Ph. Barthe

Changes in abscisic acid biosynthesis and catabolism during dormancy breaking in Fagus sylvatica embryo

Abstract. At harvest, embryos of Fagus sylvatica are dormant. A cold pretreatment without medium at 30% moisture content allowed them to germinate. A comparison of the abscisic acid (ABA) content before and after the pretreatment has no significant relevance since dormancy is expressed during the culture at 23°C. During this culture, both de novo biosynthesis and conjugate hydrolysis contributed to maintain a high level of ABA in the dormant axis. The level of conjugates and the rate of hydrolysis were not modified substantially by the cold pretreatment. In contrast, the dormancy release was associated with a strong decrease in the capacity for ABA synthesis. Moreover, feeding (+)-[3H]ABA to untreated and pretreated embryos proved that the cold treatment also induced a hastening of ABA catabolism.

Bound and Free Abscisic Acid Levels in Dormant and After-ripened Embryos of Pyrus malus L. cv. Golden delicious

Summary The endogenous levels of both cis and trans isomers of free and bound ABA, were studied in dormant and after-ripened embryos of Pyrus malus L. cv. Golden delicious. Wheat cole-optile straight-growth test and gas-liquid chromatography (GLC) were used as quantitative methods. The level of free ABA was very high in dormant embryos, mainly in cotyledons, but turned very low after three months of stratification. Bound ABA increased considerably and became dominant in after-ripened embryos. Bound trans-ABA was found only in cotyledons of after-ripened embryos. The relation between free and bound ABA levels of apple embryos and their capacity to germinate is discussed.

Oxygen availability and ABA metabolism in Fagus sylvatica seeds

At harvest, beechnuts have a very deep dormancy whichlies both in the structures surrounding the embryo andwithin the embryo itself. Covering structures preventembryo germination by interfering with water uptakeand gaseous exchange. To understand the role of thecovering structures and oxygen availability on ABAcatabolism, (+)-[3H] ABA metabolism was studied in isolated embryos as well as in intact seeds. ABAdegradation resulted essentially in oxidative products(PA, DPA). These products were more abundant inisolated embryos than in intact seeds. Theyaccumulated mainly as alkali-nonhydrolyzableconjugates of DPA. A small amount of free andesterified forms were oftenobserved. In isolated embryos a decrease in oxidativeproducts was observed either by lowering the oxygenavailability or by feeding embryos with tetcyclasis(an inhibitor of monooxygenase). In the presence of the covering structures, these oxidative products werereduced in the same manner, indicating that coveringstructures were probably responsible for limiting theoxygen supply to the embryo and for the lowgermination percentage observed in the case of intactseeds.

Regulation of ABA levels in senescing petals of rose flowers

During the vase life of a rose flower, changes in the levels of abscisic acid (ABA) were observed: a decrease during the first 3 days, followed by a steady state at a low level, and finally a sharp increase in late senescence. Feeding [2-14C]ABA to isolated petals showed that metabolism was very active despite the age of the flower, oxidation processes increased with age, whereas conjugation decreased but the level of nonmetabolized ABA remained stable. When the isolated petal was subjected to water stress, whatever its age, the ABA level increased. Hydrolysis of ABA-GE was not involved in this phenomenon. Thus, ABA synthesis occurred in the isolated petal; it could be directly correlated to the decrease in water potential. However, the ABA increase in isolated petals was limited. Moreover, on the rose tree, increases in ABA levels were not correlated to water potential changes. ABA levels seemed, therefore, mainly regulated by changes in import from leaves and other parts of the flower.

