Desensitization by red and blue light of phototropism in maize coleoptiles

The effects of pretreatments with red and blue light (RL, BL) on the fluence-response curve for the phototropism induced by a BL pulse (first positive curvature) were investigated with darkadapted maize (Zea mays L.) coleoptiles. A pulse of RL, giving a fluence sufficient to saturate phytochrome-mediated responses in this material, shifted the bell-shaped phototropic fluence-response curve to higher fluences and increased its peak height. A pulse of high-fluence BL given immediately prior to this RL treatment temporarily suppressed the phototropic fluence-response curve, and shifted the curve to higher fluences than induced by RL alone. The shift by BL progressed rapidly compared to that by RL. The results indicate (1) that first positive curvature is desensitized by both phytochrome and a BL system, (2) that desensitization by BL occurs with respect to both the maximal response and the quantum efficiency, and (3) that the desensitization responses mediated by phytochrome and the BL system can be induced simultaneously but develop following different kinetics. It is suggested that theses desensitization responses contribute to the induction of second positive curvature, a response induced by prolonged irradiation.Abbreviations BL blue light - RL red light CIW-DPB Publication No. 1001     

Phototropism of maize coleoptiles Influences of light gradients

The lateral fluence-rate gradients in unilaterally irradiated maize (Zea mays L.) coleoptiles were calculated on the basis of the proportions of P _fr (far-red-absorbing form of phytochrome) measured spectroscopically in transverse slices of the coleoptiles (top 1 cm). The results showed the occurrence of significant gradients that are wavelength-dependent. The gradient at 449 nm was steeper than those measured at 516, 534 and 551 nm, which were steeper than that measured at 665 nm. The ratios between the sides proximal and distal to the light source were, for example, 1:0.12 (449 nm), 1:0.23 (534 nm), and 1:0.28 (665 nm). Fluence-response curves for coleoptile phototropism (first positive curvature produced by less than 100 s unilateral irradiation) were measured at 449, 516, 534 and 551 nm. Comparison of the threshold fluences indicated that the responsiveness to 551 nm is about 10^4.8 less than that to 449 nm. Increasing wavelengths led to a decrease in maximal curvature, which correlated with the decrease of the fluence-rate ratios between the proximal and distal sides. Phototropic fluence-response curves were also measured using bilateral irradiation (449 nm). In one set of experiments, the fluence ratio was kept constant (either 1:1/2, 1:1/4 or 1:1/16) and the total fluence was varied, and in the other set the fluence applied to one side was kept constant and the fluence ratio was varied. A simple model based on the assumption that only one photoreaction occurs, and that the response is a function of the difference between the proximal and distal sides in the local photoreceptor action was tested. A fluence-response curve for this local photoreceptor action was calculated based on the fluence-rate ratio and the phototropic fluence-response curve measured for 449 nm. This curve was used, in conjunction with the measured fluence-rate ratios, as a basis for calculating phototropic fluence-response curves for other wavelengths and those for 449 nm obtained with bilateral irradiation. The calculated fluence-response curves showed excellent agreement with the experimental data. It is concluded that the threshold for maize coleoptile phototropism reflects the apparent photoconversion cross-section of the blue-light receptor whereas the maximal curvature depends on the steepness of the light gradient across the coleoptile.Abbreviations and symbols I(x) fluence rate at the depth x - P _fr phytochrome (far-red absorbing) - P _r phytochrome (red absorbing) - P _tot total phytochrome (P _r+P _fr) - photoconversion cross-section     

Irradiance-dependent regulation of gravitropism by red light in protonemata of the moss Ceratodon purpureus

Apical cells of protonemata of the moss Ceratodon purpureus (Hedw.) Brid. are negatively gravitropic in the dark and positively phototropic in red light. Various fluence rates of unilateral red light were tested to determine whether both tropisms operate simultaneously. At irradiances ≥140 nmol m⁻² s⁻¹ no gravitropism could be detected and phototropism predominated, despite the presence of amyloplast sedimentation. Gravitropism occurred at irradiances lower than 140 nmol m⁻² s⁻¹ with most cells oriented above the horizontal but not upright. At these low fluence rates, phototropism was indistinct at 1 g but apparent in microgravity, indicating that gravitropism and phototropism compete at 1 g. The frequency of protonemata that were negatively phototropic varied with the fluence rate and the duration of illumination, as well as with the position of the apical cell before illumination. These data show that the fluence rate of red light regulates whether gravitropism is allowed or completely repressed, and that it influences the polarity of phototropism and the extent to which apical cells are aligned in the light path. Received: 19 January 1999 / Accepted: 19 March 1999     

