Perception et réaction géotropiques de l'épicotyle d'Asparagus officinalis

Geoperception and georeaction in Asparagus officinalis epicotyls The location and the role of the statenchyma were investigated in Asparagus officinalis epicotyls 1, 2 or 3 cm in length. The statocytes represent a monocellular sheath which comprises the outer layer of the pericycle. Three phases of differentiation (S₁, S₂, S₃) of the statocytes may be distinguished by criteria of cell length, the state of vacuolisation and the sedimentation of amyloplasts and nuclei with respect to gravity. The statenchyma was found to be located only in the first 10 mm of the epicotyl 1, 2 or 3 cm in length; in effect the oldest statocytes (S₃) differentiate into sclerenchyma beneath this level. The geotropic curvature of the epicotyls begins with a positive (downward) curvature and the negative (upward) response is only realized after 15–30 min of geotropic stimulation. The upper limit of the bending is located in the zone containing the mature statocytes (S₃), while the lower limit is located much closer to the base of the epicotyl. Experiments on decapitation of the epicotyls have indicated that the S₁ and S₂ statocytes do not play an important role in the perception of gravity. However, with S₃ statocytes, it has been demonstrated that there is a statistical correlation between the initial rate of curvature of the epicotyls and the length of their sheaths. Thus, the longer the sheath, the more the epicotyls curve. From the results obtained it is proposed that the statocytes are responsible for the geotropic perception of Asparagus officinalis epicotyls.     

CALCIUM MOVEMENT,GRAVIRESPONSIVENESS, AND THE STRUCTURE OF COLUMELLA CELLS IN PRIMARY ROOTS OF AMYLOMAIZE MUTANTS OF ZEA MAYS

Primary roots of Zea mays cv. Amylomaize were less graviresponsive than primary roots of the wild-type Calumet cultivar. There were no significant differences in: 1) the partitioning of volume to organelles in columella cells, 2) the size or density of amyloplasts, or 3) rates and overall patterns of organelle redistribution in horizontally-oriented roots of the two cultivars. Amyloplasts and nuclei were the only organelles whose movement correlated positively with the onset of root gravicurvature. However, the onset of gravicurvature was not directly proportional to the average sedimentation rate of amyloplasts, since amyloplasts sedimented at equal rates in columella cells of both cultivars despite their differences in root gravicurvature. The more graviresponsive roots of Calumet seedlings were characterized by a more strongly polar movement of ⁴⁵Ca²⁺ from the upper to lower sides of their root tips than the less graviresponsive roots of Amylomaize seedlings. These results suggest that the decreased graviresponsiveness of horizontally-oriented roots of Amylomaize seedlings may be due to a delay in or decreased ability for polar transport of calcium rather than to smaller, more slowly sedimenting amyloplasts as has been suggested for their less graviresponsive coleoptiles.     

The Role of Starch in the Gravitropic Response of the Lentil Root

It is well accepted that the amyloplasts of the cap are responsible for gravisensing in primary roots. However, roots with starch-depleted plastids are able to respond to gravistimulus, but their curvature is slower than that of roots containing amyloplasts. The goal of our experiment was to analyse the effects of natural variations of statolith starch in the gravitropic response of lentil roots to a stimulation in the horizontal position. In lentil seedlings grown in the vertical position for 26 h, the volume of the amyloplasts in the statocytes differed between individual roots. The amount of starch in the cap was determined parallel to the rate of gravitropic curvature. There was no statistical correlation between the intensity of the gravitropic response and the starch content in the statocytes. Lentil roots were treated with gibberellic acid (GA₃) at 32°C in order to reduce the volume of starch in the statoliths. There was 53% less starch in the cap of GA₃treated roots as compared to the cap of control roots. But there was no relationship between starch content in the cap and the responsiveness of the root to a gravistimulus, except when the amount of starch was small.     

A Morphometric Analysis of the Ultrastructure of Columella Statocytes in Primary Roots of Zea mays L.

