The Comparative Biology of Closely Related Species Living in the Same Area: V. INTER- AND INTRASPECIFIC INTERFERENCE WITHIN CULTURES OF LEMNA spp. AND SALVINIA NATANS

Studies were made of the growth of populations of Lemna minor,L. polyrrhiza, L. gibba, and Salvinia natans under controlledlaboratory conditions. The intrinsic exponential growth-ratesof the clones were determined in un-crowded cultures, and thechanges in growth-rate of self-crowding cultures were measuredand interpreted in terms of an initial exponential growth-ratefollowed by a phase of arithmetic increase in weight and followedin turn by a phase in which the death of submerged and shadedfronds caused a decline from the arithmetic rate of growth.Mean frond weight declined in self-crowding cultures (exceptof L. gibba). Mixed cultures of two species were examined under self-crowdingconditions and changes in the proportions of the species werefollowed. Whereas the total weight of mixed cultures remainedvery constant between replicates, there was wide variation inthe proportions of components. The variation in the two componentswas most closely correlated (negatively) when the struggle forexistence was most evenly balanced. The mean frond weight ofthe losing component declined during the experiments. The order of decreasing vigour of species measured by variousparameters was as follows: Relative (intrinsic) growth-rate: L. minor > S. natans > L. gibba > L. polyrrhiza Arithmetic growth-rate when crowded: S. natans > L. polyrrhiza > L. gibba > L. minor Asymptotic yield per culture: L. polyrrhiza > L. minor > S. natans > L. gibba Success in mixed cultures: The success of a species in mixture could not be predicted fromthe parameters of growth in pure culture. Morphologic featuressuch as the gibbosity of L. gibba     

Interrelationships between Light Intensity, Temperature, and the Physiological Effects of 2: 4-Dichlorophenoxyacetic Acid on the Growth of Lemna minor

The effects of light intensity and temperature on the changesin growth and development induced by sodium 2 4-dichlorophenoxyacetatehave been studied in multifactorial experiments. Random samplesfrom a clonal population of Lemna minor were subjected to threeintensities (700, 275, and 180 foot-candles) and three temperatures(30°, 25°, and 20° C.), while varying concentrationsof the growth regulator were added to a phosphate-buffered culturesolution (pH 5:1 There were up to five sampling occasions whenthe dry weights and, for some purposes, the total frond areaswere determined. The replication was at least twofold, and thelargest experiment involved 720 dry-weight determinations. At each combination of light and temperature, sublethal concentrationsof the growth regulator induced cumulative reductions in therelative growth rate and the order of the depression increasedprogressively with concentration-3, 9, 27, and 45 p.p.m. Becauseof the cumulative effects and the great differences in the growthrates of the controls under the nine combinations of light andtemperature, the statistical analysis of the data was involved.Employing two different approaches it was concluded that therelative depression in the growth rate increased with temperaturewhile the growth of plants receiving the highest intensity wasslightly more inhibited. At a concentration (27 p.p.m.) which reduced both the relativegrowth and frond area, over all combinations of light intensityand temperature the net assimilation rate during the periodso–2 and 2–4 days was depressed by 2 and 12 per cent.respectively. There was some indication that the depressionwas larger at the highest temperature in the second period. At greatly increased concentrations the fronds exhibited chioroticsymptoms, and the concentrations required to produce a standarddegree of chiorosis at the end of 3 days were determined undertwelve combinations of light intensity and temperature. It wasfound that varying the intensity during the time of treatmentwas without effect, but that there was a highly significantinfluence of the light level prior to treatment: the lower theintensity the smaller was the concentration subsequently requiredto induce chlorosis. Likewise, with a rise in temperature themagnitude of the equi-effective concentration was reduced. These interactions are discussed in relation to changes in theratio of frond area to frond weight, the mechanism of uptake,and possible physiological effects at cell level.     

