Control of senescence in rumex leaf discs by gibberellic Acid

The kinetics of chlorophyll and protein decomposition and the effect of gibberellic acid (GA) were examined in senescing leaf discs of Rumex crispus and R. obtusifolius. Loss of Rumex total chlorophyll proceeds at a slow rate for about 2 days followed by a period of rapid logarithmic decline. Chlorophyll b is lost at a slightly faster rate than chlorophyll a during senescence in discs as well as in situ. GA causes a complete cessation of net chlorophyll and protein degradation for several days in Rumex, in contrast to the incomplete senescence inhibition generally observed with cytokinins. GA is fully effective even when added at the middle of the logarithmic phase of chlorophyll loss. Senescence inhibition by GA is apparently gradually reversed upon GA removal. The cytokinins, kinetin and 6-benzylaminopurine, were also effective in Rumex leaf discs, indicating that the senescence retarding effect was not restricted to the gibberellins.     

Hormonal control of somatic embryo development from cultured cells of caraway: interactions of abscisic Acid, zeatin, and gibberellic Acid

The effects of abscisic acid, zeatin, and gibberellic acid on the development of somatic embryos from cultured cells of caraway (Carum carvi L.) were observed.     

The effects of abscisic acid on senescence in leaf discs of radish,Raphanus sativus L.

Summary After a 6 day incubation period, abscisic acid (ABA) at 10^-4 M retarded the decline in pigment levels and promoted the decline in protein levels of radish leaf discs. ABA treatment also retarded the rise in the specific activity of the RNA fraction (calculated by counts per minute incorporated of ¹⁴C-8-adenine as a fraction of optical density at 260 nm) observed in water-treated control discs. The results indicated that ABA was primarily effective in enhancing senescence in the early stages following leaf excision. Thus the increase in RNA specific activity during an initial 24 h incubation period was especially pronounced with ABA treatment although there was no effect of the hormone on RNA level. Moreover, in contrast to control discs, the pigment levels declined markedly in ABA-treated discs in this period. When the discs had been incubated in water (preaged) for 3 or 5 days prior to ABA treatment, however, the hormone then had little effect on RNA metabolism and protein and pigment levels relative to the water control.Data are collated from different experiments to show the changes in RNA, pigment and protein with ABA treatment during a 6 day senescence period.It is considered that ABA is speeding up the natural changes in RNA metabolism possibly by affecting both RNA synthesis and degradation.     

Role of indoleacetic Acid and abscisic Acid in the correlative control by fruits of axillary bud development and leaf senescence

When fully filled pods of bean plants were deseeded, the rate of axillary bud growth and the chlorophyll content of leaves were increased. Application of 0.1% indoleacetic acid (IAA) in lanolin on the deseeded pods caused abscission of axillary buds, inhibited growth of the remaining buds, and decreased leaf chlorophyll content. The response of bud development to fruit-applied IAA was concentration dependent between 0.001 and 0.1% IAA (representing from 2 to 200 micrograms IAA per fruit) resulting in greater growth inhibition at higher IAA concentrations.     

A rapid method for the extraction and analysis of abscisic Acid from plant tissue

A simple, economical, and rapid method for the purification of plant extracts prior to abscisic acid (ABA) analysis is described. The method makes use of silica Sep-pak prepacked cartridges. The ABA extracts are loaded on to the Sep-pak cartridges which are then washed with a series of solvents resulting in the removal of pigments and other unwanted compounds. The ABA is then eluted from the cartridge and the levels of this hormone are estimated by gas chromatography. The whole technique (from maceration of the tissue to measurement of ABA levels) takes only 2 to 3 hours per sample.     

Interactions of abscisic acid and sugar signalling in the regulation of leaf senescence

Leaf senescence can be triggered by a high availability of carbon relative to nitrogen or by external application of abscisic acid (ABA). Most Arabidopsis mutants with decreased sugar sensitivity during early plant development are either ABA insensitive (abi mutants) or ABA deficient (aba mutants). To analyse the interactions of carbon, nitrogen and ABA in the regulation of senescence, wild-type Arabidopsis thaliana (L.) Heynh. and aba and abi mutants were grown on medium with varied glucose and nitrogen supply. On medium containing glucose in combination with low, but not in combination with high nitrogen supply, senescence was accelerated and sucrose, glucose and fructose accumulated strongly. In abi mutants that are not affected in sugar responses during early development (abi1-1 and abi2-1), we observed no difference in the sugar-dependent regulation of senescence compared to wild-type plants. Similarly, senescence was not affected in the sugar-insensitive abi4-1 mutant. In contrast, the abi5-1 mutant did exhibit a delay in senescence compared to its wild type. As ABA has been reported to induce senescence and ABA deficiency results in sugar insensitivity during early development, we expected senescence to be delayed in aba mutants. However, the aba1-1 and aba2-1 mutants showed accelerated senescence compared to their wild types on glucose-containing medium. Our results show that, in contrast to sugar signalling in seedlings, ABA is not required for the sugar-dependent induction of leaf senescence. Instead, increased sensitivity to osmotic stress could have triggered early senescence in the aba mutants.Abbreviations ABA Abscisic acid - aba Abscisic acid deficient - abi Abscisic acid insensitive - F_v/F_m Maximum efficiency of photosystem II photochemistry     