A morphometric analysis of the ultrastructure of columella statocytesin primary roots of Zea mays was performed to determine theprecise location of cellular organelles in graviperceptive cells.Vacuoles occupy the largest volume in the cell (11.4 per centof the protoplasm). The nucleus (9.51 per cent), amyloplasts(7.57 per cent), mitochondria (3.42 per cent), spherosomes (2.13per cent) and dictyosomes (0.55 per cent) occupy progressivelysmaller volumes of the statocytes. All organelles are distributedasymmetrically within the cell. Amyloplasts, spherosomes anddictyosomes are found in greatest numbers (and relative volumes)in the lower (i.e. ‘bottom’) third of the cell.The largest numbers and relative volumes of mitochondria arein the lower and middle thirds of the cell. Nuclei tend to befound in the middle third of the statocytes. Only the hyaloplasmis concentrated in the upper (i.e. ‘top’) thirdof Z. mays statocytes. When the sedimentation of amyloplasts(and the resulting exclusion of other organelles from the lowerthird of the cell) is corrected for, all cellular constituentsremain asymmetrically distributed within the cell. Therefore,the sedimentation of amyloplasts alone is not responsible forthe differential distribution of other cellular organelles inZ. mays statocytes. The quantitative ultrastructure of Z. maysstatocytes is discussed relative to the graviperceptive functionof these cells. Zea mays, corn, maize, root cap, stereology, columella, statocytes, graviperception, ultrastructure     

Dorsiventralität der Statocyten in plagiogeotropen Seitenwurzeln von Lepidium sativum L.

Summary The calyptra of plagiotropic lateral roots of Lepidium sativum L. is composed of three rows of cells. Movable amyloplates, possibly functioning as statoliths, are located only a few central cells of the ontogenetic youngest cell row. Beside the lateral root axis the two innermost statocytes contain a stable complex of rough endoplasmic reticulum, which is preferentially located in the central distal cell corner. In the statocytes lying above the lateral root axis the amyloplasts are sedimented on the ER-complex during growth in direction of the geotropic liminal angle. In the statocyte below the axis the ER-complex is free of amyloplasts. Thus a dorsiventrality exists in the statocytes located above and below the root axis in regard to the arrangement of their organelles.

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Herrn Professor Dr. Maximilian Steiner zum 70. Geburtstag.     

Graviperception of lentil seedling roots grown in space (Spacelab Dl Mission)

The growth and graviresponsiveness of roots were investigated in lentil seedlings (Lens culinaris L. cv. Verte du Puy) grown (1) in microgravity, (2) on a 1 g centrifuge in space, (3) in microgravity and then placed on the 1 g centrifuge for 3 h, (4) on the ground. Dry seeds were hydrated in space (except for the ground control) and incubated for 25 h at 22°C in darkness. At the end of the experiment, the seedlings were photographed and fixed in glutaraldehyde in a Biorack glove box. Root length was similar for seedlings grown in space and for the ground and the 1 g centrifuge controls. The direction of root growth in the microgravity sample deviated strongly from the initial orientation of the roots of the dry seeds. This deviation could be due to spontaneous curvatures similar to those observed on clinostats. When lentil seedlings were first grown in microgravity for 25 h and then placed on the 1 g centrifuge for 3 h, their roots bent strongly under the effect of the centrifugal acceleration. The amplitude of root curvature on the centrifuge was not significantly different from that observed on ground controls growing in the vertical position and placed in the horizontal position for 3 h. The gravisensitivity of statocytes differentiated in microgravity was similar to that of statocytes differentiated on earth. There were no qualitative differences in the ultrastructural features of the gravisensing cells in microgravity and in the 1 g centrifuge and ground controls. However, the distribution of statoliths in the gravisensing cells was different in microgravity: most of them were observed in the proximal part of these cells. Thus, these organelles were not distributed at random, which is in contradiction with results obtained with clinostats. The distal complex of endoplasmic reticulum in the statocytes was not in contact with the amyloplasts. Contact and pressure of amyloplasts on the tubules were not prerequisites for gravisensing. The results obtained are not in agreement with the hypothesis that the distal endoplasmic reticulum would be the transducer of the action of the statoliths.     