Studies in the Physiological Action of 2,2-Dichloropropionic Acid: III. FACTORS AFFECTING THE LEVEL OF ACCUMULATION AND MODE OF ACTION

The factors determining the pattern of uptake by Lemna minorof 2,2-dichloropropionic acid (DCPA), containing chlorine-36,have been examined. Entry takes place via both the roots andfrond and is largely in the ionic form. Initially, the net ratesare high and are replaced by slower but steady rates. It isconcluded that over the first 30 minutes, net uptake is dependenton the physico-chemical processes since (i) the rate is directlyproportional to the external concentration, (ii) on transferenceto buffer containing non-radioactive DCPA up to 90 per centof the radioactivity is exchangeable, (iii) the temperaturecoefficients for the rates of net influx and efflux range from1.2 to 1.4. During the second phase, the net rates of uptakeare curvilinearly related to concentration, the temperaturequotients are higher and the rates of efflux markedly lowerthan in the first phase. Phenylmercuric nitrate and cupric sulphateat 5x10^–6 M halve the net rate of uptake but higher concentrationsare demanded for dinitrophenol, arsenite, aside, and iodoacetate.The depression induced by phenylmercuric nitrate can to somedegree be reversed by the addition of either glutathione orcysteine. For pyruvic acid to halve the rate of uptake of DCPA,it requires more than a three hundredfold greater concentration.It is postulated that in the second phase, uptake is mediatedby metabolic processes involving thiol groups. The ability ofcalcium pantothenate and ß-alanine to reverse theinhibitory effects of DCPA on the growth of L. minor were investigatedin multifactorial experiments. Over a narrow range of concentrations,calcium pantothenate partially offset the inhibitions of growthbut the interactions for ß-alanine were not significant.These results do not support the view that the primary actionof DCPA is to interfere with the biosynthesis of Co-enzyme A.In a comparison of the maximal capacity of different speciesto accumulate DCPA in their roots, up to a six-fold differencewas recorded in the eight species examined. On the basis ofthese findings, the mechanisms determining selective actionand the inhibition of meristematic activity are discussed.     

Studies in the Principles of Phytotoxicity: III. THE pH FACTOR AND THE TOXICITY OF 3:5-DINITRO-o-CRESOL, A WEAK ACID

It has previously been shown that the concentration of 3:5-dinitro-ocresolrequired in the external medium to halve (a) the growth rateof the mould Trichoderma viride or (b) the respiration rateof yeast increased same 75–100 times as the pH changedfrom 4 to 7. In the present investigation it has been foundthat when solutions or suspensions of 3:5-dinitro-o-cresol areapplied as sprays to seedlings of Brassica alba the concentrationneeded to kill half the plants changes by a factor of less than2 over the same pH range. On the other hand, when Lemma minor is grown in nutrient solutioncontaining the toxicant, the concentration required to halvethe rate of frond multiplication increases sharply as the pHis raised from 5.4 to 6.8. Moreover, when leaf disks of B. albaare vacuum infiltrated with buffered solutions of 3:5-dinitro-o-cresolthe concentration needed to halve the respiration rate rises50-fold between pH 4 and 8. It is considered that the essential condition which determinesthe similarity of the result for L. minor, the leaf disks, andmicro-organisms is that a relatively large volume of solutioncontaining the toxicant is in direct contact with the tissues.In contrast, under the conditions of spraying the volume ofspray droplets relative to that of plant tissue is small. Thusthe pH effect ie masked since the buffering capacity of thecells exceeds that of the droplets, with the result that 3:5-dinitro-o-cresollargely acts upon the cells at a pH determined by the tissues,a pH which may bear little relation to that of the originalsolution. Supporting evidence is produced that the pH factor operatesin the same way for other weak acids, such as the chlorophenoxyaceticacids.     

