Does light-promoted export from Commelina benghalensis leaves result from differential light-sensitivity of the cells in the mesophyll-to-sieve tube path?

In contrast to the light-promoted uptake by mesophyll cells, light slightly inhibited sucrose uptake by stripped leaf disks of Commelina benghalensis L. This phenomenon appeared to result from a light-promoted vein-associated release, as light stimulated photosynthate release from stripped disks and inhibited that from mesophyll cells. In the -presence of the resorption-blocker p -chloromercuriphenylsulfonic acid, (PCMBS) the release of preloaded [¹⁴C]-sugars (sucrose, glucose) and [¹⁴C]-amino acids (alanine, asparagine, proline, valine, α-aminoisobutyric acid) from stripped disks was doubled in the light. Illumination enhanced by 20 to 60% the release of endogenous leaf cell compounds (sucrose, H₂PO-₄, K⁺, Mg²⁺, Ca²⁺) from stripped disks in the presence of PCMBS. Light also increased the export of [¹⁴C]-assimilates from intact leaves by 20% after pulse-labelling with ¹⁴CO₂. A model for loading is proposed, based on the differential light sensitivities of the plasma membranes in the mesophyll-to-sieve tube path.     

An acylcyclohexadione retardant inhibits gibberellin A1 metabolism, thereby nullifying phytochrome-modulation of cowpea epicotyl explants

The regulation by phytochrome of stem elongation in light-grown plants depends on gibberellins (GAs). To investigate whether this is mediated by a change in GA metabolism, the effect of the GA biosynthesis inhibitor LAB 198 999 (an acylcyclohexadione derivative) on the end-of-day far-red (FR) response in cowpea ( Vigna sinensis L.) epicotyl explants has been investigated. Growth of epicotyl explants of light-grown seedlings was enhanced when treated with far-red light before incubation in the dark (end-of-day FR effect). Low doses of LAB 198 999 (0.05 and 0.5 μg explant⁻¹) reduced the effect of FR, whereas 5 to 50 μg explant⁻¹ stimulated elongation of both red light (R)- and FR-treated epicotyl explants while nullifying the differences between R and FR treatments. In paclobutrazol-treated epicotyl explants, FR enhanced the response to applied GA₁ and GA₂₀, whereas LAB 198 999 increased the activity of GA₁ and decreased that of GA₂₀, [³H]Gibberellin A₁, injected into the basal part of the epicotyl, was transported and metabolized mainly to [³H]GA₈ in the apical 20 mm of the epicotyl. The conversion of [³H]GA₁ to [³H]GA₈ was dramatically reduced by both end-of-day FR treatments and LAB 198 999 applications. In addition, both treatments enhanced epicotyl elongation. It is proposed that the regulation of cowpea epicotyl growth by phytocrome is mediated, at least partially, by modifying GA₁ degradation.     

Biological activity, identification and quantification of gibberellins in seedlings of Norway spruce (Picea abies) grown under different photoperiods

Short photoperiod induces growth cessation in seedlings of Norway spruce ( Picea abies (L.] Karst.). Application of different gibberellins (GAS) to seedlings growing under a short photoperiod show that GA₉ and GA₂₀ can not induce growth. In contrast application of GA, and GA₄ induced shoot elongation. The results indicate that 3β-hydroxylation of GA₉ to GA₄ and of GA₂₀ to GA₁ is under photoperiodic control. To confirm that conclusion, both qualitative and quantitative analyses of endogenous GAs were performed. GA₁, GA₃, GA₄, GA₇, GA₉, GA₁₂, GA₁₅, GA₁₅, GA₂₀, GA₂₉, GA₃₄ and GA₅₁ were identified by combined gas chromatography-mass spectrometry in shoots of Norway spruce seedlings. The effect of photoperiod on GA levels was determined by using deuterated and ¹⁴C-labelled GAs as intermal standards. In short days, the amounts of GA₉, GA₄ and GA₁ are less than in plants grown in continuous light. There is no significant difference in the amounts of GA₃, GA₁₂, and GA₂₀ between the different photoperiods. The lack of accumulation of GA₉ and GA₂₀ under short days is discussed.     

