THE PHOTOTROPIC EXCITATION OF LIMAX

The photic orientation of Limax creeping geotropically upon a vertical plate is such that the phototropic vector determining the angular deflection β from the vertical path is proportional to log I. This is proved by the fact that with horizontal illumination tan β is directly proportional to log I; with non-horizontal light rays from a small source the ratio See PDF for Equation is directly proportional to log I (where A = the angle between light rays and the path of orientation), the vector diagram of the field of excitation being in this case not a right-angled triangle.     

ON THE EQUILIBRATION OF GEOTROPIC AND PHOTOTROPIC EXCITATIONS IN THE RAT

The intensity of light required to just counterbalance geotropic orientation of young rats, with eyelids unopened, is so related to the angle of inclination (α) of the creeping plane that the ratio log I/log sin α is constant. This relationship, and the statistical variability of I as measured at each value of α, may be deduced from the known phototropic and the geotropic conduct as studied separately, and affords proof that in the compounding of the two kinds of excitation the rat is behaving as a machine.     

GEOTROPIC ORIENTATION IN ARTHROPODS : III. THE FIDDLER CRAB UCA.

On an inclined surface the fiddler crab Uca pugnax, during sidewise progression, orients upward through an angle θ on the surface. The extent of negatively geotropic orientation (θ) is a rectilinear function of sin α, where α is the inclination of the surface to the horizontal. This equation differs from that describing the geotropic orientation of various other animals. The difference is traced to the fact that from an initial position with the transverse axis of the body horizontal the crab is required to turn upward to an extent such that the vertical line from its center of gravity pierces the inclined surface within the base of support provided by the legs. This leads to the equation sec θ/tan α = const., which is obeyed within the limits of precision of the measurements. This type of control of geotropic orientation represents an extension of the "muscle tension theory," and is in no sense in conflict with this view. The assumptions underlying the analytical expression connecting θ and α are verified by the asymmetry in the orientation of male fiddlers, which is shown to be due to the presence of the enlarged chela and which disappears when the claws are removed.     

GEOTROPISM OF AGRIOLIMAX

On an inclined glass plate the slug Agriolimax orients and creeps upward or downward. The angle of orientation on the plane (θ) is proportional to the logarithm of the component of gravity in the creeping plane. The coefficient of variability of the measured values of (θ) decreases linearly as the logarithm at the gravity component in the creeping plane increases. The cosine of the angle of orientation decreases almost directly in proportion to the sine of the angle of inclination of the creeping plane to the horizontal, as previously found for young rats (Crozier and Pincus). But a more satisfactory formulation for the present case shows that the sine of the angle of orientation (θ) decreases in direct proportion to the increase of the reciprocal of the sine of the angle of inclination of the creeping plane. This formulation is derived from the theory that the geotropic orientation is limited by the threshold difference between the pull of the body mass on the mutually inclined longitudinal muscles at the anterior end of the slug.     

THE GEOTROPIC RESPONSE IN ASTERINA

Upon a surface inclined at angle α Asterina gibbosa orients upward during negatively geotropic creeping until the average angle (θ) of the path is such that Δ sin θ/Δ sin α = const. This is true also in positively geotropic movement. The direction of orientation may be temporarily reversed by mechanical disturbance. The variation of θ is greater at low slopes. Tests with directed impressed pulls, due to an attached cork float, show that the pull upon the tube feet is of primary consequence for the determination of θ. When the component of gravitational pull in the direction of movement reaches a fraction of the total pull which is proportional to the gravitational vector parallel to the surface, the laterally acting component is ineffective. On this basis, it follows that Δ sin θ/Δ sin α = const.     

GEOTROPIC ORIENTATION IN ARTHROPODS : I. MALACOSOMA LARV?.

The geotropic orientation of caterpillars of Malacosoma americana during progression upon a surface inclined at angle α to the horizontal is such that the path makes an average angle θ upward on the plane, of a magnitude proportional to log sin α. More precisely, the product (sin α) (sin θ) is constant. This is traced to the fluctuation of the pull of the head region upon the lateral musculature of the upper side during the side to side swinging implicated in progression.     