Negative phototropism of rice root and its influencing factors

Some characteristics of the rice (Oryza sativa L.) root were found in the experiment of unilaterally irradiating the roots which were planted in water: (i) All the seminal roots, adventitious roots and their branched roots bent away from light, and their curvatures ranged from 25° to 60°. The curvature of adventitious root of the higher node was often larger than that of the lower node, and even larger than that of the seminal root, (ii) The negative phototropic bending of the rice root was mainly due to the larger growth increment of root-tip cells of the irradiated side compared with that of the shaded side, (iii) Root cap was the site of light perception. If root cap was shaded while the root was irradiated the root showed no negative phototropism, and the root lost the characteristic of negative phototropism when root cap was divested. Rice root could resume the characteristic of negative phototropism when the new root cap grew up, if the original cells of root cap were well protected while root cap was divested, (iv) The growth increment and curvature of rice root were both influenced by light intensity. Within the range of 0–100 μmol · m² -s⁻¹, the increasing of light intensity resulted in the decreasing of the growth increment and the increasing of the curvature of rice root, (v) The growth increment and the curvature reached the maximum at 30°C with the temperature treatment of 10–40°C. (vi) Blue-violet light could prominently induce the negative phototropism of rice root, while red light had no such effect. (vii) The auxin (IAA) in the solution, as a very prominent influencing factor, inhibited the growth, the negative phototropism and the gravitropism of rice root when the concentration of IAA increased. The response of negative phototropism of rice root disappeared when the concentration of IAA was above 10 mg · L⁻¹     

NADH-stimulated, cyanide-resistant superoxide production in maize coleoptiles analyzed with a tetrazolium-based assay

 Using the tetrazolium salt XTT (Na,3′-[(phenylamino)-carbonyl]-3,4-tetrazolium]-bis(4-methoxy-6-nitro)benzenesulfonic acid hydrate) as a sensitive and physiologically compatible probe for the determination of superoxide (O₂ ^·−) production in vivo, we have shown that maize (Zea mays L.) coleoptiles possess the capacity of generating O₂ ^·− in the apoplastic space. Our results are in agreement with the notion that this activity is localized at the plasma membrane and can be attributed to an O₂ ^·−-synthesizing enzyme with catalytic and kinetic properties similar to that of the NADPH oxidase of mammalian phagocytes, with the important exception that it utilizes NADH instead of NADPH as electron donor. When applied to the apoplastic space, NADH strongly increased the O₂ ^·−-producing activity of coleoptiles. The maize NADH-dependent O₂ ^·−-synthase activity could clearly be differentiated from peroxidase-mediated O₂ ^·−-synthesizing activity by its insensitivity to cyanide and azide, as well as by its much higher affinity to O₂. Formation of O₂ ^·−, and concomitantly appearing H₂O₂, was preferentially localized in the outer epidermis of the coleoptile. The physiological significance of O₂ ^·− and H₂O₂ production in relation to the growth-controlling function of the epidermal cell wall is discussed. Received: 14 January 2000 / Accepted: 2 May 2000     

Blue-light-induced shift of the phototropic fluence-response curve in Pilobolus sporangiophores

The effects of preirradiation with blue light on the shift of the fluence-response curve for the first and the second positive curvatures were examined in Pilobolus crystallinus (Wiggers) Tode sporangiophores. A 1-min preirradiation with blue light at 47 or 960 nmol·m^-2 lowered the fluence-response curve for the first positive curvature and shifted the peak to a higher fluence. The fluence-response curve was shifted back to a lower fluence when a dark period was inserted between the preirradiation and the curvature-inducing light. This shift back to lower fluence was biphasic when the fluence was high (960 nmol · m^-2), indicating the participation of two components in the phototropic reaction for the first positive curvature.The fluence-response curve for the second positive curvature did not seem to be shifted to a higher fluence region when fluence was varied by varying exposure time. However, the fluence-response curve obtained by varying the fluence rate of a 20-min irradiation period indicated that the second positive curvature was also shifted to a higher-fluence region by preirradiation with blue light. A small shoulder appeared on the fluence-response curve when preirradiation at a high fluence rate was given.Abbreviations BL blue light - CIL curvature-inducing light     