Growth and development of Brassica genotypes differing in endogenous gibberellin content. II. Gibberellin content, growth analyses and cell size

Three rapid cycling Brassica rapa genotypes were grown in greenhouse conditions to investigate the possible relationships between endogenous gibberellin (GA) content and shoot growth. Endogenous GA₁ GA₃ and GA₂₀ were extracted from stem samples harvested at 3 weekly intervals and analyzed by gas chromatography-mass spectrometry with selected ion monitoring, using [²H₂]-GA₁ and [²H₂]-GA₂₀ as quantitative internal standards. During the first 2 weeks, GA levels of the dwarf, rosette ( ros ), averaged 36% of levels in normal plants (on a per stem basis). Levels in the tall mutant, elongated internode (ein) , were consistently higher, averaging 305% of levels in normal plants.
Differences in shoot height across the genotypes resulted from varying internode length which resulted from epidermal cell length and number being increased in ein and decreased in ros relative to the normal genotype. The exogenous application of GA₃ to normal plants increased cell length while the application of paclobutrazol (PP333), a triazole plant growth retardant, reduced cell size. Thus, exogenous GA manipulations mimicked the influence of the mutant genes ros and ein. The dwarf, ros , had reduced shoot dry weights and relative growth rates compared to the other genotypes. Total dry weights were similar in ein and the normal genotype but stem weights were increased in ein , compensating for decreased leaf weights. Thus, the gibberellin-deficiency of ros resulted in generally reduced shoot growth. The overproduction of endogenous GA by ein did not result in enhanced shoot growth but rather a specific enhancement of internode elongation and stem growth at the expense of leaf size.     

A plasma membrane-bound enzyme oxidizes l-tryptophan to indole-3-acetaldoxime

The in vitro conversion of [¹⁴C]-tryptophan to [¹⁴C]-indole-3-acetaldoxime (IAOX) by microsomal membranes of Chinese cabbage (Brassica campestris ssp. pekinensis cv. Granat) has been studied. The reaction product was identified by thin-layer chromatography (TLC) and high performance liquid chromatography (HPLC). Furthermore. IAOX was identified as an endogenous compound of Chinese cabbage by mass spectroscopy. The tryptophan-oxidizing enzyme (TrpOxE) was characterized. MnCl₂ was required as cofactor, H₂O₂, and 2,4-dichlorophenol (DCP) stimulated the reaction. The enzyme showed a pH optimum at pH 8–9 and a K_m for l -tryptophan of 20 μ M . The membranes containing TrpOxE activity were identified as plasma membranes by means of aqueous polymer two-phase partitioning. The TrpOxE from Chinese cabbage was purified 3-fold from plasma membranes by solubilization followed by (NH₄)₂SO₄-fractionation, affinity-chromatography with concanavalin A, and native gel electrophoresis. Enzyme activity was reduced by a tunicamycin pretreatment. Several other plant species, e.g. maize (Zea mays L. Inrakorn), sunflower (Helianthus annuus L. cv. Hohes Sonnengold), tobacco (Nicotiana tabacum L. cv. White Burley), and pea (Pisum sativum L. cv. Krombeck) showed a similar conversion of [¹⁴C]-tryptophan to [¹⁴C]-IAOX by phase-partitioned plasma membranes.     