ANALYSIS OF THE GEOTROPIC ORIENTATION OF YOUNG RATS. VIII

The upward geotropic orientation (angle θ) of adult rats (race A) has been measured as a function of slope of substratum. The relative variation of orientation angle is a declining rectilinear function of θ. The fraction of the total observable variation of performance (θ) which is controlled by the intensity of excitation (56 per cent) is identical with that found for young rats of the same strain, although the total variation is a little greater. Injection of adrenin distorts the θ vs. α graph in a manner quite concordant with the effect obtained in young rats. With the adults the absolute magnitudes of the variations of θ, at corresponding intensities of excitation, are not affected by the action of adrenin, and, as with the young, the proportion of modifiable variation of θ is not altered. The variability of performance, considered as a function of the performance, must therefore be regarded as an organic invariant. Certain consequences of this finding are referred to.     

THE ORIENTATION OF ANIMALS BY OPPOSED BEAMS OF LIGHT

When orientation is attained under the influence of beams of parallel light opposed at 180° the deflection θ from a path at right angles to the beams is given by tan See PDF for Equation, where I ₁ and I ₂ are the photic intensities and H is the average angle between the photoreceptive surfaces. This expression is independent of the units in which I is measured, and holds whether the primary photosensory effect is proportional to I or to log I. When photokinetic side-to-side motions of the head occur, H decreases with increasing total acting light intensity, but increases if higher total light intensity restricts the amplitude of random movements; in each case, H is very nearly proportional to log I ₁ I ₂. For beams of light at 90°, See PDF for Equation. The application of these equations to some particular instances is discussed, and it is shown why certain simpler empirical formulæ previously found by others yield fair concordance with the experimental data. The result is thus in complete accord with the tropism theory, since the equations are based simply on the assumption that when orientation is attained photic excitation is the same on the two sides.     

THE PHOTOCHEMICAL BASIS OF ANIMAL HELIOTROPISM

1. Experiments on the heliotropic orientation of Limulus were made which confirmed Loeb''s photochemical theory of animal heliotropism proposed first in 1888 and 1889 in experiments on insects, and later in experiments on other forms of animals. 2. It is shown that these animals are oriented by light in such a way that the product I x t x cos α is the same for the symmetrical photosensitive elements of the eyes or the skin, where I is the intensity of the light, t the duration of illumination, and α the angle of incidence of the light at the surface element of the photosensitive organ. 3. When this equation holds, the products of decomposition by light must be the same in symmetrical elements of the eyes or skin, and the influence of these products of decomposition on the tension of symmetrical muscles of the locomotor organs of the animal must be the same. As a consequence the animal must move in the path of light, either to or from the source of light.     

Photoregulation of beta-Tubulin mRNA Abundance in Etiolated Oat and Barley Seedlings

The effect of light on the abundance of β-tubulin mRNA was measured in etiolated Avena sativa L. and Hordeum vulgare L. seedlings. Slot blot analysis employing an oat β-tubulin cDNA clone was used to measure β-tubulin mRNA levels. White light induced a 45% decrease in oat β-tubulin mRNA abundance by 2 hours after transfer. A saturating red light pulse induced 40 and 55% decreases in β-tubulin mRNA levels in oats and barley, respectively. Recovery of β-tubulin mRNA levels was observed after a red light pulse but not after transfer to continuous white light. The red light induced decrease in oat β-tubulin mRNA abundance was not reversible by a subsequent far-red light treatment. The mesocotyl portion of etiolated oat seedlings exhibited a more dramatic decrease in β-tubulin mRNA abundance in response to red light than did the coleoptile portion. The results indicate that the well-documented effects of red light on the growth of etiolated seedlings are accompanied by changes in the expression of the β-tubulin genes.     

GEOTROPIC ORIENTATION IN ARTHROPODS : II. TETRAOPES.