Photogravitropic equilibrium in <Emphasis Type="Italic">Avena</Emphasis> coleoptiles: fluence rate–response relationships and dependence on dark adaptation and clinostating

Phototropism of Avena coleoptiles was measured in response to blue-light irradiation lasting between 2 and 24 h. During this time the coleoptiles established a bending angle of photogravitropic equilibrium that was dependent on the time of irradiation and also on the pretreatment in light or darkness prior to stimulation. The absolute threshold for the photogravitropic equilibrium in response to blue light was 10⁻⁸ μmol m⁻² s⁻¹. Photon fluence rate–response curves, which were generated after several hours of dark adaptation, had a characteristic shape with a prominent optimum in the middle of the dynamic range. Curves which were generated without prior dark adaptation displayed no such optimum. Clinostating dark-adapted coleoptiles caused an increase of sensitivity and responsiveness during a 2-h period of unilateral irradiation. The advantages and the drawbacks of long-term irradiation experiments for the investigation of phototropism and the generation of action spectra are discussed. Received: May 14, 2001 / Accepted: December 7, 2001     

Through-flow of water in leaves of a submerged plant is influenced by the apical opening

Submerged aquatic higher plants maintain acropetal water transport to the young leaves in active growth to satisfy their demand for nutrients and hormones derived from the roots. We here present the first measurements of hydraulic properties for a submerged plant, the monocotyledon Sparganium emersum Rehman. The hydraulic conductance per unit length, K_h, was measured in leaf segments without the leaf tip and shown to be greater in old, fully developed leaves (1.5 · 10⁻¹⁰ · m⁴ · MPa⁻¹ · s⁻¹) than in young leaves (1.0 · 10⁻¹⁰ · m⁴ · MPa⁻¹ · s⁻¹). In leaves with intact leaf tips, however, K_h was significantly greater in the youngest leaves, which suggests that the leaf tip with the hydathode influences resistance and thus flow. Microscopy confirmed that the hydathodal area, which is an apical opening, undergoes structural changes with leaf age; a matrix of microorganisms develops in the older leaves and probably restricts water flow by clogging the hydathodes. The leaf specific conductivity expressing transport capacity relative to the leaf area supplied, of S. emersum (0.1 · 10⁻⁸ to 9 · 10⁻⁸ · m² MPa⁻¹· s⁻¹) was within the same range as for various species of terrestrial ferns, vines and trees. This finding does not support the traditional concept of functionally reduced vascular transport in Received: 15 July 1996 / Accepted: 30 November 1996     

Growth distribution during first positive phototropic curvature of maize coleoptiles*

Abastract Measurements of growth increments on the shaded and the irradiated sides of phototropically stimulated maize (Zea mays L.) coleoptiles, obtained over the entire fluence range of the first positive curvature, indicate that the curvature is induced by growth stimulation on the shaded side and compensating inhibition on the irradiated side (length increments on the coleoptile flanks were determined 100 min after 30 s phototropic induction with blue light). At high fluences of blue light, overall stimulation of growth takes place, but this tendency is largely eliminated when only the tip of the coleoptile is irradiated. Time courses for growth increments obtained for the maximum first positive response show that the growth stimulation on the shaded side and the growth inhibition on the irradiated side commence almost simultaneously 20-30 min after the phototropic induction. The growth on the irradiated side almost ceases, but the growth rate on the shaded side is doubled, relative to the control rate. The onset of differential growth migrates basipetally from the tip at a velocity similar to that for polar auxin transport. The first positive phototropic response of the coleoptile is concluded to be the consequence of lateral redistribution of growth, which is not necessarily accompanied by changes in the net growth. The results are consonant with the Cholodny-Went theory of tropisms, in which lateral redistribution of auxin is considered to be the cause of tropic responses.     

The tropic response of plant roots to oxygen: oxytropism in Pisum sativum L.

