Gravitropically-stimulated seedlings show autotropism in weightlessness

In a spaceflight experiment, autotropism by oat ( Avena sativa L.) coleoptiles following gravitropic responses was prominent in weightlessness: counter-reactions led to the straightening of the curved coleoptiles. This was not the case during clinorotation on earth. The autotropic reactions appeared to be related to the stimulus received during the stimulus period, i.e. the greater the response the greater the autotropic counter-reaction. Previous models of the gravitropic system which predicted that coleoptiles would not straighten in weightlessness are disproved. A modification to one of the models is proposed which includes the autotropic response observed in spaceflight. The nature of the counter-reactions in the absence of gravitropic stimulation is discussed.     

The Effect of Displacement Angle on the Gravitropic and Autotropic Growth Responses of Sunflower Hypocotyls

Sunflower hypocotyls were gravistimulated at angles of 22·5°,45° 90° 135° and 157·5° from the verticaland the gravitropically-induced growth rate changes of the upperand lower surfaces measured. The lag period before the initiation of different growth andalso the magnitude of differential growth were similar at alldisplacement angles. The fact that very small or very largeangles of displacement produce a maximal response needs to beaddressed by models of gravitropism. The autotropic straightening phase (the reduction of the gravitropically-inducedcurvature) began first in the treatments with the smaller displacementangle, consequently the initiation of autotropism was independentof the radius of curvature at the time autotropism was induced.The fact that the lag period before the onset of autotropismis not fixed illustrates the need to take into account autotropismwhen conducting studies of organs subjected to prolonged gravistimulation.Copyright1995, 1999 Academic Press Gravitropism, hypocotyls, Helianthus annuus, sunflower     

An analysis of the changes in growth rate occurring during the initial stages of geocurvature in shoots

Abstract A detailed study of the changes in growth rate on the two sides of a shoot prior to, and following geostimulation has been undertaken. Data from etiolated Zea seedlings, and cucumber hypocotyls, and from light grown sunflower hypocotyls are presented. In all cases, the differential growth which brings about geocurvature begins simultaneously along the length of the organ. This is contrary to previous reports in the literature which suggested that curvature begins first at the tip of an organ and progresses basipetally. The feature which is common to all species investigated is that growth of the upper side of the shoot ceases following geostimulation. In some cases there is also a marked acceleration in growth rate on the lower side of the shoot (sunflower) but other species show no such acceleration of growth (cucumber). It has been assumed for many years that the major factor causing upward curvature was an acceleration of growth on the lower side of the organ. The data presented here show that the cessation of growth on the upper side is a major, and in some cases the only factor bringing about geocurvature. The data are discussed in relation to the mechanisms which might control geotropic curvature.     

The role of the apex in the phototropic curvature of Avena coleoptiles: positive curvature under conditions of continuous illumination

The differential growth causing second positive phototropic curvature in intact, black-capped and decapitated Avena coleoptiles has been measured. In all cases the curvature is brought about by a cessation in growth of the illuminated side. The fact that shading the apex does not significantly alter the initial steps of differential growth means that the subapical zones can perceive and respond to unilateral illumination. Decapitation significantly reduces coleoptile growth, especially in the most apical zone. However, the fact that differential growth is still evident in the other zones of decapitated coleoptiles within 30 min of unilateral illumination requires one to conclude that the apex cannot be controlling the differential growth in those basal zones.     

Differential growth of georeacting maize roots

The growth rate of the two sides of 10-mm apical segments prepared from primary roots and of intact primary roots of maize has been analyzed in both vertical and horizontal positions, using a filming method allowing continuous growth recording. The data showed that the georeaction began by a decrease in the overall elongation rate of the roots. This inhibition is effective on the lower side of the bending zone, where the growth is practically stopped during the period of maximum rate of geocurvature. In contrast, the growth is slightly enhanced on the upper part of the elongating zone.     

