Root-to-shoot communication in Ricinus communis L. plants subjected to drying a part of the root system

This thesis examines the role of root-sourced abscisic acid in the regulation of stomatal closure and leaf expansion in response to drying approximately half of the roots of Ricinus communis L. plants. Drying part of the root system of Ricinus communis promoted stomatal closure and slowed leaf expansion in the absence of any disturbances in shoot water relations, implying the involvement of chemical rather than hydraulic signalling. Initially root-sourced ABA was believed to be responsible for these responses. Delivery rates (concentration x flow rate) of ABA out of the drying roots were calculated which took into account the effect of dilution on solutes in the well-watered and droughted plants due to different transpiration rates in these plants. The delivery term was further modified to account for the differences in sizes of their roots and shoots. ABA delivery out of the roots of plants with drying upper roots increased within the first 12 h and was maintained over the next 3 days.
However, significant decline in stomatal apertures and leaf elongation occurred only 2 - 3 days after root drying began. During the early stages of drying upper roots (2-3 d) xylem sap pH, and delivery rate of nitrate and l-aminocyclopropane-l-carboxylic acid were little changed, while hydraulic conductivity of the root system as a whole was reduced approximately 25%, and ABA accumulation (synthesis?) in roots increased. Increased ABA levels in phloem sap was not found, suggesting no enhanced re-cycling of ABA between shoots and roots was taking place in the plants during this time. Antitranspirant activity in xylem sap of droughted plants that was not ABA was sought as a possible cause of stomatal closure. However, convincing evidence of such activity was not found. Examination of ABA output by roots into shoot compared to that entering lamina of the 5th leaf in the canopy showed the attenuation of the signal as transpiration fluid moved up the plant. These observations indicate that ABA from roots is unlikely to be a highly active signal eliciting shoot responses to mild drought in Ricinus communis.