Plant roots are known to orient growth through the soil by gravitropism, hydrotropism, and thigmotropism. Recent observations of plant roots that developed in a microgravity environment in space suggested that plant roots may also orient their growth toward oxygen (oxytropism). Using garden pea (Pisum sativum L. cv. Weibul's Apollo) and an agravitropic mutant (cv. Ageotropum), root oxytropism was studied in the controlled environment of a microrhizotron. A series of channels in the microrhizotron allowed establishment of an oxygen gradient of 0.8 mmol · mol⁻¹ · mm⁻¹. Curvature of seedling roots was determined prior to freezing the roots for subsequent spectrophotometric determinations of alcohol dehydrogenase activity. Oxytropic curvature was observed all along the gradient in both cultivars of pea. The normal gravitropic cultivar showed a maximal curvature of 45° after 48 h, while the agravitropic mutant curved to 90°. In each cultivar, the amount of curvature declined as the oxygen concentration decreased, and was linearly related to the root elongation rate. Since oxytropic curvature occurred in roots exposed to oxygen concentrations that were not low enough to induce the hypoxically responsive protein alcohol dehydrogenase, we suspect that the oxygen sensor associated with oxytropism does not control the induction of hypoxic metabolism. Our results indicate that oxygen can play a critical role in determining root orientation as well as impacting root metabolic status. Oxytropism allows roots to avoid oxygen-deprived soil strata and may also be the basis of an auto-avoidance mechanism, decreasing the competition between roots for water and nutrients as well as oxygen. Received: 14 January 1998 / Accepted: 10 February 1998     

Leaf photosynthesis, plant growth and nitrogen allocation in rice under different irradiances

The photosynthetic rates and various components of photosynthesis including ribulose-1,5-bisphosphate carboxylase (Rubisco; EC 4.1.1.39), chlorophyll (Chl), cytochrome (Cyt) f, and coupling factor 1 (CF₁) contents, and sucrose-phosphate synthase (SPS; EC 2.4.1.14) activity were examined in young, fully expanded leaves of rice (Oryza sativa L.) grown hydroponically under two irradiances, namely, 1000 and 350 μmol quanta · m⁻² · s⁻¹, at three N concentrations. The light-saturated rate of photosynthesis measured at 1800 μmol · m⁻² · s⁻¹ was almost the same for a given leaf N content irrespective of growth irradiance. Similarly, Rubisco content and SPS activity were not different for the same leaf N content between irradiance treatments. In contrast, Chl content was significantly greater in the plants grown at 350 μmol · m⁻² · s⁻¹, whereas Cyt f and CF₁ contents tended to be slightly smaller. However, these changes were not substantial, as shown by the fact that the light-limited rate of photosynthesis measured at 350 μmol · m⁻² · s⁻¹ was the same or only a little higher in the plants grown at 350 μmol · m⁻² · s⁻¹ and that CO₂-saturated photosynthesis did not differ between irradiance treatments. These results indicate that growth-irradiance-dependent changes in N partitioning in a leaf were far from optimal with respect to N-use efficiency of photosynthesis. In spite of the difference in growth irradiance, the relative growth rate of the whole plant did not differ between the treatments because there was an increase in the leaf area ratio in the low-irradiance-grown plants. This increase was associated with the preferential N-investment in leaf blades and the extremely low accumulation of starch and sucrose in leaf blades and sheaths, allowing a more efficient use of the fixed carbon. Thus, morphogenic responses at the whole-plant level may be more important for plants as an adaptation strategy to light environments than a response of N partitioning at the level of a single leaf. Received: 23 February 1997 / Accepted: 8 May 1997     

Gravitropism of maize and rice coleoptiles: dependence on the stimulation angle

Gravitropism of maize and rice coleoptiles was investigated with respect to its dependence on the angle of displacement or the initial stimulation angle (ISA). Close examination of curvature kinetics and the response to a drop in stimulation angle (SA) indicated that the gravtropic response during an early but substantial part of the curvature development is directly related to the ISA, there being no effect of the reduction of SA resulting from the curvature response itself. On the basis of this finding, the relationship between the steady SA and the curvature rate was determined. In maize, the curvature rate increased linearly with the sines of SAs up to an SA of 90 degrees. Rice coleoptiles, however, showed a saturation curve in the same range of SAs. The saturation profile was nearly identical between coleoptiles grown in air and those submerged in water, although the latter elongated much faster. Rice coleoptiles appeared to be far more sensitive to gravity than maize coleoptiles. It is concluded that the sensitivity to gravity, assessed through dependence on ISA, is a property inherent to a given gravitropic organ. Long-term measurements of curvature indicated that the coleoptiles bend back past the vertical. This overshooting was marked in submerged rice coleoptiles.     

