The Production of the Striga and Orobanche Germination Stimulants by Maize Roots: II. CONDITIONS OF SYNTHESIS IN THE ROOT

Six serial segments, each 3·0 mm. in length, have beenexcised from the tip backwards along the maize root and groupsof each fragment have been aerated in water and in solutionsof sugars for varying times. The amounts of the complex of Orobancheand Striga germination stimulants which accumulate in the culturefluids have been assessed and compared with the amounts initiallypresent in the tissue at the time of excision from the root. It is shown that with each fragment more of the complex appearsexternally when sugar is supplied. 10^–2 M. solutions ofglucose, fructose, or sucrose promote an optimal effect. Theamounts in the external solutions are inversely proportionalto the initial contents of the segments. The first segment (0–3mm.) contains the highest proportion of the complex initially,but when immersed in water or the sugar solution very littleof it appears externally. The second segment (3–6 mm.)contains the least initial quantity of the complex but in wateror sugar produces considerably more than any other segment andmore than is originally present. As the series is traversedfrom segments 2 to 6, the segments have progressively largerinitial contents than has segment 2, but after aeration in thesugar solutions the amounts accumulating externally are lessthan with segment 2 and decrease with their increasing distancefrom the root tip. The significance of these observations is discussed.     

Characterization of Strigolactones, Germination Stimulants for the Root Parasitic Plants Striga and Orobanche, Produced by Maize, Millet and Sorghum

The germination stimulants for root parasitic plants Striga and Orobanche produced by sorghum (Sorghum bicolor (L.) Moench), maize (Zea mays L.), and pearl millet (Pennisetum typhoideum Rich.) were examined. Characterization of strigolactones in the root exudates from the plants grown hydroponically was conducted by comparing retention times of germination stimulants on reverse phase high performance liquid chromatography (HPLC) with those of synthetic standards, and by using HPLC linked with tandem mass spectrometry (LC/MS/MS). All the plants tested, except for a sorghum cultivar Swarna, were found to exude two major stimulants, 5-deoxy-strigol, which is known as a branching factor for arbuscular mycorrhizal (AM) fungi, and an isomer of strigol, tentatively named sorghumol. Swarna was found to exude 5-deoxy-strigol and strigol. These results imply that 5-deoxy-strigol is one of major germination stimulants of gramineous plants and that major stimulants may differ even among cultivars within the same species.     

The Movement of Ions to the Xylem Exudate of Maize Roots: I. PROFILES OF MEMBRANE POTENTIAL AND VACUOLAR POTASSIUM ACTIVITY ACROSS THE ROOT

Membrane potentials and vacuolar potassium activities of cellsin the tissues likely to be involved in the movement of ionsto the xylem vessels of maize roots have been measured. Fromthis information it has been possible to build up a detailedpicture of the profile electrochemical potential across theroots. For roots bathed in 1.0 mM KCl solution the results indicatean active movement of potassium to the vacuoles. The most strikingfinding was that this ability to accumulate potassium activelywas the same for the cells of all the tissues investigated.Consequently there appeared to be no marked gradient in thedriving force on potassium from cell to cell across the maizeroot under the conditions of our experiments. In view of the uniformity in the membrane transport propertiesof all the cells, it suggested that the endodermis does nothave any special secretory role in the radial movement of potassiumions. It is concluded that the results are consistent with thesymplasm theory of ion movement. However, contrary to the resultsof some workers, they indicate that the living stelar cellsare no more leaky than those of the cortex.     

PHYLOGENETIC TRANSITIONS IN THE CHLOROPLASTS OF THE ANTHOCEROTALES I. THE NUMBER AND ULTRASTRUCTURE OF THE MATURE PLASTIDS

Representatives of three genera of anthooerotes were examined: Phaeoceros, Notothylas, and Megaceros. Species of the first two genera were found to exemplify the typical anthocerote plastid condition. This condition is characterized by the presence in each cell of the gametophyte of only a single large chloroplast containing a “multiple” pyrenoid. The genus Megaceros, however, proved to be quite different. In two species of Megaceros the pyrenoid was observed to be composed of a highly subdivided thylakoid system of even greater complexity than the “multiple” pyrenoids of Phaeoceros. In another species only an indistinct “pyrenoid-like” area was noted while in a fourth species no evidence was found for any internal differentiation. Associated with these changes in plastid structure there are corresponding alterations in the number and the size of the chloroplasts. Together they indicate an evolutionary trend away from a primitive, algal-like condition to a more advanced land plant form.     