Polarity of statocytes in lentil seedling roots grown in space (Spacelab D1 Mission)

A morphometric analysis of root statocytes was performed on seedlings of lentil ( Lens culinaris L., cv. Verte du Puy) in order to determine the effects of microgravity on the polarity of these cells. Seedlings were grown: (1) on the ground, (2) in microgravity, (3) on a 1 g centrifuge in space, (4) first in microgravity and then placed on a 1 g centrifuge for 3 h. Dry seeds were hydrated in space (except for the ground control) for 25 h in darkness at 22°C in the Biorack facility developed by the European Space Agency. At the end of the experiment, the seedlings were photographed and fixed in glutaraldehyde in the Biorack glove box. The average shape of the statocytes and the location of endoplasmic reticulum, amyloplasts and nucleus in the cells were analysed in the four samples. By considering the cell shape, it appears that the morphology of the statocytes on the ground was different from that observed in the space samples. Cell polarity was similar in microgravity and in the centrifuged samples except for the distribution of the amyloplasts. These organelles were not distributed at random in near zero gravity, and they were more numerous in the proximal than in the distal half. Moreover, the statoliths were more voluminous in microgravity than in the centrifuged samples. The nucleus was closer to the cell center in the statocytes of roots grown in microgravity than in statocytes of roots grown in microgravity and then placed on the 1 g centrifuge for 3 h. It is hypothesized that the nucleus is attached to the cell periphery and that its location is dependent upon gravity.     

Movement of Amyloplasts in the Statocytes of Geotropically Stimulated Roots. The Pre-Inversion Effect

Previously inverted Lepidium roots were placed in a horizontal position and the amyloplasts in the statocytes of the root cap allowed to fall through their entire range of movement across the cell. Under these conditions the amyloplasts first follow a mainly downward course for 6 to 8 min at a speed between 0.5 and 0.8 μm per min. For the next 10 min they move slightly more slowly in a direction away from the apical end of the cell, still sinking somewhat, but without reaching the plasmalemma along the lower wall. Previous experiments have shown that conditions assumed to allow the amyloplasts to slide parallel to the longitudinal cell walls are those that give rise to the largest geotropic curvatures. Such conditions are for instance (1) stimulation at 135° (root tips pointing obliquely upward) and (2) inversion of roots for 16 min followed by stimulation at 45°. Treatments assumed not to permit extensive sliding of the amyloplasts produce smaller geotropic curvatures, namely (3) stimulation at 45° without pre-inversion and (4) inversion followed by stimulation at 135°. The location of the amyloplasts after these four kinds of treatment has now been determined on photomicrographs and the assumptions concerning the paths and extent of sliding of the amyloplasts confirmed. Observations on electron micrographs showed that under all conditions the amyloplasts are separated from the plasmalemma by other organelles, such as ER, nucleus or vacuoles. In roots rotated for 15 min parallel to the horizontal axis of the klinostat at 2 rpm, the amyloplasts are not clumped together as densely as in normal, inverted or stimulated roots, but they are not scattered over the entire cell volume. The statolith function of the amyloplasts is discussed in view of these and other observations.     

Comparative study of cellular structures implicated in gravisensing in statocytes of primary and lateral roots of Vigna angularis

Summary. The cellular structures of statocytes implicated in gravisensing in primary and lateral roots of Vigna angularis were compared. The statocytes of lateral roots already had small amyloplasts immediately after they emerged from the primary root. Although these amyloplasts sedimented, the lateral roots showed much weaker gravitropism than primary roots, at least until they reached a length of about 30 mm. The nuclei were usually positioned in the upper end of the statocytes in both types of roots. Electron microscopic surveys showed that many tubular elements of endoplasmic reticulum (ER) were frequently localized in the lower end of the statocyte and they sometimes diverged or curved, suggesting that the ER forms a large reticulate complex. It is worth noting that statocytes with a large ER complex were found much more frequently in primary roots than in lateral roots. The amyloplasts were not always settled on this complex but were very frequently under it, especially in the primary roots. In lateral roots, they were usually localized under the ER complex when they were present. Thus, it is suggested that the differential development and organization of the amyloplast-ER complex system is involved in the differential gravitropism of the two types of roots. Correspondence and reprints: Department of Biology, Faculty of Science, Ehime University, Bunkyo-cho, Matsuyama 790-8577, Japan.     

