Studies on Foliar Penetration: IV. MECHANISMS CONTROLLING THE RATE OF PENETRATION OF 2,4-DICHLOROPHENOXYACETIC ACID (2,4-D) INTO LEAVES OF PHASEOLUS VULGARIS

The effects of a wide range of metabolic inhibitors on the penetrationof 2,4-dichlorophenoxyacetic acid (2,4-D) into the leaf disksof Phaseolus vulgaris have been studied. While recognizing thelack of specificity of most inhibitors, compounds were chosenwhich are known to affect respiration, phosphorylation, photosynthesis,membrane permeability, protein synthesis, and the binding capacityof membrane systems. They were: fluoride, azide, arsenite, iodoacetate,arsenate, 2,4-dinitrophenol (DNP), 3-(3,4-dichlorophenyl), -I,I-dimethylurea (DCMU), phenylmercuric chloride, octenylsuccinicacid, decenylsuccinic acid, dimethyl sulphoxide, actinomycin-D,chloramphenicol, streptomycin, 5-fluorouracil, cycloheximide,and cetyltrimethylammoniumbromide (CTAB).At sub-toxic levelsall compounds had little or no influence on penetration in darknesssave for iodoacetate and decenylsuccinic acid, which causedsome enhanced entry at 10^-4M and 10^-3M respectively, and CTABwhich promoted penetration at concentrations known tolower thesurface tension of water.The much greater rate of penetrationof 2,4-D into disks exposed to bright light (16 000 lx) is unaffectedby fluoride, azide, DNP, octenylsuccinic acid, decenylsuccinicacid, dimethyl sulphoxide, or actinomycin-D. It is, however,progressively inhibited by increasing concentrations of arsenite,iodoacetate, arsenate, streptomycin, and 5-fluorouracil. Chloramphenicol,cycloheximide, and CTAB lower the rate of penetration at intermediateconcentrations but at high concentrations the affect is reversed.The most active inhibitors of light-induced penetration areDCMU and phenylmercuric chloride, compounds which block theproduction of ATP.These results are discussed in relation tomechanisms of transport, in particular the structureand stabilityof barriers likely to impede penetration.     

Studies on Foliar Penetration: III. THE EFFECTS OF CHLORINATION ON THE RATE OF PENETRATION OF PHENOXYACETIC ACID AND BENZOIC ACID INTO LEAVES OF PHASEOLUS VULGARIS

A comparative study has been made of the penetration into leafdisks of Phaseolus vulgaris of (a) phenoxyacetic acid and its2-, 4-, 2, 4-, 2, 6-, 3-5, 2, 4, 5-, and 2, 4, 6- chloro derivatives,and (b) benzoic acid and its 2-, 2, 4-, 2, 5-, 2, 3, 6-chloroand 3, 6-dichloro-2-methoxy derivatives. The methods of synthesisof each compound with ¹⁴C incorporated in the carboxyl groupare described. In the series of substituted phenoxyacetic acids it was establishedthat only for 2, 4, 5-T was there an appreciable loss of radioactivityfrom the system either in the light or darkness. In contrast,with the exception of 2, 4-DCBA, ¹⁴C is lost from each memberof the series of substituted benzoic acids. The level and pattern of penetration in the two series is differentiallyaffected by chlorination. In general progressive chlorinationof phenoxyacetic acid leads to an increase in the rate of penetrationin both light and darkness. There are, however, exceptions;for illuminated disks the rate of entry of 2,4,5-T is exceededby 2,4,6-T, 2, 4-D, and 2, 6-D whereas in the dark 2,4,5-T hasby far the highest rate of penetration. Progressive chlorinationof benzoic acid, however, depresses entry in both light anddarkness. Possible relationships between these findings and changes inselected physical properties of each series of compounds havebeen examined. Between members of the series of substitutedphenoxyacetic acids there is little variation in the dissociationconstant whereas for the substituted benzoic acids there isa marked lowering of the pK as the degree of chlorination increases.The rate of elution with chloroform of each compound from abuffered Silocel column gives a measure of the degree of partitioninto a lipid medium at a given pH. At pH 5.2 chlorination ofphenoxyacetic acid results in more rapid elution whereas chlorinationof benzoic acid causes a longer hold up. An apparatus was designed to enable measurements to be madeof the rate at which a compound moves from one aqueous phaseto another through a layer of lipid. Chlorination promotes therate of transfer of phenoxyacetic acid through octanol but retardsthe transfer of benzoic acid. The extent to which chlorinationdepresses the rate of transfer of benzoic acid matches the diminutionin the rate of leaf penetration. For the series of substitutedphenoxyacetic acids transfer rates and penetration rates followsimilar trends in the dark but in the light agreement is lessgood. These findings are discussed in relation to prior studies onthe rate of uptake and the metabolic fate of the ndividual compoundsin a number of plant tissues.     

Courtship and Mate Choice in Fishes: Integrating Behavioral and Sensory Ecology

SYNOPSIS. Sexual selection theory predicts a coevolution betweenmale sexual ornamentation and female preference. The implicationof this prediction for sensory ecology is that there shouldbe a tight coupling between the physiology of male signal productionand the physiology of female signal reception. Indicator modelsof sexual selection predict that male ornamentation is correlatedwith male condition, and that female preference is correlatedwith male ornamentation. Indicator models of sexual selectionhave a conceptual overlap with resource acquisition and investmentmodels of behavioral ecology. Empirical studies with fishes,particularlywith guppies (Poecilia reticulata) and threespinesticklebacks (Gasterosteus aculeatus), suggest a strong connectionbetween acquired resources, male condition, male ornamentation,male courtship, and female preference.     

