{alpha}Melanocyte Stimulating Hormone: Chemical Nature and Mechanism of Action

Melanotropin (-MSH) is a tridecapeptide, Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂,synthesized and secreted by the pars intermedia of the vertebratepituitary. This peptide hormone is derived from pro-opiomelanocortin,a precursor protein which contains within its structure thesequences of other melanotropic peptides (- and rß-MSH,corticotropin), and possibly other hormones. -MSH is the physiologicallyrelevant melanotropin secreted by the pituitary and in mostvertebrates plays the essential role in adaptive color changesthrough its action on integumental chromatophores. The initial actions of -MSH are mediated at the level of themelanocyte membrane and involve signal transduction from receptorto adenylate cyclase on the intracellular surface of the membrane.This results in elevated cytosolic cyclic AMP levels followedby melanosome dispersion within dermal melanocytes and melanogenesiswithin epidermal melanocytes. The action of -MSH on dermal melanocytesrequires calcium for transduction of signal and cyclic AMP production.Melanosome dispersion per se does not, however, require extracellularcalcium. Structure-function studies of -MSH analogues and fragmentshave provided important insights relative to the structuralrequirements of the hormone for receptor binding and transduction.Substitution of certain residues within -MSH has led to thedevelopment of melanotropins that exhibit extraordinary potencyand prolonged biological activity     

Contractile Proteins of a Benthic Fish.I. Effects of Temperature and Pressure on Myosin ATPase.

SYNOPSIS. Studies are described on the adenosine triphosphatase(ATPase) properties of myosin isolated from skeletal muscleof Coryphaenoides, a benthic fish captured at 2,200 meters depth.Ca²⁺-ATPase and EDTA-ATPase of Coryphaenoides myosin show thesame pH dependence as ATPase of mammalian myosin; however, ratesof ATP hydrolysis by Coryphaenoides myosin are only 5–10%of rates of ATP hydrolysis by rabbit skeletal myosin. Coryphaenoidesmyosin ATPase shows a decrease from Q₁₀ of 2.0 at 25°C toQ₁₀ of 1.4 a t 2°C, and undergoes irreversible denaturationat temperatures above 25°C. At pH 6.8 to pH 8.5, Coryphaenoidesmyosin ATPase undergoes activation by pressure at 25°C,but at 2°C shows negligible effect of pressure at valuesbelow 3,000 psi. The kinetic data on Ca²⁺-ATPase indicate valuesof 11 kcal/mole for H, –7.5 kcal/mole for TS, and –5.7cc/mole for V at 25°C, pH 7.6. Comparable data at 2°Cindicate values of 5 kcal/mole for H. –13 kcal/mole forTS, and negligible V. According to the results of 25°C,Ca²⁺-activatkm of myosin-ATP may involve disruption of fouror five hydrophobic or polar groups, presumably due to an "opening-up"of the myosin molecule at or near the site for ATP binding.It would also appear that Coryphaenoides myosin has undergonean adaptive change in the enzyme mechanism for ATPase such thatthe rate of ATP hydrolysis is relatively insensitive to pressureand temperature under conditions encountered by the living fish.     

The Ontogeny of Osmoregulation and Its Neurosecretory Control In the Decapod Crustacean, Rhithropanopeus harrisii (Gould)

