Turgor Pressure and Phototropism in Sinapis alba L. Seedlings

Rich, T. C. G. and Tomos, A. D. 1988. Turgor pressure and phototropismin Sinapis alba L. seedlings.—J. exp. Bot 39: 291-299. Phototropic responses were studied in light-grown mustard hypocotyls.Phototropism was induced by adding 0.27 µmol m^–2s^–1 unilateral blue light to a background of low pressuresodium (SOX) lamp light. Curvatures of some 6° from thevertical were reached by 60 min, the curvature rate between20 min and 60 min being 0.14° min^–1. From the axialgrowth rate and tissue geometry the local growth rates of illuminatedand shaded sides of the hypocotyl were calculated to be 1.5and 4.5 µmin^–1 respectively. Turgor pressures ofexpanding cells in control plants and in the shaded and illuminatedsides of the blue light illuminated hypocotyls were measuredto be 0.40-0.55 MPa with a pressure probe. No changes in turgorpressure were observed on initiation of curvature. The decayof pressure in the cells of non-transpiring plants followingexcision indicated that the yield stress threshold of the tissuemay be as low as 0.1 MPa. These results indicate that the phototropicgrowth response in this tissue is not mediated by changes inturgor pressure. Key words: Sinapis alba L., phototropism, turgor pressure     

Hypocotyl growth in Sinapis alba L: the roles of light quality and quantity

Abstract. A comparison is made of the relative effectiveness of light quality and light quantity on the elongation growth of Sinapis alba hypocotyls. The results show that hypocotyl extension rate in plants which have not previously been exposed to light is controlled primarily by the prevailing photon fluence rate when the phytochrome photostationary state lies between ∼0.033 and ∼0.81. Below ∼0.033, changes in photostationary state also have a marked effect on extension rate. Elongation growth in light-adapted plants is controlled by both photon fluence rate and the spectral quality of the incident radiation at all photoequilibria. Photosynthesis can modify these responses but is not essential as a prior condition for a green plant to respond to changes in light quality and quantity.     

UV-B Inhibition of Phytochrome-Mediated Anthocyanin Formation in Sinapis alba L. Cotyledons : Action Spectrum and the Role of Photoreactivation

An action spectrum was measured for ultraviolet (UV) radiation-induced damage to (inhibition of) phytochrome-induced anthocyanin formation in cotyledons of 40-hour-old Sinapis alba L. seedlings. The action spectrum showed maximum effectiveness in the 260 to 280 nanometer waveband with little effect above 295 nanometers. The damaging effect of UV could be photorepaired by subsequent exposure to sunlight or to long wavelength (360 nanometers) UV radiation. Because this form of damage is subject to photorepair (photoreactivation), it is probably due to the formation of pyrimidine dimers, and the results suggest that it would not be ecologically relevant even if there was an increase in solar UV due to a decrease in stratospheric ozone levels of about 30%. If a dark period of more than 1 hour is interspersed between the phytochrome induction and the UV irradiation, the inhibition of the phytochrome induction gradually decreases with increasing dark period.     

Ultraviolet Light Inhibition of Phytochrome-Induced Flavonoid Biosynthesis and DNA Photolyase Formation in Mustard Cotyledons (Sinapis alba L.)

In cotyledons of etiolated mustard (Sinapis alba L.) seedlings, phytochrome-far-red-absorbing form-induced flavonoid biosynthesis was found to be inhibited by short-term ultraviolet (UV) irradiations. UV inhibition was shown for the synthesis of quercetin, anthocyanin, and also for the accumulation of the mRNA for chalcone synthase, the key enzyme of this pathway. The UV effect was more pronounced on flavonoid biosynthesis, a process that selectively occurs in the epidermal layers, than on the synthesis of mRNA for chlorophyll a/b-binding protein localized in the mesophyll tissue. These UV inhibitory effects were accompanied by cyclobutane pyrimidine dimer (CPD) formation showing a linear fluence-response relationship. CPD formation and UV inhibition of flavonoid biosynthesis was found to be partially reversible by blue/UV-A light via DNA photolyase (PRE), allowing photoreactivation of the DNA by splitting of CPDs, which are the cause of the UV effect. Like flavonoid formation PRE was also induced by the far-red-absorbing form of phytochrome and induction was inhibited by UV. A potential risk of inhibition, in response to solar UV-B irradiation, was shown for anthocyanin formation. This inhibition, however, occurred only if photoreactivation was experimentally reduced. The PRE activity present in the etiolated seedlings (further increasing about 5-fold during light acclimatization) appears to be sufficient to prevent the persistence of CPDs even under conditions of high solar irradiation.     

