Ontogenetic changes in the content of endogenous gibberellins inBryophyllum crenatum leaves with respect to stem polarity

The content of endogenous gibberellins was estimated in leaves adjacent to the individual nodes ofBryophyllum crenatum in five ontogenetic periods (July 10, August 12, September 17, October 22, and December 8). Their content decreased from the stem base to the apex when 5 to 7 leaf pairs were developed (July 10 and August 12). Before transition to the generative state when 8 leaf pairs were formed (Sept. 17 and Oct. 22) the content of gibberellins was gradually increased in the apical leaves and decreased in the basal ones. This change resulted in the increasing gradient of leaf gibberellins from the stem base to the apex just before flowering (Dec. 8). The content of endogenous gibberellins increases during the leaf ontogenesis up to the beginning of its senescence. This trend occurs first in the basal leaves (I –III) where the initial increase is followed by the decline in the gibberellin content. On the other hand gibberellins in leaves derived from the apical nodes (VII–VIII) only gradually increase in the course of leaf ontogenesis.     

Dynamics of growth and the content of endogenous phytohormones during kidney bean scoto-and photomorphogenesis

The dynamics of growth and the contents of free and bound endogenous IAA, gibberellins (GA), cytokinins (zeatin and its riboside), and ABA in kidney bean plants (Phaseolus vulgaris L., cv. Belozernaya) grown in darkness or in the light was studied. Phytohormones were quantified in 5–15-day-old plants by the ELISA technique. Plant growth and phytohormone content were shown to depend on plant age and the conditions of illumination. During scotomorphogenesis, changes in the biomass and hypocotyl length were highly correlated with the content of GA, whereas during photomorphogeneses, these parameters were correlated with the content of zeatin. In darkness, epicotyl growth displayed a positive correlation with the content of GA, whereas in the light, the correlation was negative. Growth characteristics of the primary leaves were shown to correlate with IAA in darkness and with GA and zeatin in the light. At a low concentration of cytokinins in illuminated leaves, cell divisions occurred, whereas, at the higher cytokinin concentrations, cell expansion occurred. The highest content of GA was characteristic of leaves in the period of growth cessation. ABA accumulated during active leaf and root elongation and biomass increment in the light and during hypocotyl growth in darkness. After plant illumination, the ratio of auxins to cytokinins increased in bean roots and decreased in their epicotyls. Thus, light changed the developmental programs of bean plants, which was manifested in the changed rate and duration of growth of various organs (root, hypocotyl, epicotyl, and leaf). Some mechanisms of light action depended on the contents of IAA, ABA, GA, and cytokinins and the ratios between these phytohormones. Differences between scotonorphogenesis of mono-and dicotyledonous plants are discussed in relation to the levels of phytohormones in them.     

Endogenous gibberellin levels and senescence in Taraxacum officinale

Summary The level of endogenous gibberellins (GAs) in leaf tissue of Taraxacum officinale was high during leaf growth and expansion but declined progressively during leaf senescence. In the chromatographic system used, most of the GA from Taraxacum leaves moves with the R_f of GA₃. However, several other GAs were also effective in retarding senescence in Taraxacum leaves. It is concluded that ageing of dandelion leaves is associated with a deficiency of endogenous GA.     

Leaf Age as a Determinant in Stomatal Control of Water Loss from Cotton during Water Stress

The stomatal resistance of individual leaves of young cotton plants (Gossypium hirsutum L. var. Stoneville 213) was measured during a period of soil moisture stress under conditions of constant evaporative demand. When plants were subjected to increasing soil water stress, increases in stomatal resistance occurred first on the lower leaves and the stomata on the upper surfaces were the most sensitive to decreasing leaf-water potential. Stomatal closure proceeded from the oldest leaves to the youngest as the stress became more severe. This apparent effect of leaf age was not due to radiation differences during the stress period. Radiation adjustments on individual leaves during their development altered the stomatal closure potential for all leaves, but did not change the within-plant pattern. Our data indicate that no single value of leaf water potential will adequately represent a threshold for stomatal closure in cotton. Rather, the stomatal resistance of each leaf is uniquely related to its own water potential as modified by age and radiation regime during development. The effect of age on stress-induced stomatal closure was not associated with a loss of potassium from older leaves. Increases in both the free and bound forms of abscisic acid were observed in water-stressed plants, but the largest accumulations occurred in the youngest leaves. Thus, the pattern of abscisic acid accumulation in response to water stress did not parallel the pattern of stomatal closure induced by water stress.     