Lettuce seed germination: modulation of pregermination protein synthesis by gibberellic Acid, abscisic Acid, and cytokinin

Protein synthesis in gibberellin-treated lettuce (Lactuca sativa) seeds has been studied during the lag phase between the beginning of imbibition and the first signs of radicle protrusion. When compared to the water-imbibed controls, both polyribosome populations and radioactive leucine incorporation into protein increase in the embryos of GA₃- induced seeds early in the imbibition period. Since these results are contradictory to previously published studies, the reasons for the differences are outlined and various alternative possibilities eliminated. The protocol for protein extraction, particularly the speed at which the supernatant from the seed homogenate is cleared, is important for demonstrating the GA₃-mediated changes. Embryos maintained in the dormant state by abscisic acid still conduct considerable amounts of protein synthesis, and this is enhanced by concentrations of 6-benzylaminopurine which also promote germination. Therefore, the actions of GA₃, abscisic acid, and cytokinin on lettuce seed germination are mediated, directly or indirectly, via protein synthesis.     

Cytokinin biochemistry in relation to leaf senescence I. The metabolism of 6-benzylaminopurine and zeatin in oat leaf segments

Journal of Plant Growth Regulation - The metabolism of zeatin and that of 6-benzylaminopurine (BAP) have been compared in oat leaf segments in relation to the markedly differing ability of these...     

Interrelationships of ethylene and abscisic Acid in the control of rose petal senescence

The role of abscisic acid and ethylene in the senescence of rose petals cv. Golden-Wave was examined. A rise in ethylene evolution, followed by an increase in the level of abscisic acid was observed. The presence of abscisic acid in rose petals was established, using different chromatography systems, several bioassays, and immunoassay. External application of ethylene accelerated senescence and induced a rise in endogenous abscisic acid-like activity. Application of abscisic acid promoted senescence, but suppressed ethylene production. The data suggest that the participation of these two hormones in the control of senescence is via the same pathway. The possibility of interrelationship between abscisic acid and ethylene was tested and experimental evidence in favor of this hypothesis is presented. It was suggested that ethylene affects senescence in rose petals by inducing an increase in abscisic acid activity, which in turn may control ethylene evolution, via a feedback mechanism.     

Identification of abscisic acid glucose ester,indole-3-acetic acid,zeatin and zeatin riboside in receptacles of pear

The identity of abscisic acid glucose ester, indole acetic acid, zeatin, and its riboside in pear receptacles was revealed by use of chromatographic, ultraviolet and mass spectral analysis.     

Opposing effects of abscisic acid on senescence of rose flowers

Abscisic acid induced stomatal closure and reduced water lossof cut rose flower shoots bearing leaves. Its effect was verypronounced when these flowers were exposed to atmospheric stressconditions. Even short-term treatments (24 hr) with ABA extendedthe longevity of the flowers. However, in a stomata-less system(leafless flower shoots), or in leafy shoots held in darknesswhen all stomata were closed, ABA enhanced aging of the flowersand some biochemical processes associated with it (RNase activityand reduction in protein content). ¹Agricultural Research Organization, The Volcani Center.²Present address: Dept. of Environmental Horticulture, Universityof California, Davis, Calif. (Received May 2, 1974; )     

Effects of gibberellic acid and zeatin on the growth response of cucumber cotyledons

In contrast to cytokinin, gibberellic acid has no effect on the growth of the isolated cucumber cotyledon in darkness. Like cytokinins in light, gibberellic acid causes increases in fresh weight and area of the cotyledon at concentrations from 10^–7 to 10^–3 M. Radiant energies in the blue, red, and far-red regions of the spectrum all induce the growth responses to gibberellic acid. The effect of the far red is greater than that of the red, which is greater than that of the blue. Gibberellic acid is ineffective in the promotion of chlorophyll development, whereas cytokinins are very effective. Although zeatin and gibberellic acid both cause an increase in fresh weight and area of the cotyledons in light, they appear to have entirely separate actions in the growth responses.     

Effect of light and gibberellic acid on photosynthesis during leaf senescence of alstroemeria cut flowers.

