The effect of dark and light treatment on endogenous cytokinins in internodes and buds of Iris was determined. Plant material was purified by chromatographic methods and cytokinins were assayed by an immunoassay .An indirect competitive enzyme immunoassay for the determination of zeatin-and isopentenyl-adenine cytokinins was developed . This assay, which is not dependent on the titre of the antibodies raised against zeatin riboside and isopentenyl-adenosine appeared to be specific, highly sensitive and more reproducible compared to a direct competitive enzyme immunoassay for cytokinins .Isopentenyl-adenosine was the most abundant cytokinin found, followed by zeatin : the latter counteracts bud blast when injected into dark-treated plants . Smaller amounts of isopentenyl-adenine and zeatin riboside were found . Results are in agreement with the hypothesis that deficiency of growth substances like cytokinins plays an important role in the occurrence of flower-bud blasting.A possible role for the major endogenous cytokinin, isopentenyl-adenosine, which earlier was found not to be effective in counteracting bud blast when injected into buds of dark-treated plants, is discussed .
The hypersensitive reaction of Samsun NN tobacco leaves to tobacco mosaic virus (TMV) was accompanied by a large increase in ethylene production, just before necrotic local lesions became visible. Normal and virus-induced ethylene production were both largely inhibited by 0.1 milimolar aminoethoxyvinylglycine indicating that methionine is a main ethylene precursor.The contribution of methionine to ethylene production was estimated by labeling leaves with L-IU-14Clmethionine and comparing the specific activities of methionine within and ethylene produced by the leaf. When taken up through the petiole, methionine was largely retained in the veins, leading to production of ethylene with a far higher specific activity in the veins than in the interveinal tissue. After TMV infection, ethylene production increased only in the interveinal tissue, resulting in a decrease in specific activity of the ethylene produced. In the interveinal tissue, the specific radioactivity of the ethylene was lower than expected if methionine were the only precursor. After labeling by vacuum infiltration, the specific activities of the ethylene produced by water-and TMV-inoculated leaves were both identical and in accordance with the specific radioactivity of methionine. Inasmuch as the content of 1-aminocyclopropane-1-carboxylic acid was increased severalfold two days after TMV infection, methionine can be considered to be the only ethylene precursor in healthy and in TMV-infected tobacco leaves.The increase in ethylene production after TMV-infection was not accompanied by an increased concentration of free methionine within the leaf. Compartmentation of methionine does not appear to be a regulating factor since labeled methionine supplied to the leaf by vacuum infiltration is equilibrated very rapidly with any methionine pool within the leaf ceUls.
Flower-bud blasting in Iris occurs in the winter when low light intensities and short days prevail. After introduction of 14 CO, to one leaf the transport of assimilates was studied under controlled culture conditions in a control light treatment and in a treatment of 7 days darkness followed by standard light conditions . Little assimilate transport was found in the direction of the bud in dark-treated plants . However, zeatin injection into the flower buds of the plants subjected to the dark treatment clearly promoted assimilate transport to these buds. Abscisic acid levels, determined by gas chromatography, were found to increase in the buds of dark-treated plants . Zeatin injection into the flower bud resulted in a suppression of the abscisic acid level . The latter treatment also resulted in higher percentage of flowering . Removal of flower parts was found to inhibit peduncle elongation . The peduncle elongation of complete flowers started in a well defined period, and the fresh weight of buds was found to increase mainly in the last part of that period . Assimilate transport under low light intensities in relation to abscisic acid and supposed gibberellin is discussed .
SUMMARY
In the winter period with prevailing low light intensities and short days, the forcing of Iris “Wedgwood” can easily result in flower bud abortion (bud blast). Consequently a high proportion of plants fail to produce flowers.
To elucidate the cause of bud blast, plants were subjected to various treatments —12CO2, 14CO2, growth substances — in the period of rapid stem elongation about two weeks before flowering.
Plants grown under weak illumination and normal CO2 concentration (leading to 57% flowering) were compared with plants grown under strong illumination at low CO2 concentration (leading to 100% flowering). As the dry weight of whole plants of both these groups was almost the same at flowering, bud blast under low illumination cannot simply be attributed to a lack of recent photosynthetic products. The distribution pattern of photosynthates appears to be changed.
It is suggested that a hormone imbalance plays a role in the distribution pattern and consequently in bud blast, since injections of cytokinins, such as N6‐benzylaminopurine, zeatin, and kinetin, during dark treatment can increase the flowering proportion.
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