Evaluating the pattern and composition of root exudation using improved sampling and analytical techniques

Background/Question/Methods

A mechanistic understanding of how plants interact with its edaphic environment is critical for the advancement of both plant and soil ecology as these processes define and decide how a sedentary life-form adapts to its ever-changing surroundings, and dominates in a wide range of habitats. Most of these plant-soil interactions are facilitated by metabolites released by plant roots and decomposing litter to the surrounding soil environment. The overall biological function of the exudation is regulated not only by the quantity, but also by their chemical composition and the transformations that these metabolites undergo in soil matrix. Also because of the varied bioactivity of individual metabolites, the overall physiological function of exudation could be driven by metabolites that are less abundant in exudates and/or those that exhibit pulsatile release. Hence the time of sampling and the sensitivity of sampling procedures are major determinants in understanding the chemical composition and biological/ecological functions of root exudates. Since plant metabolites could be subjected to a diurnal rhythm of light intensity, which can be linked to the diurnal regulation of photosynthetic carbon metabolism, I hypothesized that metabolites could exhibit a pulsatile exudation pattern. I tested the efficiency of various polymeric probes for the active sorption of ionic and organic compounds around the plant roots.

Results/Conclusions

Using modified sampling techniques and various chromatography–spectroscopy-mass spectrometry coupling systems, I was able to identify and quantify metabolites in the sub-picomolar range from the growing medium. Diurnal rhythms in exudation were less evident under growth-chamber condition with fluorescent lighting, mainly due to the lack of spectral variation resulting substantially lower production of exudates, and possibly due to the better stability of compounds under relatively axenic conditions.  Exudation/accumulation of phenolic compounds exhibited a possible diurnal rhythm with respect to natural light intensity with the highest concentration between 5-7 hours after exposure to direct sunlight, after which the concentration decreased partly due to the potential re-uptake of the compounds by the plants, and partly due to the degradation as evident from the detection of break-down products. Hypercrosslinked co-polymeric adsorbents exhibited a higher sorption-desorption efficiency of metabolites both from hydroponics and soil matrices compared to the various traditional polymeric adsorbents that followed relatively less robust equilibrium sorption.  Enhancement of exudate-capturing efficiency of the polymer probes by imparting cationic, anionic and hydrophobic moieties through photografting were further investigated.