Effects of red light on the fluence–response relationship for pulse-induced phototropism of maize coleoptiles

Dark-adapted coleoptiles of maize (Zea mays L.) were treated with red light (3min at 10.5 μmol m^?2S^?1) and were Stimulated, after a dark interval, with a pulse of unilateral blue light to induce phototropism. Phototropic fluence-response curves were obtained in this way for different dark intervals. It was confirmed that the bell-shaped fluence-response curve for the first pulse-induced positive phototropism (FPIPP) shifts to higher fluences following the red-light treatment, the maximal shift being achieved at a dark interval of 2h. We found, however, that the two arms of the Fluence-response curve do not shift synchronously. The shift of the descending arm to higher fluences began at 15 min. The ascending arm showed a slight shift to lower fluences before a greater shift to higher flucnces. the change of the shift direction occurring at 30–40min. Accordingly, the fluence-response curve obtained for a 30 min dark interval was comparatively wide. Although dark-adapted coleoptiles showed only fPIPP, another bell-shaped fluence-response curve, representing the second pulse-induced positive phototropism (sPIPP), appeared gradually after the red-light treatment. These changes of the phototropic fluence– respnse curve following exposure to red light are likely to have adaptive values because they favour phototropism under brighter light.     

Failure to obtain a unique threshold on the blood lactate concentration curve during exercise

The purpose of this study was to compare various methods and criteria used to identify the anaerobic threshold (AT), and to correlate the AT obtained with each other and with running performance. Furthermore, a number of additional points throughout the entire range of lactate concentrations [La⁻] were obtained and correlated with performance. A group of 19 runners [mean age 33.7 (SD 9.6) years, height 173 (SD 6.3) cm, body mass 68.3 (SD 5.4) kg, maximal O₂ uptake (V˙O_{2 max} ) 55.2 (SD 5.9) ml · kg⁻¹ · min⁻¹] performed a maximal multistage treadmill test (1 km · h⁻¹ every 3.5 min) with blood sampling at the end of each stage while running. All AT points selected (visual [La⁻], 4 mmol · l⁻¹ [La⁻], 1 mmol · l⁻¹ above baseline, log-log breakpoint, and 45° tangent to the exponential regression) were highly correlated one with another and with performance (r > 0.90) even when there were many differences among the AT (P < 0.05). The additional points (ranging from 3 to 8 mmol · l⁻¹ [La⁻], 1 to 6 mmol · l⁻¹ [La⁻] above the baseline, and 30 to 70° tangent to the exponential curve of [La⁻]) were also highly correlated with performance (r > 0.90). These results failed to demonstrate a distinct AT because many points of the curve provided similar information. Intercorrelations and correlations between AT and performance were, however, reduced when AT were expressed as the percentage of maximal treadmill speed obtained at AT or percentage of V˙O_{2 max} . This would indicate that different attributes of aerobic performance (i.e. maximal aerobic power, running economy and endurance) are measured when manipulating units. Thus, coaches should be aware of these results when they prescribe an intensity for training and concentrate more on the physiological consequences of a chosen [La⁻] rather than on a “threshold”. Accepted: 22 October 1997     

Phototropic stimulation does not induce unequal distribution of indole-3-acetic acid in maize coleoptiles

Distribution of endogenous diffusible auxin into agar blocks from phototropically stimulated maize coleoptile tips was studied using a bioassay and a physicochemical assay, to clarify whether phototropism in maize coleoptiles involves a lateral gradient in the amount of auxin. At 50 min after the onset of phototropic stimulation, when the phototropic response was still developing, direct assay of the blocks with the Avena curvature test showed that the auxin activity in the blocks from the shaded half-tips was twice that of the lighted side, at both the first and second positive phototropic curvatures. However, physicochemical determination following purification showed that the amount of indole-3-acetic acid (IAA) was evenly distributed in the blocks from lighted and shaded coleoptile half-tips at both the first and second positive phototropic curvatures. The even distribution of the IAA was also confirmed with the Avena curvature test following purification by HPLC. These results indicate that phototropism in maize coleoptiles is not caused by a lateral gradient of IAA itself and thus cannot be described by the Cholodny-Went theory. Furthermore, the lower auxin activity in the blocks from the lighted half-tips suggests the presence of inhibitor(s) interfering with the action of auxin and their significant diffusion from unilaterally illuminated coleoptile tips.     

