Studies on the involvement of abscisic acid in the regulation of dormancy in fruits of Lactuca sativa L

• Main Author: McWha, James Alexander
• Format: Thesis
• Language: English
• Published: ProQuest Dissertations & Theses 1973
• Subjects: Arctic
• Online Access: http://theses.gla.ac.uk/73922/; http://theses.gla.ac.uk/73922/1/10662700.pdf

In a study of structure-activity relationships, ABA and thirty-two of its analogues have been tested in the lettuce germination, lettuce hypocotyl and pen epicotyl bioassays. ABA delays germination of lettuce, a delay which cannot be overcome by the application of GA3 although kinetin is capable of causing at least a partial reversal. In the lettuce hypocotyl and pea epicotyl assays, ABA inhibits GA3-induced growth but is less effective as an inhibitor in the absence of GA3. This results in the existence of a statistical interaction between GA3 and ABA in these assays, although the relevance of this observation to an understanding of physiological mechanisms is debatable. Twelve of the analogues are more active than ABA as inhibitors of lettuce germination. Assessment of the molecular requirements of ABA for activity revealed that the requirements are similar in all three bioassay systems. The carbonyl and/or hydroxyl groups in the ring are necessary for inhibitory activity as is the unsaturated bond. The double bond at C-2 in the pentadienoic acid side chain is also an important feature of the molecule for ABA-like activity, although other changes in the degree of saturation and to the terminal moiety can be made without destroying the ability of the molecule to act like ABA. The side chain, however, if retaining the pentadiene structure must be the cis, trans-isomer, the trans, trans-isomer being inactive. There are two phases of 2-[14C]-ABA uptake in lettuce fruits. Although the first phase appears to relate to the imhibition of water, uptake continues for three hours following complete imhibition. The second phase corresponds with the commencement of radicle extension. Studies of the distribution after uptake of radioactive ABA by soluble compound microautoradiography were affected by the appearance of light reflecting bodies within the cotyledonary cells. The bodies were similar in size and optical properties to silver grains, rendering the results of automatic grain counting techniques and visual comparisons of the treatments misleading. The appearance in the tissues of these artefacts was caused by fixation in the proprietory fixers, "Kodafix", "Motafix" and "Unifix", but not in either sodium or ammonium thiosulphate. The source of artefacts has been shown, by light and electron microscopy, to be protein storage bodies, many of which contain small inclusions. Treatment of the tissue with photographic fixer causes these inclusions to be released. The distribution of silver grains, and thus of the radioactivity, could be ascertained by removing the section from the slide and examining the latent image which remained in the emulsion. This revealed that fruits imbibed in 14C-ABA had radioactivity uniformly distributed throughout the cotyledons, although after radicle extension began, a slight accumulation was apparent immediately behind the radicle tip. Exogenously applied radioactive ABA is metabolized in lettuce fruits to form an unidentified substance which has chromatographic properties different to previously described ABA metabolites. When ABA is present in the bathing solution, the quantity of metabolite never significantly exceeds that of radioactive ABA in the tissues. Transference to water of fruits imbibed in ABA results in a rapid loss of ABA from the tissues, although no leaching of metabolite occurs. Germination begins shortly after transference and may relate to the lowering of ABA levels within the tissues by either leaching or metabolism. ABA has been shown to occur endogenously in lettuce fruit tissues wars. Great Lakes and Arctic King by techniques which include circular dichroism, gas liquid chromatography and mass spectrometry. The level of ABA in tissues of Great Lakes has also been shown to fall dramatically up to the time when radicle emergence occurs; the fall cannot be explained in terms only of leaching.