Gravitropic bending of cress roots without contact between amyloplasts and complexes of endoplasmic reticulum

The polar arrangement of cell organelles in Lepidium root statocytes is persistently converted to a physical stratification during lateral centrifugation (the centrifugal force acts perpendicular to the root long axis) or by apically directed centrifugation combined with cytochalasin-treatment. Lateral centrifugation (10 min, 60 min at 10\g or 50\g) causes displacement of amylplasts to the centrifugal anticlinal cell wall and shifting of the endoplasmic reticulum (ER) complex to the centripetal distal cell edge. After 60 min of lateral centrifugation at 10\g or 50\g all roots show a clear gravitropic curvature. The average angle of curvature is about 40° and corresponds to that of roots stimulated gravitropically in the horizontal position at 1\g in spite of the fact that the gravistimulus is 10-or 50-fold higher. Apically directed centrifugation combined with cytochalasin B (25 g\ml^-1) or cytochalasin D (2.5 g\ml^-1) incubation yields statocytes with the amyloplasts sedimented close to the centrifugal periclinal cell wall and ER cisternae accumulated at the proximal cell pole. Gravitropic stimulation for 30 min in the horizontal position at 1\g and additional 3 h rotation on a clinostat result in gravicurvature of cytochalasin B-treated centrifuged (1 h at 50\g) roots, but because of retarded root growth the angle of curvature is lower than in control roots. Cytochalasin D-treatment during centrifugation (20 min at 50\g) does not affect either root growth or gravicurvature during 3 h horizontal exposure to 1\g relative to untreated roots. As lateral centrifugation enables only short-term contact between the amyloplasts and the distal ER complex at the onset of centrifugation and apically directed centrifugation combined with cytochalasin-treatment even exclude any contact the integrity of the distal cell pole need not necessarily be a prerequisite for graviperception in Lepidium root statocytes.Abbreviations CB cytochalasin B - CD cytochalasin D - ER endoplasmic reticulum - g gravitational acceleration     

Lentil root statoliths reach a stable state in microgravity

 The kinetics of the movement of statoliths in gravity-perceiving root cap cells of Lens culinaris L. and the force responsible for it have been analysed under 1 g and under microgravity conditions (S/MM-03 mission of Spacehab 1996). At the beginning of the experiment in space, the amyloplasts were grouped at the distal pole of the statocytes by a root-tip-directed 1-g centrifugal acceleration. The seedlings were then placed in microgravity for increasing periods of time (13, 29, 46 or 122 min) and chemically fixed. During the first 29 min of microgravity there were local displacements (mean velocity: 0.154 μm min⁻¹) of some amyloplasts (first at the front of the group and then at the rear). Nevertheless, the group of amyloplasts tended to reconstitute. After 122 min in microgravity the bulk of amyloplasts had almost reached the proximal pole where further movement was blocked by the nucleus. After a longer period in microgravity (4 h; experiment carried out 1994 during the IML 2 mission) the statoliths reached a stable position due to the fact that they were stopped by the nucleus. The position was similar to that observed in roots grown continuously in microgravity. Treatment with cytochalasin D (CD) did not stop the movement of the amyloplasts but slowed down the velocity of their displacement (0.019 μm min⁻¹). Initial movement patterns were the same as in control roots in water. Comparisons of mean velocities of amyloplast movements in roots in space and in inverted roots on earth showed that the force responsible for the movement in microgravity (F_c) was about 86% less (F_c = 0.016 pN) than the gravity force (F_g = 0.11 pN). Treatment with CD reduced F_c by two-thirds. The apparent viscosity of the statocyte cytoplasm was found to be 1 Pa s or 3.3 Pa s for control roots or CD treated roots, respectively. Brownian motion or elastic forces due to endoplasmic reticulum membranes do not cause the movement of the amyloplasts in microgravity. It is concluded that the force transporting the statoliths is caused by the actomyosin system. Received: 22 March 1999 / Accepted: 18 December 1999     

Kinetics of phosphate absorption by mycorrhizal and non-mycorrhizal Scots pine seedlings