Control of Seed Germination by Abscisic Acid: I. Time Course of Action in Sinapis alba L

The germination process of mustard seeds (Sinapis alba L.) has been characterized by the time courses of water uptake, rupturing of the seed coat (12 hours after sowing), onset of axis growth (18 hours after sowing), and the point of no return, where the seeds lose the ability to survive redesiccation (12 to 24 hours after sowing, depending on embryo part). Abscisic acid (ABA) reversibly arrests embryo development at the brink of radicle growth initiation, inhibiting the water uptake which accompanies embryo growth. Seeds which have been kept dormant by ABA for several days will, after removal of the hormone, rapidly take up water and continue the germination process. Seeds which have been preincubated in water lose the sensitivity to be arrested by ABA after about 12 hours after sowing. This escape from ABA-mediated dormancy is not due to an inactivation of the hormone but to a loss of competence to respond to ABA during the course of germination. The sensitivity to ABA can be restored in these seeds by redrying. It is concluded that a primary action of ABA in inhibiting seed germination is the control of water uptake of the embryo tissues rather than the control of DNA, RNA, or protein syntheses.     

Control by Light of Hypocotyl Elongation and Levels of Endogenous Gibberellins in Seedlings of Lactuca sativa L.

Four 13-hydroxygibberellins, gibberellin A₁ (GA₁), 3-epi-GA₁,GA₁₉ and GA₂₀ were identified by full-scan GC/MS in extractsof lettuce seedlings (Lactuca sativa L. cv. Grand Rapids). Theresults suggest that the early-13-hydroxylation biosyntheticpathway to GA₁ functions in the lettuce seedlings. It was alsofound that GA₁ is active per se in the control of hypocotylelongation in lettuce seedlings. To investigate the relationshipbetween control by light of hypocotyl elongation and levelsof endogenous GAs in lettuce, endogenous levels of GAs werequantified by radioimmunoassay in seedlings that had been grownfor 5 days in the dark (5D) and in those that had been grownfor 4 days in the dark and then under white light for 1 day(4D1L). The endogenous level of GA₁ in the upper and lower partsof hypocotyls in 5D seedlings was about four times higher thanthat in 4D1L seedlings. The response of explants (hypocotylsegments with cotyledons) from dark-grown seedlings to GA₁ isknown to be similar in the dark and under white light when theexplants are treated with inhibitors of the biosynthesis ofGA. Therefore, the above information suggests that the highlevel of GA₁ in hypocotyls of dark-grown seedlings is responsiblefor the rapid elongation of hypocotyl, while irradiation bywhite light decreases the endogenous level of GA₁ in the hypocotylswith a resultant decrease in the rate of hypocotyl elongation. (Received March 13, 1992; Accepted May 21, 1992)     

Bee visits and the nectar of Echium vulgare L. and Sinapis alba L.

Abstract. 1. The time-course of anthesis of Echium vulgare is described.
2. Diel changes in the sugar concentration of the nectar, the quantity of nectar and the quantity of sugar per flower are illustrated for E.vulgare and for Sinapis alba.
3. These changes are interpreted in terms of (a) the periodicity of secretion and (b) the influence of microclimate and insect visits on post-secretory changes in the composition and volume of nectar.
4. There was hour-to-hour and day-to-day variation in the species composition and the proportion of workers taking nectar rather than nectar plus pollen among the social bees visiting E.vulgare.
5. Honeybee visits to E.vulgare were more numerous in humid weather, when there was enough nectar per flower for their relatively short tongues to reach, and in an area where the corollas grew shorter than they did elsewhere.
6. The significance of changes in the caloric content, volume, concentration, viscosity and sugar composition is discussed from the points of view of insects and ecologists.     