The Movement of Ions to the Xylem Exudate of Maize Roots: III. THE LOCATION OF THE ELECTRICAL AND ELECTROCHEMICAL POTENTIAL DIFFERENCES BETWEEN THE EXUDATE AND THE MEDIUM

Trans-root and membrane potentials have been measured simultaneouslyin the same maize root by using microelctrodes inserted in theexuding sap, external bathing solution, and a vacuole of anepidermal cell. On rapidly increasing the KCl concentrationof the external solution, the membrane and trans-root potantialsfell simultaneously. This initial rapid phase of depolarizationwas complete within 20s of changing the external solution whenthe membrane potential had reached a new stable value. However,the trans-root potential continued to fall slowly and this phaseof depolarization lasted for about 25 min. Then followed a riseto a stable value at 1.5–2.0 h. This secondary rise wasrelatively small compared with the initial fall. The major part(approx. 80 per cent) of the depolarization of the trans-rootpotential occured during the initial rapid phase. These results indicate that the major component of the trans-rootpotential resides at the plasmalemma of the epidermal cellswith a smaller contribution from the cells underlying the epidermis.The rise in the trans-root potential after 25 min suggestedthat this back potential was associated with the plasmalemmaof the xylem parenchyma. From knowledge of the elelctrical propertiesof these cells this back potential could be calculated and trans-rootpotentials accurately predicted from values of the membranepotentials of root cells. It is concluded that in maize roots, ion movement to the xylemvessels is mainly symplasmic, that the outer boundery of thesymplasm is the plasmalemma of the epidermal cells and thatthe inner boundary is the plasmalemma of the xylem parenchyma.This hypothesis has enabled trans-root electrochemical potentialdifferences to be predicted accurately from vacuolar values.     

THE ABPHYL SYNDROME IN ZEA MAYS. I. ARRANGEMENT. NUMBER AND SIZE OF LEAVES

A range of phyllotactic variation in Zea mays has been obtained from progeny derived from a single ‘opposite-leaved’ plant. Some segregants exhibit a form of alternate phyllotaxy with poor separation between nodes, while others duplicate the original decussate condition. Numerous intermediate examples have also been observed. Some have both spiral and opposite arrangements on the same plant; others, which begin their ontogeny with the normal distichous arrangement, switch at different stages of maturity to spiral or decussate arrangement. Leaves from ABPHYL plants are up to one-half the width of comparable normal leaves although their length is similar. Since ABPHYL plants may have twice as many leaves as do normal siblings, total area and dry weight of leaf blades per ABPHYL plant are greater than in normal siblings. Leaf width of both ABPHYL and normal plants correlates well with the number rather than the width of epidermal cells.     

Studies on the Germination of the Seeds of Striga hermonthica: II. THE EFFECT OF THE STIMULATING SOLUTION ON SEED RESPIRATION

1. Data are presented which show the effect of the natural stimulatingsolution on the respiration of the seeds of Striga hermonthica.
2. The stimulating solution has been shown to enhance the aerobicrespiration of seeds exposed to it, compared with that of seedsto distilled water at the same temperature. This effect on therespiration of the seeds is quite independent of any previousmoisture-treatment of the seeds, and thus of germination.
3. Themaximum anaerobic rate of carbon-dioxide output of air-dryseedsafter treatment with the stimulating solution may be upto fifteentimes greater than that of seeds moistened with distilledwater.The anaerobic respiration of air-dry seeds after exposuretothe stimulating solution is greatly reduced by 0?025 M. sodiumfluoride and 0?001 M. sodium monoiodoacetate.
4. The stimulatingsolution has been found to have little effecton the anaerobicoutput of carbon dioxide from seeds which hadbeen moisture-treatedfor 6 days at 22? C. before treatmentwith the stimulating solution.
5. No correlation could be established between the effects ofthestimulating solution on germination and respiration.