The functioning of the photosynthetic apparatus during leaf senescence was investigated in alstroemeria cut flowers by a combination of gas-exchange measurements and analysis of in vivo chlorophyll fluorescence. Chlorophyll loss in leaves of alstroemeria cut flowers is delayed by light and by a treatment of the cut flowers with gibberellic acid (GA₃). The maximal photosynthesis of the leaves was approximately 6 μmol CO₂ m⁻² s⁻¹ at I 350 μmol m⁻² s⁻¹ (PAR) which is relatively low for intact C₃ leaves. Qualitatively the gas-exchange rates followed the decline in chlorophyll content for the various treatments, i.e. light and GA₃-treatment delayed the decline in photosynthetic rates. However, when chlorophyll loss could not yet be observed in the leaves, photosynthetic rates were already strongly decreased. In vivo fluorescence measurements revealed that the decrease in CO₂ uptake is (partly) due to a decreased electron flow through photosystem II. Furthermore, analysis of the fluorescence data showed a high nonphotochemical quenching under all experimental conditions, indicating that the consumption of reducing power in the Calvin cycle is very low. The chlorophyll, remaining after 9 days incubation of leaves with GA₃ in the dark should be considered as a 'cosmetic' pigment without any function in the supply of assimilates to the flowers.     

The effects of gibberellic acid,fluridone, abscisic acid and paclobutrazol on anther culture of Brussels sprouts

Both gibberellic acid (GA₃) and fluridone, a non-specific inhibitor of ABA biosynthesis, promoted embryo production in anther cultures of Brussels sprouts cv. Hal, but not in cv. Gower. Abscisic acid (ABA) and the gibberellin-biosynthesis inhibitor paclobutrazol inhibited embryo production in both cultivars.     

Effects of early-fruit removal on endogenous cytokinins and abscisic acid in relation to leaf senescence in cotton

Numerous studies have shown that early-fruit removal enhances vegetative growth and development of cotton (Gossypium hirsutum L.). However, few studies have examined changes in leaf senescence and endogenous hormones due to fruit removal. The objective of this study was to determine the correlation between some endogenous phytohormones, particularly the cytokinins and abscisic acid (ABA), and leaf senescence following fruit removal. Cotton was grown in pots and in the field during 2005 and 2006. Two early-fruiting branches were excised from plants at squaring to form the fruit removal treatment while the non-excised plants served as control. Plant biomass, seed cotton yield, cytokinins and ABA levels in main-stem leaves and xylem sap as well as main-stem leaf photosynthetic rate (Pn) and chlorophyll (Chl) concentration were determined after removal or at harvest. Fruit removals increased the leaf area, root and shoot dry weight and plant biomass at 35 days after removal (DAR), whether in potted or field-grown cotton; under field conditions, it also improved plant biomass and seed cotton yield at harvest. The Pn and Chl concentration in excised plants were significantly higher than in control plants from 5 to 35 DAR, suggesting that fruit removal considerably delayed leaf senescence. Fruit-excised plants contained more trans-zeatin and its riboside (t-Z + t-ZR), dihydrozeatin and its riboside (DHZ + DHZR), and isopentenyladenine and its riboside (iP + iPA) but less ABA in both main-stem leaves and xylem sap than control plants from 5 to 35 DAR. These results suggest that removal of early fruiting branches delays main-stem leaf senescence, which can be attributed to increased cytokinin and/or reduced ABA. Cytokinin and ABA are involved in leaf senescence following early fruit removal.     

The effect of sucrose on the levels of abscisic acid, indoleacetic acid and zeatin/zeatin riboside in wheat ears growing in liquid culture

The influence of a varied sucrose supply on grain size and hormonal contents of detached wheat ears ( Triticwn aestivum L. cv. Schirokko) was investigated throughout grain development. In ears led limited amounts, or no sucrose, grain weights in both proximal and distal grain positions of the ear were reduced. Radioimmunoassay for abscisic acid, indoleacetic acid and zeatin/zeatin riboside showed that the changes in the levels of these hormones in grains and bracts were comparable to intact ears when detached ears were well supplied with sucrose. Under conditions of limited sucrose supply, higher abscisic acid levels in the distal and proximal grains of detached ears were found compared to ears supplied with adequate sucrose. Limiting sucrose supply to the ear did not alter the levels of indoleacetic acid or zeatin/zeatin riboside in either the grains or bracts of detached ears.     

Effect of obstructed translocation on leaf abscisic Acid, and associated stomatal closure and photosynthesis decline

Pod removal or petiole girdling, which causes obstruction of translocation, was found in our previous study to cause reduced rates of photosynthesis in soybean leaves due to stomatal closure. The purpose of this research was to determine the involvement of photoassimilate accumulation and leaf abscisic acid (ABA) levels in the mechanism of stomatal closure induced by such treatments.