Kinetics of net phosphate (P_i) uptake was measured on intact ectomycorrhizal and non‐mycorrhizal Pinus sylvestris seedlings using a semihydroponic cultivation method. The depletion of P_i in a nutrient solution was assessed over a 160–0.2 μM P_i gradient. Growth of the pine seedlings was P limited and measurements were performed 7 and 9 weeks after inoculation. Three ectomycorrhizal fungi were studied: Paxillus involutus, Suillus bovinus and Thelephoraterrestris. P_i uptake was extremely fast in plants colonised by P. involutus. The P_i concentration dropped below 0.2 μM within 4–5 h. In plants colonised with S. bovinus this occurred in 5–6 h and in plants associated with T. terrestris 8 h were needed to run through the whole concentration range. Non‐mycorrhizal plants of similar size and nutrient status decreased P_i to a concentration between 1 and 2 μM in 18 h. Data were curve fitted to a two‐phase Michaelis‐Menten equation. The apparent kinetic constants, K_m and V_max, for the high affinity P_i uptake system of the pine roots could be estimated accurately. V_max of this system was up to 7 times higher in pines associated with P. involutus than in non‐mycorrhizal seedlings. The intact extraradical mycelium greatly increased the absorption surface area of the roots (V_max). Non‐mycorrhizal plants had a K_m between 7.8 and 16.4 μM P_i. Plants mycorrhizal with P. involutus had K_m values between 2.4 and 7.2, plants colonised with S. bovinus had a K_m between 5.1 and 12.3, and seedlings associated with T. terrestris had a K_m from 4.6 to 10.1 μM P_i. All 3 ectomycorrhizal fungi had a strong impact on the P_i absorption capacity of the pine seedlings. The results also demonstrated that there is substantial heterogeneity in kinetic parameters among the different mycorrhizal root systems.     

Phosphate starvation‐inducible pyrophosphate‐dependent phosphofructokinase occurs in plants whose roots do not form symbiotic associations with mycorrhizal fungi

Previously, we reported that phosphate (P_i) starvation of suspension cells or seedlings of Brassica nigra results in a large elevation in the activity of pyrophosphate-dependent phosphofructokinase (EC 2.7.1.90) (PFP). However, other researchers have found that P_i deprivation either causes a significant reduction or no change in extractable PFP activity of Catharanthus roseus suspension cells, or roots of Nicotiana tabacum and Phaseolis vulgaris seedlings. The present study was undertaken to examine the prevalence of P_i starvation-inducible PFP in seedlings, root cultures, or suspension cells of a variety of plant species differing in phylogenetic relatedness to B. nigra. In all species examined, fresh weights were decreased and acid phosphatase (EC 3.1.3.2) activities were increased by P_i limitation. Brassica napus suspension cells, Arabidopsis thaliana seedlings, and roots of B. napus, B. carinata, B. oleracea, Beta vulgaris, Fagopyrum esculentum, Sinapis alba, and S. arvensis seedlings grown with P_i-limited media contained 170–510% greater PFP activity than did nutrient-sufficient controls. In five of these species the induction of PFP activity by P_i limitation was based in part upon an increased susceptibility of the enzyme to its allosteric activator, fructose-2,6-bisphosphate. By contrast, the PFP activity in P_i-deprived Lycopersicon esculentum root cultures and Nicotiana silvestris suspension cells decreased by 45–65% relative to P_i-sufficient controls. Immunoblotting of extracts from A. thaliana seedlings, S. arvensis, F. esculentum and B. oleracea roots, and B. napus suspension cells probed with potato tuber PFP antibodies indicated that the upregulation of PFP activity by P_i stress in these species was not correlated with an alteration in the amount or subunit composition of PFP. Our findings suggest that induction of PFP during long-term P_i starvation may be characteristic of members of the Cruciferae, Chenopodiaceae and Polygonaceae families whose roots do not form symbiotic associations with mycorrhizal fungi.     

Root growth and statocyte polarity in lentil seedling roots grown in microgravity or on a slowly rotating clinostat

The ability of clinostats to simulate microgravity was evaluated by comparing lentil ( Lens culinnrias L. cv. Verte du Puy) seedlings grown in space (Spacelab D1 Mission) with seedlings grown on a slowly rotating elinostat. Seeds were germinated and incubated for 25.5 h at 22°C (1) in microgravity, (2) on a 1g-centrifuge in space. (3) on a slowly rotating elinostat and (4) on the ground. Morphological (root length and orientation) and ultrastructural (distribution of amyloplasts, location of the nucleus in statocytes) parameters were studied. For clinostat experiments, two different configurations were employed: the longitudinal axis of the root was parallel (horizontal elinorotation) or perpendicular (vertical elinorotation) to the axis of rotation. the same configurations were used for the lg-controls. Root length and orientation were similar for roots grown on the clinostat and in microgravity. The amyloplasts were identically distributed in statocytes of horizontally clinorolated roots and in statocytes differentiated in microgravity. However, the location of the nucleus was similar in vertically rotated roots and microgravity samples. Since the involvement of the nucleus in graviperception is not known, it can be concluded that horizontal clinorotation simulates microgravity better than vertical elinorotation.     