Stimulatory Effects of Protein Kinase C and Calmodulin Kinase II on N-Methyl-d-Aspartate Receptor/Channels in the Postsynaptic Density of Rat Brain

Abstract: To clarify the regulatory mechanism of the N -methyl- d -aspartate (NMDA) receptor/channel by several protein kinases, we examined the effects of purified type II of protein kinase C (PKC-II), endogenous Ca²⁺/calmodulin-dependent protein kinase II (CaMK-II), and purified cyclic AMP-dependent protein kinase on NMDA receptor/ channel activity in the postsynaptic density (PSD) of rat brain. Purified PKC-II and endogenous CaMK-II catalyzed the phosphorylation of 80–200-kDa proteins in the PSD and l -glutamate-(or NMDA)-induced increase of (+)-5-[³H]methyl-10, 11-dihydro-5 H -dibenzo[a, d]cyclohepten-5, 10-imine maleate ([³H]MK-801; open channel blocker for NMDA receptor/channel) binding activity was significantly enhanced. However, the pretreatment of PKC-II-and CaMK-II-catalyzed phosphorylation did not change the binding activity of l -[³H]glutamate, cis -4-[³H](phospho-nomethyl)piperidine-2-carboxylate ([³H]CGS-19755; competitive NMDA receptor antagonist), [³H]glycine, α-[³H]-amino-3-hydroxy-5-methyl-isoxazole-4-propionate, or [³H]-kainate in the PSD. Pretreatment with PKC-II-and CaMK-II-catalyzed phosphorylation enhanced l -glutamate-induced increase of [³H]MK-801 binding additionally, although purified cyclic AMP-dependent protein kinase did not change l -glutamate-induced [³H]MK-801 binding. From these results, it is suggested that PKC-II and/or CaMK-II appears to induce the phosphorylation of the channel domain of the NMDA receptor/channel in the PSD and then cause an enhancement of Ca²⁺ influx through the channel.     

Localization of ornithine decarboxylase and changes in polyamine content in root meristems of Zea mays

Polyamine content and the activity of arginine decarboxylase (EC 4.1.1.19) and ornithine decarboxylase (EC 4.1.1.17) were studied with respect to meristematic activity in primary roots and in developing lateral roots of Zea mays L. (cv. Neve Ya'ar 170) seedlings. Comparative localization of active ornithine decarboxylase and of meristematic activity were determined by labelling roots either with α-[5-¹⁴C]-difluoromethyl ornithine or with [³H]-thymidine, respectively.
Lateral roots were formed during the 72 h post-decapitation period, accompanied by an initial decline in putrescine content and by a significant increase in spennidine con-tent at 48–72 h. High levels of spermidine and lower levels of putrescine were found in the primary root apex as well. A marked increase in ornithine and arginine decarboxylase activity, as measured by ¹⁴CO₂ release, was found during the 72 h post-decapitation period of lateral root development. This increase in ornithine decarboxylase activity was confirmed also by a parallel rise in the incorporation of α-[5-¹⁴C]-difluoromethyl ornithine into trichloroacetic acid-insoluble fractions. Microautoradiographs of longitudinal and cross sections of roots, labelled with α-[5-¹⁴C]-difluoromethyl ornithine, showed that ornithine decarboxylase is localized mainly in the meristematic zones, as evidenced by [³H]-thymidine incorporation. A close correlation between meristematic activity and polyamines was demonstrated in situ , suggesting that polyamine content and biosynthesis may have a role in meristematic activity in corn roots.     

Uptake kinetics of iron-phytosiderophores in two maize genotypes differing in iron efficiency