Glyceraldehyde 3-Phosphate Dehydrogenases and Glyoxylate Reductase: I. Their Regulation Under Continuous Red and Far Red Light in the Cotyledons of Sinapis alba L

The development of NADP- and NAD-dependent glyceraldehyde 3-phosphate dehydrogenase and NADH-specific glyoxylate reductase was followed in Sinapis alba cotyledons grown in the dark or under continuous red and far red light. All three enzyme activities are promoted by light, continuous far red light being more than twice as effective as continuous red light. The activities of the NADP-glyceraldehyde 3-phosphate dehydrogenase and glyoxylate reductase increase in the far red light from 36 to 96 hours. They remain constant until at least 120 hours after sowing and are respectively 11 and 6 times higher than the maximum dark activities. Contrary to this, the activity of the NAD-glyceraldehyde 3-phosphate dehydrogenase is scarcely more than doubled under continuous far red irradiation relative to its maximal dark level, and its time course curve is displaced along the time axis, with the activity increasing between 24 and 72 hours after sowing.     

The mosaic of ancestral karyotype blocks in the Sinapis alba L. genome

The organisation of the Sinapis alba genome, comprising 12 linkage groups (n = 12), was compared with the Brassicaceae ancestral karyotype (AK) genomic blocks previously described in other crucifer species. Most of the S. alba genome falls into conserved triplicated genomic blocks that closely match the AK-defined genomic blocks found in other crucifer species including the A, B, and C genomes of closely related Brassica species. In one instance, an S. alba linkage group (S05) was completely collinear with one AK chromosome (AK1), the first time this has been observed in a member of the Brassiceae tribe. However, as observed for other members of the Brassiceae tribe, ancestral genomic blocks were fragmented in the S. alba genome, supporting previously reported comparative chromosome painting describing rearrangements of the AK karyotype prior to the divergence of the Brassiceae from other crucifers. The presented data also refute previous phylogenetic reports that suggest S. alba was more closely related to Brassica nigra (B genome) than to B. rapa (A genome) and B. oleracea (C genome). A comparison of the S. alba and Arabidopsis thaliana genomes revealed many regions of conserved gene order, which will facilitate access to the rich genomic resources available in the model species A. thaliana for genetic research in the less well-resourced crop species S. alba.     

Biosynthesis of wax esters in tissues of Sinapis alba L. seeds

The biosynthesis of wax esters has been investigated in maturing seeds of Sinapis alba. Exogenous long-chain alcohols are incorporated exclusively into alkyl moieties of wax esters. Oxidation of the long-chain alcohols is not detected. Exogenous fatty acids are incorporated into acyl moieties of wax esters to a low extent. A reduction of fatty acids to alcohols is not observed. Synthesis of wax esters is localized exclusively in the testa; both outer and inner integument are equally active in wax ester biosynthesis. The biosynthesis of wax esters is specific with regard to both chain length and degree of unsaturation of long-chain alcohols. Exogenous and endogenous sterols are not esterified.     

Correlative Growth in Seedlings of Phaseolus vulgaris L. : Inhibition of Stem Growth by the Primary Leaves

Primary leaves of one-week-old seedlings of dwarf beans effectivelyinhibit stem elongation either in natural daylight or in continuousdarkness. Stem elongation is promoted by exogenous gibberellicacid and kinetin and inhibited by indol-3-ylacetic acid (IAA)or abscisic acid, and it is suggested that IAA is the inhibitorysubstance emanating from the primary leaf blades which affectsthe growth of the stem.     

New evidence from Sinapis alba L. for ancestral triplication in a crucifer genome.

We present clear evidence of ancestral genome triplication in Sinapis alba, a close relative of the cultivated Brassica species. Exceptionally high levels of heterozygosity in the parents of an F1 intercross permitted the mapping of an estimated 87% of all detected restriction fragment length polymorphism (RFLP) loci, with each RFLP probe typically detecting 2 or 3 loci. These duplicated loci were arranged in 8 triplicated homologous linkage blocks and 2 small, duplicated, homologous linkage blocks covering the majority of the S. alba genome. Several large-scale inversions and translocations appear to have rearranged the order of loci within homologous blocks. The role of successive polyploidization events on the evolution of crucifer species is discussed.     

The development of protein and oil bodies in the seed of Sinapis alba L.