Cryo-Preservation of Roots for Scanning Electron Microscopy

Fully hydrated roots can be examined in the scanning electronmicroscope after cryo-preservation. Shrinkage associated withdehydration by freeze-drying or critical point drying, to whichroot hairs and secreted mucigel are particularly vulnerable,is avoided. Roots, Lepidium sativum, scanning electron microscopy, cryo-preservation, fully hydrated     

Studies on Foliar Penetration: VI. FACTORS CONTROLLING THE PENETRATION OF 4-AMINO-3,5,6-TRI-CHLOROPICOLINIC ACID (PICLORAM) INTO THE LEAVES OF PHASEOLUS VULGARIS

The evaluation of the factors controlling the penetration of4-amino-3,5,6-trichloropicounic acid (picloram), containing¹⁴C in the carboxyl group, into leaves of Phaseolus vulgarisis complicated by the ability of the leaf tissue to decarboxylatethe compound rapidly. The degradation system becomes fully activatedonly after some hours following treatment with picloram whilethe capacity for decarboxylation declines with the age of theleaf. When allowance for decarboxylation is made, the pattern of penetrationfor picloram resembles in particular that of 2,2-dichloropropionicacid (dalapon). At the abaxial surface penetration continuesover 24 h at a fairly constant rate but in light, which enhancespenetration, a ‘surge’occurs between the secondand sixth hour. Raising the external concentration promotespenetration and in light entry is directly proportional to concentrationup to at least 2.5 x 10^–l M. The response to varying lightintensity resembles that of dalapon rather than 2,4-D. Evenlow intensities promote penetration; the effect is maximal atabout 10 000 lx at the abaxial surface but continues at leastup to 20 000 lx at the adaxial surface. Penetration is retarded at 4°C but the decarboxylation systemremains functional. In light, but not in darkness, penetrationis strikingly reduced by 3-(3,4-dichlorophenyl)-l,l-dimethylurea(DCMU). Raising the external pH from 4.2 to 7.2 retards entry. It is concluded that the factors which determine the penetrationof picloram into Phaseolus leaves also control the entry ofa wide range of organic compounds and inorganic ions.     

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.     

Effects of Acetazolamide (Diamox), Ethoxzolamide and High Levels of CO2 on Carbonic Anhydrase, Photosystem Activity, and Oxygen Evolution in Euglena gracilis

Investigations using steady-state culture conditions indicate that carbonic anhydrase activity is correlated to the photosynthetic rate in Euglena in some but not all circumstances. When cultures grown with 5% CO₂ were changed to air growth, the photosynthetic rate was independent of the carbonic anhydrase activity. While experiments using the inhibitor acetazolamide indicated a close correlation between photosynthetic capacity and carbonic anhydrase activity, the inhibitor was found to be nonspecific. Acetazolamide altered photosystem activities directly as measured by the photoreduction of DCPIP in chloroplast preparations, whole-cell fluorescence transients of chlorophyll a, and by whole chain photoelectron flow. Ethoxzolamide, another inhibitor of carbonic anhydrase, was also found to inhibit photosystem activities, i.e., the photoreduction of DCPIP, and in vivo photoelectron flow, at high concentrations. Cells grown in 5% CO₂ were less sensitive to the effects of acetazolamide than cells exposed to air. The rate of electron flow in chloroplasts from cells grown with 5% CO₂ and exposed to 10 mM acetazolamide was 2.5-fold faster than that of chloroplasts from air-grown cells exposed to the same concentration of inhibitor. The whole cell chlorophyll a fluorescence transients of cultures grown with high CO₂ were completely different from those of air-grown cells and also showed fewer effects on exposure to acetazolamide. These results suggest a reevaluation of the hypothesis that carbonic anhydrase activity regulates photosynthesis. It is also apparent that results from air-grown and 5% CO₂-grown cultures cannot be directly compared in such studies.     

The Occurrence of a Secondary Cuticle in Libertia elegans (Iridaceae)

A study of the formation of cuticle in Libertia elegans hasshown that the cuticle develops centripetally and is producedcontinuously at its juncture with the underlying pectic layer.Peroxidase activity may be demonstrated in this region duringdevelopment. Growth occurs in two discrete phases; initiallywhilst the underlying epidermal cells are expanding and laterwhen such expansion is complete. The phases are morphologicallydistinct and it is proposed that the terms primary and secondarybe applied to them. Cuticular channels with a pit-like appearancein T.S. are described here for the first time as is the fissuringand active replacement of inelastic secondary cuticular materialto accommodate costal expansion and the growth of papillae.     

The scutellum of Avena: a structure to maximize exploitation of endosperm reserves

NEGBI, M. & SARGENT, J. A., 1986. The scutellum of Avena : a structure to maximize exploitation of endosperm reserves . The scutella of graminaceous embryos possesses a well developed vascular system and a surface epithelium of closely packed cells. The epithelial cells typically elongate during germination and separate to form papillae, thereby increasing the interfacial area for exchange between the endosperm and embryo. Avena and a dozen non-crop genera are distinguishable by a scutellum which grows into and through the endosperm. Thus diffusion pathways for mobilized reserves in Avena are kept short. The course of elongation of the scutellum and of its epithelial cells in A. Jatua was studied during germination and seedling establishment. A comparison was made with with the fully developed scutellum of Secale cereale which does not elongate but only forms papillae. It is proposed that the competitive success of A.Jatua as a weed is attributable in part to an ability, conferred by its scutellum, to exploit its seed reserves steadily and fully during germination and seedling development.