Iron inefficiency in the maize ( Zea mays L.) mutant ysl is caused by a defect in the uptake system for Fe-phytosiderophores. To characterize this defect further, the uptake kinetics of Fe-phytosiderophores in ysl was compared to the Fe-efficient maize cultivar Alice. Short-term uptake of ⁵⁹Fe-labeled Fe-deoxymugineic acid (Fe-DMA) was measured over a concentration range of 0.03 to 300 μM. Iron uptake in Fe-deficient plants followed Michaelis-Menten kinetics up to about 30 μM and was linear at higher concentrations, indicating two kinetically distinct components in the uptake of Fe-phytosiderophores. The saturable component had similar K_m (∼ 10 μM) in both genotypes. In contrast. V_max was 5.5 μmol Fe-DMA g⁻¹ dry weight [30 min]⁻¹ in Alice, but only 0.6 μmol Fe-DMA g⁻¹ dry weight [30 min]⁻¹ in _ysl. Uptake experiments with double-labeled ⁵⁹Fe-[¹⁴C]DMA suggest that in both cultivars Fe-DMA was taken up by the roots as the intact chelate. The results indicate the existence of a high-affinity and a low-affinity uptake system mediating Fe-phytosiderophore transport across the root plasma membrane in maize. Apparently, the mutation responsible for Fe inefficiency in ysl affected high-affected uptake and led to a decrease in activity and/or number of Fe-phytosiderophore transporters.     

SUBCELLULAR DISTRIBUTION OF ENDOGENOUS AND [3H] γ-AMINOBUTYRIC ACID IN RAT CEREBRAL CORTEX

Abstract— Slices of rat cerebral cortex were labelled by incubation with [³H]γ-aminobutyric acid (GABA) and homogenized in isotonic sucrose. The subcellular distributions of endogenous GAB A, [³H]GABA and glutamate decarboxylase (GAD) were studied by density gradient centrifugation. The subcellular distributions of the labelled and endogenous amino acid were remarkably similar, indicating that [³H]GABA is taken up into the endogenous GABA pool. About 40 per cent of both endogenous and [³H]GABA were recovered in particles which were tentatively identified as synaptosomes from their equilibrium density and sensitivity to osmotic shock. In slices labelled with [³H]GABA and [¹⁴C]α-aminoisobutyric (AIB) acid, significantly more [³H]GABA was recovered in paniculate fractions than [¹⁴C]AIB. About 80 per cent of the enzyme GAD was also recovered in the same particle fractions which contained [³H]GABA and endogenous GABA. Evidence is presented which suggests that a loss of particle-bound GABA occurs during subcellular fractionation procedures.     

Endogenous Noradrenaline Activates α2-Adrenoceptors on Serotonergic Nerve Endings in Human and Rat Neocortex

Abstract: Slices from human neocortex preincubated with [³H]serotonin ([³H]5-HT) were superfused and stimulated electrically to investigate whether the α₂-adrenoceptors on serotonergic terminals can be stimulated by endogenous noradrenaline (NA) released from neighboring noradrenergic fibers. The stimulation-evoked ³H overflow, representing action potential-induced, exocytotic release of 5-HT, was depressed by the NA uptake blocker (+)-oxaprotiline. Rauwolscine (a mixed α₂-adrenoceptor antagonist/5-HT autoreceptor agonist) or phentolamine [a combined α- adrenoceptor/5-HT autoreceptor antagonist; the latter drug in the presence of (+)-oxaprotiline] enhanced the release when the 5-HT autoreceptors had previously been blocked by metitepine. Under hypothermia the release of 5-HT was found to be decreased and that of NA to be increased; under these conditions idazoxan (an α₂-adrenoceptor antagonist) enhanced the release of 5-HT. In neocortex slices from rats (+)-oxaprotiline similarly depressed the release of 5-HT (measured with the same methods) as in human tissue. When rats were pretreated with 6-hydroxydopamine, the inhibitory effect of exogenous NA on 5-HT release was increased, and in slices from rats pretreated with desipramine, it was decreased. In conclusion, α₂-heteroreceptors can be activated by endogenous NA released from neighboring noradrenergic fibers. Because regulatory processes analogous to those in rats probably occur in humans as well, an up- or down-regulation of α₂- heteroreceptors in depressed patients with a (pathological) decrease or a (therapeutic) enhancement of the noradrenergic neurotransmission may also be assumed to occur.     