Mechanotransduction in root gravity sensing cells

The analysis of the dose-response curve of the gravitropic reaction of lentil seedling roots has shown that these organs are more sensitive when they have been grown in microgravity than when they have been grown on a 1 g centrifuge in space before gravistimulation. This difference of gravisensitivity is not due to the volume or the density of starch grains of statoliths, which are about the same in both conditions (1 g or microgravity). However, the distribution of statoliths within the statocyte may be responsible for this differential sensitivity, since the dispersion of these organelles is greater in microgravity than in 1 g. When lentil roots grown in microgravity or in 1 g are stimulated at 0.93 g for 22 min, the amyloplasts sediment following two different trajectories. They move from the proximal half of the statocytes toward the lower longitudinal wall in the microgravity grown sample and from the distal half toward the longitudinal wall in the 1 g grown sample. At the end of the stimulation, they reach a similar position within the statocytes. If the roots of both samples are left in microgravity for 3 h, the amyloplasts move toward the cell centre in a direction that makes an average angle of 40 degrees with respect to the lower longitudinal wall. The actin filaments, which are responsible for this movement, may have an overall orientation of 40 degrees with respect to this wall. Thus, when roots grown in microgravity are stimulated on the minicentrifuge the amyloplasts slide on the actin filaments, whereas they move perpendicular to them in 1 g grown roots. Our results suggest that greater sensitivity of seedling roots grown in microgravity should be due to greater dispersion of statoliths, to better contacts between statoliths and the actin network and to greater number of activated mechanoreceptors. One can hypothesize that stretch activated ion channels (SACs) located in the plasma membrane are responsible for the transduction of gravistimulus. These SACs may be connected together by elements of the cytoskeleton lining the plasma membrane and to the actin filaments. They could be stimulated by the action of statoliths on the actin network and/or on these elements of the cytoskeleton which link the mechanoreceptors (SACs).     

A Morphometric Analysis of the Redistribution of Organelles in Columella Cells in Primary Roots of Normal Seedlings and Agravitropic Mutants of Hordeum vulgare

Moore, R. 1985. A morphometric analysis of the redistributionof organellcs in columella cells in primary roots of normalseedlings and agravitropic mutants of Hordeum vulgare.—J.exp. Bot. 36:1275–1286. The redistribution of organeUes m columella cells of horizontally-orientedroots of Hordeum vulgare was quantified in order to determinewhat structural changes in graviperceptive (i.e, columella)cells are associated with the onset of root gravicurvature.The sedimentation of amyloplasts is the only major change incellular structure that correlates positively with the onsetof root gravicurvature, which begins within 15 min after re-orientation.There is no consistent contact between sedimented amyloplastsand any other organelles. Nuclei are restricted to the proximalends of columella cells in vertically-oriented roots, and remainthere throughout gravicurvature after roots are oriented horizontally.Root gravicurvature does not involve significant changes in(1) the volume of columella cells, (2) the relative or absolutevolumes of organelles in columella cells, or (3) the distributionof endoplasmic reticulum (ER). The size, number and sedimentationrates of amyloplasts in columella cells of non-graviresponsiveroots of mutant seedlings are not significantly different fromthose of graviresponsive roots of normal seedlings. Similarly,there is no significant difference in (1) cellular volume, (2)distribution or surface area of ER, (3) patterns or rates oforganelle redistribution in horizontally-oriented roots, or(4) relative or absolute volumes of organelles in columellacells of graviresponsive and non-graviresponsive roots. Theseresults suggest that the lack of gravi-responsiveness by rootsof mutant seedlings is probably not due to either (1) structuraldifferences in columella cells, or (2) differences in patternsor rates of organelle redistribution as compared to that characteristicof graviresponsive roots. Thus, the basis of non-graviresponsivenessin this mutant is probably different from other agravitropicmutants so far studied. Key words: Agravitropic mutant, barley, columella cell, gravitropism (root), Hordeum vulgare, ultrastructure     

Amyloplast sedimentation and organelle saltation in living corn columella cells

Amyloplast sedimentation during gravistimulation and organelle movements was studied in living central rootcap cells of Zea mays L. cv. Merit. Cells from sectioned roots were viewed with a horizontally-mounted videomicroscope. The kinetics of gravity-induced amyloplast sedimentation were comparable to those calculated from experiments using fixed material. Individual amyloplasts fell at an average velocity of 5.5 micrometers min-1; the maximal velocity of fall measured was 18.0 micrometers min-1. Amyloplasts often rotated, sometimes rose in the cytoplasm, and occasionally underwent sudden rapid movements as fast as 58 micrometers min-1. Saltations of other organelles were frequently observed. This appears to be the first report of cytoplasmic streaming in the presumptive statocytes of roots.     