The Uptake of Growth Substances: II. THE ABSORPTION AND ACCUMULATION OF 2:3:5-TRIIODOBENZOIC ACID BY THE ROOT AND FROND OF LEMNA MINOR

The absorption by Lemna minor of 2 : 3 : 5-triiodobenzoic acid(TIBA), labelled in the 2 position with iodine-131, has beeninvestigated. Under constant conditions of light (300 foot candles)and temperature (25?C.) the course of uptake by the whole plantis dependent on the concentration of TIBA in the culture solution(pH 5.I). Up to circa 1 mg./l. a high rate of uptake in thefirst 30 minutes is followed by a slower but steady accumulationduring the next 4.5 hours. When the concentration is increasedto 5 mg./1. uptake, at first rapid, falls off progressivelyso that after 1-2 hours the content of the tissues starts todecrease because TIBA moves out in to the external solutionfaster than it enters. The magnitude of this reversal from apositive to a negative rate of uptake,.partitularly for theroots, is even more pronounced at 10 mg/l. When plants, first placed in labelled solutions for 2 to 2.5hours are transferred to culture solution the loss of the labelledcompound to the external solution, especially from the roots,is very rapid : the decrease in root content may reach 90 percent. in the first 80 minutes. The initial rate of egress islittle affected either by the hydrogen ion concentration orthe presence of unlabelled TIBA in the external solution. Between5, 15, and 25? C. the temperature quotients range from 1.29to 1.70 for the root and 1.66-1.37 for the frond. Comparable experiments on the initial phase of uptake duringthe first 2o to 30 minutes demonstrate that (a) the rates aregreatly dependent on the external pH i.e. uptake is largelyin the molecular form and (b) between 5 and 15?C. the ratesare not temperature dependent but between 15 and 25?C. theyare: the quotients for roots and frond are 2.6 and 2.0. Comparisons of the course of uptake of normal plants and plantspreviously frozen in liquid air showed that at both I? and 25?C.'pre-frozen' fronds continue to accumulate TIBA and may containat the end of 5 hours 8 times as much as control fronds. Incontrast, the uptake by the pre-frozen roots is depressed. Theaddition of high (30-50 mg./l.) but not low (10 mg./1.) concentrationsof indole-3-acetic acid (IAA) to the external solution alsocauses a striking accumulation of TIBA in the frond but uptakeby the roots is not changed. Lastly, the presence of TIBA in the external solution whilenot affecting the uptake of cerium-144. in the first 2 hourssubsequently depresses uptake by the root but not by the frond. These findings are compared with the similar pattern of uptakefor z,4-dichlorophenoxyacetic acid described in the previouspaper. The nature of the processes controlling the inward andoutward flow of TIBA, their location within the cell and theirbearing on the physiological actions of auxins are discussed.     

Influence of nutrients on the development of gibbosity in fronds of the duckweed Lemna gibba L.

The duckweeds Lemna gibba L. and Lemna minor L. only grew wellin undisturbed culture under axenic conditions in low lightintensity when provided with a suitable energy source such asglucose. In media containing N0₃-N gibbosity (a convex ventralsurface) was induced in the presence of the chelating agentethylene-diamine-di-o-hydroxyphenylacetic acid (EDDHA). In nutrientsolutions containing NO₃-N as the only N source, but withoutEDDHA, L. gibba occasionally exhibited gibbosity in culturesolutions of 40 cm³ volumes. More fronds were induced to exhibitgibbosity when the volume of the culture medium was increasedfrom 40 cm³ to 200 cm³. Gibbosity was never induced in L. minor,neither was it induced in L. gibba in media containing NH₄-N,even in the presence of NO₃-N. There was no direct correlationbetween the occurrence of gibbosity and frond growth rate, butgibbosity occurred only when there was good frond growth. In the absence of a sugar, frond growth was enhanced by bubblingair through the culture solution in the light. Increasing theCO₂ concentration in the air up to 1% enhanced growth and inducedgibbosity. Carbon dioxide did not induce gibbosity in mediacontaining NH₄-N. Key words: Ammonium-N, carbon dioxide, gibbosity, Lemna, nitrate-N     

Studies on Foliar Penetration: V. FACTORS CONTROLLING THE PENETRATION OF 2,2-DICHLOROPROPIONIC ACID (DALAPON) INTO THE LEAVES OF PHASEOLUS VILGARIS