Laboratory-hatched larvae of this estuarine crab were rearedat 25°C in seawater of 25 salinity for 18 days coveringzoeal Stages I to IV and a megalops. Three-day periods betweenzoeal stages represent intermolt stages of circadean metecdysis,diecdysis, and proecdysis.Larvae were exposed to either a seriesof seawater salinities from 5-40 in 5 increments or of 10-40in 10 increments for one hour during each day of their development.The osmoconcentrations of 20-80 nanoliter hemolymph samplesfrom each of four larvae were measured separately by determinationsof freezing point depression. Eyestalkless larvae in metecdysis of zoeal Stage II were exposedto the same osmoconcentrations as unoperated controls to testfor osmoregulation by eyestalk nerve tissue. Larvae tend to be isosmotic with seawater of 30-40 salinity(S) and to hyperregulate in more dilute media except for larvaein their first diecdysis which remain isosmotic. Larvae in thelast few hours of proecdysis hyperregulate against 40 S as well,presumably to insure inflow of water to establish a greaterbody volume during hardening of the exoskeleton. They are consequentlyisosmotic in the very early metecdysis. The presence of eyestalks at the first metecdysis (Stage II)keeps zoeas hyperosmotic to 5-30 S, but prevents them from hyperregulationagainst 40 S. Eyestalkless zoeas become isosmotic with 5-30S and hyperregulate against 40 S like late proecdysal larvae.Lack of eyestalks makes diecdysal animals hyperregulate againsta medium with which normal animals are isosmotic. The eyestalkinfluence affects second metecdysal (Stage III) larvae in away similar to those in first metecdysis except that it apparentlyalso prevents a curious tendency to hyporegulate in 5-30 S.Similarly, in this stage, the eyestalks prevent hyperosmosityin 40 S seawater as they do during the first day of zoeal StageII. Eyestalk nerve tissue reduces the degree to which diecdysallarvae of this stage remain hyperosmotic to media of 10 S and20 S and apparently causes larvae to be hypoosmotic at 40 S. Preliminary data indicate that removal of eyestalks has littleeffect on proecdysal larval osmoregulation or on regulationof Stage IV zoeas. In other experiments ablation of eyestalks caused Stage II larvaeto lose the ability to osmoregulate against 10-30 S seawaterwithin two hours after the operation. The same zoeas did nothyperregulate against 40 seawater until four hours after removalof eyestalks.     

A Comparative View of Alpha Crystallins: The contribution of comparative studies to understanding function

Integration between comparative biology and cellular/molecularbiology has helped advance understanding of the structure, functionand physiology of the vertebrate small heat shock proteins A-and B-crystallin. These proteins are expressed at high concentrationin the eye lens where they contribute to transparency and refractivepower. But they also function similarly to molecular chaperonesby preventing the aggregation of denatured proteins that cancause opacities, or cataracts. -crystallins also serve a numberof other roles in and out of the lens that are still not completelyunderstood. Comparative examination of -crystallins and closelyrelated small heat shock proteins from diverse taxa has helpedprovide insights into the proteins' three-dimensional shapeand structure/function relationships. Until recently, no studieshad examined the tissue specific expression or chaperone-likeactivity of -crystallins from a non-mammalian vertebrate. Ihave been investigating the -crystallins of the zebrafish, Daniorerio, as a first step towards utilizing the bony fishes asa model group for understanding the evolution of -crystallinfunction. Zebrafish A-crystallin displays similar structureand expression and increased chaperone-like activity comparedto its human orthologue. Zebrafish B-crystallin, however, hasa truncated C-terminal extension, more limited expression andlower chaperone-like activity than its human orthologue. Thesedata suggest that A-crystallin physiological function may beconserved between zebrafish and mammals, while B-crystallinphysiological function has diverged. Understanding zebrafish-crystallin physiology is necessary before this species canbe used for developmental and genetic studies, and providesa foundation for further comparative studies.     

{delta}15N and {delta}13C Measurements of Antarctic Peninsula Fauna: Trophic Relationships and Assimilation of Benthic Seaweeds

