The Heterotrophic Nutrition of Chlorella vulgaris (Brannon No. 1 Strain): With two Figures In the Text

A range of sugars, sugar alcohols, sugar phosphates, organicacids, and monohydric alcohols have been tested as carbon sourcesfor growth and as respiratory substrates using Chlorella vulgaris,Brannon I, grown in darkness. Much higher rates of growth and respiration were obtained withd-glucose than with any other substance tested. Ethanol (at0·005 M.) sustained both growth and respiration at c.50 per cent, of the level with glucose (0·028 M. or higher).Evidence was obtained that the organism can become ‘adapted’to utilize d-galactose and sucrose as effective carbon sources.Sustained growth was not obtained with any of the other substancestested. The glucose monophosphates, methanol and certain organic acids(oxalacetate, -ketoglutarate, cis-aconitate, and pyruvate) clearlystimulated oxygen uptake but to a less extent than ethanol.The other substances tested were either inhibitory to respirationor inactive or of very low activity as substrates. The growth in darkness and in liquid culture of Chlorella whensupplied with d-glucose was insensitive to pH over the range4·5 to 7·0 and was markedly enhanced by a highlevel of aeration. Gains in cellular dry weight ranging from45 to 90 per cent, of the weight of d-glucose disappearing fromthe culture medium were recorded in growth experiments; measurementsof CO₂ evolution in the Warburg indicated retention of up totwo-thirds of the glucose-C in cell material.     

The Carbohydrate Nutrition of Tomato Roots: VII. Sugars, Sugar Phosphates, and Sugar Alcohols as Respiratory Substrates for Excised Roots

The respiratory responses of substrate-depleted excised roottips to a range of sugars, sugar phosphates, and sugar alcoholshave been determined by measuring oxygen uptake by the directmethod of Warburg. Sucrose, dextrose, and laevulose are the only sugars which promotea high level of oxygen uptake. The effects of azide and DNP on the oxygen uptake promoted bysucrose and by dextrose are described. Mannose is a strong inhibitor of respiration. This inhibitionis reversed by the simultaneous addition of those sugars whichalso reverse the growth inhibition caused by mannose. Mannoseinhibits the respiration of sucrose and of glycolytic intermediates.Galactose is slowly respired and does not, even at high concentration,inhibit the respiration of sucrose. The results are discussed in relation to the growth effectsof the sugars tested.     

The Carbohydrate Nutrition of Tomato Roots: VI. The Inhibition of Excised Root Growth by Galactose and Mannose and its Reversal by Dextrose and Xylose

The addition of 0.01–0.015 per cent. galactose or 0.005–0.01per cent, mannose reduces by 50 per cent, the linear growthof excised tomato roots cultured in a I per cent, sucrose medium.An addition of 0.03–0.04 per cent, of either sugar causesnot less than a 90 per cent, inhibition of growth. The survivalof meriste-matic activity is higher in presence of fully inhibitoryconcentrations of mannose than of galactose. Roots inhibitedby galactose are distinguishable from those inhibited by mannose. The inhibitory effect of concentrations of galactose up to 0.15per cent, and of mannose up to 0.4 per cent, can be fully antagonizedby the simultaneous addition to the culture medium of dextrose.The minimum ratio of dextrose: inhibitory sugar for maximumantagonism of the growth inhibition is with galactose 5: 1 andwith mannose 3.5: 1. Growth of roots in a dextrose-containingmedium does not protect them from subsequent inhibition by eithergalactose or mannose. d-xylose has significant activity as an antagonist of mannoseinhibition and even more so of galactose inhibition. However,the restoration of growth achievable from the addition of xyloseis not comparable with that resulting from the addition of dextrose.The inhibition of growth by xylose is not alleviated by thesimultaneous addition of dextrose. Maltose has low activityas an antagonist of galactose and mannose inhibition. All othersugars tested and the sugar alcohols corresponding to galactoseand mannose were quite inactive as antagonists of the growthinhibition by these two sugars. Mixtures of partially inhibitory concentrations of galactoseand mannose were less inhibitory than their more inhibitorycomponent. The concentration of dextrose required to reversethe inhibitory activity of such mixtures was not greater thanthe minimum concentration required to antagonize the actionof the more inhibitory component. The antagonism of galactose inhibition by dextrose is not dueto dextrose impeding galactose uptake.     