Phytochrome is required for the occurrence of time-dependent phototropism in maize coleoptiles

Time-dependent phototropism (TDP), sometimes called second positive curvature, occurs when the duration of phototropic stimulation with blue light (B) exceeds a few minutes. TDP was characterized in maize (Zea mays L.) coleoptiles raised under continuous red light (R). Subsequently, coleoptiles adapted to darkness were used to investigate the effect of R on TDP. It was found that TDP, which is induced in R-grown coleoptiles, does not occur in dark-adapted coleoptiles and that dark-adapted coleoptiles begin to show TDP after treatment with R. The TDP responsiveness became maximal 1-2 h after treatment with a R pulse and decreased during the next few hours. At least 10 min was required after a short pulse of R before the coleoptile began to respond to B for the induction of TDP. The effect of R in establishing the TDP responsiveness was totally suppressed by a pulse of far-red light given immediately after an inductive pulse of R. It is concluded that the mechanism of TDP requires for its establishment a R signal perceived by phytochrome. The TDP of R-grown and R-pretreated coleoptiles showed relationships to stimulation times and fluence rates that are similar to those reported for oat coleoptiles, except that TDP of maize showed a sharp increase in its magnitude within a narrow range of stimulation times as short as 5-10 min.     

Temperature dependence of habituation of the initial responses to cold-water immersion

The initial responses to cold-water immersion, evoked by stimulation of peripheral cold receptors, include tachycardia, a reflex inspiratory gasp and uncontrollable hyperventilation. When immersed naked, the maximum responses are initiated in water at 10°C, with smaller responses being observed following immersion in water at 15°C. Habituation of the initial responses can be achieved following repeated immersions, but the specificity of this response with regard to water temperature is not known. Thirteen healthy male volunteers were divided into a control (C) group (n = 5) and a habituation (H) group (n = 8). Each subject undertook two 3-min head-out immersions in water at 10°C wearing swimming trunks. These immersions took place at a corresponding time of day with 4 days separating the two immersions. In the intervening period the C group were not exposed to cold water, while the H group undertook another six, 3-min, head-out immersions in water at 15°C. Respiratory rate (f _R), inspiratory minute volume (V˙ _I) and heart rate (f _H) were measured continuously throughout each immersion. Following repeated immersions in water at 15°C, the f _R, V˙ _I and f _H responses of the H group over the first 30 s of immersion were reduced (P < 0.01) from 33.3 breaths · min⁻¹, 50.5 l · min⁻¹ and 114 beats · min⁻¹ respectively, to 19.8 breaths · min⁻¹, 26.4 l · min⁻¹ and 98 beats · min⁻¹, respectively. In water at 10°C these responses were reduced (P < 0.01) from 47.3 breaths · min⁻¹, 67.6 l · min⁻¹ and 128 beats · min⁻¹ to 24.0 breaths · min⁻¹, 29.5 l · min⁻¹ and 109 beats · min⁻¹, respectively over a corresponding period of immersion. Similar reductions were observed during the last 2.5 min of immersions. The initial responses of the C group were unchanged. It is concluded that habituation of the cold shock response can be achieved by immersion in warmer water than that for which protection is required. This suggests that repeated submaximal stimulation of the cutaneous cold receptors is sufficient to attenuate the responses to more maximal stimulation. Accepted: 6 February 1998     

On the relation between photo- and gravitropically induced spatial memory in maize coleoptiles