Analysing branching pattern in plantations of young red oak trees (Quercus rubra L., Fagaceae)

Branching patterns of the growth units of monocyclic or bicyclic annual shoots on the main axis of 5-year-old red oaks were studied in a plantation in south-western France. For each growth unit, the production of axillary structures associated with each node was described in the form of a sequence. For a given category of growth units, homogeneous zones (i.e. zones in which composition in terms of type of axillary production does not change substantially) were identified on such sequences using a dedicated statistical model called a hidden semi-Markov chain. For instance, on the first growth unit of bicyclic annual shoots, a zone with 1-year-delayed branches was found systematically below a zone with buds and one-cycle-delayed branches. Branching patterns shown by the growth unit of monocyclic annual shoots and on the second growth unit of bicyclic annual shoots were very similar. Branches with a 1-year delay in development tended to be polycyclic at the top of the growth unit and monocyclic lower down. The number of nodes shown by the branched zone of the growth unit of monocyclic annual shoots was stable, irrespective of the total number of nodes of the growth unit. In contrast, the second growth unit of bicyclic annual shoots exhibited a correlation between the number of nodes in the branching zone and the total number of nodes. The contribution made by this method to understanding plant functioning is discussed.     

Autotropism, automorphogenesis, and gravity

Segments of organs that have undergone gravitropic curvature later straighten during the course of gravitropism or after the g ‐vector becomes randomized on a clinostat. Little is known about the mechanisms underlying these and perhaps related phenomena which have been described with various overlapping terms such as autotropism, autotropic straightening, automorphosis, automorphogenesis, automorphic curvature, and gravitropic straightening. The types of phenomena that historically have been named by the above terms are reviewed critically with respect to an interaction with gravitropism. We suggest that the term "autotropism" should not be applied to the phenomenon of organ straightening that occurs during the course of gravitropism, since this straightening is part of a complex series of local growth adjustments overall through time, and since this phenomenon is not itself a tropistic response to a directional exogenous stimulus. It is suggested that the term autotropism should be used only for the phenomenon of organ straightening that occurs after the g ‐vector is randomized on a clinostat or withdrawn in the microgravity conditions of spaceflight. Usage of the term automorphogenesis is most appropriate for describing curvatures or orientations that result from morphological relationships such as in nastic curvatures.     

Some aspects of the control of root growth and georeaction: the involvement of indoleacetic Acid and abscisic Acid

Apical segments of roots of Zea mays L. cv. Orla and cv. Anjou show a strong georeaction during 7 hours geostimulation. This is abolished by detipping the segments and restored by replacing the tips upon the apical cut surfaces. After exodiffusion of endogenous indoleacetic acid (IAA) the retipped segments showed a significantly lower geocurvature. Application of low concentrations of IAA to the basal cut surface of root segments from which endogenous IAA had not been allowed to exodiffuse increased the geocurvature of retipped Orla segments but decreased geocurvature of Anjou segments. At appropriate concentration basally applied IAA restored the georeaction capacity of root segments from which the endogenous auxin had exodiffused. The implications of the interaction between exogenous and endogenous IAA in the control of root georeaction are discussed with special reference to the normal role of endogenous IAA in the regulation of root georeaction and the variation in endogenous IAA content of roots of different cultivars of maize. The probability that the normal control of root growth and georeaction involves concomitant actions in the elongation zone of IAA moving preferentially in the acropetal direction and basipetally transported growth inhibitors (such as abscisic acid) produced in the cap cells is stressed.     

The role of nodal and internodal responses in gravitropism and autotropism in Galium aparine L

Abstract This time course and location of gravitropically induced curvatures in stems of goosegrass (Galium aparine L.), a member of the Rubiaceae, have been investigated. In the early stages of the response (0–5 h), curvature develops throughout the growing region, and is followed by an autotropic straightening which affects the internodes only, leading to the production of essentially straight internodes some 15 h after the onset of gravistimulation. Curvatures developing in the nodal regions, however, continue to increase over this period, and are not subject to reversal by autotropism. The nodal curvatures are not entirely dependent on the presence of any other part of the plant, since marked curvatures can be induced in isolated nodal segments. This pattern of response leads ultimately to correction of the growth direction of the plant by means of curvature responses confined exclusively to the nodes, despite the initial participation of both nodes and internodes in the gravitropic reaction.     

Differences in proleptic and epicormic shoot structures in relation to water deficit and growth rate in almond trees (Prunus dulcis)

Background and Aims

Shoot characteristics differ depending on the meristem tissue that they originate from and environmental conditions during their development. This study focused on the effects of plant water status on axillary meristem fate and flowering patterns along proleptic and epicormic shoots, as well as on shoot growth rates on ‘Nonpareil’ almond trees (Prunus dulcis). The aims were (1) to characterize the structural differences between proleptic and epicormic shoots, (2) to determine whether water deficits modify shoot structures differently depending on shoot type, and (3) to determine whether shoot structures are related to shoot growth rates.