Hormonal Control of Sucrose Phosphate Synthase and Sucrose Synthase in Sugar Beet

We studied the effects of synthetic analogs of phytohormones (benzyladenine, IAA, and GA) on the activities of the enzymes catalyzing sucrose synthesis and metabolism, sucrose phosphate synthase (SPS, EC 2.4.1.14) and sucrose synthase (SS, EC 2.4.1.13), and on the content of chlorophyll and protein during the sugar-beet (Beta vulgaris L.) ontogeny. Plant spraying with phytohormonal preparations activated SPS in leaves; direct interaction between phytohormones and the enzyme also increased its activity. The degree of this activation differed during the ontogeny and in dependence on the compound used for treatment. Analogs of phytohormones maintained high protein level in leaves, retarded chlorophyll breakdown, and, thus, prolonged leaf functional activity during development. Phytohormonal preparations practically did not affect the SS activity both after plant treatment and at their direct interaction with the enzyme. It is supposed that the SS activity in sugar-beet roots is controlled by sucrose synthesized in leaves rather than by phytohormones. The effects of hormones on leaf metabolism were mainly manifested in growth activation.     

Ontogenetic changes in response of adaxial and abaxial epidermal conductances to water stress

The sensitivity of stomata to water stress increased during ontogeny of primary bean leaves growing under constant environmental conditions: the decrease in both adaxial and abaxial epidermal conductances was induced by slightly more negative values of leaf water potential in young than in mature and especially old leaves. However, these differences were less expressive than the differences mentioned in the literature for leaves of different insertion level, where adaptations to microclimatic conditions in different canopy layers in addition to ontogeny may occur.     

Polarity of the stem and ontogenetic changes in sunflow (Helianthus annuus L.) leaves in relation to endogenou cytokinins and ethylene

The content of endogenous cytokinin-like substances and the release of ethylene were determined in leaves of different insertion of sunflower plants during their ontogeny. The content of cytokinin-like substances was highest in the leaves on the middle part of the stem (that is in leaves just before full expansion), with a decrease occurring both towards the base and the apex of the stem, when followed at four growth phases (vegetative plants, plants with inflorescence diameter up to 0.5 cm, plants with inflorescence diameter up to 3 cm, and plants in flower). Changes in the content of cytokinin-like substances during the ontogeny of the leaf also corresponded to this pattern. Data obtained with the leaf at the third node from the basis of the stem showed that the level of cytokinin-like substances first sharply increased, and then after reaching maximal value (at the time when leaf blade area reached approximately 70 % of the final value) slowly and continuously decreased. The highest amount of ethylene released from the leaves was recorded in basal leaves and then also in apical leaves, whereas the leaves with the largest blade area situated at the central part of the stem released the lowest amount of ethylene. This pattern was repeatedly found at all four selected growth phases of sunflower plants.     

Patterns of asymmetry in the twining vine Dalechampia scandens (Euphorbiaceae): ontogenetic and hierarchical perspectives

We studied patterns of fluctuating asymmetry (FA) in leaves of four populations of the neotropical vine Dalechampia scandens to obtain insight into the origin of leaf FA and the level at which it is controlled. We analysed correlations in signed and unsigned asymmetry at different organizational levels. We also analysed the ontogeny of FA during leaf expansion to test whether asymmetry is regulated during cell expansion, and whether fast-expanding leaves are more or less asymmetrical. Signed asymmetry was negatively correlated between successive leaves, that is, when the right side of a leaf was larger than the left side, the next leaf on the shoot tended to show the opposite pattern. The magnitude of FA, however, was very weakly correlated among successive leaves or among leaves measured on different shoots. The direction of asymmetry did not change during leaf expansion, but the relative asymmetry, that is, asymmetry corrected for difference in trait size, decreased during expansion. We found a weak negative relationship between leaf expansion rate and relative asymmetry on the fully expanded leaves. These results suggest that leaf asymmetry in Dalechampia originates from perturbations in cell proliferation in the stem, creating asymmetries in opposite directions in successive leaves. These asymmetries persist during leaf expansion, but tend to be reduced by unknown mechanisms.     

Hormonal changes associated with leaf senescence in cotton (Gossypium barbadense)

To study the hormonal regulation of foliar senescence in cotton ( Gossypium barbadense L. cv. Giza 68, long staple), the sequential changes in gibberellins (GAs), indoleacetic acid (IAA) and abscisic acid (ABA) were examined in the cotyledons from the completion of expansion through senescence (days 12-24 after sowing). The onset of senescence could be detected on day 20, the stage of maximum accumulation of leaf metabolites. At this stage, free GAs quickly lost more than 40% of their initial activity. Further decrease of free GAs was then characteristically observed in the senescent leaves. A remarkable increase in free IAA and free ABA between days 18 and 20 immediately followed by nutrient depletion, suggests the contribution by both hormones to the senescence system. The definite drop in free IAA below its initial level occurred on day 24, when most of the leaf protein and chlorophyll were already broken down.     