Summary The cotyledons of Sinapis alba L. seed are the storage organs and first photosynthetic organs. The development of the cotyledon cell contents was studied using electron and light microscopy. From the heart shaped embryo (11 days from petal fall) to the mature seed, nine stages were examined.Both types of protein grains (designated aleurone grains and myrosin grains) were found to form within vacuoles, but the mode of protein accumulation differed with each type of grain.Oil bodies were apparent with the EM from 18 days onwards, but could not be seen to arise from the ER. They were granular in appearance at early stages, but later became electron transparent.     

The appearance of competence for phytochrome-mediated anthocyanin synthesis in the cotyledons of Sinapis alba L.

Summary It is demonstrated that phytochrome-mediated anthocyanin synthesis in the epidermal cells of mustard seedling cotyledons takes place only 27 h after sowing onwards (at 25°C). This starting point cannot be shifted by light treatments or by nutrients. The late appearance of competence for P _fr (P _r and P _fr, red- and far-red absorbing forms of phytochrome, respectively) with regard to anthocyanin synthesis is not related to the phytochrome system per se (P _rP _fr) as this is fully functional immediately after sowing of the seed; nor is it related to the primary reaction of phytochrome: P _fr+XP _fr XP _fr X (X, X, two forms of a receptor for P _fr) or to the initial action of P _fr X:P _fr X+KY (K, coupling element, leading to the product Y, which is no longer photoreversible). Rather, the starting point is determined by internal factors only and is thus not accessible to any specific control by external factors. On the other hand, however, the beginning of the initial action of P _fr X (coupling point) can be shifted by light via phytochrome under high irradiance conditions. Moreover, it is shown that there is no phytochrome-independent effect of blue light on photomorphogenesis in the young mustard seedling and that there is no rapid dark reversion of P _fr which can be detected by physiological means, at least duringAbbreviations P _r red-absorbing forms of phytochrome - P _fr far-red-absorbing forms of phytochrome - P ₁ total spectrophotometrically detectable phytochrome - HS Hoagland's nutrient solution - HIR high irradiance response     

Photoperiodic Control of Flowering in Dark-Grown Seedlings of Pharbitis nil Choisy : The Effect of Skeleton and Continuous Light Photoperiods

The control of night-break timing was studied in dark-grown seedlings of Pharbitis nil (Choisy cv. Violet) following a single continuous or skeleton photoperiod. There was a rhythmic response to a red (R) interruption of an inductive dark period, and the phasing of the rhythm was influenced by the preceding light treatment.

Following a continuous white light photoperiod of 6 hours or less, the points of maximum inhibition of flowering were constant in real time. Following a continuous photoperiod of more than 6 hours, maximum inhibition occurred at 9 and 32.5 hours after the end of the light period. The amplitude of the rhythm during the second circadian cycle was much reduced following prolonged photoperiods.

Following a skeleton photoperiod, the time of maximum sensitivity to a R interruption was always related to the second pulse of the skeleton, R₂, with the first point of maximum inhibition of flowering occurring after 12 to 18 hours and the second after 39 hours. Without a second R pulse, the time of maximum sensitivity to a R interruption was related to the initial R₁ pulse. A `light-off' or dusk signal was not mimicked by a R pulse ending a skeleton photoperiod; such a pulse only generated a `light-on' signal and initiated a new rhythm.

It is concluded that the timing of sensitivity to a R interruption of an inductive dark period in Pharbitis nil is controlled by a single circadian rhythm initiated by a light-on signal. After 6 hours in continuous white light, the phase of this rhythm is determined by the transition to darkness. Following an extended photoperiod, the timing characteristics were those of an hourglass; this seemed to be due to an effect on the coupling or expression of a single circadian timer during the second and subsequent cycles, rather than to the operation of a different timing mechanism.

In addition to the effects on timing, the photoperiod affected the magnitude of the flowering response.

     

The Effects of Light Irradiation on the Orientation of Microtubules in Seedlings of Avena sativa L. and Pisum sativum L.