Methods

A hidden semi-Markov model of the axillary meristem fate and number of flower buds per node was built for two shoot types growing on trees exposed to three plant water status treatments. The models segmented observed shoots into successive homogeneous zones, which were compared between treatments. Shoot growth rates were calculated from shoot extension measurements made during the growing season.

Key Results

Proleptic shoots had seven successive homogeneous zones while epicormic shoots had five zones. Shoot structures were associated with changes in growth rate over the season. Water deficit (1) affected the occurrence and lengths of the first zones of proleptic shoots, but only the occurrence of the third zone was reduced in epicormic shoots; (2) had a minor effect on zone flowering patterns and did not modify shoot or zone composition of axillary meristem fates; and (3) reduced growth rates, although patterns over the season were similar among treatments.

Conclusions

Two meristem types, with different latency durations, produced shoots with different growth rates and distinct structures. Differences between shoot type structure responses to water deficit appeared to reflect their ontogenetic characteristics and/or resource availability for their development. Tree water deficit appeared to stimulate a more rapid progression through ontogenetic states.     

Proton Efflux from the Outer Layer of the Peduncle of Tulip in Gravitropism and Circumnutation*

Epidermal plus hypodermal peels from tulip peduncles produced bands of acidity on agar containing bromocresol purple. Peels from horizontally oriented peduncles gave rise to an acidity band which corresponded to the lower side of the peduncle. The band began 3–6 cm beneath the flower and extended basipetally within the region of gravitropic bending. No corresponding band appeared in an agar layer laid on the cortical surface exposed by peeling. Peduncles growing in the normal vertical position showed circumnutations with a period in the range of 4 h. The peels from these stalks produced one or two bands more acid than the remaining part of the peel. Since the acidity band in horizontally positioned stalks corresponds to the zone of faster growth causing gravitropic bending, we infer that the band(s) produced by vertical stalks also correspond to zones of differential growth involved in circumnutation. On the basis of a previous finding that tulip leaves give rise to an oscillating acidity pattern, we infer that vertical stalks also show such a pattern. This inference fits the model proposing the involvement of an internal oscillator in circumnutation. However, the ratio of the circumnutation period to the gravitropic lag phase in tulip peduncles is such as predicted by the gravitropic-feedback model of circumnutation.     

Seasonal changes in growth and metabolic activity of underground shoots of yarrow

Growth and metabolic activity of underground shoots of a long-rhizome perennial herbaceous species yarrow (Achillea millefolium L.) were studied. The active growth of rhizomes and the formation of new meristematic zones were observed during the second half of the growing season after termination of aboveground shoot growth. In this period, underground shoots had a rather high rate of respiration (1.3 mg CO₂/(g dry wt h)), a considerable content of nonstructural carbohydrates (15% of dry weight), and the elevated activities of IAA, cytokinins, and ABA. In autumn, the rate of respiration of underground shoots decreased to 1.0 mg CO₂/(g dry wt h), soluble sugars accumulated, the ratio between unsaturated and saturated fatty acids rose as well as the ratios GA/ABA and cytokinins/ABA. Temperature optimum for the rhizome growth lay in the range of low and moderate above-zero temperatures (5–20°C), and the freezing point of water in the apices of under-ground shoots was about ?10°C. It is concluded that rhizome quiescence predominantly depends on low temperatures and is not associated with the accumulation of growth inhibitors. In the course of plant preparation to winter, morphogenetic transformations in underground shoots depend on changes in the hormonal balance directed in favor of growth hormones and relatively high respiratory activity in the apical zones of the rhizomes.     

Cryobiology of articular cartilage: ice morphology and recovery of chondrocytes

The cryopreservation of articular cartilage chondrocytes has been achieved with cells isolated from the cartilage matrix but has found only limited success when the tissue is left intact. Previous work with ovine cartilage has shown that cryopreservation of the chondrocytes of the superficial and deep zones is possible, but the cells of the intermediate zone have not been successfully cryopreserved. This finding led to the suggestion that there might be biological differences between chondrocytes of the different morphological zones that were responsible for this differential recovery. This study investigates the hypothesis that the cells of the intermediate zone are more sensitive to cryoinjury by introducing cuts in the cartilage so that cells of the intermediate zone have the same proximity to the outer surface of the tissue as the cells of the superficial zone. When this was done, it was found that cells of the intermediate zone could survive cryopreservation as well as the cells of the superficial zone when they were near a surface, but not when they were embedded deep within the tissue. Thus the hypothesis of a biological difference between the cells of the two zones being responsible for the differential recovery is disproved. It is further hypothesized that physical proximity to a surface leads to higher recovery as a result of planar ice growth into the cartilage.     