The Effect of Inorganic Nutrients on the Hormonal Requirements of Normal, Habituated, and Crown-gall Tissue 4

The addition of Braun and Wood's inorganic supplements (845mg l^–1 KCl, 1800 mgl^–1 NaNO₃, 300 mg l^–1 NaH₂PO₄.2H₂O,790 mg l^–1 (NH₄)₂SO₄) to White's medium caused markedincreases in the growth of normal tissues of Helianthus annuus,Nicotiana rustica, Daucus carota, and Vinca rosea and crown-galltumour tissues of H. annuus. However, no evidence was obtainedwhich suggested that the presence of these extra salts markedlyinfluenced the essential requirements of normal callus for auxinsand kinetin. In contrast their presence significantly influencedthe hormonal requirements of certain habituated cultures ofH. annuus and V. rosea. These habituated cultures had specificauxin requirements on White's medium while either an auxin orkinetin was sufficient on high-salts medium. These results arediscussed in relation to previous reports which suggested thatthe biosyntheses of auxins and other growth factors in normaland crown-gall cultures are specifically activated by certaininorganic ions.     

Changes in the Sugar Content of the Leaf Sheath of Dwarf Maize Seedlings Associated with Gibberellin-Induced Growth

Changes in the time in sugar and starch content of the firstleaf sheath were compared among normal, d₅, and GA₁-treated(1µg/plant) d₅ seedlings of Zea mays L., from which endospermshad been removed. The patterns of changes in the levels of thesecarbohydrates were more or less similar to one another amongthe three groups of seedlings. Generally, the levels decreasedtoward the time of maturation of the leaf sheath except forthe level of sucrose which showed a sharp increase followedby a decrease. There was an inverse relationship over time betweengrowth and levels of glucose and fructose. Although treatmentwith GA₁ resulted in a decrease in sucrose content, it had practicallyno significant effect on the levels of the other sugars. Inthe leaf sheath of d₅, growth is probably not limited by thelevels of these sugars, and GA-induced elongation does not seemto have any significant correlation with the changes in thelevels of sugars. ¹Present address: Department of Botany, Suri Vidyasagar College,Suri, Dt. Birbhum, West Bengal, India. (Received March 18, 1988; Accepted November 21, 1988)     

The Effects of Gibberellins on the Growth of Excised Tomato Roots

1. At appropriate concentrations both gibberellic acid (GA) and1-naphthalene-acetic acid (NAA) enhance the main axis growthof excised tomato roots grown in culture media containing sucroseat concentrations below 1 per cent. Lateral root extension growthis enhanced by GA at all sucrose concentrations tested; onlyat the lower sucrose concentrations is this effect observedwith NAA. Both GA and NAA increase the number of emergent lateralroots and this effect is most marked in media of low sucrosecontent. Both GA and NAA at higher concentrations inhibit rootgrowth but NAA exhibits its full range of growth effects overa much narrower concentration range than GA.
2. GA, like NAA,speeds up the loss of meristematic activity whichoccurs whenindividual meristems are repeatedly subculturedin media containing1 per cent, or higher concentrations ofsucrose.
3. The promotionof main axis growth by both GA and NAA involvesenhanced cellelongation and cell division. At a moderatelyinhibitory concentrationGA reduces both cell elongation andcell division; this is notthe case with NAA.
4. Gibberellins A₁, A₂, and A₄ resemble GA(gibberellin A₃) intheir growth effects. Allogibberic acidlike G A promotes lateralroot extension growth but causes markedinhibition of root growthat a much lower concentration thanGA.

