Upregulating of organic acids in contrasting plant species results in moderation of high pH stress

Abstract

Development of vegetative cover on the surface of polluted waste deposits provides a physical protective layer and an input of organic compounds via root exudation. The monocot yorkshire fog (Holcus lanatus) and eudicot red clover (Trifolium pratense) are plant species that have been observed to grow successfully in alkaline waste, managed with phytostabilization. In this work the effects of pH-stress conditions (pH ∼10) on root exudation of both these species is studied in a hydroponic set up. Both species, when exposed to stress conditions, were able to neutralize the pH in the collection solution, with a reduction of ∼2 pH units. For both yorkshire fog and red clover, the net exudation had no statistical difference between non-stressed and pH-stressed conditions (263 vs 220 μg DOC (g-root)⁻¹(hrs)⁻¹ for yorkshire fog and 158 vs 118 μg DOC (g-root)⁻¹(hrs)⁻¹ for red clover). GC-MS analysis of the exudate solutions, however, shows a shift from a sugars and sugar alcohols-dominated exudate solution in non-stressed conditions, to an exudate solution with upregulated organic acids in pH stressed yorkshire fog and organic acids plus amino acids in red clover. These results show a similar stress response for these two species, contrary to the general assumption that grass species are less efficient than eudicots in producing organic acids in response to stress. The prevalence of organic acids in exudates under stress conditions, could favour metal and nutrient mobilization in growth substrates, though the lack of an increase in overall exudation limits the extent of their potential impact.