Metabolism and Biological Activity of Gibberellin A4 in Vegetative Shoots of Zea mays,Oryza sativa,and Arabidopsis thaliana

[17-13C,3H]Gibberellin A4 (GA4) was injected into the shoots of tall (W23/L317), dwarf-1 (d1), and dwarf-5 (d5) Zea mays L. (maize); tall (cv Nipponbare), dwarf-x (dx), and dwarf-y (dy) Oryza sativa L. (rice); and tall (ecotype Landsberg erecta), ga4, and ga5 Arabidopsis thaliana (L.) Heynh. [13C]GA4 and its metabolites were identified from the shoots by full-scan gas chromatography-mass spectrometry and Kovats retention indices. GA4 was metabolized to GA1 in all nine genotypes. GA4 was also metabolized in some of the genotypes to 3-epi-GA1, GA2, 2[beta]-OH-GA2, 3-epi-GA2, endo-GA4, 16[alpha], 17-H2-16, 17-(OH)2-GA4, GA34, endo-GA34, GA58, 15-epi-GA63, GA71, and 16-epi-GA82. No evidence was found for the metabolism of GA4 to GA7 or of GA4 to GA3. The bioactivities of GA4 and GA1 were determined using the six dwarf mutants for assay. GA4 and GA1 had similar activities for the maize and rice mutants. For the Arabidopsis mutants, GA4 was more active than GA1 at low dosages; GA4 was less active than GA1 at higher dosages.     

Lymantria dispar herbivory induces rapid changes in carbon transport and partitioning in Populus nigra

We tested for rapid changes in photosynthate transport and partitioning in response to Lymantria dispar (L.) (Lepidoptera: Lymantriidae) (gypsy moth) herbivory in Populus nigra L. (Salicaceae). Transport and partitioning of [¹¹C]-photosynthate from young mature leaves were measured in vivo before and 18 h after leaf chewing by gypsy moth larvae, which were caged on three older leaves. Following herbivory, there was an increase in export speed of recently fixed carbon from younger mature leaves. The increased export speed was due to a quicker transit time of ¹¹C through the leaf, rather than a change in transport speed through the phloem. Additionally, basipetal partitioning of [¹¹C]-photosynthate was increased following herbivory. Neither of these changes was observed in control plants. This enhancement of export occurs even though herbivores are well known to induce increases in carbon allocation to secondary metabolites within leaves. Our results demonstrate that the use of non-destructive imaging of ¹¹C tracer is a powerful tool for examining plant responses to herbivory. Although the mechanisms underlying the rapid increase in carbon flux to stems and roots remain to be elucidated, our results raise the possibility of a coordinated whole plant response to herbivory. Thus, even when the herbivore specializes on only one plant tissue type, a whole plant approach may be key to understanding how plants respond to herbivory.     

Influence of Cannabinoids on Electrically Evoked Dopamine Release and Cyclic AMP Generation in the Rat Striatum

Abstract: Using the endogenous cannabinoid receptor agonist anandamide, the synthetic agonist CP 55940 {[1α,2β( R )5α]-(−)-5-(1,1-dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]phenol}, and the specific antagonist SR 141716 [ N -(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1 H -pyrazole-3-carboxamide hydrochloride], second messenger activation of the central cannabinoid receptor (CB1) was examined in rat striatal and cortical slices. The effects of these cannabinoid ligands on electrically evoked dopamine (DA) release from [³H]dopamine-prelabelled striatal slices were also investigated. CP 55940 (1 µ M ) and anandamide (10 µ M ) caused significant reductions in forskolin-stimulated cyclic AMP accumulation in rat striatal slices, which were reversed in the presence of SR 141716 (1 µ M ). CP 55940 (1 µ M ) had no effect on either KCl- or neurotransmitter-stimulated ³H-inositol phosphate accumulation in rat cortical slices. CP 55940 and anandamide caused significant reductions in the release of dopamine after electrical stimulation of [³H]dopamine-prelabelled striatal slices, which were antagonised by SR 141716. SR 141716 alone had no effect on electrically evoked dopamine release from rat striatal slices. These data indicate that the CB1 receptors in rat striatum are negatively linked to adenylyl cyclase and dopamine release. That the CB1 receptor may influence dopamine release in the striatum suggests that cannabinoids play a modulatory role in dopaminergic neuronal pathways.     