The factors which control the penetation of 2,2-dichloropropionicacid (dalapon), containing ³⁸Cl, into leaf disks of Phaseolusvulgris have been investigated. In many respects the patternof penctration resembles that already reported for 2,4-dichlorophenoxyaceticacid (2,4-D) but in others the results dispaprate. In darknessthe rate of penetration is proportional to the external concentration,remains constant over at least 24 h, is unaffected by 3-(3,4-dichlorophenyl)-1,1-dimethlyrea(DCMU), and has a temperature coefficient ranging from 1.6 to2.3. Light intensities as low as 1500 lx enhance penetrationat both surfaces but, whereas the responses at the abaxial surfacea maximum at 10.000 lx, at the adaxial surface penetration continuesto increase slightly as the intensity is raised to 21 500 lx.After 4 or 8 h at the higher light intersities an accelertedphase of penetration is initiated, which is sensitive to bothlow temperature and DCMU. When leaf disks, exposed to dalaponin darkness,are translated to buffer, there is no outward diussusion,nor on adding non-radioactive dalapon does exchange take place.Dalapon taken up in the light is less firmly bound: it failsto diffuse out, but partial exchange is found. While penetrationfalls as the external pH is raised to 5.2, significant amountscontinue to enter which are unaffected by p H as the latteris raised to values which cause almost complete dissociation.It is concluded that, as with 2,4-D, uptake in light is ATP-drivenand both polar and non-polar pathways appear to be involvedduring penetration.     

TWO LIGHT REACTIONS IN THE PHOTOPERIODIC CONTROL OF FLOWERING OF LEMNA PERPUSILLA AND L. GIBBA

The effects of light quality on the photoperiodic control inthe flowering of a SD duckweed, Lemna perpusilla strain 6746,and a LD duckweed, L. gibba strain G3, were investigated withspecial reference to the interaction between R and B or FR lights. In the diurnal alternation of R or G light and dark periods,L. perpusilla responded as a SDP, but in that of B or FR lightit was almost daylength-indifferent. On the other hand, L. gibbaresponded as a LDP under B, R or FR light, although the criticallight length was altered by the light quality. In the diurnal alternation of R and B or FR light periods containingno dark period, L. perpusilla flowered with the shortening ofthe optimal and critical R light lengths, compared with theplant exposed to that of R light and dark period. The floweringresponse of L. gibba to the R light length showed double peaks,that is, the first peak at the R duration less than 9 hours,and the second at the R duration longer than 9 hours. The firstpeak corresponds to the optimal R light length in L. perpusilla. Under the CL with a mixture of R and B or FR lights, the floweringand frond production were influenced by the intensity ratioof two light given. In both plants, the optimal ratio of B toR or FR to R for the flowering was always greater than thatfor the frond production. It is suggested that the B or FR light interacts with the Rlight in the photoperiodic process in the plants and this interactionbetween the R and B or FR lights should be of importance forobtaining a better understanding of photoperiodism. (Received August 28, 1965; )     

Studies in the Principles of Phytotoxicity: IV. THE EFFECTS OF THE DEGREE OF NITRATION ON THE TOXICITY OF PHENOL AND OTHER SUBSTITUTED BENZENES