     

Characteristics of Nectar Secretion by the Extrafloral Nectaries of Ricinus communis

The characteristics of nectar secretion by excised extrafloralnectaries of Ricinus have been examined. Secreted nectar wasfound to contain three sugars: sucrose, glucose and fructose,with glucose and fructose occurring in a 1: 1 ratio. All threesugars supported secretion when used in the culture medium andthe yield of nectar sugar was found to be concentration-dependent.Other sugar sources failed to support secretion. Experimentsusing ¹⁴C-sugars and ¹⁴CO₂ fed to intact plants allowed themovement of sugars through the nectary to be examined. Sucrosesynthesis occurs when excised glands are fed glucose and thisoccurs very early in the transport through the nectary. Themain sugar transported was sucrose, with little hydrolysis occurringuntil the final step of secretion. There was no evidence thatsucrose hydrolysis occurs either by invertase in the nectaror by a microbial flora. Inhibitors of respiration were foundto inhibit secretion as did anaerobiosis. Temperature also hada marked effect, with a temperature coefficient of 1.8. However,secretion of sucrose was not affected by anaerobic conditions,low temperatures or inhibitors of respiration as markedly asthat of glucose and fructose. Electron microscopy revealed the presence of a thickened andheavily stained wall at the inner border of the secretory epidermallayer. This wall contained numerous plasmodesmata at a frequencyof 14 per µm² and may represent an apoplastic barrier.Light microscope cytochemistry revealed that acid phosphataseis primarily located in the nectiferous tissue, while ATPaseis concentrated in the epidermis. The possibility that the nectarycontains two pathways for sucrose secretion, both apoplasticand symplastic, is discussed. Key words: Invertase, nectary, plasmodesmata, Ricinus communis, sucrose     

Investigations into the Role of Sucrose in Potato cv. Estima Microtuber Production in vitro

Sucrose has been the carbohydrate traditionally used for potatomicrotuber production. Added to nutrient media, sucrose canact solely as a carbon source, or as an osmoticum, or both.Preliminary tests showed that the osmolarity of sucrose solutionswas increased by autoclaving, indicating some breakdown of thesugar. This was taken into consideration in experiments whichinvolved supplementing 4% sucrose media with sucrose, maltose,glucose or fructose, while keeping the osmotic potential ofthe media constant. A medium concentration of about 400 mM withonly sucrose was more suitable for microtuber production thanmedia supplemented with maltose, glucose or fructose. However,a better microtuber yield was obtained when hexoses were addedthan with unsupplemented 4% sucrose media. When glucose wassupplied at concentrations which had the same number of carbonatoms as 8% sucrose, the high osmolarity inhibited microtuberisation.Sugar movement in the tubering plantlet was followed using radio-labelledsucrose, glucose and fructose. The sucrose was translocatedand used at a faster rate than the other sugars, which tendedto remain in the roots of the plantlets. Furthermore, therewas no difference in microtuber production on media to whichthe sucrose was added before or after autoclaving, indicatingthat levels of breakdown were not severe enough to affect theprocess. Therefore, it is concluded that sucrose acts primarilyas a suitable carbon source for uptake and utilization by theplantlets, but, at 8%, it also provides a favourable osmolarityfor the development of microtubers.Copyright 1995, 1999 AcademicPress Solanum tuberosum (L.), potato, microtuber, media, sugar, sucrose, osmolarity, pH     

The Location of the Transporting System for Inorganic Carbon and the Nature of the Form Translocated in Chlamydomonas reinhardtii

