The hormonal control of stomatal aperture in Commelina communis L. by abscisic acid

• Main Author: Weyers, Jonathan D. B.
• Published: University of Glasgow 1978
• Subjects: 571.1; QK Botany
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.476935

The experiments of this thesis were concerned with the involvement of abscisic acid (ABA) in the control of stomatal closure following water stress. The hypothesis that this control is exerted in a truly hormonal manner was introduced by considering the evidence implicating wilt-induced ABA in certain post-stress stomatal responses in relation to present concepts of phytohormone action. The results are presented in seven sections, each dealing with a specific aspect of the investigation. Section 1 was concerned with the development and characterisation of experimental systems whereby the effects of ABA on stomata of isolated leaf epidermis segments of Commelina communis could be studied. In Section 2, 14C-labelled ABA was presented to epidermis tissue floating on buffer solution. The subsequent uptake of radioactivity was assayed quantitatively and qualitatively using the techniques of scintillation spectrometry, TLC, and soluble-compound microautoradiography. Section 3 involved the use of 86Rb+ as a tracer for K+, the major cation involved in stomatal movements. 86Rb+ was taken up by tissue into the guard cell region, and both the pattern of this uptake and the rate of subsequent efflux was found to be affected by ABA treatment. The fourth section described the development of a whole-leaf incubation system which was then used to examine the effects and movement of 14C-ABA supplied via the transpiration stream. Autoradiographic evidence indicating guard-cell uptake of the honnone corroborated the conclusions of Section 2. In addition, it was possible to obtain 'upper limit estimates' of the sensitivity of Commelina stomata to ABA utilising the ability of this plant to yield epidermal peels. Thus, it was found that significant stomatal closure could be effected by as little as amol ABA per stomatal complex. Complete closure (within 40 min) apparently required over 230 amol per complex. Section 5 provided a measure of the ability of Commelina leaf tissue to synthesise ABA during a short-term (0-5 h) water stress. Levels of the hormone estimated by GLC-ECD rose from 31 to 178 ng per gramme fresh weight after 3 h incubation of tissue which had lost 10% of its fresh weight. Investigation of optimal ABA purification procedures was also carried out. An attempt was made in Section 6 to examine the ultrastructure of Commelina epidermis. Plasmodesmatal junctions between cells were observed in all cell walls except those between guard cells and subsidiary cells. The relevance of this finding in relation to intercellular transport routes and mechanisms was discussed. The seventh and final section described the transport of radioactivity from ABA in the phloem sap of Ricinus communis; Double-label techniques were used to study co-transport of ABA and sucrose. The hormone appeared to be readily translocated. GLG-MS was used to identify free ABA in Ricinus phloem sap. In the General Discussion the results were considered in relation to the concept of the hormonal nature of wilt-induced ABA action, and areas of research deserving of further attention were identified.