1. The relative toxicity of phenol, o- and p-nitro, 2:4-dinitro-,and 2:4:6- trinitrophenol was measured by up to five techniques.Determinations were made of the concentrations required to reduceby half (i) the growth rate of Trichoderma viride at pH 3, (ii)the frond multiplication rate of Lemna minor at pH 5·1–5·4,(iii) the respiration rate of yeast at pH, (iv) the respirationrate of infiltrated leaf disks of Brassica alba at pH 3, and(v) the number of B. alba seedlings surviving after sprayingwith buffered solutions of the compounds. Whenever possibletoxicity was measured at a pH level below pK so as to eliminatethe effect of pH on toxicity, but in some of the measurementswith 2:4-dinitro- phenol (pK = 4·0) and with picric acid(pK = 0·8) this was not feasible. It was then necessaryto measure toxicity at a pH at which the compound was much dissociated;from the measured values of toxicity of dinitrophenol the toxicityat a lower pH was estimated from the generalized pH-activityrelationship of Simon and Beevers (5952). 2. The relative toxicity of the nitrophenols is very similarwhichever test procedure is employed. The first two nitrationsare accompanied by large increases in toxicity, but with thethird successive nitration there is a decrease. 3. The toxicity of the corresponding mono, di, and trinitroderivatives of anisole, phenetole, toluene, and nitrobenzenewas determined using T. viride. In each series the first andthird stages of nitration brought about large increases in toxicitybut the intermediate one had little effect. 4. All the p-nitro compounds were more toxic than their o- isomers. 5. On the basis of the concepts put forward by Ferguson (1939)and Albert (1951) it is concluded that the toxic action of thenitroanisoles and nitrotoluenes is ‘structurally non-specific’,i.e. the effect is not dependent on the presence or absenceof a particular chemical group in the molecule. With both thenitrobenzene and the phenol series the results suggest thatthe exact chemical structure is an important factor.     

Stomatal Characteristics of Four Native Herbs Following Exposure to Elevated CO2

Contrasting effects on the stomatal index (SI), stomatal density,epidermal cell size and number were observed in four chalk grasslandherbs (Sanguisorba minor Scop., Lotus corniculatus L., Anthyllisvulneraria L. and Plantago media L.) following exposure to elevatedcarbon dioxide concentrations (CO₂) in controlled environmentgrowth cabinets. SI of S. minor increased for both leaf surfaces,whilst in A. vulneraria and P. media SI decreased on one surfaceonly. In L. corniculatus , no differences in SI were observedas epidermal cell density changed in parallel with stomataldensity. In L. corniculatus and S. minor stomatal density increasedon both surfaces, whereas in P. media it decreased; in A. vulnerariastomatal density decreased on the abaxial leaf surface alonefollowing exposure to elevated CO₂. In the latter three species,SI changed because stomatal density did not change in parallelwith epidermal cell density. The results suggest elevated CO₂is either directly or indirectly affecting cell differentiationand thus stomatal initiation in the meristem. In S. minor and P. media leaf growth increased in elevated CO₂,because of increased cell expansion of epidermal cells, whereasin L. corniculatus, epidermal cell size decreased and greaterleaf growth was because of an increase in epidermal cell divisions.In A. vulneraria, leaf size did not change, but increased cellexpansion on the adaxial surface suggests CO₂ affects leaf surfacesdifferently, either directly or indirectly at the cell differentiationstage or as the leaf grows. These results suggest component species of a plant communitymay differ in their response to elevated CO₂. Predicting theeffect of environmental change is therefore difficult.Copyright1994, 1999 Academic Press Elevated CO₂, Sanguisorba minor (salad burnet), Lotus corniculatus (birdsfoot trefoil), Anthyllis vulneraria (kidney vetch), Plantago media (hoary plantain), stomatal index, stomatal density, epidermal cell size     

环境因子对角倍蚜秋迁蚜生殖和雌性蚜发育的影响

研究了环境因子对角倍蚜Schlechtendalia chinensis (Bell) 秋迁蚜生殖和雌性蚜发育的影响。温、湿度单因子试验表明,秋迁蚜在26℃和80%RH条件下有最大生殖量;温、湿度对秋迁蚜生殖量的影响均符合开口向下的二次抛物线变化趋势,极端温、湿度会导致生殖量的下降。采用三元一次正交组合设计,研究了环境温度（X₁）、湿度（X₂）和光照强度（X₃）三因子不同水平组合对雌性蚜发育的影响,表明温度是影响发育历期的主要因子,其次是光照强度,最后是湿度。因此,适当高温、强光照条件可以加快雌性蚜发育;而适当高湿条件可以降低雌性蚜的发育速率而延长其发育历期。在人工培育角倍蚜生产中,创造有利于秋迁蚜生殖的温、湿度条件可以使秋迁蚜产下较多的越冬侨蚜;在适当降低温度、增加湿度的阴暗条件下贮留雌性蚜可以适当延长其发育,以使角倍蚜与盐肤木在物候上达到最佳吻合。     