ABA Involvement in the Psychrolabile Dormancy of Fagus Embryo

Embryos of Fagus sylvatica isolated from fruits at harvest were dormant. After a cold-treatment in restricted water conditions, embryos were able to germinate at a percentage which increased with chilling duration. Embryo ABA content decreased during the dormancy-releasing treatment at 4°C; it also decreased in the same proportions during embryo culture at 23°C, a temperature allowing dormancy to be strongly expressed. It thus appears that embryo ABA content was not correlated with the physiological potentialities of the embryo. The significant decrease in ABA level observed during culture at 23°C could be explained by a very rapid ABA metabolism, revealed by the results of [3H]ABA feeding experiments. Also, when fluridone, an inhibitor of carotenoid synthesis, was applied directly to axes, dormant embryos were able to germinate after a one-week culture. The comparison between axis ABA content in the absence or in the presence of fluridone after a 6-day culture gave an estimation of the axis capacity for a de novo ABA synthesis. Consequently, it appears that the psychrolabile dormancy of Fagus is associated with the ability of the axis to synthesize its own ABA.

Metabolism of [2-14C]abscisic acid in dormant and Afterripened Apple Embryos

Summary Dormant and after-ripened embryos of Pyrus malus L. var. Golden delicious were incubated with (±) [2- 14 C]abscisic acid at 10 -5 M (947 MBq mmol -1 ) for various periods (2, 6, 9,12 days) at 24 °C in darkness. The embryos were extracted in 80 % ethanol. Two acidic diethyl-ether phases which contained the free acids and the acids released after mild alkaline hydrolysis, respectively, were analysed as well as the radioactivity which remained in the final aqueous phase. The results obtained indicated that ABA metabolism was very active in both cases since, despite continuing absorption, the percentage of the radioactivity represented by abscisic acid decrease as a function of time. Greater amounts of abscisic acid remained in after-ripened embryos than in dormant embryos. The surplus of abscisic acid metabolized was found almost exclusively in the form of a-D-glucopyranoside abscisate while distribution of the radioactivity corresponding to other metabolites was not modified. There is, therefore, a tendency in dormant embryos towards the formation of β-D-glucopyranoside abscisate, a substance the role of which as a potential source of ABA has already been demonstrated in this material.

Changes in the buffering capacity of cell sap in senescing rose petals

Abstract An increase in the pH of cell sap (pHs) was observed during the ageing of rose petals (Rosa hybrida L.) cultivar ‘Sweet Promise’. Changes in the buffering capacity of cell sap (βs) were determined during petal life time, in order to correlate the increase in pHs and the variation of the βs. The βs was minimal in a region slightly more alkaline than that of the pHs. The minimum value of βs increased with age from 2 to 10 μmol H+ pH unit−1 ml−1 cell sap. In the acidic region, the βs, which was relatively low in young petals, increased sharply later. In the alkaline region the βs of young petals (1 day) was very low (less than 4 μmol H+ pH unit−1 ml−1 at pH 7.5). In senescing petals, on the other hand, a very high buffering capacity could be observed. The increase in βs in the low range of pH values was correlated with a marked increase in the malate content in senescing petals. It has been generally assumed that the pHs of cell sap extracted by a freeze-thaw method is representative of the vacuolar pH (pHv). The changes with time of pHv, determined by a labelled chemical probe, were compared with those of pHs. In senescing petals, pHv, unlike pHs, showed a slow but clear decrease. The variations observed in βs and the increase of the difference (pHs–pHv) in ageing petals are discussed. They are consistent with increasing contamination of vacuolar sap by cytoplasm during cell sap extraction.

Cytokinines fibres et liées dans les axes embryonnaires et les cotylédons des embryons dormants et non dormants de pommier

Summary The endogenous levels of free and bound cytokinins were studied in dormant and afterripened embryos of Pyrus malus L. cv. Golden delicious. The postmaturation treatment was a three months stratification at 4 °C in the seeds. Qualitative and quantitative studies were made using paper chromatography, sephadex gel chromatography and tobacco callus bioassay. In the embryonic axes from dormant or after-ripened embryos the cytokinins levels were found very similar: relatively important for the free forms, negligeable for the bound ones. In the cotyledons of dormant embryos the bound forms were found in very high amounts, whereas the free forms existed in negligeable quantities. The postmaturation treatment induced a spectacular decrease of the bound forms with an important increase of the free forms, the latter being found in lesser quantities than could be deduced from the losses of the bound forms. Our results showed that complex qualitative changes were also induced under the influence of the postmaturation treatment.