The permeability of the plasmalemma of Chlamydomonas reinhardtiicells was increased by treatment with poly-L-lysine or dimethylsulphoxideas indicated by 3-phosphoglyceric acid dependent O₂ evolution.These treatments decreased the ability of the cells to accumulateinorganic carbon internally and hence their photosynthetic affinityfor inorganic carbon in the medium. With saturating light andinorganic carbon, the photosynthetic rate was less affectedby the poly-L-lysine and dimethylsulphoxide treatments. Thusthe poly-L-lysine and dimethylsulphoxide did not alter the activityof the chloroplasts but rather made the intracellular inorganiccarbon pool more freely exchangeable with the medium. It isconcluded that the transporting system for inorganic carbonis located at the plasmalemma. Treatment with Diamox, an inhibitor of carbonic anhydrase, didnot affect photosynthetic rate and accumulation of inorganiccarbon when CO₂ was supplied but strongly inhibited both parameterswhen HCO^–₃ was supplied. In a mutant of Chlamydomonasreinhardtii lacking a cell wall, carbonic anhydrase leaks tothe medium and uptake of inorganic carbon is much faster whenCO₂ is supplied than when HCO^–₃ is supplied. These resultssuggest that CO₂ rather than HCO^–₃ is the inorganic carbonspecies that is actively translocated across the plasmalemma. Key words: Chlamydomonas, Inorganic carbon uptake     

Root Respiration and Carbohydrate Status of Two Wheat Genotypes in Response to Hypoxia

To investigate root respiration and carbohydrate status in relationto waterlogging or hypoxia tolerance, root respiration rateand concentrations of soluble sugars in leaves and roots weredetermined for two wheat (Triticum aestivum L.) genotypes differingin waterlogging-tolerance under hypoxia (5% O₂) and subsequentresumption of full aeration. Root and shoot growth were reducedby hypoxia to a larger extent for waterlogging-sensitive Coker9835. Root respiration or oxygen consumption rate declined withhypoxia, but recovered after 7 d of resumption of aeration.Respiration rate was greater for sensitive Coker 9835 than fortolerant Jackson within 8 d after hypoxia. The concentrationsof sucrose, glucose and fructose decreased in leaves for bothgenotypes under hypoxia. The concentration of these sugars inroots, however, increased under hypoxia, to a greater degreefor Jackson. An increase in the ratio of root sugar concentrationto shoot sugar concentration was found for Jackson under hypoxicconditions, suggesting that a large amount of carbohydrate waspartitioned to roots under hypoxia. The results indicated thatroot carbohydrate supply was not a limiting factor for rootgrowth and respiration under hypoxia. Plant tolerance to waterloggingof hypoxia appeared to be associated with low root respirationor oxygen consumption rate and high sugar accumulation underhypoxic conditions.Copyright 1995, 1999 Academic Press Oxygen consumption rate, sugar accumulation, Triticum aestivum L., waterlogging tolerance     

Gibberellins and the Growth of Excised Tomato Roots: Comparison of gib-1 Mutant and Wild Type and Responses to Applied GA3 and 2S, 3S Paclobutrazol

Clones of excised roots of wild type tomato (Lycopersicon esculentum,Mill., cv. Moneymaker) and a near-isogenic GA-deficient mutant(gib-1/gib-1) were cultured in modified White's medium containing1.5% w/v sucrose. The linear elongation rate of the main axisof the gib-1 mutant was 40% less than that of the wild type.In addition, the main axis of the gib-1 mutant was thicker thanthat of the wild type but main axis volume growth was the samein both genotypes, indicating that the gib-1 allele was affectingthe orientation of root expansion. There was no evidence tosuggest that the gib-1 allele affected either the pattern ofemergence or the density of lateral roots. Elongation rate andthickness of gib-1 mutant roots were restored to those of thewild type by the addition of low concentrations (0.1–1.0µM) of gibberellic acid (GA3). These concentrations ofGA3 caused a slight reduction in extension growth of wild typeroots, indicating that endogenous GAs were not limiting elongationof normal roots in culture. The GA biosynthesis inhibitor, 2S,3S paclobutrazol, at 0.1 µM, significantly reduced elongationof wild type roots and this inhibition was counteracted by 0.1µM GA₃. It is concluded that the difference in growthbetween the gib-1 mutant and the wild type represented GA-dependentgrowth. Low concentrations of 2S, 3S paclobutrazol caused onlya small (5%) reduction in growth of the gib-1 mutant and thisgrowth inhibition was not reversed by GA₃. This observation,and the fact that gib-1 mutant roots grow in the absence ofadded GA₃, suggested that part of root growth was GA-independent.However, the possibilities that the gib-1 mutant is ‘leaky’and that paclobutrazol does not inhibit GA biosynthesis completelycannot be excluded. Key words: gib-1 mutant, gibberellic acid, Lycopersicon esculentum, 2S, 3S paclobutrazol, root growth     

