism of N6-benzyladenine and 1-naphthaleneacetic acid and dynamics of indole-3-acetic acid and cytokinins in two callus lines of Actinidia deliciosa differing in growth and shoot organogenesis

Uptake, metabolism and accumulation of N⁶‐benzyladenine (BA) and 1‐naphthaleneacetic acid (NAA) as well as changes in endogenous indole‐3‐acetic acid (IAA) and several isoprenoid‐type cytokinins (Cks) were characterized in two callus lines of Actinidia deliciosa cv. Hayward showing different growth and shoot organogenic responses to exogenously applied 2.7 µM NAA and 4.4 µM BA. Studies were carried out in callus 0, 1, 12, 24 and 48 h after the onset of their fifth subculture on a medium containing [³H]NAA or 8‐[¹⁴C]BA. Kiwifruit callus of line A presented high caulogenic response and lower growth that was positively associated with faster BA uptake, with transient accumulation of BA and isoprenoid‐type Cks, mainly zeatin, exceeding three‐ and four‐fold that of the non‐caulogenic callus, and with values of the BA/NAA ratio exceeding 1, in fact higher than the BA/NAA ratio in the culture medium. The accumulation of BA took place in both callus lines during the first 24 h of subculture and before the re‐initiating of callus proliferation. The higher growth and the low or null caulogenic response shown by line B callus were correlated with faster NAA uptake, with endogenous NAA levels two‐fold higher than in A calli, with higher IAA amounts and with values of the BA/NAA ratio below 1. Moreover, at 48 h free NAA in both kinds of callus reached levels close to those found after 35 days of subculture. Results suggest that temporal accumulation of BA and endogenous Cks is involved in the initiation of cell division leading to callus growth, whereas the maintenance of high NAA and IAA levels are related to the support of long‐term callus development. It also appears that for callus cells to become committed to shoot organogenesis, they must have a concentration of active Cks higher than a threshold value during the first 2 days of culture on fresh medium, while at the same time the concentration of auxins must not exceed a certain maximum.     

18F-FDG Is a Surrogate Marker of Therapy Response and Tumor Recovery after Drug Withdrawal during Treatment with a Dual PI3K/mTOR Inhibitor in a Preclinical Model of Cisplatin-Resistant Ovarian Cancer

AIM: Targeting the phosphoinositide 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) pathway is a potential means of overcoming chemoresistance in ovarian cancer. We investigated the capability of ¹⁸F-fluororodeoxyglucose (¹⁸F-FDG) small-animal positron emission tomography (SA-PET) to predict the effects of a dual PI3K/mTOR inhibitor (BEZ-235) in a cisplatin-resistant ovarian cancer model. METHODS: In a first experiment, nude rats bearing subcutaneous SKOV3 tumors received BEZ-235 for 3 days given alone or after paclitaxel and were compared to controls (either untreated or that were given the excipients of paclitaxel and BEZ-235). SA-PET was performed at baseline, on day 3, and day 7. In a second experiment aiming at further exploring the kinetics of ¹⁸F-FDG tumor uptake during the first 48 hours following drug cessation, untreated controls were compared to rats receiving BEZ-235, which were imaged at baseline, on day 3, on day 4, and on day 5. SA-PET results were compared to cell proliferation assessment (Ki-67), PI3K/mTOR downstream target expression studies (pAKT and phospho-eukaryotic translation initiation factor 4E-binding protein 1), and apoptosis evaluation (cleaved caspase-3). RESULTS: In the first experiment, BEZ-235, compared to untreated controls, induced a marked decrease in ¹⁸F-FDG uptake on day 3, which was correlated to a significant decrease in cell proliferation and to a significant PI3K/mTOR pathway inhibition. No tumor necrosis or apoptosis occurred. Four days following treatment cessation, tumor recovery (in terms of PI3K/mTOR inhibition and cell proliferation) occurred and was identified by ¹⁸F-FDG SA-PET. Paclitaxel plus BEZ-235 showed results similar to BEZ-235 alone. In the second experiment, PI3K/mTOR pathways exhibited partial recovery as early as 24 hours following treatment cessation, but both ¹⁸F-FDG SA-PET and cell proliferation remained unchanged. CONCLUSIONS:¹⁸F-FDG SA-PET is a surrogate marker of target inhibition during treatment with BEZ-235 and predicts tumor recovery 4 days after drug withdrawal, but not during the first 48 hours following drug cessation, when a lag between PI3K/mTOR pathway recovery and metabolic recovery is observed. ¹⁸F-FDG SA-PET could be used for therapy monitoring of PI3K/mTOR inhibitors, but our results also raise questions regarding the potential impact of the delay between PET imaging and the last drug intake on the accuracy of FDG imaging.     