The creeping of the beetle Tetraopes tetraopthalmus during negatively geotropic orientation shows the angles of orientation (θ) on a surface inclined at α° to the horizontal to be proportional to sin α. The direction of orientation easily suffers temporary reversal to positive as result of handling. Mechanical stability during upward progression should be just possible when K ₁ cot α = K ₂ sin θ + K ₃ cos θ, the weight of the body being supported on the tripod formed by the legs on either side and by the posterior tip of the abdomen. Lack of this stability produces tensions on the legs through (1) the bilaterally distributed pull of the body mass on the legs, and (2) the torque on the legs due to the weight of the abdomen. The downward gravitational displacement of the tip of the abdomen causes K ₂ and K ₃ in the preceding formula to be functions of α. These relations have been tested in detail by shifting the location of the center of gravity, by attaching additional masses anteriorly and posteriorly, and by decreasing the total load through amputation of the abdomen; the latter operation changes the conditions for stability. Different formulæ are thus obtained (cf. earlier papers) for the orientation of animals in which the mechanics of progression and the method of support of the body weight on an inclined surface are not the same. This demonstrates in a direct way that the respective empirical equations cannot be regarded as accidents. The results are in essence the same as that already obtained with young mammals. The diversity of equations required for the physically unlike cases merely strengthens the conception of geotropic orientation as limited by the tensions applied to the musculature of the body (caterpillars, slugs) or of appendages (beetles, and certain other forms) when the body is supported upon an inclined surface, since equations respectively pertaining to the several instances, and satisfactorily describing the observations, are deduced on this basis.     

Mode of Action of the Massively Accumulated beta-Carotene of Dunaliella bardawil in Protecting the Alga against Damage by Excess Irradiation

When grown under defined conditions Dunaliella bardawil accumulates a high concentration of β-carotene, which is composed primarily of two isomers, all-trans and 9-cis β-carotene. The high β-carotene alga is substantially resistant to photoinhibition of photosynthetic oxygen evolution when compared with low β-carotene D. bardawil or with Dunaliella salina which is incapable of accumulating β-carotene. Protection against photoinhibition in the high β-carotene D. bardawil is very strong when blue light is used as the photoinhibitory agent, intermediate with white light, and nonexistent with red light. These observations suggest that the massively accumulated β-carotene in D. bardawil protects the alga against damage by high irradiation by screening through absorption of the blue region of the spectrum. Irradiation of D. bardawil by high intensity blue light results in the following temporal sequence of events: photoinhibition of oxygen evolution, photodestruction of 9-cis β-carotene, photodestruction of all-trans β-carotene, photodestruction of chlorophyll and cell death.     

Cell Wall and Cytoplasmic Isozymes of Radish beta-Fructosidase Have Different N-Linked Oligosaccharides

When 36-hour-old dark grown radish seedlings are transferred to far-red light, there is a decrease in cytoplasmic β-fructosidase (βF) and an increase in cell wall βF compared to the dark controls. Cytoplasmic and cell wall-bound β-fructosidase are both glycoproteins and exhibit high antigenic similarities, but differ according to charge heterogeneity and carbohydrate microheterogeneity. Growth of radish seedlings in the presence of tunicamycin results in a partial inhibition of βF glycosylation but nonglycosylated βF still accumulates in the cell wall under far-red light. Thus, glycosylation is not necessary for intracellular transport, for correct targetting, or for wall association of an active βF. The nonglycosylated cytoplasmic and cell wall βF forms have the same relative molecular mass but glycosylated forms have different oligosaccharide side-chains, with respect to size and susceptibility to α-mannosidase and endoglycosidase D digestion. The oligosaccharides of both forms are partly removed by endoglycosidase H when βF is denatured. Isoelectric focusing analysis of βF shows that the cell wall-associated isozymes are more basic than the cytoplasmic isozymes, and that the charge heterogeneity also exists within a single plant. A time course of changes in βF zymograms shows a far red light stimulation of the appearance of the basic forms of the enzyme. However, the more basic cell wall specific βF forms are not present when N-glycosylation is prevented with tunicamycin. These results indicate that cytoplasmic and cell wall βF probably have common precursor polypeptides and basic cell wall forms arise via processing events which are tunicamycin sensitive.     