Soya Bean Seed Growth and Maturation In vitro without Pods

Immature Glycine max (L.) Merrill seeds, initially between 50and 450 mg f. wt, were grown and matured successfully in vitro.Excised seeds were floated in a liquid medium containing 5 percent sucrose, minerals and glutamine in flasks incubated at25 °C under 300 to 350 µE m^–2 s^–1 fluorescentlight. During 16 to 21 d in culture, seeds grew to a matured. wt of 100 to 600 mg per seed at an average rate of 5 to 25mg d. wt per seed d^–1 depending on initial size. Growthrates were maximal during the first 8 to 10 d in vitro but declinedwith loss of green colour in the cotyledons. Seed coats rupturedwith rapid cotyledon expansion during the first 2 d in culture.Embryos were tolerant to desiccation and 80 to 90 per cent germinatedif removed from culture before complete loss of green colour.The growth of excised seeds in vitro exceeded the growth ofseeds in detached pods, but when windows were cut in pods topermit direct exposure of seeds to the medium, seed growth wascomparable. Glycine max (L.) Merrill, soya bean, seed culture, seed growth, seed maturation, germination     

Distinct Cellular and Organismic Responses to Salt Stress

We have compared metabolic effects of high salinity betweenplants and cell suspension cultures from the facultative halophyteMesembryanthemum crystallinum (common ice plant). This plantshows developmentally-programmed inducibility for a switch fromC₃-photosynthesis to CAM (Crassulacean Acid Metabolism). Themetabolic switch is enhanced by environmental factors such asdrought, low temperature, and, most effectively, soil salinity.CAM induction is dependent on organized leaf tissue and cannotbe elicited by salt stress in suspension culture cells. In contrast,the accumulation of proline [Thomas et al. (1992) Plant Physiol.98: 626] is induced by NaCl in cultured cells as well as inplants and must be considered a cellular response to stress.We have extended our observations to include another trait ofsalt- and low-temperature-stress responses in the ice plant,the accumulation of putative osmoprotective sugars and sugaralcohols. In whole plants the cyclic sugar alcohol, pinitol,accumulates to amounts that approach 1 M during stress, whilein suspension cells no increase in sugar alcohols is observed.The distribution of carbon to different sugars is markedly differentbetween cells and plants under stress. Particularly obviousis the distinction between cell types in the different compositionof sugars and polyols, as exemplified by the epidermal bladdercells of ice plants. Ion contents and the content of sugarsand sugar alcohols of bladder cells indicate that Na⁺, Cl^–,pinitol and an unknown carbohydrate compound provide osmoticpressure in these cells, while organic anion concentrationsare low. With the ice plant, we conclude that cells in culturemimic only partly the stress response mechanisms of intact plantsand we hypothesize that communication between different tissuesis required to mount a complete environmental stress response. ⁴ Present address: Department of Botany Oklahoma State University,Stillwater, OK, 74078, U.S.A.     

Effects of Light Quality on Photosynthetic Carbon Metabolism in C4 and C3 Plants: Rapid Movements of Photosynthetic Intermediates Between Mesophyll and Bundle Sheath Cells