     

The Movement of Ions to the Xylem Exudate of Maize Roots: II. A COMPARISON OF THE ELECTRICAL POTENTIAL AND ELECTROCHEMICAL POTENTIALS OF IONS IN THE EXUDATE AND IN THE ROOT CELLS

The electrical potential differences (E_ex) and the electrochemicalpotential differences for potassium and chloride in the xylemexudate of excised maize roots were determined and the valuesobtained indicated active transport of both ions to the xylemexudate. The potential differences were depolarized by increasesin the external KCl concentration ([KCl]_o), and E_ex was a linearfuction of log[KCl]_o. The corresponding data for the cell vacuolesof the same roots were also determined and trends similar tothose found for the exudate were obtained. The electrochemicalpotential differences between the cell vacuoles and the externalsolution for potassium (µ_rac.k) and chloride (µ_vac.Cl)indicated that both ions were actively transported to the vacuoles.Both µ_vac.k and µ_vac.cl decreased in a linear manneras [KCl]_o was increased. However, in spite of the similar trendsin both sets of data, the values of the electrical potentialand chloride electrochemical potential differences were alwaysmarkedly larger for the vacuoles than for the exudate. For potassium,there were no significant differences. It is concluded thatthe discrepancy between the data for the vacuoles and the wholeroots does not support the idea that the exuding root systembehaves like a single cell, as has been suggested by some workers.     

The strigolactone germination stimulants of the plant-parasitic Striga and Orobanche spp. are derived from the carotenoid pathway

The seeds of parasitic plants of the genera Striga and Orobanche will only germinate after induction by a chemical signal exuded from the roots of their host. Up to now, several of these germination stimulants have been isolated and identified in the root exudates of a series of host plants of both Orobanche and Striga spp. In most cases, the compounds were shown to be isoprenoid and belong to one chemical class, collectively called the strigolactones, and suggested by many authors to be sesquiterpene lactones. However, this classification was never proven; hence, the biosynthetic pathways of the germination stimulants are unknown. We have used carotenoid mutants of maize (Zea mays) and inhibitors of isoprenoid pathways on maize, cowpea (Vigna unguiculata), and sorghum (Sorghum bicolor) and assessed the effects on the root exudate-induced germination of Striga hermonthica and Orobanche crenata. Here, we show that for these three host and two parasitic plant species, the strigolactone germination stimulants are derived from the carotenoid pathway. Furthermore, we hypothesize how the germination stimulants are formed. We also discuss this finding as an explanation for some phenomena that have been observed for the host-parasitic plant interaction, such as the effect of mycorrhiza on S. hermonthica infestation.     

Studies on the Geotropism of Roots: I.GROWTH-RATE DISTRIBUTION DURING RESPONSE AND THE EFFECTS OF APPLIED AUXINSI

Experiments are described in which the normal geotropic responsesof the roots of Pisum sativum seedlings have been compared withthose obtained in the presence of auxins (indole-3-acetic acidand 2:4-dichlorophenoxyacetic acid) in the external medium.The courses of positive curvature resulting from short exposures(40 minutes) and also subsequent recovery phenomena on a horizontalklinostat have been followed. A photographic recording techniqueallowed the determination of absolute growth-rates of both upperand lower sides of the root during the course of each experiment. Positive curvature started at its maximum rate (0.30–0.32deg./min.) after a reaction time of 11.5 minutes and continuedconstant at that rate for about 60 minutes after stimulationceased. Recovery took place at a similar rate of curvature andwas complete after a further 150–200 minutes. During thephase of positive curvature overall root growth-rates were considerablyreduced and were slowly restored to normal during recovery. Low concentrations (1 part in 10¹¹) of both auxins increasedthe rate of positive curvature by 30–40 per cent. andshortened the reaction time roughly in proportion. The growth-ratesof both sides of the root were increased to the same extentduring both curvature and recovery. High concentrations (10^–8IAA and 3.10^–8 2: 4-D)reduced the rates of curvature by 50 per cent., lengthened thereaction time, and inhibited the growth of both sides of theroot during both curvature and recovery. Neither concentration of either auxin otherwise affected thetime course of response and recovery. It is suggested that geotropic response is due to the de novoproduction of an endogenous inhibitor in the extending cellsof the lower side of the root whence it may later spread tothe upper side. The complete independence of the growth actionsof this inhibitor and of the applied auxins suggests that itis not indole-3-acetic acid or any similar compound. Recoverymay be very largely independent of both inhibitor and auxinsand due to the action of another growth factor limiting celllength. The implications of these findings and of the attendant theoriesare fully discussed.     