The interaction of photo- and gravitropic stimulation was studied by analysing the curvature of maize (Zea mays L.) coleoptiles subjected to rotation on horizontal clinostats. Gravitropic curvature in different directions with respect to the stimulation plane was found to be transient. This instability was caused by an increasing deviation of response direction from the stimulation plane towards the caryopsis. The bending angle as such, however, increased steadily. This reorientation of the gravitropic response towards the caryopsis is thought to be caused by the clinostat-elicited nastic curvature found in maize coleoptiles. In contrast, the response to phototropic stimulation was stable, in both, orientation and curving. Although stimulation by gravity was not capable of inducing a stable tropistic response, it could inhibit the response to opposing phototropic stimulation, if the counterstimulation was given more than 90 min after the onset of gravistimulation. For shorter time intervals the influence of the phototropic stimulus obscured the response to the first, gravitropic stimulation. For time intervals exceeding 90 min, however, the phototropic effects disappeared and the response was identical to that for gravity stimulation alone. This gravity-induced inhibition of the phototropic response was confined to the plane of gravity stimulation, because a phototropic stimulation in the perpendicular direction remained unaffected, irrespective of the time interval between the stimulations. This concerned not only the stable phototropic curving, but also the capacity of the phototropic induction to elicit a stable directional memory as described earlier (P. Nick and F. Schäfer, 1988b, Planta 175, 380–388). This was tested by a second bluelight pulse opposing the first. It is suggested that gravity, too, can induce a directional memory differing from the blue-light elicited memory. The mechanisms mediating gravi- and phototropic directional memories are thought to branch off the respective tropistic signal chains at a stage where photo- and gravitropic transduction are still separate.This work was supported by the Deutsche Forschungsgemeinschaft and a grant of the Studienstiftung des Deutschen Volkes to P. Nick.     

Red-light-induced positive phototropism in Arabidopsis roots

The interaction between light and gravity is critical in determining the final form of a plant. For example, the competing activities of gravitropism and phototropism can determine the final orientation of a stem or root. The results reported here indicate that, in addition to the previously described blue-light-dependent negative phototropic response in roots, roots of Arabidopsis thaliana (L.) Heynh. display a previously unknown red-light-dependent positive phototropic response. Both phototropic responses in roots are considerably weaker than the graviresponse, which often masks phototropic curvature. However, through the use of mutant strains with impaired gravitropism, we were able to identify a red-light-dependent positive phototropic response in Arabidopsis roots. The red-induced positive phototropic response is considerably weaker than the blue-light response and is barely detectable in plants with a normal gravitropic response. Received: 22 May 2000 / Accepted: 3 July 2000     

Effects of 3 days of carbohydrate supplementation on muscle glycogen content and utilisation during a 1-h cycling performance

This study compared the effects of supplementing the normal diets of six trained cyclists [maximal oxygen uptake O_2max) 4.5 (0.36)l · min⁻¹; values are mean (SD)] with additional carbohydrate (CHO) on muscle glycogen utilisation during a 1-h cycle time-trial (TT). Using a randomised crossover design, subjects consumed either their normal diet (NORM) for 3 days, which consisted of 426 (137) g · day⁻¹ CHO [5.9 (1.4) g · kg⁻¹ body mass (BM)], or additional CHO (SUPP) to increase their intake to 661 (76) g · day⁻¹ [9.3 (0.7) g · kg⁻¹ BM]. The SUPP diet elevated muscle glycogen content from 459 (83) to 565 (62) mmol · kg⁻¹ dry weight (d.w.) (P < 0.05). However, despite the increased pre-exercise muscle glycogen stores, there was no difference in the distance cycled during the TT [40.41 (1.44) vs 40.18 (1.76) km for NORM and SUPP, respectively]. With NORM, muscle glycogen declined from 459 (83) to 175 (64) mmol · kg⁻¹ d.w., whereas with SUPP the corresponding values were 565 (62) and 292 (113) mmol · kg⁻¹ d.w. Accordingly, both muscle glycogen utilisation [277 (64) vs 273 (114) mmol · kg⁻¹ d.w.] and total CHO oxidation [169 (20) vs 165 (30) g · h⁻¹ for NORM and SUPP, respectively] were similar. Neither were there any differences in plasma glucose or lactate concentrations during the two experimental trials. Plasma glucose concentration averaged 5.5 (0.5) and 5.6 (0.6) mmol · l⁻¹, while plasma lactate concentration averaged 4.4 (1.9) and 4.4 (2.3) mmol · l⁻¹ for NORM and SUPP, respectively. The results of this study show that when well-trained subjects increase the CHO content of their diet for 3 days from 6 to 9 g · kg⁻¹ BM there is only a modest increase in muscle glycogen content. Since supplementary CHO did not improve TT performance, we conclude that additional CHO provides no benefit to performance for athletes who compete in intense, continuous events lasting 1 h. Furthermore, the substantial muscle CHO reserves observed at the termination of exercise indicate that whole-muscle glycogen depletion does not determine fatigue at this exercise intensity and duration. Accepted: 25 November 1996