Photoassimilates Transport and Distribution in Sugar Beet Taproots affected by Biotic Stress and Shoot Manipulations

Abstract

<strong>Background and Motivation:</strong> Exogenous factors such as diseases, herbivory damage and mechanical manipulation may disrupt photoassimilates transport and distribution between source and sink organs in plants, possibly affecting the economic value of the plants. A current economically relevant example is the syndrome "basses richesses" (SBR) disease, which is prevalent in sugar beet and already spreads to other crops. The thesis investigates how photoassimilates transport and allocation dynamics in sugar beet taproot are altered spatially and temporally by SBR, partial defoliation and partial shading; aiming to provide mechanistic understanding of how exogenous factors affect source-sink dynamics in sugar beet.<br/> <strong>Materials and Methods:</strong> In this thesis, a combination of two tomographic methods, magnetic resonance imaging (MRI) and positron emission tomography (PET), using ¹¹C as tracer was employed to non-invasively determine SBR effects on structural growth and photoassimilates distribution within the developing taproot over several weeks. Compartmental modeling for long-distance tracer transport in plants was used to characterize photoassimilates transport properties in sugar beet taproots. In follow up experiments, the effect of shoot manipulations like partial defoliation and partial shading on the distribution of recently fixed photoassimilates was investigated.<br/> <strong>Results:</strong> MRI analysis revealed a deformed cross-sectional anatomical structure, a reduction in taproot volume and width of inner cambium ring structures upon SBR disease. PET analysis revealed a heterogeneous distribution of labeled photoassimilates for diseased plants, but transport velocities were not affected by SBR disease. However further analysis of tracer transport properties revealed that transport velocities vary locally inside the taproot and over the developing sugar beet taproot. Partial defoliation had no distinct effects on recently fixed photoassimilates distribution homogeneity. On the contrary, partial shading induced a sequential redistribution of recently fixed photoassimilates, towards attaining homogeneity in the distribution of photoassimilates resources within sugar beet taproot.<br/> <strong>Conclusions:</strong> The findings indicate that healthy sugar beet taproot redistributes photoassimilates when parts of connecting shoot vasculature is disturbed. But when the vasculature of the taproot is altered by a biotic stressor (as in the case of SBR), photoassimilates distribution becomes limited at the infected sectors, thereby affecting sink capacity and the development of taproot sink organ.