The effects of light irradiation on the arrangement of corticalmicrotubules (MTs) were examined in etiolated A vena mesocotylsand coleoptiles, and in Pisum epicotyls. Elongation of A venamesocotyls ceased as a result of irradiation with white lightwithin 1 h. The predominant orientation of MTs became more longitudinalwithin 1 h in epidermal cells and changed from transverse tooblique, after the elongation ceased, in parenchymal cells.Irradiation with red and with blue light also caused cessationof cell elongation and the same changes in the orientation ofMTs. Elongation of Avena coleoptiles ceased as a result of irradiationwith white light within 24 h. The predominant orientation ofMTs became more longitudinal in epidermal cells and changedfrom transverse to oblique in parenchymal cells. The changein orientation of MTs in epidermal cells preceded that in parenchymalcells. In Pisum epicotyls, elongation ceased as a result ofirradiation with white light within 1 h. Although the orientationsof MTs in epidermal cells did not show any remarkable change,those in parenchymal cells changed from transverse to obliqueafter cell elongation ceased. The change in orientation of MTs and the cessation of cell elongationof A vena mesocotyls induced by white-light irradiation wereboth significantly retarded by treatment with IAA. This resultsuggests that IAA is involved in maintaining the transverseorientation of MTs in Avena mesocotyls. (Received February 22, 1989; Accepted August 2, 1989)     

Stimulation by Ethylene of Axis and Hypocotyl Growth in Bean and Cocklebur Seedlings

Effects of ethylene on the elongation of bean (Phaseolus vulgaris) embryonic axes and hypocotyls, and of cocklebur (Xanthium pennsylvanicum) hypocotyls were studied. In the bean axes, exogenous ethylene was promotive in stimulating longitudinal growth during the early germination period, but thereafter it turned inhibitive. This transition of the ethylene action is likely involved in the appearance of newly differentiated tissues in the hypocotyl, which are negatively sensitive to the gas. The ethylene stimulated elongation of the axes was hardly affected by light or by the presence of the cotyledons. In the bean hypocotyl segment unit, elongation was stimulated by ethylene in its limited zone, when the concentration of ethylene and the exposure times to ethylene were adequate (0.3 to 30 μl/l, 6 to 8 h): Elongation in the much younger region near to the elbow was inhibited by ethylene treatment, whereas the treatment of the upper region of the shank with ethylene finally resulted in significantly increased growth as compared to the untreated controls. In the continuing presence of ethylene over 3 days, the elongation of every region was retarded markedly and radial growth was induced in most regions of the shank from just below the elbow. These ethylene responses occurred independently of red light irradiation, but the ethylene promotion of elongation was lost by shortening the segment height, by removing the hook portion from the segment unit, or with its natural disappearance as a result of ageing. Fundamentally, similar effects of ethylene was observed in cocklebur hypocotyls.     

Blue Light Mediated Regulation of {beta}-Amylase Activity in Mustard (Sinapis alba L.) Cotyledons

In the cotyledons of mustard (Sinapis alba L.) seedlings grownunder continuous blue light, ß-amylase activity increasedbetween 42–96 h from sowing and thereafter the ß-amylaseactivity abruptly declined. Preirradiation with blue light didnot increase the responsivity of the subsequent phytochrome-mediatedß-amylase increase in the cotyledons. The run-offkinetics of ß-amylase increase in seedlings transferredfrom blue light to darkness indicated that the components ofthe blue light-triggered signal chain are kinetically identicalto those of the phytochrome-mediated signal chain. Far-red reversibilityexperiments showed that the above blue light response is eithermediated by phytochrome directly or the blue light photoreceptorrequires the coaction of phytochrome. (Received November 11, 1987; Accepted March 23, 1988)     

Studies on the Photoreceptors for the Promotion and Inhibition of Flowering in Dark-Grown Seedlings of Pharbitis nil Choisy

Three-day-old etiolated seedlings of Pharbitis nil were exposedto red light for 10 min and sprayed with N⁶-benzyladenine beforetransfer to a 48-h inductive dark period, after which they weregrown under continuous white light. A second red irradiationpromoted flowering when given at the 5 and 24th hour of theinductive dark period but inhibited flowering at the 10 and15th hour. Far-red light inhibited flowering when given at anytime during the first 24 h of the dark period. Red/far-red reversibilitywas clearly observed at the 0, 5, 10 and 24th hour, but notat the 15th hour when both red and far-red lights completelyinhibited flowering. The action spectrum for the inhibition of flowering at the 15thhour of the inductive dark period had a sharply defined peakat 660 nm and closely resembled the absorption spectrum of theP_R form of phytochrome. The photoreceptors involved in thesephotoreactions are discussed. (Received June 10, 1983; Accepted July 6, 1983)