Measurements of ¹³C, ¹⁵N, and C/N for a variety of Antarcticpeninsula fauna and flora were used to quantify the importanceof benthic brown algae to resident organisms and determine foodweb relationships among this diverse littoral fauna. ¹³C valuesranged from–16.8 for benthic algal herbivores (limpets)to –29.8 for the krill, Euphausia superba; the averagepooled value for brown macroalgae, including their attachedfilamentous diatoms, was–20.6. There was no correlationbetween biomass ¹³C or ¹⁵N with C/N content, and consequentlyboth ¹³C and ¹⁵N values were useful in evaluating trophic relationships.¹⁵N values of the fauna ranged from 3.1 to 12.5, with lowestvalues recorded in suspension feeders (e.g., bryozoans) andhighest values in Adelie penguins (12.5) collected in 1989.The comparatively lower ¹⁵N value for a Chinstrap penguin (6.9)collected in 1997 is attributed to the different dietary foodsources consumed by these species as reflected in their respective¹³C values. Significant amounts of benthic macroalgal carbonis incorporated into the tissues of invertebrates and fishesthat occupy up to four trophic levels. For many benthic andepibenthic species, including various crustaceans and molluscs,assimilation of benthic algal carbon through detrital pathwaysranges from 30 to 100%. Consequently, the trophic importanceof benthic brown algae may well extend to many pelagic organismsthat are key prey species for birds, fishes, and marine mammals.These data support the hypothesis that benthic seaweeeds, togetherwith their associated epiphytic diatoms, provide an importantcarbon source that is readily incorporated into Antarctic peninsulafood webs.     

Comparative Biochemistry of the Pineal Gland

The pineal gland is biochemically very active. In mammals, ithas the unique capacity to synthesize the hormone melatonin(N-acetyl-5-methoxy-tryptamine). Although the synthesis of melatoninis confined mainly to the pineal gland of all vertebrates, theeyes and brains of amphibians and fish also can form melatonin.Melatonin is synthesized in the pineal as follows: tryptophan 5-hydroxy-tryptophan serotonin N-acetylserotonin melatonin.The final step is catalyzed by the enzyme, hydroxyindole-O-methyltransferase (HIOMT), which is highly localized in the pinealof all vertebrate species examined. The activity of HIOMT ischanged when animals are kept in constant darkness or light.In rats, highest HIOMT activity is present in constant darkness,while the reverse occurs in avian species. In mammals, informationabout lighting reaches the pineal via the retina inferior accessoryoptic tract preganglionic sympathetic fibers superior cervicalganglia postganglionic fibers pineal parenchymal cells. Lightingmessages reach the hen's pineal via a nonretinal pathway. Studieswith tissue culture indicate that noradrenaline liberated fromsympathetic nerves stimulates synthesis of melatonin. Thereare circadian rhythms in pineal serotonin content which areendogenous and abolished by removal of superior cervical gangliaor by decentralization. There is also a 24-hour rhythm in pinealnoradrenaline. This rhythm is exogenous and is abolished byblinding or cutting the inferior accessory optic tract.     

The Meaning of Matric Potential

The commonly used equation, = P - + , which describes thepartitioning of plant water potential, , into components ofhydrostatic pressure, P, osmotic pressure, , and matric potential,, is misleading. The term , which is supposed to show the influenceof a solid phase on , is zero if a consistent definition ofpressure is used in the standard thermodynamic derivation. However,it can be usefully defined by = + _D, where _D is the osmoticpressure of the equilibrium dialysate of the system. The practicaland theoretical significance of this definition is discussed.     

Pumpkin (Cucurbita sp.) seed globulin IV. Terminal sequences of the acidic and basic peptide chains and identification of a pyroglutamyl peptide chain

Pumpkin seed globulin is composed of heterogeneous polypeptidechains, acidic and chains and basic ₁ and ₂ chains (12). This study showed that the basicchains had similar N-terminal sequences, Gly-Leu-Asp-Glu-Thr-Ile-for the ₁ chain and Gly-Leu-Glu-Glu-Thr-Ile- for ₂. On the contrary,the N-terminal sequences of the acidic and chains were dissimilar, Ile-Gln-Gly-Tyr- for the chain and no N-terminal residue for the chain, according to routine terminal analysis. Pyrrolidonylpeptidase digestion of the chain and its thermolysin digestion followed by Edman degradationsrevealed that the N-terminal sequence of the chain was < Glu-Ile-Glu-Gln-Gln-Glu-Pro(Trp,Ser)-. The N-terminal sequences and the C-terminal residuesindicated that the acidic and chains were more heterogeneous than the basic ₁ and ₂ chains.A preliminary study on the degradation of storage globulin isalso presented. (Received November 9, 1979; )     