The influence of varying light intensity and quality on thecarbon labelling patterns in Rumex vesicarius (a C₃ plant),Setaria italica (a malate-formingC₄ plant), and Amaranthus paniculatus(an aspartate-forming C₄ plant) was studied. In A. paniculatusand B. vesicarius blue light decreased the transfer of radioactivityto sugars and starch but in S. italica only slightly decreasedradioactivity in sugar phosphates, sucrose, and insolubles.Negligible transfer was observed from the C₄ acids to sugarphosphates, sucrose, and starch under dim blue-green and blue-yellowlights in S. italica and A. paniculatus. Blue light favouredthe formation of malate, aspartate, and alanine in all threeplants. The differential effect of blue and red light suggesteda variation in the mechanisms of C₄-photosynthesis in Setariaand Amaranthus. Leaves of S. italica and A. paniculatus were allowed to photosynthesizein ¹⁴CO₂ for 5 s and then the distribution of the labelled productsbetween the mesophyll and the bundle sheath cells was determinedduring subsequent photosynthesis in ¹²CO₂. Malate and aspartatewhich appeared initially in the mesophyll layer moved rapidlyinto the bundle sheath cells. Phosphoglyceric acid originatingin the bundle sheath moved swiftly to the mesophyll layer. Sugarphosphates were recovered from both the mesophyll and the bundlesheath cells. Most of the starch was found in the bundle sheathcells while sucrose and alanine were localized in the mesophyllcells.     

The Effect of Isonicotinyl Hydrazide on the Photosynthetic Incorporation of Radioactive Carbon Dioxide into Ethanol-Soluble Compounds of Chlorella

The effect of 10^-2M. isonicotinyl hydrazide (isoniazid) on theincorporation of radioactive carbon dioxide by Chlorella duringphotosynthesis has been studied under steady-state conditionsat two carbon dioxide concentrations. Isoniazid treatment resultsin increased radioactivity in sucrose, glycollic acid, and glycineand decreased radioactivity in sugar monophosphates, serine,and alanine. An unidentified compound which is strongly radioactiveafter short-term exposures to ¹⁴CO₂ is present in isoniazid-treatedcells. It is suggested that isoniazid pre-dominantly inhibitsthe conversion of glycine to serine.     

Localization and Kinetic Properties of {beta}-Glycerophosphatase in Barley Roots

The study of ß-glycerophosphatase activity in cell-wallpreparations and in excised root tips from barley seedlingssupports the view that the former, which constitutes about 20per cent of the activity of the whole homogenate, representsthe fraction located at the surface of the roots in vivo. Theactivities of the cell-wall suspension and intact roots arevirtually identical, and further show identical relations topH, substrate concentration (K_m), and competitive inhibitionby molybdate and inorganic phosphate (K_i). The enzyme must thereforebe freely exposed to the external solution without any permeabilitybarrier separating it from either substrate or inhibitors. Theabsence of any lag phase in the hydrolysis in excised root tipssuggests that the surface enzyme may be limited to the outermostlayers of the root. The solubilization of some of the activityof the cell-wall preparation by treatment with sodium chlorideand ammonium sulphate suggest that surface activity may havebeen lost from these preparations rather than adsorbed duringhomogenization and extraction. The K_m and pH-activity curveof the supernatant activity remaining after centrifugation ofthe cell-wall fraction indicate that only this enzyme and noother detectable glycerophosphatase exists in the roots.     

Sucrose and Hexose Release by Excised Stem Segments of Vicia faba L.: The Sucrose-Specific Stimulating Influence of Cuscuta on Sugar Release and the Activity of Acid Invertase

By washing out solutes in 0.5 mM CaSO⁴at 25 °C during aperiod of 5–6 h, the release of sugars by excised stemsegments of Vicia faba L. was measured. The stem parasite Cuscutaeuropaea strongly stimulated the release of sucrose into theefflux medium; this effect was most marked during the last hoursof each experiment but this stimulating effect of the parasitecould not be detected for glucose and fructose. The fact thatparasitized stem segments released higher than normal hexoseamounts during the last hours of several experiments, couldbe explained as the result of extracellular hydrolysis of sucroseby free space invertase. A high free space acid invertase activitywas present in young stem segments of Vicia faba and in tissuesof Cuscuta. The stimulating influence of Cuscuta on sugar releaseby cells of stem segments appears sucrose-specific, supportingthe idea that the stimulating influence of Cuscuta on sugarrelease is restricted to the sieve-tube system. When metabolic inhibitors were added to the washing solutionor when segments were incubated at low temperature, no cleareffect of the parasite could be observed and for all segments(parasitized and non-parasitized) a strongly enhanced releaseof sucrose into the efflux medium was found during the lasthours of an experiment. These data support the idea that anintensive resorption of sucrose occurs within stem segments,after its release into the apoplast. Key words: Cuscuta europaea, Parasitic relationship, Phloem unloading     