ON THE GEOTROPIC ORIENTATION OF HELIX

The snail Helix nemoralis in negatively geotropic creeping orients upward upon an inclined surface until the angle of the path of progression (θ) is related to the tilt of the surface (α) as (Δ sin θ) (Δ sin α) = – const.; θ is very nearly a rectilinear function of log sin α. The precision of orientation (P.E._θ) declines in proportion to increasing sin α, P.E._θ/^θ in proportion to θ. These facts are comprehensible only in terms of the view that the limitation of orientation is controlled by the sensorial equivalence of impressed tensions in the anterior musculature.     

Steady-State Phototropism in Phycomyces

The steady-state phototropic bending of Phycomyces sporangiophores was studied using apparatus designed to keep the growing zone vertical and the angle of illumination constant over long periods of time. The bending speed is quite constant if the intensity and angle of illumination are fixed. A phototropic inversion occurs in response to a sudden change in intensity, either an increase or a decrease. A bending component lateral to the illumination direction is strongly evident at normal incidence. It is shown that this component is due to a rotation between the stimulus and response loci about the axis of the growing zone, which is probably related to the spiral growth of the cell. The steady-state bending speed is at a maximum value for illumination directions ranging from normal incidence to about 45°. From 45 to 14° the bending speed decreases linearly with angle, reaching zero at 14°. Angles less than 14° elicit a weak negative phototropic response. Using an optical model of the growing zone, the intracellular intensity distribution was determined as a function of the angle of illumination. Several hypotheses relating the intensity distribution to the phototropic response are discussed.     

Stereoisomers of beta-Carotene and Phytoene in the Alga Dunaliella bardawil

Dunaliella bardawil, a halotolerant green alga, was previously shown to accumulate high concentrations of β-carotene when grown outdoors under defined conditions. The β-carotene of algae cultivated under high light intensity in media containing a high salt concentration is composed of approximately 50% all-trans β-carotene and 40% 9-cis β-carotene. We show here that the 9-cis to all-trans ratio is proportional to the integral light intensity to which the algae are exposed during a division cycle. In cells grown under a continuous white light of 2000 microeinsteins per square meter per second, the ratio reached a value of around 1.5, while in cells grown under a light intensity of 50 microeinsteins per square meter per second, the ratio was around 0.2. As previously shown, algae treated with the herbicide norflurazon accumulate phytoene in place of β-carotene. Electron micrographs showed that the phytoene is accumulated in many distinct globules located in the interthylakoid spaces of the chloroplast. Here too, two isomers are present, apparently all-trans and 9-cis phytoene, and their ratio is dependent upon the integral light intensity to which the algae are exposed during a division cycle. In the presence of norflurazon, Dunaliella bardawil grown under a light intensity of 2000 microeinsteins per square meter per second contained about 8% phytoene with a 9-cis to all-trans ratio of about 1.0. This ratio decreased to about 0.1 when the light intensity was reduced to 50 microeinsteins per square meter per second. These data suggest that the isomerization reaction which leads to the production of the 9-cis isomer occurs early in the path of carotene biosynthesis, at or before the formation of all-trans phytoene. The presence of the 9-cis isomer of β-carotene and the dependence of its preponderance on light intensity seem to be a common feature of many plant parts. Thus carrots which are exposed to minimal light contain no 9-cis isomer while sun-exposed leaves, fruits, and flowers contain 20 to 50% of the 9-cis isomer.     