Genotypic Differences in Leaf Water Relations between Brassica juncea and B. napus

Various kinds of measurement of tissue water status were madeseveral times during water stress and recovery in Brassica juncea(cv Canadian Black) and B napus (cv Drakkar) Unstressed plantsof the two species had similar leaf water potentials (_w), solute(_s) and turgor potentials (_p) Values of relative water content(RWC) and the slope of the linear relationship between _p andRWC (_p/RWC) were greater in B napus than in B juncea Statistical correlations of pooled data for the watered andstressed treatments differentiated the relationships among RWC,_w and its components in the two species The major statisticaldifference was that _p/RWC was related to RWC in B napus andto _w and _s in B juncea A decline in _p/RWC with decreasing _sin B juncea may be a mechanism for maintaining _p at low soilwater potentials through maintenance of more elastic cell walls. Brassica juncea, Brassica napus, osmotic adjustment, tissue elasticity, water relations     

Alterations in the Components of Peanut Leaf Water Potential during Desiccation

Changes in components of leaf water potential during soil waterdeficits influence many physiological processes. Research resultsfocusing on these changes during desiccation of peanut (Arachishypogeae L.) leaves are apparently not available. The presentstudy was conducted to examine the relationships of leaf water_l, solute _s and turgor _p potentials, and percent relative watercontent (RWC) of peanut leaves during desiccation of detachedleaves and also during naturally occurring soil moisture deficitsin the field. The relationship of _p to _l and RWC was evaluated by calculating_p from differences in _l and _s determined by thermocouple psychrometryand by constructing pressure-volume (P-V) curves from the _land RWC measurements. Turgor potentials of ‘Early Bunch’and ‘Florunner’ leaves decreased to zero at _l of–1.2 to –1.3 MPa and RWC of 87%. There were no cultivardifferences in the _l at which _p became zero. P-V curves indicatedthat the error of measuring _s after freezing due to dilutionof the cellular constituents was small but resulted in artefactualnegative _p values. Random measurements on two dates of _l, _s, and calculation of_p from well-watered and water-stressed field plots consistingof several genotypes indicated that zero _p occurred at _l of–1.6 MPa. It was concluded that the relationships of _p,_l, _s, and RWC of peanut leaves were similar to leaves of othercrops and that these relationships conferred no unique droughtresistance mechanism to peanut.     

Hydrophobic mannosides act as acceptors for trypanosome {alpha}-mannosyltransferases

A series of hydrophobic mannosides were synthesized and testedfor their ability to act as acceptor substrates for mannosyltransferasesin a Trypanosoma brucei cell-free system. The thiooctyl -mannosidesand octyl -mannosides all accepted single mannose residues in-linkage, as judged by thin layer chromatography of the productsbefore and after jack bean -mannosidase digestion. The mannosylationreactions were inhibited by amphomycin, suggesting that theimmediate donor was dolicholphosphate-mannose (Dol-P-Man) inall cases. The transferred -mannose residues were shown to beboth 1-2 and 1-6 linked by Aspergillus phoenicis -mannosidaseand acetolysis treatments, respectively. These data suggestthat the compounds can act as acceptor substrates for the Dol-P-Mandependent 1-2 and 1-6 mannosyltransferases of the GPI biosyntheticpathway and/or the dolichol-cycle of protein N-glycosylation.One of the compounds, Man1-6Man1-O-(CH₂)₇CH₃, inhibited endogenousGPI biosynthesis in the cell-free system, suggesting that itcould be a substrate for the trypanosome Dol-P-Man:Man₂GlcN-Pl1-2 mannosyltransferase. dolichol glycosylphosphatidylinositol mannosyltransferase trypanosome     

Isolation of Subunits of Coupling Factor 1 from Maize and Spinach Chloroplasts and Properties of Combinations of Subunits with ATPase Activity