The Initial Uptake of Ions by Barley Roots: I. UPTAKE OF ANIONS

Experiments have been carried out to examine the applicabilityof the Goüy–Chapman electrical double-layer theoryto the initial entry of anions into excised barley roots. Anattempt was made to separate the total quantity of labellediodide or sulphate ions which had entered the roots under varyingconditions into three fractions: the two fractions which couldbe successively eluted with water and extracted with an exchangingsolution and that which remained in the roots after these treatments.Except at a low temperature and a low hydrion concentration,the three fractions could not clearly be distinguished, andthere were indications of the presence of metabolically maintainedpositive sites. However, the effect of varying pH on the rateat which anions were eluted with water was consistent with thepresence in the roots of a system of pores, the walls of whichcarried a varying number of negative charges depending on thehydrion concentration. The effect of calcium ions on the magnitudeof the water extractable fraction also accorded qualitativelywith the double-layer theory.     

GROWTH OF THE BARLEY COLEOPTILE. I. ITS RELATIONSHIP TO CELL NUMBER AND LENGTH IN NORMAL AND γ-IRRADIATED SEEDLINGS

The number of cells per vertical column in barley coleoptiles differs in various growth classes; it is highest in tall coleoptiles, intermediate in medium ones, and lowest in short ones. In those that elongate early and grow rapidly, cells per column increased from 88–218 between 12 and 44 hr after the seeds were placed on water; in short coleoptiles they increased to only 85 per column after 115 hr because elongation and division are restricted in these. Cell number does not increase in coleoptiles from seeds irradiated with 250 krad. Variation in the growth pattern of irradiated coleoptiles was similar to that of normal ones, although the range in lengths was reduced. Although the number of cells is much higher in tall controls than in irradiated coleoptiles, the latter can become tall; therefore an increase in cell number during germination does not seem to be a prerequisite for tallness. Coleoptiles 32 mm long, from irradiated seeds, have the same number of cells per column as the shortest ones (6 mm) after 119 hr.     

Light Actions in the Germination of Cocklebur Seeds: I. DIFFERENCES IN THE LIGHT RESPONSES OF THE UPPER AND LOWER SEEDS

Germination responses to light were studied in the upper andlower seeds of cocklebur (Xanthium pennsylvanicum Wallr.). Thelower seed was dark-germinating and negatively photoblastic;the upper one had a red-light (R) requirement and was positivelyphotoblastic. Germination of the lower seeds was inhibited bya prolonged single irradiation with R, blue (B) or far-red (FR)light applied during imbibition. The maximal inhibitory effectof a single irradiation occurred 9 h and 13 h after the startof soaking at 33 °C and 23 °C, respectively. However,the inhibitory effect of R differed from that of B and FR, byonly delaying germination. A single exposure to B or FR lightcould be replaced by intermittent B or FR irradiation, and theireffects were repeatedly reversible by the following R irradiation.If the upper seeds were not exposed to R during imbibition,they failed to germinate even at 33 °C which was optimalfor germination, and the promotive effect of R increased withdelay of its application time. The photoperceptive locus incocklebur seeds was the axial tissue for all B, R and FR. Lightreceived by the cotyledonary tissue had little effect. Germinationdimorphism in response to light is discussed with respect tothe phytochrome content and the ageing of axial tissues. Key words: Blue light, Dimorphism, Far red light, Germination, Red light, Xanthium seed     