Comparative effects of gibberellic acid and N6-benzyladenine on dry matter partitioning and osmotic and water potentials in seedling organs of dwarf watermelon

Apical applications of 0.2 g N⁶-benzyladenine (BA), a synthetic cytokinin, or 5 g of gibberellic acid (GA₃) significantly enhanced hypocotyl elongation in intact dwarf watermelon seedlings over a 48-h period. Accompanying the increase in hypocotyl length was marked expansion of cotyledons in BA-treated seedlings and inhibition of root growth by both compounds. A study on dry matter partitioning indicated that both growth regulators caused a preferential accumulation of dry matter in hypocotyls at the expense of the roots; however, GA₃ elicited a more rapid and greater change than did BA. In comparison to untreated seedlings, BA decreased total translocation of metabolites out of the cotyledons. Water potentials of cotyledons and hypocotyls were determined by allowing organs to equilibrate for 2 h in serial concentrations of polyethylene glycol 4000. Osmotic potentials were determined by thermocouple psychrometry. During periods of rapid growth in cotyledons and hypocotyls of BA-treated seedlings and in hypocotyls of GA-treated seedlings, the osmotic potential increased and the turgor pressure decreased in relation to untreated seedlings, indicating that cell wall extensibility was being increased. Osmotic potentials were lower in hypocotyls of GA-treated than in those of BA-treated seedlings, even though growth rates were higher in GA-treated seedlings, indicating that the latter treatment was generating more osmotically active solutes in hypocotyls.Scientific Contribution No. 1219 from the New Hampshire Agricultural Experiment Station.     

Polyamines and Root Formation in Mung Bean Hypocotyl Cuttings : II. Incorporation of Precursors into Polyamines

The incorporation of [¹⁴C]arginine and [¹⁴C]ornithine into various polyamines was studied in mung bean (Vigna radiata [L.] Wilczek) hypocotyl cuttings with respect to the effect of indole-3-butyric acid on adventitious root formation.

Both [¹⁴C]arginine and [¹⁴C]ornithine are rapidly incorporated into putrescine, spermidine, and spermine, with similar kinetics, during 5- to 24-hour incubation periods. The incorporation of arginine into putrescine is generally higher than that of ornithine. The biosynthesis of putrescine and spermidine from the precursors, in the hypocotyls, is closely related to the pattern of root formation: a first peak at 0 to 24 hours corresponding to the period of root primordia development, and a second peak of putrescine biosynthesis at 48 to 72 hours corresponding to root growth and elongation. Indole-3-butyric acid considerably enhances putrescine biosynthesis in both phases, resulting in an increase of the putrescine/spermidine ratio.

It is concluded that the promotive effect of indole-3-butyric acid on putrescine biosynthesis, from both arginine and ornithine, supports the hypothesis that auxin-induced root formation may require the promotion of polyamine biosynthesis.

     

Transport of purine and pyrimidine bases and nucleosides from endosperm to cotyledons in germinating castor bean seedlings

During germination and early growth of castor bean (Ricinus communis), all cellular constituents of the endosperm are eventually transferred to the growing embryo. The present results bear on the transport of breakdown products of nucleic acids. The total content of nucleic acids and nucleotides declines rapidly between day 4 and day 8 of seedling development. Concomitant with this decline, a secretion of adenosine, guanosine, and adenine from excised endosperms into the incubation medium takes place, accompanying a much more extensive release of sucrose and amino acids. Release of nucleotides could not be detected. The rates of release were linear for at least 5 hours for all compounds measured, indicating that they were liberated due to a coordinated metabolism. Uptake studies with cotyledons removed from the seedling showed that these have the ability to absorb all the substances released from the endosperm. Besides sucrose and amino acids, both nucleosides and free purine and pyrimidine bases were taken up by the cotyledons with high efficiency. AMP was also transported whereas ATP was not. Kinetic analyses were carried out to estimate the maximal uptake capacities of the cotyledons. Rates of uptake were linear for at least 1 to 2 hours and saturation kinetics were observed for all substances investigated. It is concluded that nucleosides can serve best as transport metabolites of nucleic acids, inasmuch as they are taken up by the cotyledons with the highest efficiency, the V_max/K_m ratios being considerably higher than those found for free purine and pyrimidine bases. For both adenosine and adenine transport, the V_max was about 2 micromoles per hour per gram fresh weight, and the K_m values were 0.12 and 0.37 millimolar, respectively. The rates of metabolite release from the endosperm and the capacity of the absorption system in the cotyledons are shown to account for the observed rates of disappearance of nucleic acids from the endosperm and efficient transport to the growing embryo.     

Role of ethylene in chlorophyll degradation in radish cotyledons

The effect of age of radish seedlings on changes in chlorophyll concentration caused by ethylene was examined. Ethylene was produced at 2–4 nl g^–1 h^–1 following excision of cotyledons from 5-to 20-day-old seedlings. The youngest cotyledons maintained this rate, whereas ethylene synthesis declined by as much as 80% during a 24-h period in older cotyledons. The youngest cotyledons continued to accumulate chlorophyll in the dark, but after 7 days cotyledons lost chlorophyll and the proportion of chlorophyll lost increased with age. Ethylene promoted, and norbornadiene inhibited, this loss of chlorophyll; in combined treatments the effects of ethylene and norbornadiene were competitive. The maximal rate of chlorophyll loss occurred in 1l L^–1 ethylene; extrapolation of the response to concentration indicated that half-maximum loss would occur at 0.005–0.01 l L^–1 ethylene. In cotyledons from 20-day-old seedlings, chlorophyll degradation occurred mainly after 24 h from excision and transfer to the dark. Chlorophyll degradation during 48 h in the dark was affected by norbornadiene or ethylene applied from 0–24 h or from 24–48 h.