Subunits (, ß, ) and mixtures of subunits ( ß, , ß , ß ) were isolated without denaturationfrom a chloroform extract of chloroplast coupling factor 1 (CF₁)from maize (Zea mays var. Ushiku 5-4) and from spinach by fastprotein liquid chromatography (FPLC), on an anion-exchange columnof Mono-Q in the presence of n-octylglucoside (OG) and on achromatofocusing column of Mono-P. The ß -subunitcomplex (CF¹ ß ) was the minimum unit required forATPase activity, as was confirmed by the reconstituted complexof ß and subunits. An subunit isolated from maizeinhibited the ATPase activity of CF₁ ß from bothmaize and spinach. CF₁ ß was found to contain anOG-dependent Mg²⁺-ATPase. The ATPase activity of CF₁ ß required divalent cations, such as Mg²⁺ or Mn²⁺, for its expressionin the presence of OG; its optimum pH was 8.0 and it was markedlyinhibited by NaN₃. The enzyme hydrolyzed ATP in prefernece toGTP but not CTP, UTP, ADP, AMP or pNPP. Lineweaver-Burk plotsof its activity were curvilinear in the range of 0.6–0.7mM ATP.Mg²⁺. ¹Present address: Department of Biology, School of Education,Waseda University, Shinjuku-ku, Tokyo, 160 Japan. (Received February 15, 1989; Accepted April 20, 1989)     

Pumpkin (Cucurbita sp.) seed globulin III. Comparison of subunit structures among seed globulins of various Cucurbita species and characterization of peptide components

Various Cucurbita seed globulins showed patterns similar toone another on SDS-gel electrophoresis, and ß bandsfor unreduced globulins and , ', and ' bands for reduced ones.On gel electrophoresis in 6 M urea, reduced globulin gave twoacidic and two basic bands. These corresponded to and ' chainsand ₁ and ₂ chains, respectively, identified by two-dimensionalurea-SDS gel electrophoresis. The compositions of the and ßsubunits were proposed. (Received September 8, 1977; )     

The Derivation of the Cell Wall Elasticity Function from the Cell Turgor Potential

An equation is derived expressing average turgor pressure ofa leaf (_p) as a function of relative water content (RWC). Basedon this derivation, the relationships of the bulk elastic modulus(_v) and both RWC and _p, are formulated and discussed. The bulkelastic modulus (_v) becomes zero for _p = 0, that is at the turgorloss point for the leaf. At full water saturation the valueof ev is proportional to the water saturation turgor potential_p(max). The factor relating _P and _v (structure coefficient ,Burstrom, Uhrstr?m and Olausson, 1970) changes only very littlefor values of _p, which are not too close to zero. An exampleis given for the calculation from experimental data of the turgorpressure function, the structure coefficient function, and the_v function. Key words: Cell wall, Turgor pressure, Bulk elastic modulus     

Wilting of Leaves in Vicia faba L.

Wilting of leaves of Vicia faba L., which occurs when the pressurepotential (_p) is zero, and the leaf-water potential () at wiltingboth depend entirely upon the solute potential at incipientplasmolysis (_so) and not on soil-water status. Wilting in V.faba is acropetal; this is consistent with the hypothesis thatthere is a gradient of decreasing _so up the plant and that wateris transferred from the lower to the upper leaves, hasteningthe overall water loss from the lower leaves to the point when_p is zero. The gradient in _so up the plant is of the order of3–8 bar. It is proposed that wilting when _p>0 (i.e. > _so) shouldbe ‘apparent wilting’ and that when _p0 (i.e. _so),‘true wilting’.     