Influence of cold on organization of actin filaments of different types of root cells in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

The effect of the low temperature (+4°C) on the organization of actin filaments (microfilaments) of cells from different growth zones has been studied in the roots of Arabidopsis thaliana (L.). It was found that cold treatment inhibited the growth of the primary root and changed its morphology, causing a formation of large number of deformed (ectopic) root hairs in differentiation zone. The temporal relationship between the disorientation and the organization of actin filaments and the detected changes of growth and morphology of roots after cold treatment was shown. It has been found that actin filaments of root hairs, meristematic cells, cells of elongation zone, and epidermal cells of all root zones of A. thaliana are the most sensitive to the cold.     

Photoregulation of hypocotyl growth: geotropic evidence for the operation of two photosystems

Abstract. The rate of curvature of etiolated cress ( Lepi-dium sativum L. ) hypocotyls in response to gravity (negative geotropism) was retarded by red or blue light; far-red irradiation was without effect. The timing of the irradiation period in relation to the presentation for geostimulus markedly affected the response. When seedlings were irradiated during the 1–2 h period of geostimulus, blue light was more effective than red at retarding curvature; when seedlings were irradiated prior to geostimulus, only red light affected geocurvature. These results are interpreted as a further example of the kinetically distinct effects of red and blue light on hypocotyl development. Blue light elicited a rapid, immediate response effective only during the period of irradiation; red light induced a response characterized by a lag period and persistence in subsequent darkness. Etiolated mustard seedlings showed similar responses to light and gravity. The results are discussed in relation to the possibility that two photosystems operate in hypocotyl growth.     

Effects of Temperature and Leaf Position on Leaf Area Expansion of Kiwifruit (Actinidia deliciosa) Shoots: Development of a Modelling Framework

This paper describes mathematically the effects of temperatureand position on the expansion of leaves along a kiwifruit (Actinidiadeliciosa) shoot, taking into account shoot morphology. Theleaves were grouped into three zones along the shoot: initialcluster leaves (first zone); the rest of the leaves that werepreformed during the previous season (second zone); and leavesthat were initiated during the current season (third zone).After opening of the initial cluster, the leaves appeared atconstant rates for each of the two temperature treatments considered.The expansion of individual leaves was modelled by a growthfunction with the parameters: final area; duration of the growthwindow; centre of the growth window (timing of expansion); andlower asymptote. Within the first two zones, the pattern ofleaf expansion was affected by nodal position, with basal leaveshaving higher initial rates of expansion than distal leaves.The timing of expansion was linear with respect to the nodalposition within each of the zones, with the slope independentof temperature for the first zone. The slopes of the timingof expansion for the second and third zones depended on temperatureand were correlated for each temperature treatment. Final leafarea was a function of leaf position in the first zone and afunction of timing of leaf expansion for distal leaves startingfrom leaf 10. Temperature had no effect on final leaf area inthe first zone. For the rest of the leaves, temperature affectedfinal leaf area indirectly, through the timing of leaf expansion.The effect of temperature on the growth window of individualleaves within the first zone was less than that for the restof the leaves. However, simulated values for the total leafarea of shoots using the average shoot growth window showedgood agreement with experimental values.Copyright 2001 Annalsof Botany Company Actinidia deliciosa‘Hayward’, shoot development, individual leaf area, temperature effect, positional effects, modelling     

Defoliation effects on carbon and nitrogen substrate import and tissue-bound efflux in leaf growth zones of grasses

Grassland plants suffer regular defoliation, causing loss of photosynthetic activity and internal resources. Consequently, re‐foliation may be substrate‐limited. The present study was undertaken to test the hypothesis that decreased C import in leaf growth zones is (partially) compensated by: (i) mobilization of substrate within growth zones; and (ii) increased efficiency of substrate use in leaf area expansion; but (iii) that these processes depend on the C status of growth zones at defoliation. Mixtures of a C₃ (Lolium perenne L.) and a C₄ grass (Paspalum dilatatum Poir.) were grown at 15 °C (C₃ dominance) and 23 °C (C₄ dominance). Individual plants thus grew in contrasting (light and temperature) environments before being defoliated. Defoliation caused a drastic and immediate decrease in C import, but effects on leaf area expansion were buffered by biomass mobilization in the growth zone and increases in specific leaf area of produced tissue. Thus, over the first 2 d post‐defoliation, the amount of leaf area produced per unit imported C increased by 39 to 102% depending on treatment. The magnitude of these buffering responses was correlated with the concentration of water soluble carbohydrates in the growth zone at defoliation. Similar responses were observed for N, although defoliation effects were smaller and delayed relative to those on C import. This study demonstrates refoliation is sustained by short‐term mobilization of reserves within the growth zone and reduced costs of produced leaf area, but that these mechanisms depend on growth zone C status at defoliation.     