Semicontinuous lactic fermentation of whey byLactobacillus bulgaricus I. Experimental results

Summary A Monod-like equation correlates the lactic acid productivity and the volume fraction of inoculum in semicontinuous fermentation of whey byLactobacillus bulgaricus. The volume of the inoculum varied from 10% to 80% of the reactor working volume.Nomenclature N number of fermentation cycles after the first fermentation - P lactic acid productivity - S_w average total sugars concentration of the whey as lactose (the standard deviation is indicated) - T average fermentation time (the standard deviation is indicated) - V_a average total consumption of NH₄OH solution (the standard deviation is indicated) - V_i volume of recently fermented medium used as inoculum of the next fermentation cycle - V_w volume of whey added to the reactor at the beginning of each fer mentation cycle - volume fraction of inoculum=V_i/(V_i+V_w)     

The effect of centrifugal accelerations on the polarity of statocytes and on the graviperception of cress roots

The structural polarity of statocytes of Lepidium sativum L. is converted to a physical stratification by a root-tip-directed centrifugal acceleration. Sedimentation of amyloplasts and nucleus to the centrifugal (distal) cell pole and the lateral displacement of the distal endoplasmic reticulum (ER) complex occur after centrifugation for 20 min at an acceleration of 50 g. With higher doses (20 min, 100-2,000 g), smaller organelles become increasingly displaced. From the centrifugal to the centripetal cell pole, the following stratification is observed: 1) amyloplasts with mitochondria; 2) nucleus with mitochondria and a few dictyosomes, as well as laterally located ER; 3) dictyosomes with a few mitochondria; 4) vacuoles; and 5) lipid droplets. Within the first 7.5 min, after the roots have been returned to 1 g, the original arrangement of the amyloplasts sedimented on the underlying ER complex is reestablished in 66% of the statocytes. When roots previously centrifuged in an apical direction are exposed in a horizontal position to 1 g, the latent period of the graviresponse is increased by 7.5 min relative to the non-centrifuged controls. The kinetics of the response are identical to the controls. Roots centrifuged first in an apical direction and then for 2 h in a lateral direction (1,000 g) have statocytes with a physical stratification perpendicular to the root axis. A gravitropic curvature does not take place during the lateral centrifugation. These results support the hypothesis that the distal ER complex is necessary and sufficient for graviperception.     

A morphometric analysis of the redistribution of organelles in columella cells in primary roots of normal seedlings and agravitropic mutants of Hordeum vulgare

The redistribution of organelles in columella cells of horizontally-oriented roots of Hordeum vulgare was quantified in order to determine what structural changes in graviperceptive (i.e., columella) cells are associated with the onset of the root gravicurvature. The sedimentation of amyloplasts is the only major change in cellular structure that correlates positively with the onset of root gravicurvature, which begins within 15 min after re-orientation. There is no consistent contact between sedimented amyloplasts and any other organelles. Nuclei are restricted to the proximal ends of columella cells in vertically-oriented roots, and remain there throughout gravicurvature after roots are oriented horizontally. Root gravicurvature does not involve significant changes in (1) the volume of columella cells, (2) the relative or absolute volumes of organelles in columella cells, or (3) the distribution of endoplasmic reticulum (ER). The size, number and sedimentation rates of amyloplasts in columella cells of non-graviresponsive roots of mutant seedlings are not significantly different from those of graviresponsive roots of normal seedlings. Similarly, there is no significant difference in (1) cellular volume, (2) distribution or surface area of ER, (3) patterns or rates of organelle redistribution in horizontally-oriented roots, (4) relative or absolute volumes of organelles in columella cells of graviresponsive and non-graviresponsive roots. These results suggest that the lack of graviresponsiveness by roots of mutant seedlings is probably not due to either (1) structural differences in columella cells, or (2) differences in patterns or rates of organelle redistribution as compared to that characteristic of graviresponsive roots. Thus, the basis of non-graviresponsiveness in this mutant is probably different from other agravitropic mutants so far studied.