Overcoming Incompatibility in Brassica campestris L. by Carbon Dioxide, and Dark Fixation of the Gas by Self- and Cross-pollinated Pistils

Supplementing pollen suspension cultures with CO₂ (3–5per cent) caused a marked increase in germination and tube growthin vitro in Brassica campestris L. cv. toria. A weakening ofself-incompatibility by increased CO₂ levels from 3–5per cent was observed. The percentage of pollen tubes whichpenetrated the cuticle layer of stigmatic papilla cells in self-pollinatedpistils was high when CO₂ level was 5 per cent. Phosphoenolpyruvate (PEP) carboxylase activity was greater in the pollengerminated in 4 per cent CO₂ as compared to air (0.03 per cent).A possible role of CO₂ for self-recognition and control of pollentube growth is proposed, proposed. Brassica campestris L., carbon dioxide, self-incompatibility, phosphoenol pyruvate carboxylase     

Some Effects of Sucrose Concentration on Primordium Development in Excised Primary Roots

The effects of varying the amount of sucrose used to supplementthe culture medium maintaining the growth of excised roots ofPisum sativum L., Vicia faba L., Zea mays L. and Phaseolus vulgarisL., on the rates of primordium initiation and subsequent emergenceas lateral roots and on the duration of the interval betweenprimordium inception and emergence as a secondary root throughthe tissues of the primary have been investigated. Variation in the exogenous concentration of sucrose from 0.5to 8 per cent had little effect on the rate of primordium inceptionin Pisum and Vicia and the rates never reached the values obtainedfor the roots of the corresponding intact plants. Moreover,over the 6 day culture period lateral root emergence did notoccur in any of the excised roots of these two species. In contrast,each of the aspects of primordium development examined in theexcised roots of Zea and Phaseolus was markedly affected bythe amount of sucrose used to supplement the culture medium.In addition, in the presence of about 6 per cent sucrose, primordiumdevelopment in these cultured roots was very similar to thatin roots of the corresponding intact plants. The results indicate either that some factor necessary for primordiumdevelopment is present in adequate amounts in excised rootsof Zea and Phaseolus, but not in those of Pisum and Vicia, orthat the factors controlling such development are differentin the former and latter two species. Vicia faba L., Pisum sativum L., Zea mays L., Phaseolus vulgaris L., broad bean, garden pea, maize, dwarf bean, primordium development, sucrose concentration, cultured roots     

The Time Factor in Studies of Growth Inhibition in Excised Organ Sections

1. Apprehension over the adequncy of current techniques stimulateda detailed study of the time factor in the arsenate inhibitionof growth and respiration in excised stem and root sectionsof Pisum sativum.
2. Growth inhibition by arsenate sets in veryslowly, its rateof onset being related to the molar concentration(C) of arsenateate by the relation where T₅₀ is the time taken in hours to reduce the growthrateto 50 per cent of the control and K is a constant. An explanationof the physiological basis of this relationship is attempted.
3. Estimates were made of the final steady growth rate (relativeto control) in various arsenate concentrations. The inhibitionscalculated from this rate are held to approximate to the truearsenate effect and are shown to be very different from thosecalculated from ‘total growth’ measures.
4. Respirationof growing stem sections is not inhibited by thelow arsenateconcentrations that inhibit growth. Some inhibitionis indicatedat high concentrations (3 ? 10^–4M. and over)but onlyafter 15-20 hours of exposure.
5. Two per cent sucrose has noeffect on the arsenate inhibiitionof stem growth. Sucrose,however, markedly stimulates respirationin stem sections, butthis stimulation is prevented by arsenate.
6. The misinterpretationswhich may arise as a result of ignoringthe time factor in inhibitionstudies in excised organ sectionsare discussed and the desirabilityof constructing completegrowth curves in all such studies isstressed.