Sialylation of n-alkyllactosides, galactosides and glucosides by sialyltransferases from rat liver Golgi vesicles

nAlkyl - and -lactosides, galactosides and glucosides with differentalkyl chain lengths (C₂, C₈, C₁₄, and C₂₀) were synthesizedand used as acceptors for sialyltransferases from rat liverGolgi vesicles. The -galactosides, -glucosides, and both - and-lactosides, were sialylated. Keeping the acceptor concentrationconstant, sialylation rates reached a maximum for the n-octyl- and -lactosides, n-Octyl -galactoside and noctyl -glucoside,respectively. noctyl -glucoside, respectivwly. n-Octyl -galactosideand n-octyl -glucoside were not sialylated. The reaction productswere characterized by TLC. With n-octyl lactoside and galactosideas acceptors, two major sialylation products were formed. Thjeycould be separated by preparative TLC, and their structureswere identified as 2–3 and 2–6 sialylated acceptors,respectively, by a combination of periodated oxidation, NaBD₄reduction,permethylation and subsequent analysis by fast atombombardment mass spectrometry (FAB-MS). The structure of thesingle product obtained from n-ictyl -glucoside was determinedto be the 2–6 sialylated glucoside. Competition experimentswith n-octyl lactoside and lactosylceramide and gangliosideGal1-3GalNAc1-4(NeuAc2–3)Gal1–4Glcbeeta1–1Cer(GM1) as acceptors for sialyltransferases suggested that SAT-I[NeuAc2–3Gal1–4Glc1-1Cer (GM3) synthase] was atleast in least in part responsible for the 2–3 sialylationof n-octyl lactoside. alkylgalactosides alkylglucosides alkyllactosides neoglycolipids sialytransferases     

Molecular dynamics simulations of oligosaccharides and their conformation in the crystal structure of lectin-carbohydrate complex: importance of the torsion angle {psi} for the orientation of {alpha}1,6-arm

The conformation of the heptasaccharide Man-1,6-(Man-1,3)(Xyl-ß1,2)-Man-ß,4-GlcNAc₂-ß1,4-(L-Fuc-1,3)-GlcNAc₁,the carbohydrate moiety of Erythrina corallodendron lectin (EcorL),the hexasaccharide Man-1,6-(Man-1,3) (GlcNAc-ß1,4)-Man-ß1,4-GlcNAc-ß1,4-GlcNAcand their disaccharide fragments have been studied by moleculardynamics (MD) simulations for 1000 ps with different initialconformations. In the isolated heptasaccharide, the most frequentlyaccessed conformation during MD has a value of 180° aroundMan-1,6-Man linkage. This conformation is stabilized by theformation of a hydrogen bond between the carbonyl oxygen ofGlcNAc₂ with the O3/O4 hydroxyls of the 1,6-linked mannose residue.The conformation of the heptasaccharide found in the crystalstructure of the EcorL-lactose complex (Shaanan et al., Science,254, 862, 1991), that has a value of 76° around Man-1,6-Manlinkage, is accessed, although less frequently, during MD ofthe isolated oligosaccharide. The ,, = 58°,–134°,–60°conformation around Man-₁,6-Man fragment observed in the crystalstructure of the Lathyrus ochnrs lectin complexed with a biantennaryoctasaccharide (Table I in Homans,S.W., Glycobiology, 3, 551,1993) has also been accessed in the present MD simulations.These values for the 1,6-linkage, which are observed in theprotein-carbohydrate crystal structures and are accessed inthe MD simulations, though occasionally, have not been predictedfrom NMR studies. Furthermore, these different values of leadto significantly different orientations of the 1,6-arm for thesame value of . This contrasts with the earlier predictionsthat only different values of can bring about significant changesin the orientation of the ₁,6-arm. The MD simulations also showthat the effects of bisecting GlcNAc or ß1,2-xyloseare very similar on the ₁,3-arm and slightly different on the₁,6-arm. bisecting GlcNAc carbohydrates glycoprotein lectinsaccharide complex     

Liquid-Tissue Mechanics in Amphibian Gastrulation: Germ-Layer Assembly in Rana Pipiens