Carbon Assimilation at Low Carbon Dioxide Levels: II. THE PROCESSES OF APPARENT ASSIMILATION

The relation of the net uptake or output of carbon dioxide bydetached leaves of Pelargonium zonale or wheat to ambient carbondioxide concentration was investigated at two or three lightintensities. The statistics of fitted curves were used as datain analysis of variance. For Pelargonium the results were consistentwith a hypothesis of a ‘straight balance’ betweena rate of photosynthetic uptake proportional to concentrationand a constant rate of respiratory production. For wheat, however,the curve was steeper near than at lower carbon dioxide levels;this disproved the ‘straight balance’ hypothesisand was consistent with an increase in carbon dioxide fixationor a reduction in output over a limited concentration rangeon either side of . The characteristics of light respiration, possibly carbon dioxidedependent, were investigated further in an experiment on theeffect of oxygen concentrationupon r values for two species(P. zonale and Hydrangea sp.) at two light intensities; theywere also studied by measuring the ‘burst’ of carbondioxide output when leaves were darkened after illuminationat four different light intensities and at two temperatures. was linearly related to oxygen tension up to 610 mm partialpressure of mercury and a fivefold increase in light intensityhad only a very small effect indicating that photoxidation wasnot important. The carbon dioxide burst on darkening showedrelations to temperature and previous light intensity quitedifferent from those of which should be proportional to lightrespiration. These results, therefore, do not support the viewthat the burst represents the persistence of an enhanced lightrespiration.     

Studies on Foliar Penetration: IX. PATTERNS OF PENETRATION OF 2,4-DICHLOROPHENOXYACETIC ACID INTO THE LEAVES OF DIFFERENT SPECIES

The comparative patterns of penetration of 2,4-dichlorophenoxyaceticacid (2,4-D) into the leaves of Phaseolus vulgaris, Zea mays,Pisum sativum, Beta wlgaris, Helianthus annuus and Gossypiumhirsuium have been examined. Save for Zea and Gossypium where there is little change withthe stage of leaf development the rates of penetration intoboth surfaces decrease as the leaf matures. The relative ratesare dependent on the species and the age of the leaf but thereare differences between the surfaces. In Phaseolus the characteristicsof primary leaves differ from those of trifoliate leaves sinceonly in immature trifoliate leaves is penetration into the adaxialsurface greater. In darkness the rates of penetration over 24 h remain constantor fall but slightly for all species. Light consistently promotespenetration but with Beta there is a lag before entry is acceleratedinto the abaxial surface as has previously been reported foryoung primary leaves of Phaseolus. For the remaining speciesthe courses of penetration in both light and darkness into bothsurfaces follow similar patterns. As the light intensity isincreased entry is enhanced but the limit of response variesbetween species, between surfaces within species, and in trifoliateleaves of Phaseolus with age. For the six species the order of the relative rates of entryis closely similar whether comparisons are made in light ordarkness or between abaxial and adaxial surfaces: viz. Zea >Helianthus > Phaseolus (primary) > Phaseolus (trifoliate)> Pisum = Beta = Gossypium. The observed specific differencesare discussed in relation to variations in leaf structure, theproperties and thickness of the cuticle and the physiologicaland metabolic processes which influence transport within theepidermal tissues after it has passed through the cuticle bydiffusion.     

