Root exudation pattern of sugar beet (Beta vulgaris L.) as influenced by light intensity and P deficiency

Sugar beet is known as a P uptake efficient plant species due to its ability for increasing P availability in soil. This is done by changing rhizosphere chemistry through exudation of e.g. citric, malic, oxalic, citramalic and/or salicylic acids. All of these acids are able to improve P availability, but unfortunately, exuded amounts are far too low to explain the P efficiency of sugar beet quantitatively. The aim of this PhD study was to analyze root exudates of P-deficient sugar beet plants for compounds which haven't been in focus, yet, and which might take part in chemical mobilization of soil P. Furthermore, most of previous published experiments have been performed in growth chambers with relative low light intensity and might not be representative for field conditions. Hence, a second topic was to evaluate the impact of light intensity on root exudation pattern of sugar beet under P deficiency. Sugar beet plants were grown in hydroponic culture in green house with two P levels (high and low) and with either high light intensity (without shading) or low light intensity (with shading). Root exudates were collected by dipping method at different plant growth stage. The results showed a critical role of light intensity on both quantity and quality of root exudates. The root exudation rate was increased by enhancing light intensity, regardless of P supply. Moreover, the profile of root exudates was significantly changed by light intensity alteration. It is suggested that in the studies concerning the function of root exudates in plants physiology, a special attention should be given to the light conditions. The major objective of the work was the identification of organic acids different from the common ones (e.g. citric, malic or oxalic acid) in sugar beet root exudates which may be responsible for mobilizing P. Root exudation rate was roughly 4-times higher under low P compared to high P supply. To get a comprehensive knowledge of the composition of root exudates, a full-scan (non-targeted) metabolic profiling based on HPLC-ESI-MS was used in this study. Root exudates collected from high and low P conditions were compared and signals that meet following criteria were screened (i) the signal was not detected in water control; (ii) the signal collected present in at least 5 of 6 replicates of root exudate samples; (iii) signal intensity must be at least 5 folds higher with low P supply plants compared to well-supply; (iv) standard deviation (SD) should be lower than 100% of corresponding mean value. After data processing, 69 signals were selected for further investigations. Among these signals, 16 putative metabolites were achieved from databases depending on their functional groups, i.e. carboxyl. Seven of putative metabolites were tested by co-elution root exudates with pure standards, however, none of them has been definitively confirmed to date. Malonic acid (C3H4O4) corresponded to m/z 103; fumaric acid (C4H4O4), maleic acid (C4H4O4) and choline (C5H14NO) corresponded to m/z 115; succinic acid (C4H6O4) corresponded to m/z 117; 4-aminobutanoic acid (GABA; C4H9NO2) and 3-aminobutanoic acid (BABA; C4H9NO2) corresponded to m/z 104 were shown to be involved in plant physiological processes and pure standard compounds could be used for following confirmation by co-elution with root exudates. Other signals were so far omitted from further investigations because they are not mentioned in KEGG or other databases, they play unknown role in plant physiology or they cannot achieved as a pure compound for further investigations. In addition, the amount of collected root exudates was not ample for structural elucidation by nuclear magnetic resonance spectroscopy (NMR- spectroscopy).