During amphibian gastrulation, migrating subsurface germ layersmay flow past one another like immiscible viscous liquids inresponse to tissue surface tension forces. We describe heretwo physical tests for liquid-tissue morphogenesis in culturedaggregates of subsurface ectoderm (E), mesoderm (M) and endoderm(N) excised from mid-yolk-plug Rana pipiens gastrulae. (i) Liquidsare coherent substances in which subunits can slip past oneanother to relax internal shear stresses. We find, in cross-sectionsof cell aggregates fixed during compression, that cells withinflattened aggregates are intially deformed, but do, as predicted,gradually reassume their original, undistorted shapes, (ii)Surface tensions ('s) govern ordinary liquid-droplet spreading;e.g., if equal-sized droplets A and B fuse in medium O, B spreadsaround A when _{AO BO}. When pairs of subsurface aggregates arecultured together, N surrounds M and E, and M surrounds E. Tosee if _EO > _MO > _NO. we flatten aggregates with quartzfibers calibrated to measure the force of compression. As predicted,under the same flattening force, E aggregates are rounder thanM aggregates, which are rounder than N aggregates. Furthermore,a second surface tension relationship can account for the autonomousinvolution of M between E and N; and these surface tension relationshipscan also explain the inversion of E, M and N by coated ectodermto produce normal gastrular germ-layer arrangements. We concludethat, combined with active cell shape changes in solid-likesurface cell layers and also with autonomous elongation of dorsallip mesoderm, tissue surface tension control of liquid-tissueflow in subsurface germ layers is a key morphogenetic mechanismin amphibian gastrulation which might be regulated by changesin intercellular adhesiveness.     

Effects of Priming and Endosperm Integrity on Seed Germination Rates of Tomato Genotypes: II. GERMINATION AT REDUCED WATER POTENTIAL

Seed germination rates (GR =inverse of time to germination)are sensitive to genetic, environmental, and physiological factors.We have compared the GR of tomato (Lycopersicon esculentum Mill.)seeds of cultivar T5 to those of rapidly germinating L. esculentumgenotypes PI 341988 and PI 120256 over a range of water potential(). The influence of seed priming treatments and removal ofthe endosperm/testa cap enclosing the radicle tip on germinationat reduced were also assessed. Germination time-courses atdifferent 's were analysed according to a model that identifieda base, or minimum, allowing germination of a specific percentage(g) of the seed population (_b(g)), and a ‘hydrotime constant’(_H) indicating the rate of progress toward germination per MPa.h.The distribution of _b(g) determined by probit analysis was characterizedby a mean base (_b) and the standard deviation in _b among seeds(_b). The three derived parameters, _b, _b) and _H, were sufficientto predict the time-courses of germination of intact seeds atany . A normalized time-scale for comparing germination responsesto reduced is introduced. The time to germination at any (t_g())can be normalized to be equivalent to that observed in water(t_g(0)) according to the equation t_g(0)=[l–(/_b(g))]t_g().PI 341988 seeds were more tolerant of reduced and had a morerapid GR than T5 seeds due to both a lower _b and a smaller _H.The rapid germination of PI 120256, on the other hand, couldbe attributed entirely to a smaller _H. Seed priming (6 d in–1.2 MPa polyethylene glycol 8000 solution at 20 ?C followedby drying) increased GR at all >_b(g), but did not lower theminimum allowing germination; i.e. priming reduced _H withoutlowering _b. Removing the endosperm/testa cap (cut seeds) markedlyincreased GR and lowered the mean required to inhibit germinationby 0.7 to 0.9 MPa. However, this resulted primarily from downwardadjustment in _b during the incubation of cut seeds at low inthe test solutions. The difference in _b between intact and cutseeds incubated at high was much less (0.l MPa), indicatingthat at the time of radicle protrusion, the endosperm had weakenedto the point where it constituted only a small mechanical barrier.In the intact seed, endosperm weakening and the downward adjustmentin embryo _b ceased at < –0.6 MPa, while the reductionin _H associated with priming proceeded down to at least –1.2MPa. Based on these data and on the pressure required to pushthe embryos from the seeds at various times after imbibition,it appears that the primary effect of priming was to shortenthe time required for final endosperm weakening to occur. However,as priming increased GR even in cut seeds, priming effects onthe embryo may control the rate of endosperm weakening. Key words: tomato, Lycopersicon esculentum Mill., water potential, germination rate, seed priming, genetic variation