不同光照强度下物种组合对沉水植物苦草种间关系的影响

为了解种间关系对沉水植物群落结构的影响，在不同光照(20%自然光和50%自然光)和不同物种组合下，研究了长江中下游常见优势沉水植物苦草(Vallisneria natans)与黑藻(Hydrilla verticillata)和穗状狐尾藻(Myriophyllum spicatum)的相互作用。结果表明，在低光下，苦草与穗状狐尾藻混种时，苦草生物量、株高和叶数均没有明显变化，当穗状狐尾藻的比例较高时，苦草根长生长受到抑制，根叶比呈下降趋势；在高光下，穗状狐尾藻比例的增加会促进苦草单株生物量和叶生物量的增加，而对苦草株高、根长和叶数无显著影响；与黑藻混种相比，苦草与穗状狐尾藻混种时，苦草的株高、根长和叶数均无显著差异，而苦草的单株生物量和叶生物量均呈降低趋势。因此，物种组合和混种比例均会影响苦草与其他物种的相互作用关系，进而影响沉水植被的群落动态。     

Changes in the Growth of Salvinia natans Induced by Cycles of Light and Darkness of widely Different Duration

The growth of Salvinia natans has been examined when clonalmaterial, maintained at a constant temperature (30 ^°C) receivesthe same amount of light energy per day but where the light-darkcycle of equal intervals varies in eight steps from 1 min to12 h. The area per leaf, the rate of leaf production, the netassimilation rate, and the relative growth-rate increase withthe lengthening of the cycle, but the leaf-area ratio is reduced.The magnitude of the changes differs between the criteria whilethe order of the response may be disparate between consecutiveintervals. In supporting experiments the same measurements weremade on plants subjected to a wide range of light intensitieswith a common photoperiod of 12 h. Here decreasing light intensityproduces similar trends to those recorded for increasing thelength of the cycle but the patterns of response may diverge.It was also established that the size of the stomatal pore isthe same in the light and in the dark. It is postulated thatvanations in the light-dark cycle may influence both the levelof photosynthetic activity and the pathways.     

Interactions in the Physiological Effects of Growth Substances on Plant Development

The interrelated effects of 3-indolylacetic acid and sodium2 : 4-dichlorophenoxyacetate on the growth and development ofLemna minor and Helianthus annuus have been investigated bymeans of multifactorial experiments, involving three to fivelevels of each compound. A clonal population of L. minor wasmaintained under constant conditions of light (550 foot-candles)and temperature (25 C.) and the growth regulators were addedto a phosphate-buffered culture solution (pH 51), which waschanged every 2 days. In the H. annuus experiments the compounds,in aqueous solution, were placed by means of a micro-pipetteon the leaves of young plants grown in a greenhouse or in theopen. Indolylacetic acid, at concentrations up to 25 p.p.m., increasesthe relative growth rate of L. minor on either a dry-weightor frond-area basis. Sodium dichlorophenoxyacetate at 0025p.p.m. has little effect on growth, but at and above concentrationsof 02 p.p.m. growth is depressed. The ratio of frond area tototal dry weight is increased by indolylacetic acid but is depressedby the phenoxyacetate. In the presence of indolylacetic acid,more particularly at the higher levels, the relative inhibitionof growth caused by sodium dichlorophenoxyacetate is reduced. When sunflowers are treated daily with indolylacetic acid (0,6, 30, 150 µg./ plant) or initially with sodium dichlorophenoxyacetate(0, 20, 100 µ.g./plant) in all possible combinations,then the relative growth rates of the treated leaves, the leavesabove the treated leaves, the first internode, and the rootsare depressed when each compound is applied alone. On the otherhand, the growth rate of the hypocotyl is increased by bothgrowth regulators, while for the second internode indolylaceticacid decreases the growth rate but the phenoxyacetate at thelower level accelerates it. The combined action of the two compoundson the different parts tends to be cumulative, save for thehypocotyl, where the effect of indolylacetic acid is dependenton the level of the phenoxyacetate. It is concluded that these results cannot be explained on theconcept of wholly additive effects. There is evidence that thecompounds can mutually interfere in their actions and it wouldseem that more than one mechanism is involved. Many of the resultscan be satisfactorily interpreted on the basis that the twocompounds act on a common system and that they compete withone another, but the results are not compatible with the postulatethat the dichlorophenoxyacetic acid directly inactivates indolylaceticacid.     