Growth-promoting activity of gibberellins on shoot elongation in Salix pentandra is reduced by 16,17-dihydro derivatisation

The metabolism of GA₁₀ is thought to be under photoperiodic control in the woody plant Salix pentandra . However, in a recent study using 16,17-[³H₂]GA₁₉ as a mimic of Ga₁₀, no effect of photoperiod was found on its metabolism to 16,17-dihydro-GA₂₀ and 16,17-dihydro-GA₁. To investigate if this was due to differential action of exogenous 16,17-dihydro-GAs and GAs, the effects of the 16,17-dihydro-derivatives of the gibberellins GA₁₉, GA₁, and GA₁ as compared with their parent GAs, on shoot elongation in seedlings of S. pentandra were studied. 16,17-Dihydro-GA₁₉, and -GA₂₀ were both almost inactive, while 16,17-dihydro-GA₁ induced some shoot elongation in seedlings treated with ancymidol as well as under short days. GA₁₉, GA₂₀ and GA₁ were all able to counteract the inhibitory effect of ancymidol under continuous light, while inhibition induced by a 12-h photoperiod was antagonised only by GA₂₀ and GA₁. Thus, the growth-stimulating activity of the tested GAs is significantly reduced by 16,17-dihydro derivatisation, but the derivatives do not inhibit stem elongation in S, pentandra , as has been found in monocotyledons.     

Translocation of photoassimilate from leaves of two popyploid genotypes of tall fescue differing in photosynthetic rates

The photosynthetic rate of a decaploid genotype (1-16-2) of tall fescue ( Festuca arundinacea Schreb.) is about twice that of a common hexaploid genotype (V6-802) (Plant Physiol. 72: 16–21, 1983). Translocation of photosynthate out of the leaves is a possible means of regulating carbon assimilation. To evaluate this possibility, we have examined a) translocation velocity, b) time course of translocation from leaves, c) photoassimilate partitioning pattern into whole plants in pulse and chase experiments, and d) interveinal distances between two ploidy genotypes. Most of the ¹⁴C accumulated in sucrose, and the labelled carbon moved down the leaf blades at similar velocities (6 to 10 cm h⁻¹) in both genotypes. Recent ¹⁴C assimilate was rapidly translocated from the fed area of the leaf blade. For example, the decaploid and the common hexaploid had translocated 40 and 26% of the ¹⁴C, respectively, at 6 h, and 79 and 49% of the ¹⁴C, respectively, at 24 h. Partitioning of ¹⁴C among plant organs was considerably different between the genotypes after a 24 h chase. For example, out of the total ¹⁴C recovered from the whole plant, the decaploid had retained 40% in the labelled leaf with 10, 33 and 29% in other leaves, stem bases and roots, respectively; whereas the hexaploid had retained 91% in the labelled leaf with 4, 3 and 2% in other leaves, stem bases and roots, respectively. However, the higher rate of translocation was correlated with greater interveinal distances in the decaploid genotype. These results suggested that the higher translocation percentage in the decaploid than the hexaploid genotype was due to greater sink activity.     