Ecological Consequences of Long-Term Exposure of Anabaena variabilis (Cyanophyceae) to Shifts in Environmental Factors

Cultures of Anabaena variabilis were exposed to different light intensities, and the time course of photoadaptation was measured by photosynthetic rate and changes in pigmentation. A shift down in intensity of 284 μEin · m⁻² · sec⁻¹ caused a temporary decrease in the photosynthetic response followed by gradual adaptation to the new conditions. Final chlorophyll a and carotenoid concentrations were reached after 1 day, although other physiological indicators showed that adaptation required 4 days. The parameter I_k was shown to be the best indicator of photoadaptation. A shift up in light intensity of the same magnitude also required 4 days for complete photoadaptation by the culture, although chlorophyll and carotenoid concentrations stabilized within 1 day. A shift down in light intensity of 392 μEin · m⁻² · sec⁻¹ resulted in a temporary attempt to adapt followed by collapse of the population. This demonstrates an apparent threshold in the magnitude of the shift in light intensity which will permit successful adaptation. Simultaneous changes in light intensity and temperature also adversely affected culture populations. Our observations present a possible cause for the decline or prevention of an algal bloom under a fluctuating light regime and suggest that it may be possible to predict this decline as a result of synoptic weather patterns or hydrodynamic influences.     

Formation of Dynamic Soluble Surfactant-induced Amyloid β Peptide Aggregation Intermediates

Intermediate amyloidogenic states along the amyloid β peptide (Aβ) aggregation pathway have been shown to be linked to neurotoxicity. To shed more light on the different structures that may arise during Aβ aggregation, we here investigate surfactant-induced Aβ aggregation. This process leads to co-aggregates featuring a β-structure motif that is characteristic for mature amyloid-like structures. Surfactants induce secondary structure in Aβ in a concentration-dependent manner, from predominantly random coil at low surfactant concentration, via β-structure to the fully formed α-helical state at high surfactant concentration. The β-rich state is the most aggregation-prone as monitored by thioflavin T fluorescence. Small angle x-ray scattering reveals initial globular structures of surfactant-Aβ co-aggregated oligomers and formation of elongated fibrils during a slow aggregation process. Alongside this slow (minutes to hours time scale) fibrillation process, much faster dynamic exchange (k_ex ∼1100 s⁻¹) takes place between free and co-aggregate-bound peptide. The two hydrophobic segments of the peptide are directly involved in the chemical exchange and interact with the hydrophobic part of the co-aggregates. Our findings suggest a model for surfactant-induced aggregation where free peptide and surfactant initially co-aggregate to dynamic globular oligomers and eventually form elongated fibrils. When interacting with β-structure promoting substances, such as surfactants, Aβ is kinetically driven toward an aggregation-prone state.     

On the Factors Which Determine Massive beta-Carotene Accumulation in the Halotolerant Alga Dunaliella bardawil

Dunaliella bardawil, a β-carotene-accumulating halotolerant alga, has been analyzed for the effect of various growth conditions on its pigment content, and compared with Dunaliella salina, a β-carotene nonaccumulating species. In D. bardawil, increasing light intensity and light period or inhibiting growth by various stress conditions such as nutrient deficiency or high salt concentration caused a decrease in the content of chlorophyll per cell and an increase in the amount of β-carotene per cell. As a result, the β-carotene-to-chlorophyll ratio increased from about 0.4 to 13 grams per gram and the alga changed its visual appearance from green to deep orange. D. salina grown similarly decreased in content of both chlorophyll and β-carotene per cell and the culture turned from green to yellowish. Low chlorophyll-containing cells of D. bardawil or D. salina exhibit very high photosynthetic rates when expressed on a chlorophyll basis (~600 micromoles O₂ evolved per milligram chlorophyll per hour).

Variation of pigment content in D. bardawil by a large variety of environmental agents has been correlated with the integral irradiance received by the algal culture during a division cycle. The higher the integral irradiance per division cycle, the lower the chlorophyll content per cell; the higher the β-carotene content per cell, and therefore the higher the β-carotene-to-chlorophyll ratio. The results are interpreted as indicating a protecting effect of β-carotene against injury by high irradiance under conditions of impairment in chlorophyll content per cell.

     

PHOTOTROPISM OF DIXIPPUS MOROSUS

1. Local differences in the effects of stimulation of parts of the eye by light are expressed in Dixippus morosus by differential circus movements. 2. The angle of inclination of the body axis toward one source of light when the animal is on a vertical plane with light from one side is inversely proportional to the logarithm of the intensity of the light.