Endogenous light-on rhythm in respiration of a long-day duckweed, Lemna gibba G3 III. Relationship to frond production

Relationship between the C>2-uptake rhythm and frond productionin a long-day duckweed, Lemna gibba G3, was investigated. The rate of frond production and the amplitude of the rhythmwere dependent on light intensity. Photosynthetic inhibitors,CMU and DCMU, at concentrations effective in stopping frondproduction, abolished the O₂-uptake rhythm after a lag of 1day. In the presence of inhibitors of protein and RNA syntheses,ETH, CH and TU, at concentrations which brought about completeinhibition of frond production, the O₂-uptake rhythm disappeared.FUdR, an inhibitor of DNA synthesis, did not eliminate the rhythmalthough it suppressed frond production. This indicates a ratherindirect correlation between the rhythm and the rate of frondproduction which, in turn, is probably related to photosynthesis.The rhythm may be more directly correlated with the cell expansion. (Received July 27, 1971; )     

Studies on Foliar Penetration: I. FACTORS CONTROLLING THE ENTRY OF 2, 4 - DICHLOROPHENOXYACETIC ACID

A technique, using leaf disks, has been developed to study thepenetration of isotopically labelled compounds into leaves underconditions where there is no appreciable change in the concentrationof the external solution and no subsequent translocation. Inthis preliminary survey, the leaves of Phaseolus vulgaris andColeus Blumei were employed to investigate the entry of 2,4-dichlorophenoxyaceticacid (2,4-D), labelled in the carboxyl group with ¹⁴C. Over3 days there is no loss of ¹⁴C to the atmosphere from treatedleaves of Phaseolus. The rate of penetration is enhanced when(a) the leaves are young, (b) the water status is lowered, (c)the temperature is raised (Q₁₀=2.3–2.8), and (d) a surface-activeagent is added to the external solution. Penetration is alsofavoured by a decrease in the pH, the relation indicating thatboth ions and molecules enter. Penetration is greater in thelight and prior illumination of the tissues positively affectsthe subsequent rate in the light, but not in the dark. In boththe light and the dark considerably more 2,4-D penetrates theabaxial surface of Phaseolus leaves. For Coleus an even greaterdifference between surfaces is found in the light but not inthe dark. For both species in the light the rates of entry intoboth surfaces are proportional to their respective stomataldensities. The simultaneous addition of indoleacetic acid tothe external solution caused more 2,4-D to enter Phaseolus leaves,but the addition of triiodobenzoic acid restricts entry. Therate of penetration remains constant over 24 hours and between0.1 and 200 mg./l. the rate is linearly related to concentration.Subsequent to entry, the 2,4-D is in a form which does not diffusefrom the tissue into buffer or exchange with unlabelled 2,4-D.Moreover, no outward movement takes place from treated tissuewhich has been frozen and thawed. These findings are discussedin relation to previous work on foliar penetration. It is concludedthat at least with Phaseolus penetration largely takes placethrough the guard cells and/or accessory cells.     

MORPHOGENESIS IN SPIEODELA OLIGORRHIZA: EFFECTS OF PYRIMIDINE BASE ANALOGUES ON INITIATION, ELONGATION AND DIFFERENTIATION OF FRONDS

The effect of four pyrimidine base analogues and their antidoteson S. oligorrhiza was studied. FUdR stopped cell division at concentrations of 4 10^–7M and higher. This effect could be nullified by the additionof 4 10^–6 M thymidine. Neither uridine nor uracil hadan antidotal effect on FUdR. FU (8 10^–6 M or higher concentration) affected celldivision, frond elongation and differentiation, and could notbe counteracted by either thymidine or uracil. TU (8 10^–4 M) rather specifically inhibited differentiationof frond tissues, while not preventing cell division or theinitiation of new generations. Uracil and uridine at about equimolarconcentrations completely counteracted the TU effect. AzU (10^–3 M) suppressed cell division, frond elongationand frond differentiation. When thymidine (10^–3 M) wasadded simultaneously with AzU only cell elongation and differentiationof fronds were inhibited, but cell division proceeded. 10^–3M uracil (but not uridine) counteracted all effects of AzU. ¹ Based on a portion of the senior author's Ph.D. Thesis.