The Initial Uptake of Ions by Barley Roots: III. THE UPTAKE OF CATIONS

Experiments have been carried out to determine the extent towhich rubidium and strontium, recently absorbed by excised barleyroots, can be divided into discrete fractions, namely, ionswhich are extractable with water, those which are exchangeable,and those remaining in the tissue at the end of the experiment.The object of this study was to determine whether the behaviourof cations could be validly interpreted in terms of the electricaldouble-layer theory as had been previously demonstrated foranions. To facilitate the interpretation of the results obtained, comparableexperiments were carried out on a cation-exchange resin andon gelatin. The behaviour of strontium accorded qualitatively with the theoryproviding allowance was made for effects due to the hydrolysisof negative sites and for the presence of positive charges inthe roots. For rubidium, on the other hand, the situation was much morecomplex. Not only was there evidence even at 0.2°C for siteswhich retained a high specificity for this ion, but also movementof the ion due to physical diffusion was masked by concurrentmetabolic activity. It was concluded that a meaningful separation of the over-alluptake of rubidium into successive steps of exchange absorptionand metabolic accumulation cannot be made.     

Physiological Studies on Germination of Phaseolus mungo Seeds: I. DEVELOPMENT OF RESPIRATION AND CHANGES IN THE CONTENTS OF CONSTITUENTS IN THE EARLY STAGES OF GERMINATION

Changes in respiration rate and in the contents of various constituentsduring the early period of germination of Phaseolus mungo seedswere studied. The course of the respiration developed in threephases. A sharp rise was observed in the first phase (Phasea), followed by the second phase (Phase b) of fairly constantrespiration rate. The respiration rate increases again in thethird phase (Phase c). The O² uptake in Phases a and b was notinhibited by iodoacetate and fluoroacetate, while that in Phasec was inhibited. The contents of aspartic and glutamic acidsand amides were very high. A decrease of aspartic acid contentwas notable during the first few hours of germination. Citricand malic acids were the major organic acid constituents. Citricacid content in the seeds decreased, while that of malic acidremained unchanged. The leaching of malic acid into the soakingmedium was remarkable during the first 6 h of imbibition     

Experiments on the Growth and Geotropism of Roots: PART I. TECHNIQUE FOR ACHIEVING REGULAR GROWTH, AND A STUDY OF THE EFFECTS OF DECAPITATION AND REHEADING ON THE GROWTH OF VJCIA FABA ROOTS

A technique is described whereby regular growth of roots isobtainable with good aeration. Decapitation at 05, 1, or 2 mm. behind the root-apex does notaccelerate the growth of the stump. An obvious retardation ofgrowth is recorded when 2 mm. of the root-tip is removed, aless conspicuous retardation, or no effect at all, is recordedwhen shorter parts are removed. The amount of retardation isaffected by the extent to which the apical meristem is removed. The growth of stumps reheaded with their own tips is similarto that of unreheaded stumps.     

LABYRINTH AND EQUILIBRIUM : I. A COMPARISON OF THE EFFECT OF REMOVAL OF THE OTOLITH ORGANS AND OF THE SEMICIRCULAR CANALS.

1. A dogfish from which all six ampullæ have been removed maintains its equilibrium; the righting reactions occur promptly; compensatory movements of the eyes occur in response to rotations in all planes except the horizontal; the compensatory position of the eyes is retained if the animal is held in an abnormal position. Both the static and dynamic functions of equilibrium continue, therefore, after complete removal of all the semicircular canals and all the ampullæ. 2. After complete removal of the otoliths from the vestibules without injury to the ampullæ the animal maintains its equilibrium in the water, rights itself promptly, and makes compensatory motions to rotations in all planes. If held in an abnormal position the compensatory position of the eyes is maintained. Both static and dynamic functions of equilibrium continue. 3. Destruction of both the semicircular canals and the otolith organs completely abolishes all compensatory movements and equilibrium reactions of labyrinthine origin. 4. It is pointed out that these observations do not justify the theory of Mach and Breuer that the ampullæ and semicircular canals are the organs for the dynamic functions of equilibrium, and that the otoliths are the organs for the static functions of equilibrium. 5. The new experiments recorded in this paper show that the ampullæ alone (without the otoliths) suffice for all the dynamic and all the static functions of equilibrium of the ear; and that the otolith organs (without the ampullæ) suffice for all the static and for all the dynamic functions of equilibrium of the ear with the exception of the response to a rotation of the animal in a horizontal plane.