Threats to pine stands by the pine shoot beetles Tomicus piniperda (L.) and Tomicus minor (Hart.) (Col., Scolytidae) around a sawmill in southern Poland

Forest stands growing around sawmills are especially exposed to damage due to the feeding by the pine shoot beetles Tomicus piniperda (L.) and Tomicus minor (Hart.) (Col., Scolytidae) in the pine shoots, causing a considerable loss of increment. The purpose of this study was to determine the relation of the distance zone of a stand from a sawmill to the density of the pine shoot fall, width of annual rings, and pine stand quality. The investigations were carried out between 1992 and 1996 in a Pinus silvestris – Abies alba stand adjacent to a sawmill in the Zagnańsk Forest District. In the investigated stand, eight zones situated at different distances from the sawmill were marked out, in which the fallen pine shoots were collected and dendrometric measurements taken. The study results showed that the pine shoot fall in the area up to 300 m away from the sawmill was significantly greater than the fall in zones situated further away. In addition, the mean current annual and 10-year radial increments at breast height were significantly smaller, and the pine stand quality class was lower within that area. The radial increment at breast height in the area up to 300 m away from the sawmill was almost twice as low as that in the control area, and about 40% lower than the increment in the area situated 300–900 m away. The increment pattern of pine in this area differed from that of fir, suggesting that the incremental loss and lower pine stand quality are the result of long-term feeding of the pine shoot beetles in the pine shoots. These results indicate that other causes, such as differences in site class, climatic factors or air pollution, can be excluded.     

Analysis of growth during geotropic curvature in seedling hypocotyls

Abstract. The patterns of growth in organs curving under the influence of gravity were analysed by time-lapse photography of cress and cucumber hypocotyls which were delimited into 1 mm zones by ion-exchange beads. Geotropic curvature resulted from changes in growth rate on both sides of the organ. Growth inhibition of varying degrees of intensity occurred in all the previously growing zones of the upper (concave) side. An absolute reduction in length due to compression frequently occurred in some zones. Also, in both species growth stimulation was observed on the lower (convex) side. The disparity in growth rate between the upper and lower surfaces varied with time, being more apparent in the subapical region in the first hour of curvature. A later promotion of growth rate on the lower surface subsequently increased the curvature of the more basal zones. Autotropic straightening occurred as a consequence of growth changes, both inhibitory and stimulatory, in the apical zones. These events indicate a polarity of response in which apical zones have precedence over basal zones.     

The role of actin filaments in the gravitropic response of snapdragon flowering shoots

The involvement of the actin and the microtubule cytoskeleton networks in the gravitropic response of snapdragon ( Antirrhinum majus L.) flowering shoots was studied using various specific cytoskeleton modulators. The microtubule-depolymerizing drugs tested had no effect on gravitropic bending. In contrast, the actin-modulating drugs, cytochalasin D (CD), cytochalasin B (CB) and latrunculin B (Lat B) significantly inhibited the gravitropic response. CB completely inhibited shoot bending via inhibiting general growth, whereas CD completely inhibited bending via specific inhibition of the differential flank growth in the shoot bending zone. Surprisingly, Lat B had only a partial inhibitory effect on shoot bending as compared to CD. This probably resulted from the different effects of these two drugs on the actin cytoskeleton, as was seen in cortical cells. CD caused fragmentation of the actin cytoskeleton and delayed amyloplast displacement following gravistimulation. In contrast, Lat B caused a complete depolymerization of the actin filaments in the shoot bending zone, but only slightly reduced the amyloplast sedimentation rate following gravistimulation. Taken together, our results suggest that the actin cytoskeleton is involved in the gravitropic response of snapdragon shoots. The actin cytoskeleton within the shoot cells is necessary for normal amyloplast displacement upon gravistimulation, which leads to the gravitropic bending.