Characterization of [3H]CP-96,501 as a Selective Radioligand for the Serotonin 5-HT1B Receptor: Binding Studies in Rat Brain Membranes

Abstract: 3-(1,2,5,6-Tetrahydro-4-pyridyl)-5- n -propoxyindole (CP-96,501) was found to be a more selective ligand at the serotonin 5-HT_1B receptor than the commonly used 5-HT_1B agonist, 3-(1,2,5,6-tetrahydro-4-pyridyl)-5-methoxyindole (RU 24969). In rat brain membranes, the tritiated derivative, [³H]CP-96,501, was found to bind with a high affinity ( K _D, 0.21 n M ) to a single binding site ( n _H, 1.0). The receptor density of this site ( B _max, 72 fmol/mg of protein) matched that of the 5-HT_1B receptor determined with [³H]5-HT. Competition curves of 16 serotonergic compounds in [³H]CP-96,501 binding also indicated a single binding site. The rank order of their binding affinities with this new radioligand showed a high degree of correlation with their affinities at the 5-HT_1B receptor determined with [³H]5-HT or [¹²⁵I]iodocyanopindolol. Serotonergic compounds displayed competitive inhibition of [³H]CP-96,501 binding. In the presence of 5'-guanylylimidodiphosphate [Gpp(NH)p], [³H]CP-96,501 binding was reduced, while the potency of CP-96,501 to displace [¹²⁵I]iodocyanopindolol binding was also decreased. These findings are consistent with the agonist nature of CP-96,501. The results of this study suggest that [³H]CP-96,501 is a useful agonist radioligand for the 5-HT_1B receptor.     

Interaction of gibberellins and phytochrome in the control of cowpea epicotyl elongation

The physiological response of cowpea ( Vigna sinensis L.) epicotyl explants to far‐red light (FR) and its interaction with gibberellins (GAs) have been investigated. The effect of FR and GA₁ varied with the age of the seedlings from which the explants were made: for FR, it decreased progressively with age (though the sensitivity of the epicotyls to FR did not change significantly until at least day 11), whereas it remained essentially constant for applied GA₁ between days 5 and 9 after sowing. This indicates that the loss of response to FR may be due to a decrease in endogenous GA levels in the epicotyl. For a range of GA₁ and GA₂₀ (0.01–1 µg explant⁻¹), both hormones were more active in FR than in R irradiated epicotyls, suggesting that phytochrome may affect GA sensitivity besides GA metabolism. The location of the epicotyl region most sensitive to FR (between 5 and 20 mm below the apex) was different from that to GAs (the upper 10 mm). Nevertheless, FR extended the region responsive to applied GAs, even in paclobutrazol‐treated epicotyls where elongation was due entirely to exogenous GAs. This means that modulation of epicotyl elongation by phytochrome, that occurs in a zone different from though overlapping with the GA‐sensitive subapical zone, is also mediated by GAs. Growth in the most FR‐sensitive region of the epicotyl stimulated by FR or GA₁ was due to cell elongation, and in the most GA‐sensitive region to both cell division and elongation. The effect of FR and GA₁ was negated by colchicine, indicating that microtubules may be involved in the response to both factors.     

The role of sink demand in carbon partitioning and photosynthetic reinvigoration following shoot decapitation

Photosynthesis, growth, and carbon partitioning of vigorous coppice shoots were compared with the slower growing intact shoots of Populus maximowiczii × nigra L. MN9 to determine the relationship between carbon partitioning and photosynthetic rate. Relative height growth rate of coppice shoots was 2.2 times that of intact shoots with net photosynthetic rate 1.9 times that of intact shoots. Coppice leaves exported a larger proportion of newly-fixed assimilate (11% compared with 6%) after a 4-h chase. The greater export from coppice leaves was correlated with a greater proportion of [¹⁴C]-labelled photosynthate deposited as starch in stems 4 cm below the point of label application. Coppice leaf assimilate levels were reduced to 15% that of leaves on intact plants, but coppice leaves had twice the concentration of labelled sucrose. Carbohydrates constituted 55% of the water-soluble [¹⁴C]-labelled photosynthate in leaves of coppice shoots compared with 40% in intact shoots. The results suggest that carbon allocation and partitioning in coppice shoots were altered towards production and export of new assimilate, and support the hypothesis that photosynthetic rate is responsive to sink demand for assimilates.