The role of hormonal balance in plant adaptation to flooding

The effect of flooding on the growth parameters and hormonal dynamics (anxins, abscisic acid, cytokinins, gibberellins, and ethylene) has been studied in a vegetation experiment on the leaves of wheat (Triticum aestivum L.) and oat (Avena sativa L.). Growth inhibition during flooding in both species was due to the accumulation of abscisic acid and ethylene, while the repair processes were due to the increased level of auxins, cytokinins, and gibberellins. The difference in the hormonal response in wheat and oat to flooding, in particular, the degree and timing of accumulation of abscisic and indoleacetic acids and different dynamics of the level of cytokinins and gibberellins, induced their different physiological response, which determined the level of their resistance. The growth control of cereals during flooding as well as the hormonal dynamics are proposed to rely on the strategy of plant ontogenetic adaptation.     

Effect of Leaf Age on Potassium Efflux and Net Flux in Tomato Leaf Slices

The effects on leaf age on K (⁸⁶Rb) efflux, influx and net fluxinto lamina slices from leaf 7 on a tomato plant (Lycopersiconesculentum Mill.) were determined. The ontogenetic trend inK efflux was dependent on the external K concentration. At externalKCI concentrations between 0.5 and 10.0 mM, K efflux rates increasedduring leaf elongation. Only a small increase in efflux occurredin mature leaves with increasing age. It is suggested that thetonoplast retains its structural integrity through the initialstages of leaf senescence. In fully expanded leaves, a zeronet K flux (a balance between influx and efflux) was achievedat external KCI concentrations between 1.0 and 3.5 mM. The Kcontent of lamina slices from leaves 5 and 13 remained constantwhen bathed in a solution containing 2 to 3 mM K. It is suggestedthat the decline in K concentration in mature tomato leaf tissueis due to a decline in leaf free space K concentrations below1 to 3 mM which would result in a net efflux out of leaf cells. Lycopersicon esculentum Mill., tor ato, free space, ion fluxes, leaf age, leaf ontogeny, potassium     

Gibberellin-induced delay of leaf senescence of Alstroemeria cut flowering stems is not caused by an increase in the endogenous cytokinin content

The effects of various chemically pure gibberellins and cytokinins on leaf yellowing of Alstroemeria were described. The loss of chlorophyll was measured both in leaves of cut flowering stems and in a model system consisting of detached leaf tips. It was demonstrated that plant growth substances affected chlorophyll loss in both systems to the same extent. Leaf senescence was delayed by various gibberellins and cytokinins. The results demonstrated that some of the gibberellins (GA₄ and GA₇) are far more effective in delaying chlorophyll loss than GA₃, which is commonly used as a postharvest treatment for Alstroemeria cut flowering stems. Immunoassays were used to demonstrate that the effect of gibberellins on leaf yellowing does not involve an increase in the endogenous cytokinin concentrations in the leaves as an intermediate step.Abbreviations GA gibberellin A - HPLC high performance liquid chromatography - GA_3Mc GA₃-methyl ester - ZR zeatin riboside - IPAR isopentenyl adenine riboside.     

Effects of Leaf Age and Position on the Shoot Axis on Potassium Absorption by Tomato Leaf Slices

The effect of leaf age on K (⁸⁶Rb) influx into tomato (Lycopersiconesculentum Mill.) leaf lamina slices was determined for leaves5, 9 and 13 counting acropetally. Potassium influx rates expressedon a leaf fresh weight basis declined rapidly during leaf elongationat external KCI concentrations between 0.5 and 20.0 mM. In fullyexpanded leaves, K influx rates declined more slowly with age.The onset of senescence in mature leaves did not result in alarge loss in K uptake capability. Leaf position on the shootaxis and the stage of whole plant development had little influenceon K influx into leaf cells. It is suggested that the rapiddecrease in K influx in growing leaves is related to a dilutionin the concentration of K transporter sites resulting from anincrease in cell volume and weight. Lycopersicon esculentum Mill, tomato, free space, potassium, influx rate, ion uptake, leaf slices, leaf age leaf ontogeny     

Proline involvement in the common sage antioxidant system in the presence of NaCl and paraquat

To elucidate proline antioxidant properties in common sage (Salvia officinalis L.) plants, they were treated with paraquat (a producer of superoxide radical) and/or NaCl and also with paraquat and proline at the stage of 4–5 true leaves. The paraquat solution (1 ml containing 0.1 μmol of the agent) was applied to the leaf surface; NaCl (200 mM) and proline (the final concentration of 5 mM) were added to nutrient medium. Experimental plants were firstly kept in darkness for 12 h, then illuminated, and in 3, 6, and 12 h, leaves and roots were fixed for biochemical analyses. The results obtained are in agreement with the supposition of proline antioxidant properties. In particular, it was established that paraquat induced a slight increase in the proline level in the leaves during dark period of plant growth and also during subsequent 3 h after light switching on. This transient proline accumulation in the leaves was accompanied by its level decrease in the roots. Proline addition to the nutrient medium of paraquat-treated plants neutralized paraquat damaging action on the leaves. In the presence of paraquat, proline treatment reduced the accumulation in the roots of hydrogen peroxide and malondialdehyde, the product of membrane lipid peroxidation. It also affected indirectly the activities of superoxide dismutase (SOD) and free, covalently bound, and ionically bound peroxidases. Keeping in mind that, in the presence of paraquat, superoxide-induced changes in SOD activity in the roots were negatively correlated with the level of proline, which content was the highest during the last hours of experiments, we can conclude that proline antioxidant effects are manifested only after 12 h of stressor action, whereas antioxidant enzymes are involved in ROS scavenging during the earlier stage of damaging factor action.     

A dynamic analysis of the shade-induced plasticity in Arabidopsis thaliana rosette leaf development reveals new components of the shade-adaptative response

BACKGROUND AND AIMS: It is well known that plant aerial development is affected by light intensity in terms of the date of flowering, the length of stems and petioles, and the final individual leaf area. The aim of the work presented here was to analyse how shade-induced changes in leaf development occur on a dynamic basis from the whole rosette level to that of the cells. METHODS: Care was taken to ensure that light intensity was the only source of micro-meteorological variation in the study. The dynamics of leaf production, rosette expansion, individual leaf area expansion and epidermal cell expansion were analysed in Arabidopsis thaliana plants grown under two light intensities in three independent experiments. KEY RESULTS: The total area of rosette leaves was reduced by the shading treatment. Both the number of leaves produced and their individual leaf areas were reduced. The reduction in leaf number was associated with a reduction in leaf initiation rate and the duration of the phase of leaf production. The reduction in individual leaf area was associated with a reduction in leaf expansion rate and an increase in the duration of leaf expansion. The changes in leaf expansion dynamics were accompanied by a decrease in epidermal cell number which was partly compensated for by an increase in epidermal cell area. Overall, the whole rosette leaf expansion rate was reduced by shading, whereas the total duration of rosette leaf expansion was unaffected. This was mainly due to the accumulation of the increases in the durations of expansion of each individual leaf which was associated with an increase in cell expansion. CONCLUSIONS: The dynamic analysis presented here reveals a new shade-adaptative response mediated via the control of area expansion at the cell, organ and whole plant levels.     

Photosynthesis on individual leaves of sugar beet (Beta vulgaris) during the ontogeny at variable water regimes

It is well known that the extent of yield reduction depends not only on the severity of water stress but also on the stage of plant development. Assessing photosynthetic response of individual leaves to water deficit during the ontogeny may, therefore, offer a clue to better understand the whole plant behaviour. This research aimed at investigating the influence of early and late water stress on net photosynthesis (P_n), carbon‐isotope discrimination and other related traits on individual leaves during ontogeny. Sugar beet plants were grown in rain‐sheltered soil columns of relevant volume (300 L), subdivided into well‐watered (WW); early (S1) and late (S2) stress. In general, water stress significantly reduced leaf lifespan and P_n. Relieving the stress at about one‐third and two‐thirds of potential leaf life substantially restored P_n at the levels of WW. Stressing a previously WW leaf brought about a comparatively heavier loss than stressing a leaf since the beginning. As for leaves at different phenological times, the early leaves had higher initial photosynthetic peaks but steeper falls during their lives. An insight into the relationships between P_n and substomatal CO₂ concentration (C_i) shows that in mature leaves the photosynthetic restoration following stress relief did not entail a full recovery of the electron transport rate, the parameter most severely affected by the stress. The partial reversibility of the effects of water deficiency, associated to the anticipated leaf senescence and to the natural slow‐down of net assimilation during leaf life, may be seen as a key factor in predicting to what extent the plant can tolerate drought and the damages caused by water stress.     

Variation in heteroblastic succession among populations of Crepis tectorum

Eleven populations of the monocarpic species, Crepis tectorurn (Asteraceae) in South Sweden differed in the extent to which different leaf characters changed during plant ontogeny. Multivariate analyses on sequences of leaf samples collected from greenhouse-grown plants of different ages revealed a group structure of populations that was different from that revealed by analysing variation in leaves from adult plants. Ontogenetic data suggested that populations within a weed and an alvar ecotype constituted more natural groups than combinations of populations from both eco-types that had similar leaves at adult stages, supporting the hypothesis that these ecotypes may have different migration histories in this area. Variation among the populations in leaf characteristics was partly due to differences that were present at all ontogenetic stages and partly due to variation in the rate of development of some of the leaf characters during plant ontogeny. Plants in some populations appeared to have more 'juvenile' leaves at adult stages than plants in other populations, at least in some characters.